cementation has been well characterized the influence of pore-filling by other common ..... Grids were then overlain on the images using Adobe Illustrator TM.
FACTORS AFFECTING THE PRECIPITATION OF QUARTZ UNDER HYDROTHERMAL CONDITIONS
Chris Pepple
A Thesis Submitted to the Graduate College of Bowling Green State University in partial fulfillment of The requirements for the degree of MASTER OF SCIENCE August 2007
Committee: John Farver, Advisor Charles Onasch, Co-Advisor Kurt Panter
ii ABSTRACT John R. Farver, Advisor Charles M. Onasch, Co-Advisor
Natural Pocono Sandstone and synthetic quartz crystals, crushed and sieved (125250um), have been used to experimentally evaluate changes in the rate and nature of quartz cementation under hydrothermal conditions. Experiments were carried out from 1 hour to 5 weeks at temperatures of 300-600°C and at 150 MPa confining pressure to simulate cementation conditions analogous to a quartz reservoir at depth. Experimental charges consisted of AlCl3, amorphous silica, NaCl brine, and one of the quartz sample materials weld-sealed in a gold tube. Interactions of quartz systems with and without the addition of iron oxides (goethite) were conducted to determine the influence of iron oxides on cementation. After the experiments, samples were impregnated with epoxy and then analyzed using cathodoluminescence (CL), plain polarized light (PPL), cross polarized light (XPL), and scanning electron microscopy (SEM). Mosaic images were constructed for node counting of grains, cement, and porosity for synthetic quartz samples, and grains and porosity measures for Pocono Sandstone experiments. The mosaic images illustrate precipitation and dissolution occurred at sample bases and tops due to a saturation gradient formed by the interactions of AlCl3 and amorphous silica. Measured cement and porosity values of synthetic quartz samples were used to calculate precipitation and transport rates detailing differences between pure and goethite system experiments. Consistently, synthetic quartz goethite experiments showed greater cementation in response to increased silica solubility and goethite’s pervasive and adsorptive nature towards quartz. In addition, it is shown that significant amounts of quartz cementation
iii can occur due to saturation gradients between quartz and amorphous silica even in the absence of a temperature or pressure gradient.
iv AKNOWLEDGMENTS
Through out these last two years I’ve been given much support and encouragement from family, friends, and colleagues. I owe my advisor Dr. John Farver a great deal of thanks for his persistence, patience, and unfaltering support of myself and this project through times when direction and end seemed no where in sight. He provided the necessary assistance and encouragement for success of which I cannot thank enough. A great thanks also to Dr. Charlie Onasch whose guidance, persistence, and “what if” questions proved a great resource in shaping these experiments and this project which I am indebted to. Many thanks also to Dr. Kurt Panter for being on my committee and asking those fundamental and application questions that helped me keep perspective and direction on this project. Special thanks and recognition for financial support go out to Francis Furman, Katzner Bookstore Grant Committee, Geological Society of American Grant (8370-06), and to the Department of Geology. Additionally, thanks goes to Shaun Wallace for his assistance in the laboratory and photo editing. I also would like to thank my family and friends for their support, advice, and good times which helped me keep perspective. Thanks to my siblings Dave and Ellen for patience, and an open ear when I really needed it. Finally I would like to thank most my parents. Without their consistent encouragement and support, none of this would be possible.
v TABLE OF CONTENTS Page 1. INTRODUCTION………………….……………………………………………………..
1
1.1 Previous research………………………………………………………………..
2
1.2 Nature and rate of cement formation…………………………………………....
6
1.2.1 Silica dissolution………………………………………………………
7
1.2.2 Silica transport………………………………………………………...
7
1.2.3 Silica precipitation……………………………………………………..
9
1.2.4 Changes in porosity and permeability………………………………….
10
1.3 Questions to be answered………………………………………………………..
10
2. METHODS………………………………………………………………………………..
12
2.1. Starting materials………………………………………………………………..
12
2.1.1. Pocono sandstone…………………………………………………….
12
2.1.2. Synthetic quartz sandstone…………………………………………...
13
2.1.3. Iron oxides……………………………………………………………
13
2.2. Experimental methods……………………………………………………….....
14
2.2.1. Preparation of experimental charges …………………………………
14
2.2.2. Method of post experimental preparation…………………………….
15
2.3. Analytical methods………………………………………………………………
16
2.3.1. Cathodoluminescence imaging………………………………………..
16
2.3.2. Scanning electron microscopy………………………………………..
16
2.3.3. Thin section petrography …………………………………………….
17
2.3.4. Image processing……………………………………………………...
17
2.3.5 Rate calculations……………………………………………………….
18
vi 3. RESULTS………………………………………………………………………………….
19
3.1. Control samples………………………………………………………………….
19
3.2. 300° to 600° C Pocono sandstone experiments………………………………….
20
3.3. 300° and 450° C Non-goethite synthetic quartz experiments……………………
24
3.4. 450° C Variable experiments…………………………………………………….
29
3.5. 450° C Step down temperature experiments…………………………………….
31
3.6. 450° C Goethite experiments …………………………………………………… 32 3.7. Precipitation and transport rates ………………………………………………...
34
3.8. Base and top grain long axis measures in goethite and non-goethite exp………. 36 3.9 450° C Sandwich experiment…………………………………………………….
38
4. DISCUSION………………………………………………………………………………. 39 4.1. Effects of time and temperature on cementation ……………………………….. 39 4.2. Effect of AlCl3 on cement growth ………………………………………………
39
4.3. Evidence for cement within the Pocono sandstone ……………………………..
41
4.4. Evidence for sources of cement………………………………………………….
42
4.5. Amorphous silica alteration and trace elements incorporation………………….
43
4.6. Nature of goethite on silica mobility, solubility, and cementation……………...
45
4.7. Comparison of goethite vs. non-goethite experiments ………………………….
46
4.8. Rates of precipitation and transport……………………………………………..
48
4.9. Rate limiting and driving forces in goethite and non-goethite experiments…….
49
5. CONCLUSIONS…………………………………………………………………………..
54
6. REFERENCES………………………………………………………………...................
56
7. APPENDICIES…………………………………………………………………………… 62
vii LIST OF FIGURES Figure 1
Page CL image of CH-22 showing the Pocono Sandstone sample/amorphous silica powder interface…………………………………………………………….....
25
The change in percent cement over time for both synthetic quartz experiments with and without goethite shown by the pink line, and the goethite system experiments shown by the blue line………………………………..
27
Changes in porosity for both synthetic quartz experiments with and without goethite shown respectively by pink and blue lines……………………….
27
SEM photomicrographs of A Micropore in CH-27 (2 weeks at 450° C) illustrating pit dissolution of synthetic quartz grains near charge top………………
28
5
Charge CH-30 run four weeks at 450°C and 150MPa……………………………..
30
6
Synthetic quartz and SiO2 powder interface using PPL (a) and CL (b) of CH-35 showing Fe-oxides surrounding synthetic quartz grains (A), and infilling pore space (B)………………………………………………………....
33
The calculated precipitation rates, in mol/sec, for one, two, and four-week experiments for systems with (blue) and without (pink) the addition of goethite……………………………………………………………………………..
35
The calculated transport rates, in m/sec, for one, two, and four week experiments for systems with (blue) and without (pink) the addition of goethite……………………………………………………………………………..
35
Median long axis lengths of top and base grains in system experiments without goethite……………………………………………………………............
37
Median long axis lengths of top and base grains in experiments loaded with goethite………………………………………………………………………..
37
2
3
4
7
8
9
10
viii LIST OF TABLES Table 1
2
Page Results of % grains, cement, and porosity sorted by descending cement in synthetic quartz samples and ascending porosity in Pocono Sandstone samples…………………………………………………………………….............
21
Experimental charge run conditions and list of reagents added……………………
23
ix LIST OF APPENDICIES App. A
Page Illustration of Au-tube showing the location of amorphous silica, cementation, and synthetic quartz grains…………………………………………..
62
B (1) Synthetic quartz starting experiments CH-37 (A) and CH-38 (B) cathodoluminescence (CL) transect images……………………………………….
63
B (2) Cathodoluminescence (CL) transect of CH-41 Pocono Sandstone starting experiment…………………………………………………………………............
64
B (3) Cathodoluminescence (CL) transect of Pocono experiments CH-11 (A) and CH-21 (B) Pocono Sandstone experiments run at 300º C for 4 and 35 days respectively…………………………………………………………………………
65
B (4) Cathodoluminescence (CL) transect of Pocono experiments CH-15 (A) and CH-18 (B) Pocono Sandstone experiments run at 450º C for 7 and 28 days respectively…………………………………………………………………………
66
B (5) Cathodoluminescence (CL) transect of CH-14 (A), CH-20 (B), and CH-22 (C) Pocono Sandstone experiments run at 600º C for 7 days, 1 hour, and 7 days respectively……………………………………………………………………
67
B (6) Cathodoluminescence (CL) transect of CH-29 synthetic quartz experiment run at 300º C for 34.1 days…………………………………………………………
69
B (7) Cathodoluminescence (CL) transect under 5X (A) and 10X (B) of synthetic quartz experiment CH-26 run one week at 450º C………………………………...
70
B (8) Cathodoluminescence (CL) transect under 5X (A) and 10X (B) of synthetic quartz experiment CH-23 run two weeks at 450º C…………………………….....
71
B (9) Cathodoluminescence (CL) transect under 5X of synthetic quartz experiment CH-27, a duplicate experiment of CH-23, run two weeks at 450º C………………………………………………………………………………
72
B (10) Cathodoluminescence (CL) transect under 5X (A) and 10X (B) of synthetic quartz experiment CH-30 side cut run four weeks at 450º C………………………
73
B (11) Cathodoluminescence (CL) transect under 5X (A) and 10X (B) of synthetic quartz experiment CH-30 center cut run four weeks at 450º C……………............
74
B (12) Cathodoluminescence (CL) transect of CH-31 (A) and CH-42 (B) synthetic quartz experiments run for two weeks at 450º C…………………………………..
75
x B (13) Cathodoluminescence (CL) transect of CH-32 (A) and CH-33 (B) synthetic quartz and Pocono Sandstone temperature drop experiments run two days at 450º C and two days at 200º C…………………………………………………….
76
B (14) Cathodoluminescence (CL) transect under 5X (A) and 10X (B) of synthetic quartz experiment CH-44 run with goethite for one week at 450º C………..........
77
B (15) Cathodoluminescence (CL) transect under 5X (A) and 10X (B) of synthetic quartz experiment CH-34 run with goethite for two weeks at 450º C…………….
78
B (16) Cathodoluminescence (CL) transect under 5X (A) and 10X (B) of synthetic quartz experiment CH-35 run with goethite for two weeks at 450º C…………….
79
B (17) Cathodoluminescence (CL) transect under 5X (A) and 10X (B) of synthetic quartz experiment CH-45 run with goethite for two weeks at 450º C…………….
80
B (18) Scanned transect image under plain light of synthetic quartz sandwich experiment CH-39 run for two weeks at 450º C………………………………….
81
1 1. INTRODUCTION Knowledge of the nature and rate of quartz dissolution, transport, and precipitation is of fundamental importance in understanding a number of diverse processes including the evolution of porosity and permeability within veins and reservoir rocks. Differences in the rates of dissolution, transport, and precipitation are commonly attributed to changes in pressure, temperature, and/or fluid composition, which drive reactions and dictate rates. What is not clearly understood are the factors influencing changing in porosity, permeability, cementation, fluid pathways and connectivity in quartz-rich rocks under static temperature and pressure conditions. A better understanding of dissolution, transport, and precipitation (DTP) mechanisms has application in modeling development of quartz precipitation refining exploration and development strategies for more efficient hydrocarbon recovery. In addition, these investigations have application in determining fluid/rock interactions, providing models for ore body deposition, predicting the mechanical behavior and dominant mechanism of quartz-rich rock deformation, and evaluating rock retentiveness for isolation and confinement of nuclear and chemical waste. Quartz is one of the most abundant rock-forming minerals in the upper crust where it commonly occurs as monomineralic rocks (sandstone and quartzite) or as a major constituent in quartzo-feldspathic rocks. Quartz-rich rocks and veins constitute major reservoirs for hydrocarbon and ore deposits; as such, the value of understanding fluid migration and silica cement formation through and within quartz-rich rocks, particularly with respect to the evolution of porosity and permeability in these rocks, is obvious. In addition, the strengths of quartz-rich rocks and mechanisms of deformation in quartz-rich rocks, as a function of changing pressure and temperature (P/T) and fluid conditions, are important in understanding rock behavior as quartz cementation dictates fluid paths, rates, and presence of transport including dissolved
2 minerals and heat, and rock strength (Newton 1990, Ferry and Dipple 1991). Fuid migration and evolution at deep crust conditions is responsible for deformation mechanisms, faulting, and important in defining the brittle-ductile transition zone. Understanding quartz dissolution, transport, and precipitation processes in economically significant vein and faults systems has wide reaching applications for better understanding mineral reaction rates and fluid pathways. Fault dilation and motion is largely caused by pore fluid pressure exceeding rock strength resulting in the upward migration of hot crustal fluids to shallow depths. Quartz precipitation occurs in response to lowering pressures and temperatures as saturated fluids flow along fracture walls or fault breccias forming crack-seal textures as either a thin film or crystal growth bridging fault wall rock (Sibson et al. 1975, Sibson 1990). Precipitation of quartz within fault zones serves to illustrate DTP processes allowing temperatures and pressure of precipitation to be recorded by fluid inclusions formation providing a history of fluid chemistry and evolution through the fault history (Sibson 1987). These vein deposits serve two purposes; first to act as a proxy for crustal fluids at depth and their evolution as transport to shallow depths and lower temperatures occurs, and second to allow modeling of fault activity and conditions of precipitation and behavior in response to movement or dilation along the fault zone. The continued study of quartz vein formation is essential and depending on the degree of fault dilation may serve to further our understanding of quartz precipitation within microfractures of quartz grains at P/T conditions.
1.1. Previous Research Previous research on quartz cementation has focused on quartz-rich hydrocarbon reservoir rocks, characterizing digenetic conditions, silica sources (internal or external) (Worden
3 and Morad 2000) and solubilities of both quartz and amorphous silica at (P/T) conditions (Dove and Rimstidt 1994). However, DTP mechanisms are often studied at micron scales via healing of microfractures in single quartz crystals or sand grains. Crack healing studies involving fluid chemistry effects (Brantley and Voigt 1989) pressure solution deformation rates (Bas den Brok 1998) and fracture healing rates (Brantley et al. 1990) as well as synthetic fluid inclusion studies involving brine and CO2 systems (Schmidt et al. 1995), brine and CH4 systems (Lamb et al. 2002), and quartz growth in the presence of hydrocarbons along microfractures (Teinturier and Pironon 2004) characterize precipitation and cementation changes at sub-micron to micron scales along fractured quartz grains. Quartz cementation involved in microfracture healing occurs faster than quartz overgrowth formation as healing is driven by the minimization of free energy along newly formed narrow fractures, whereas overgrowth formation occurs on grains with low surface energies (Brantley et al. 1990). Microfracture formation in quartz grains occurs as stress exceeds grain strength. Grains accommodate this stress increase by fracture followed by infilling with quartz cement. Cementation of microfractures through either crack healing or crack sealing involves internal or external sources of silica to minimize boundary free energy and applied grain stress. Experimental crack healing rates obtained by Brantley et al. (1990) indicate cementation in microfractures measuring approximately 10 μm wide and 100 μm long from silica-saturated solutions along fracture walls within 4 hours at 600° C and 200 MPa. Single crystal quartz crack healing experiments by Brantley and Voigt (1989) showed the healing rate changes as a function of temperature and fluid chemistry, with faster microfracture healing in brine solutions and temperatures above 400º C. In addition, the results of that study showed the important role of
4 crack apertures and geometries where narrow apertures healed much faster than microfractures with large apertures. Studies of synthetic fluid inclusion formation along quartz microfractures involving CH4, CO2, and petroleum brine systems conducted by Lamb et al. (2002), Teinturier et al. (2003), and Teinturier and Pironon (2004) provide constraints on factors affecting quartz precipitation in the presence of hydrocarbons and carbon-rich fluids. The formation of methane-rich fluid inclusions along quartz microfractures (Lamb et al. 2002) as well as the formation of naturally occurring fluid inclusions in quartz with greater than 98% CH4 (Cook et al. 2006) suggest silica precipitation can occur within H2O-poor fluid systems. Hydrocarbon rich fluid inclusions generated by Teinturier and Pironon (2004) show that even with 100% oil, precipitation of silica occurred and petroleum inclusions were trapped. Their experiments show quartz precipitation and cementation can occur along quartz microfractures from non-aqueous or aqueoushydrocarbon mixed fluids. Behavior observed at grain surfaces and pore spaces by Worden et al. (1998), and Worden and Morad (2000) suggest non-armored or hydrocarbon wetted surfaces must be present in order for quartz cement growth to occur on grain boundaries. While these studies provide insight into microfracture healing and fluid inclusion formation, questions still remain about cementation in aggregate by quartz overgrowth formation. Differences in precipitation rates exist between microfractures and pore space as narrow crack aperture, geometry, and surface area have a large effect on silica mobility within microfractures. As Brantly and Voigt (1989) demonstrated these variable effects have a greater influence on crack healing than the presence of NaCl brine or elevated temperatures (up to 600° C). Microfracture and fluid inclusions studies, as noted above, show the nature and rate of quartz precipitation and cementation in microfractures is not necessarily analogous or applicable
5 to conditions resulting in large volumes of overgrowth cementation. Grain scale cementation is, however, known to be influenced by variables such as grain armoring by accessory minerals (Webber and Ricken 2005, Dewers and Ortolewa 1991, Pittman et al. 1992) or non-aqueous fluids (Worden and Morad 2000, Worden et al. 1998). Grain substrates play a primary role in predicting the extent of cement formation just as lithics and clays have been shown to hinder silica cementation compared to quartz substrates (Laubach et al. 2004). As feldspars and accessory silicate-rich minerals breakdown or alter to more stable forms during weathering they often provide a source of silica and newly altered minerals, such as clays, that may influence quartz cementation (Girard et al. 2001, Lacharpange et al. 1999, Weber and Ricken 2005). The occurrence of accessory minerals like clay along quartz grains are known to behave in a bimodal fashion by both facilitating quartz pressure solution where the clay minerals impinge upon quartz grain boundaries and allowing for precipitation of cement on partially armored grains. Quartz grains fully armored by clay have been inhibited from grain boundary cementation as a quartz substrate is unavailable for nucleation and precipitation (Rossi et al. 2002). While the influence of clay on quartz cementation has been well characterized the influence of pore-filling by other common accessory minerals like the iron oxides hematite and goethite, are not well known. Iron oxides, in particular goethite, are abundant and chemically-stable minerals that are easily adsorbed to themselves and other grain surfaces (Dzomback and Morel 1990, Schwertmann and Cornell 2000, Cornell and Schwertmann 2000). Numerous studies including synthesis experiments have characterized goethite metal adsorption (Xu and Axe 2005), pHdependent reactions and analytical identification (Scheidegger et al. 1993), tannic acid retention (Kaal et al. 2005), synthetic preparation and solubilities (Schwertmann and Cornell 2000, 2003),
6 and iron reduction in hydrocarbon contaminated quartz sands (Tuccillo et al. 1999). Experimental goethite-quartz interactions studied by Morris and Fletcher (1987), show increases in quartz grain dissolution and solubility. Their experiments used ferrous iron that oxidized to goethite forming rims around quartz grains, which in turn enhanced quartz dissolution. This demonstrated the sorptive nature of goethite for silica. Similar observations by O’Kane et al. (2007) indicate that the presence of goethite increases quartz solubility based on extensive quartz dissolution in the Tuscarora Sandstone along the Cove Fault, Pennsylvania. As a result of goethite’s low temperature stability and sorbent nature, grain surface coatings on quartz and soil particulates are difficult to remove (Schwertmann and Cornell 2000, Dzombak and Morel 1990). Goethite’s low solubility and small grain size suggest that goethite rims result not from precipitation, but rather adsorption onto quartz surfaces (Cornell and Schwertmann 2000, Dzombak and Morel 1990). The mobility of goethite is controlled by changing mechanical, chemical (Schwertmann and Cornell 2003), and biological (Styriakova et al. 2003) conditions. In studies noted above, these goethite interactions occur at low P/T conditions with quartz sediments exposed to meteoric waters which do not represent goethitequartz interactions at depth. While the interactions of goethite with quartz and other Fe-oxides during diagenesis have been extensively studied, its behavior is still not completely understood. Because of its potential effects on the evolution of porosity in reservoir rocks, further study is warranted.
1.2. Nature and Rate of Cement Formation The nature and rate of silica cement formation is dictated by silica dissolution, transportation, and precipitation, any of which can be rate-limiting. Several variables, including pH,
7 temperature, pressure, and site locations, play integral roles in determining which of the three processes (dissolution, transportation, precipitation) is rate-limiting for a particular setting. Each of these processes will be discussed in greater detail below. 1.2.1. Silica Dissolution Quartz dissolution is driven by disequilibrium conditions including; silica-undersaturated of fluids (Dove and Rimstidt 1994), minimized surface energy through pressure solution and water film diffusion (Renard et al. 1999, Weyl 1959, and Rutter 1976), and increased stress on grain contacts or fracture walls (O’Kane et al. 2007, Farver and Yund 2000). Grain dissolution is enhanced by increases in temperature and pressure. When the temperatures in quartz systems exceed 300ºC, changes in quartz rheology arising from water become chemically, rather than mechanically, driven (Dove and Rimstidt 1994, O’Kane 2005). Increases in temperature and pressure during diagenesis also accelerate reaction kinetics by increasing solubilities of quartz and amorphous silica providing a greater amount of soluble silica available for quartz cementation (Dove and Rimstidt 1994). The presence of NaCl brines at temperatures exceeding 300ºC increases the silica solubility through manipulation of water molecules amplifying hydrogen bonding, thus resulting in further quartz dissolution (Brantley and Voigt 1989, Dove and Rimstidt 1994, Fournier and Potter III 1982, Xie and Walther 1993). Dissolution does differ; however, as the solubility of amorphous silica is an order of magnitude greater than quartz which is the result of the additional lattice bound water (Dove and Rimstidt 1994). 1.2.2. Silica Transport Transport of dissolved silica occurs though advective and diffusional processes caused by temperature, pressure, and silica activity gradients, which may lead to wide-reaching cementation of quartz-rich reservoirs (Worden and Morad 2000). Investigations into
8 mechanisms of silica transport as discussed by Farver and Yund (2000) and Brantley et al. (1990) indicate that water is the critical transport agent for movement of silica whether by diffusional or advective means through primary porosity. According to Dove and Rimstidt (1994), Fournier and Potter III (1982), and Manning (1994) increases in solution pH, temperature, and pressure result in increased solubilities of quartz which potentially allows for greater transport (i.e. flux) of material. Transport via large-scale fluid flow (advection) follows fault zones or fractures as well as stratigraphic pathways, but this type of flow is highly channelized and restricted to narrow zones (Lonergan et al. 1999, Deming et al. 1990, Ramsey and Onasch 1999). Pervasive transport along grain boundaries or microfractures is necessary to reach the interior of un-fractured blocks and individual grains. Grain-scale fluid pathways are driven by pressure gradients include primary porosity, microfractures in the form of fluid inclusion planes (FIPs), microveins or cataclastic bands, grain-grain and transgranular dissolution surfaces, and grain boundaries. Most grain boundaries and many microcracks may be too narrow for significant volumes of fluid flow even if there is fluid along them. Furthermore, transport along microcracks is limited as they tend to heal rapidly especially when an aqueous phase is present (Brantley et al., 1990; Hickman and Evans, 1987, Brantley and Voigt 1989). This suggests that large interconnected fluid pathways are necessary to attain pervasive cementation in large quartz-rich reservoirs. It is evident however, that during diagenesis fluid pathways and transportation rates will evolve as silica precipitates from solution along microfractures and as grain overgrowth changes reservoir porosity and permeability. This may cause fluid transport to be rate-limiting.
9 1.2.3. Silica Precipitation Silica precipitation is driven by fluid over saturation caused by decreasing temperature and pressure, increasing pH, changing fluid chemistry, or by increasing precipitation site availability (Dove and Rimstidt 1994). The extent and rate of precipitation along quartz grain boundaries is influenced by the volume and rate (i.e. flux) of advected or diffused silica from internal or external dissolution sources to precipitation sites (Barclay and Worden, 2000). Furthermore, grain wettability within quartz-rich rocks influences transport and site precipitation on grain surfaces. However, Worden et al. (1998) noted the presence of wetting fluids, like petroleum, can poison or inhibit precipitation on grain boundaries. Silica precipitated within reservoir rocks as overgrowths and fracture fill can be derived from internal (Barclay and Worden 2000) or external (Weber and Ricken 2005) sources, including the weathering of feldspars (Girard et al. 2001, Lacharpange et al. 1999, Weber and Ricken 2005), and conversion of smectite to illite (Lynch et al. 1997, Towe 1962, Hower et al. 1976, Boles and Franks 1979). However, the nature, rate and timing (episodic or gradual), of quartz precipitation are not well understood (Worden and Morad 2000). Despite extensive work on characterizing paragenesis and conditions, few experimental precipitation studies in aggregates have been done. Changes in quartz overgrowth CL color indicate precipitation may be gradual or episodic depending upon fluid source, composition, and changes in other variables (Barclay and Worden 2000, Laubach et al. 2004, Girard et al. 2001). Extensive precipitation results in very low porosity and permeability, which not only affects transport, but also the number or density of sites available for quartz precipitation to occur. As precipitation of silica increases it becomes the rate-limiting step in quartz cementation.
10 1.2.4. Changes in Porosity and Permeability The evolution of porosity and permeability in quartz-rich rocks is dictated by dissolution, transport, and the precipitation of silica (i.e., quartz-cementation). In porous rocks, transport is related to permeability, fluid pathways, porosity, and in particular, to the pore geometry (size and shape) and connectivity (e.g., Bernabe 1991; Fredrich et al., 1993). The evolution of pore geometry, grain fracturing, and fluid connectivity during compaction is an important parameter defining changes in permeability and the strength of rocks (Lemee and Gueguen 1996, Chester et al. 2007). Increasing compaction results in porosity loss through grain crushing and rearrangement, affecting pore size and connectivity (Fawad et al. 2002, Farver and Yund 2000, Chester et al. 2007). The healing of microfractures has a direct impact on reducing fluid connectivity within quartz-rich rocks. The sealing of the fracture network through cementation leads to a reduction in the volume of fluid that can be transported along fractures, which affects silica transport and cementation (Worden and Morad, 2000). Additional cementation infilling of pore spaces and fracture networks reduces overall porosity and permeability (Worden and Morad 2000). As porosity decreases, transport can become the rate-limiting step in quartz cementation.
1.3. Questions to be Answered Despite extensive characterization of quartz cement within sandstone reservoirs, the timing, sources, physical, and chemical interactions that occur during cementation are not yet clearly understood (Worden and Morad 2000). A necessary step forward lies in characterizing cementation along grain boundaries in controlled experiments designed to simulate pressure and temperature conditions of diagenesis and cement formation. A study of porosity loss due to silica precipitation from saturated fluids within microfractures and along grain boundaries has
11 greater application in understanding silica dissolution, transport, and precipitation mechanisms involved in reservoir porosity loss. In addition, much research has focused on interactions of clay and feldspar minerals as well as CO2, CH4, and petroleum rich fluids as they are abundant and commonly associated with quartz-rich reservoirs; however, Fe-oxides such as goethite and hematite, while abundant, have not been studied as extensively. Investigations into what, if any, significance Fe-oxides (goethite in particular) have within these quartz-rich hydrocarbon reservoirs is necessary, as few experimental studies pertaining to aggregate characterization has created a need for further understanding of their mechanical and chemical interactive nature with quartz. This leaves two primary questions to be addressed. Are pressure or temperature gradients as well as advection necessary for precipitation of silica and quartz overgrowth formation, and what influence goes goethite have on quartz precipitation and cement development?
12 2. METHODS 2.1. Starting Materials This study focused on two sample types: a natural quartz-rich sandstone and a synthetic sandstone. The Mississippian Pocono Sandstone was selected because it represents a natural sample of a typical sandstone reservoir rock. However, as noted below, the impurities and mixed lithologic heterogeneity of quartz grains greatly complicated quantitative analysis of cement formation during the experiments based on CL images. The second sample type consisted of crushed clasts of synthetic quartz representing sandstone. This sample type was selected to provide a chemically and mineralogically clean monomineralic end member with an initial homogeneous grain size, porosity, and texture. Detailed descriptions of these materials follow. 2.1.1. Pocono Sandstone The natural material was selected from samples of the Pocono, Tuscarora, and Berea Sandstones on the basis of porosity, friability, and degree of contamination from accessory lithics, clays, and iron oxides. Thin section analysis of the Tuscarora showed low porosities, limited fluid pathways, and original quartz overgrowths under CL, thereby limiting available space for transport and precipitation of silica, although accessory minerals were minimal. The Berea Sandstone has large pore spaces with discontinuous grain boundary connections, allowing for rapid transport and sufficient space for silica to precipitate; however, due to high clays and iron oxides content, as well as sample friability, it was rejected. The Pocono sandstone is competent with low friability and has large connected pore spaces showing fewer accessory mineral contaminants than Berea. Petrographic analysis of lower Mississippian Pocono Sandstone conducted by Swales (1988) revealed the sandstone as being of high porosity, moderate to poorly sorted unit containing accessory lithics, feldspars, illite, kaolinite, and
13 dominantly chlorite micas. The Pocono has been used as a proxy for petroleum reservoirs yet limited hydrocarbons have been found (Swales 1988). Because of these properties the Pocono Sandstone was selected for this study. 2.1.2. Synthetic Quartz Sandstone The material for synthetic sandstone is comprised of uncemented 125-250 μm sized fragments of synthetic quartz. The synthetic quartz was grown hydrothermally and is high purity thus ensuring minimal particulate and trace element contamination. The synthetic quartz crystal was crushed , ultrasonically cleaned in ethanol, rinsed in distilled deionized water (DDW), subjected to magnetic separation, and mechanically inspected under a binocular microscope removing contaminants (magnetite, feldspar, organic material, and natural quartz contamination from sieves) prior to charge loading. While use of synthetic quartz as a natural quartz analog may not be widely practiced, it is an acceptable analog to high purity natural quartz like Brazil, readily available, and easily prepared. The similar solubilities between natural and synthetic quartz, along with the ability to maintain system purity, should yield similar cementation rates for natural quartz systems as a function of time, temperature, and mineral interactions. 2.1.3. Iron Oxides Understanding iron oxide behavior within quartz-rich rocks is important, as its abundance, solubility, and sorbent nature may influence reservoir cementation rates (Cornell and Schwertmann 2000, 2003). Iron oxide in the form of goethite was added to a series of experiments using the synthetic quartz. Goethite was selected because of its abundance and widespread occurrence in sediments and quartz-rich rocks. Goethite for these experiments was harvested from goethite mineralization found within the Tuscarora Sandstone in the Cove fault
14 zone of southeast Pennsylvania (O’Kane, 2005). Quartz grains were mechanically separated from goethite deposits before samples were crushed using a mortar and pestle. Goethite used for experiments was identified through X-Ray Diffraction (XRD) analysis.
2.2. Experimental Methods 2.2.1. Preparation of Experimental Charges Experiments used to mimic natural conditions of quartz cementation in samples were carried out using cold-seal reaction vessels and prepared using adapted methods outlined by Lamb et al. (1996) and Frantz et al. (1989). Pocono Sandstone samples were cut into bricks approximately 3mm x 3mm x 10mm using a thin-blade wafer saw, the sharp edges were polished off and the pieces were ultrasonically cleaned in ethanol, and then dried in a one atmosphere oven at 95°C for at least one hour. A typical experimental charge, as illustrated in Appendix A, consists of sequential loading of approximately 3.0 mg AlCl3 (for synthetic quartz experiments) or 5.0-20.0 mg Al2O3 (for Pocono Sandstone experiments) for use as a CL tracer, 25.0 mg amorphous silica powder used as a source of soluble silica for cementation, 25.0 mg NaCl brine (25 wt% NaCl) to increase quartz solubility, and 100.0-130.0 mg of either Pocono Sandstone or synthetic quartz depending upon how much space remained for crimping and welding of the Autube. These materials were then tamped down using a steel rod, and the Au-tubes were weldsealed. The contents and order of loading the charges varied when testing for the influence of different components such as AlCl3 and amorphous silica where these components were not added to Au-tubes. The Fe-oxide experiments were prepared by adding goethite amongst preweighted synthetic quartz grains, mixing both minerals mechanically on a watch glass with acetone, and drying at room temperature causing goethite and synthetic quartz to weakly adhere.
15 This weakly consolidated material was then added to a Au-tube pre-loaded with standard amounts of AlCl3, amorphous silica, and NaCl brine solution. Null experiments were conducted at room temperature, 27° C, to evaluate the effect of pressurization on compaction and grain crushing. Gold tubing with approximately 5 mm diameter, 20-25 mm lengths, and 0.2 mm wall thicknesses were used for sample containment during pressure and temperature experiments. Tubes were consistently loaded vertically into cold seal reaction vessels with the amorphous silica powder end of the experiment positioned at the base of the reaction vessel. Charges were weighed, put into a 95° C drying oven for at least 30 minutes, and re-weighed to test the weld. The Gold tubes were placed in cold-seal reaction vessel then inserted into a pre-heated furnace at temperatures between 300-600º C and pressures of 100-150 MPa with reporting errors for temperature of less than 5° C. Experimental run times ranged from 1 hour to 5 weeks to determine volume of cement formation, rate of precipitation and transport, and what variables enhance or diminish cementation rates and volumes. 2.2.2. Method of Post-Experimental Preparation After the experiment, the charge was removed from the cold-seal vessel and washed to evaluate whether if it had leaked. In addition, Au tubes were cut using a razor blade and the presence of fluid confirmed by visual release of fluid and by weight loss upon heating in a 95° C drying oven. The sample was then removed from the Au tube and vacuum impregnated with epoxy containing a blue dye. Once cured, samples were sliced using a thin bladed wafer saw into approximately 1 mm-thick sections, polished, mounted onto glass slides, and ground down to 30-60 μm thickness using diamond lap wheels of 400, 600, and 1000 grit. Samples were then used for thin section petrography, CL microscopy, and SEM analysis to characterize their
16 textures and to assess crack healing and cementation rates following conventional methods (Brantley et al., 1990; Hart 2006). 2.3. Analytical Methods 2.3.1. Cathodoluminescence Imaging Cold cathodoluminescence microscopy was used to characterize cement, crack healing textures, and porosity infill in aggregate samples. The Al2O3 and AlCl3 powders were added to the experimental charges to provide an Al tracer that could be observed in CL. A Technosyn cathode luminescence system on a Nikon microscope was used to collect grain-scale images of cementation within the aggregate sample, and healing textures not readily observed in plane or cross-polarized light. Variations in color and/or intensity of luminescence of grains and quartz cement as healed microfractures and overgrowth is attributed to trace element and bond structure defects within samples (Laubach et al. 2003; Pagel et al. 2000). Synthetic quartz aggregate appears pink to red in color with newly formed cement containing a blue to purple luminescent color. It is notable however that subtle differences in luminescence of amorphous silica powder, epoxy, synthetic quartz grains, and newly formed cement occurred within the same sample at separate times, which is attributed to variation in CL gun current and image exposure times. 2.3.2. Scanning Electron Microscopy A Hitachi scanning electron microscope (SEM) was used to characterize mineral, cement, and healed microfractures to confirm or compliment data gathered in CL. Welch and Banfield (2001) outline sample preparation for use of the SEM to analyze grain textures begins by rinsing the sample with distilled deionized water and placing it on carbon tape. Silver paint is then applied to the samples coating the surface. Samples placed in SEM are imaged using 1.0 μm to
17 200 μm scales along grain boundaries, and within pore spaces to determine growth and mineral species. 2.3.3. Thin Section Petrography Petrographic analysis was conducted on the Pocono Sandstone and synthetic quartz aggregates using plane polarized light (PPL) and cross polarized light (CPL) to determine quartz and accessory minerals. Images of areas identified as cement using CL were confirmed using CPL and PPL, and display undulatory extinction with small fluid inclusions observable in PPL. Because syntaxial cement cannot be differentiated from the host grain, petrographic analysis must be used in conjunction with CL. 2.3.4. Image Processing Microsoft Office Picture Manager TM was used to process and edit CL, petrographic, and SEM photomicrographs. Adjustments were made to brightness, contrast and image color saturation in order to enhance the contrast between new cement, porosity, and the original material. A mosaic of the resulting images was then created in Panaview software, creating a profile of samples from the base silica powder end to the opposite top end (see Appendix A of representative mosaic). Grids were then overlain on the images using Adobe Illustrator TM software for point-counting. For the Pocono Sandstone images only grain size and porosity were measured, while grain size, porosity and cement were measured on images of synthetic quartz samples. From these measurements, two-dimensional profiles were created showing the variation in porosity from one end of the capsule to the other. Point counting of the first 50 columns of the grid were taken, the grid was translated to an area adjacent to the original counts and recounting was done using a 200-400% viewing window ensuring 400 points counted for statistical significance. Criteria used for point count data include 40 and 60 pixel grid system for
18 low and high magnification images. Omitted point count data included non luminescent grains below surficial epoxy, yellow, blue, or red orbs resulting from embedded grit during the finishing process, plucking of grains during the finishing process, and any node falling within the amorphous silica. Feldspars, iron oxides, clays, micas, or magnetite were categorized as grain material in Pocono Sandstone samples. Cement is counted as blue, purple, pink, or red luminescent material occurring as overgrowth or infill material along synthetic quartz grain boundaries. Calculated uncertainties were determined by counting the same sample twice then comparing the difference in measured percentages. 2.3.5. Rate Calculations The rate of quartz precipitation and transport in the synthetic sandstone samples was determined using the calculated percentage of cement from point count data, how far it infiltrated the sample, and the duration of the experiment. These calculations require two assumptions; cement formation is consistent in the third dimension, and the maximum extent of cement represents the total transport distance. Sample volumes were determined by measuring the field of view lengths and widths which represents the sample size. Length measures used the total distance represented by the 50 columns used in node counting, which are consistent for every sample. Cement percentages were converted to mass in grams, then moles to determine the total amount of material transported. Assuming constant rates, total calculated moles for a sample were then divided over experiment duration or distance to determine rate of precipitation or transport. This method is consistent with rate calculations by Ganor et al. 2005.
19 3. RESULTS Experiments involving both synthetic quartz and the Pocono Sandstone showed differences in cementation, dissolution, and silica precipitation and transport rates as a function of time, temperature, pressure, and fluid composition. Detailed below are descriptions of each of the experiments along with characterization of the resulting sample.
3.1. Control Samples Experimental charges CH-37 and 38 (Appendix B1 (A) and (B)) were run on synthetic quartz samples to determine the initial luminescence of the samples prior to heating, and to evaluate the amount of compaction and fracture formation induced when the sample was tamped down during loading the charge (CH-38) and during the initial pressurization of the sample (CH37). Changes in porosity and grain size distribution during the hydrostatic cold compaction in CH-37 were used as a baseline for comparison with subsequent experiments run at higher temperatures and for longer times. Appendix B shows CL images for all the experimental samples. Notice, grain boundaries in sample CH-37 show no evidence for dissolution and contrast sharply with the encasing epoxy (Appendix B1). The synthetic quartz grains in CL have a distinct dark red to pink luminescence, however there are anomalous grains having dark purple luminescence attributed to applied stress, inducing differences in CL rather than compositional, as the starting material is homogenous, and stress was applied to synthetic quartz during sample preparation. The amorphous silica powder in sample CH-37 and 38, which served as the source of silica, has a 0.01-0.07 mm grain size with light blue luminescence, contrasting to a moderatedark blue CL color observed in charges taken to temperature at 300° to 600° C. Measured grain size and percentage of cement and porosity for all samples are shown in Table 1. The initial
20 porosity for the experimental samples is approximately 26.6% based on the porosity of the cold pressurized sample CH-37. The starting grain size and porosity of the Pocono Sandstone samples were determined using sample CH-41 (Appendix B2) which was not exposed to experimental P/T conditions. Sample CH-41 is poorly sorted having sub rounded grains showing sutured grain boundaries marking localized areas of grain dissolution. Quartz grains are mono and polycrystalline connected by quartz cements and matrix material identified in CPL; however, polycrystalline quartz is virtually indistinguishable from monocrystalene in CL. Accessory minerals feldspar, kaolinite, magnetite, muscovite, and calcite have respective CL luminescent colors of light bluebright white, blue, orange, opaque, and red to green. Point count calculations yield 20.6% porosity for the Pocono starting material, which is similar to sandstone reservoir porosities reported by Weber and Ricken (2005) and Rossi et al. (2002). Pocono Sandstone measure of error for porosity was determined by the measured differenced between two separate point counting data sets for porosity of CH-41. This method was also applied to determine the measure of error for percent cement amongst synthetic quartz samples using CH-30.
3.2. 300° to 600° C Pocono Sandstone Experiments Samples of Pocono Sandstone were run at 150 MPa confining pressure and temperatures of 300°, 450°, and 600° C to determine the effects of temperature on quartz cementation and microfracture healing. Charges included CL tracers Al2O3, TiO2, and AlCl3 in order to distinguish new (i.e. precipitated during the experiment) verses preexisting cement. However, the observed similar CL colors within the Pocono Sandstone did not allow a clear identification of new cement. Measured differences in porosity amongst Pocono samples run at different
21 Table 1. Results of % grains, cement, and porosity sorted by descending cement in synthetic quartz samples and ascending porosity in Pocono Sandstone samples. % Grain
% Cement
% Porosity
Time
Temp C
Image Scale
CH 45*
72.8%
13.3%
13.9%
4 Weeks
450
High Mag 10 X
CH 30 (Center)
77.7%
11.9%
10.5%
4 Weeks
450
High Mag 10 X
CH 30 (Side)
72.9%
9.8%
17.3%
4 Weeks
450
High Mag 10 X
CH 35
65.4%
8.3%
26.3%
2 Weeks
450
High Mag 10 X
CH 23
80.3%
6.4%
13.3%
2 Weeks
450
High Mag 10 X
CH 34
72.9%
6.2%
20.9%
2 Weeks
450
High Mag 10 X
(Fe-Oxide in Pwd)
CH 44
68.7%
4.6%
26.7%
1 Weeks
450
High Mag 10 X
(Fe-Oxide in Pwd and Qtz)
CH 29
81.4%
3.2%
15.4%
5 Weeks
300
High Mag 10 X
CH 26
71.1%
2.5%
26.4%
1 Weeks
450
High Mag 10 X
CH 32
74.4%
1.8%
23.7%
2-2 Days
400-200
High Mag 10 X
(Quench)
CH 42
72.1%
1.4%
26.5%
2 Weeks
450
High Mag 10 X
(No AlCl3)
CH 31**
74.1%
0.2%
25.8%
2 Weeks
450
Low Mag 5 X
CH 37 (Control)
73.4%
0.0%
26.6%
0
27
High Mag 10 X
Syn Qtz
* Experimental charge lost seal upon quench and de-pressureization ** Sample damage prohibited accurate recount at higher magnification ***Sample contained natural vug included in point count measures
Notes
(Fe-Oxide in Pwd and Qtz)
(Fe-Oxide in Pwd and Qtz)
(No SiO2 Pwd) Control (Hydrostatic Crush)
22 temperatures and durations indicate there was new cement formation and therefore the changes in porosity are used as a proxy for cement growth. Samples CH-11 and CH-21 are 4-day and 35 day experiments, respectively, conducted at 300° C, and have measured porosities of 21.7% and 17.5% respectively and both contain evidence of grain dissolution in the form of corroded grain boundaries. In addition, sample CH-21 (Appendix B3 (B)) appears to have locally developed dark blue luminescent overgrowth cement, whereas sample CH-11 (Appendix B3 (A)) has no cement. The porosity measured for CH-21 is 3.1% less than the starting material (sample CH41) which exceeds the calculated uncertainty of 0.5% for the Pocono Sandstone experiments. The relatively low silica solubility at 300° C required unreasonably long run durations to yield observable cementation. Hence, experiments were run at 450° C yielding higher quartz solubilities as a function of temperature promoting measurable amounts of silica precipitation and cementation in reasonable time. Samples CH-15 and CH-18 (Appendix B4 (A) and (B)) run at 450° C and 150 MPa for 7 and 28 days, (Table 2), have 21.0% and 22.7% porosity, respectively, indicating a 1.7% increase in CH-18 porosity as time progressed. Sample CH-18 (Appendix B4 (B)) shows dark blue cement overgrowths, and crack healing throughout the sample, whereas sample CH-15 (Appendix B4 (A)) has only sporadic dark blue overgrowths. Adding increased volumes of amorphous silica powder and Al2O3 to CH-18 (Table 2) was done in an attempt to increase availability of silica and CL tracer incorporated into new cement growth; however, little differences were seen between CH-15 and 18. With no significant differences in cement, additional experiments were run at 600° C to determine if higher temperatures would result in increased cement formation or reduction in sample porosity. Samples CH-14 and CH-20, were run at 600° C and 150 MPa for 7 days and 1 hour, respectively, to force silica precipitation through longer run conditions at elevated temperatures
23 Table 2. Experimental charge run conditions and list of reagents added.
Sample
Temp: (C)
Press: (Mpa)
CH-11 CH-14 CH-15 CH-18 CH-20 CH-21
300 600 450 450 600 300 600 / 550 / 500 / 450 / 400 / 350 / 300 450 450 0 450 450 450 300 450 450 450 / 200 450 / 200 450 450 450 0 0 450 450 0 450 450 450
100 150 150 150 150 150 150 / 137 / 125 / 112 / 100 / 87 / 75 150 150 150 150 150 150 150 150 150 150 / 50 150 / 50 150 150 150 150 0 150 150 0 150 150 150
CH-22 CH-23 CH-24 CH-25 CH-26 CH-27 CH-28 CH-29 CH-30 CH-31 CH-32 CH-33 CH-34 CH-35 CH-36 CH-37 (Control) CH-38 (Control) CH-39 CH-40 CH 41 (Control) CH-42 CH-44 CH-45
SiO2
4 Days 7 Days 7Days 28 Days 1Hr 35 Days
9.1 mg 14.0 mg 14.2 mg 24.0 mg 24.5 mg 19.6 mg
11.7 mg 6.5 mg 5.5 mg 24.4 mg 20.3 mg 20.4 mg
28.7 mg 25.4 mg 31.6 mg 24.3 mg 24.8 mg 28.7 mg
29.6 mg 21.9 mg 20.8 mg 25.9 mg 27.2 mg 24.7 mg 24.3 mg 26.8 mg 26.8 mg
20.8 mg
23.9 mg 17.7 mg 16.5 mg 20.6 mg 22.8 mg 21.9 mg 20.5 mg 26.0 mg 23.3 mg 26.4 mg 28.9 mg 23.6 mg 21.9 mg 26.4 mg 20.4 mg
1/1/1/1/1/ 1 / 1 Day 14 Days 7 Days 2 Hrs 7 Days 14 Days 14 Days 34.1 Days 28 Days 14 Days 2D / 2D 2D / 2D 14 Days 14 Days 14 Days 35 min 0 Days 14 Days 14 Days 0 Days 14 Days 7 Days 28 Days
29.2 mg 23.7 mg 22.7 mg 23.2 mg 21.0 mg 27.9 mg 20.1 mg 53.4 mg 152.2 mg 25.4 mg 20.7 mg 22.4 mg
FeOOH
Al2O3
TiO2
NaCl 25 wt%
Time:
2.3 mg
6.5 mg 17.7 mg 7.2 mg
17.1 mg 17.1 mg
AlCl3
Pocono Sandstone
Syn Qtz
127.0 mg 119.7 mg 111.6 mg 120.0 mg 127.3 mg 112.6 mg
150.7 mg 3.3 mg
130.4 mg 117.3 mg 123.5 mg 110.7 mg 126.4 mg 134.0 mg 135.1 mg 129.0 mg 132.3 mg 139.5 mg
3.5 mg 3.5 mg 4.2 mg
40.5 mg 20.4 mg
3.0 mg 3.2 mg 3.1 mg 3.5 mg 2.8 mg 3.0 mg 3.5 mg 3.1 mg 3.6 mg 2.2 mg 5.1 mg 3.1 mg
26.5 mg 20.9 mg 26.3 mg
3.2 mg 3.8 mg
138.5 mg 112.8 mg 104.5 mg 111.3 mg 111.1 mg 105.0 mg 260.5 mg
130.4 mg 97.2 mg 100.4 mg
24 and greater temperature changes upon sample quench. Samples CH-14 and CH-20 (Appendix B5 (A) and (B)) have porosities of 30.0%, and 24.7%, respectively, which represents a 4.1 and 9.4% increase compared to starting material. (CH-41). Greater porosity in CH-14 is attributed to a naturally occurring vug 0.3 mm in width above the base of the sample observed when the sample was cut open after the experiment. Dissolution is observed in both samples, and is most evident towards sample tops. Sample CH-22, a 7-day step-wise drop experiment (Appendix B5 (C)), experienced (50° C) temperature and (12-13 MPa) pressure drops every day starting from 600° C and 150 MPa to 300° C and 75 MPa. Multiple short drops in P/T conditions were done to mimic episodic faulting and vein formation to encourage where precipitation and incremental cement growth occur in response to drops in fluid pressure. Synthetic quartz grains at the base of CH-22 in (Figure 1) show dark blue cement as the dominant overgrowth material up to 0.04 mm thick along grains up to 1 mm away from the powder/sample interface, and grain dissolution is not as apparent as in sample CH-14. The porosity for sample CH-22 is 27.0% which is 6.4% above starting material values and outside error of measure 0.5%.
3.3. 300° and 450° C Non-goethite Synthetic Quartz Experiments Due to the heterogeneity of the natural Pocono Sandstone material and the inability to distinguish new cement from pre-existing cement using CL analysis, the bulk of the experiments were done using crushed grains of synthetic quartz. In addition, the source of the Al tracer was changed from Al2O3 to AlCl3 powder. The AlCl3 proved a much greater (nearly infinite) solubility than the Al2O3 under the experimental conditions selected providing a much greater amount of Al+3 for incorporation into the cement. The AlCl3 powder also yields an aqueous fluid
25
a c
d b
Figure 1. CL image of CH-22 showing the Pocono Sandstone sample/amorphous silica powder interface. a. Dark blue luminescent material is new cement growth. b. Blue luminescent material is altered amorphous silica at base of Au tube. c. Dark red-brown material is iron oxides leached from sample. d. Al2O3 powder appears grey-white in CL.
26 that has a much stronger ionic strength that the NaCl brine alone which increases the solubility of silica in the fluid during the experiments. Sample CH-29 run at 300° C and 150 MPa for 5 weeks (Appendix B6) shows a zone of cementation approximately 0.4 mm thick, 0.8 mm above the powder/sample interface. The boundaries of all grain sizes outside the cemented zone show smooth or straight edges with little to no evidence of grain dissolution. The percent cement and porosity are calculated to be 3.2% and 15.4%, respectively, which fall above and below 1.2% the margin of error values indicating porosity loss is significant in comparison to sample CH-37, the starting material. Cement formation within sample CH-29, indicated by light pink luminescence, is found along grain boundaries and infilling microfractures, but is not as extensive as the values measured for the 450° C experiments. Sample CH-26, CH-23, CH-27, and CH-30, the sides and center, (Appendix B7, B8, B9, B10 and B11) represent synthetic quartz experiments run for one, two, and four weeks, respectively, at 450° C and 150 MPa. These samples show a consistent increase in percent cement and a decrease in porosity over time as shown in Figures 2 and 3. The majority of the cement appears as bright blue to dark blue-purple in CL; however, localized areas show crack healing cement as pink-red luminescence with synthetic quartz grains appearing red to bright pink. These four samples show dissolution at the top end with a zone of cementation extending approximately 0.3-0.4 mm above the powder/sample interface. SEM imaging of CH-27, a duplicate of experiment CH-23, in Figure 4, shows open pore spaces at sample base and top ends with pit dissolution and micron sized terminated growth crystals lining pore walls, supporting observed growth and dissolution in CL. For one to two-week experiments the silica powder has a dark purple, to opaque luminescence, but becomes a brighter blue CL luminescence in fourweek experiments. In samples CH-23 and CH-30, the luminescence of cement infilling
27
20.0% C H-45
15.0% % Cement
C H-35
10.0%
C H-44
C H-30
5.0%
C H-23
0.0%
C H-26
-5.0% 0
0.5
1
1.5
M e a s ure o f Erro r = 1.2%
2 2.5 Duration (Weeks)
3
3.5
4
4.5
Figure 2. The change in percent cement over time for both synthetic quartz experiments with and without goethite shown by the pink line, and the goethite system experiments shown by the blue line. Corresponding sample numbers are given in boxes. The margin of error for both systems is 1.2%
40.0% 35.0% C H-44
% Porosity
30.0%
C H-35
25.0% C H-26
20.0% C H-45
15.0% C H-23
10.0%
C H-30
5.0% 0.0% 0 M e a s ure o f Erro r = 1.2%
0.5
1
1.5
2
2.5
3
3.5
4
Duration (Weeks)
Figure 3. Changes in porosity for both synthetic quartz experiments with and without goethite shown respectively by pink and blue lines. Initial porosity at time-zero is taken from the hydrostatic crushing experiment CH-37 for comparison.
4.5
28
a
b Figure 4. SEM photomicrographs of a. micropore in CH-27 (2 weeks at 450° C) illustrating pit dissolution of synthetic quartz grains near charge top. b. Terminated quartz crystals found along the pore walls of a sample base micro pore, up to 5 μm in size, indicating cement growth.
29 microfractures and pore space changes from bright blue to dark purple towards the top of the charge over a 0.2 mm interval. Grain dissolution is confined to approximately the top 1/3rd of the sample resulting in porosity increases further from the sample/powder interface (Appendix B11)). Images gathered from central and side cross sections of CH-30 (Appendix B10) show differences in both percent cement and porosity indicating differences in dissolution and cementation within samples. Porosity in the sample center and sides measures 10.5% and 17.3%, respectively, indicating a 6.8% decrease in volume moving from the outer side margins towards the sample center. The cement in the center and sides is 11.9% and 9.8%, respectively, observed as dense concentrations occurring 0.9 mm up from sample/powder interfaces in both side and center cuts diminishing over 0.6 mm to the sample top where little to no cement is found (Appendix B11 and B10). Changes in cement CL colors at the base of sample CH-30 in (Figure 5) illustrates the transition from light blue to deep purple luminescence cement further up from charge base. Grain dissolution in CH-30 center cut is prevalent within the top 1/3rd of the charge: however no dissolution of cement is observed surrounding base grains (Figure 5). Little to no dissolution is apparent in CH-30 side cut sample top, which is supported by observations of fine synthetic quartz fragments from hydrostatic crushing.
3.4. 450° C Variable Experiments To evaluate the role of the amorphous silica powder and AlCl3, on silica cementation, two samples (CH-31 and 42) were run at 450°C, 150MPa over two weeks either without the AlCl3 (CH-42) or without the silica powder (CH-31). Sample CH-31 (Appendix B12 (A)) shows
30
a
Figure 5. Charge CH-30 run four weeks at 450°C and 150MPa. (a) shows cementation indicated by blue-purple luminescence occurring along grain boundaries and in filling microfractures. The bottom of the photomicrograph is at the sample/powder interface as cement extends upward into the sample showing preservation of fine synthetic quartz fragments lacking dissolution surfaces.
grain boundaries with straight or smooth edges where little dissolution has occurred. The cement in healed fractures and in shattered zones has a pink-red CL color. Note areas of sample CH-31 (Appendix B12 (A)) that lack luminescence or are out of focus represent grain plucking during sample preparation, and the bright blue and yellow luminescent colors represent halite from the brine and diamond fragments embedded from sample preparation, respectively.
Sample CH-42
(Appendix B12 (B)) has a thin zone of cementation at the sample/powder interface representing most of the 1.4% cement measured for this sample. This cement volume is 5% less than a comparable two-week experiment (CH-23) illustrating the influence of AlCl3 on cementation. The cement appears to have white-pink-red CL colors making it difficult to discern from
31 reflections of internal grain fractures or grain surface planes at depth; therefore, it is understood some recognition of cement is arguable. Notably different from all other samples is the pink CL color of the amorphous silica powder at the base of the sample, and lack of dissolution features throughout the entire sample. As with the margin of CH-30 the presence of fine synthetic quartz fragments, compared to original grain size, are found throughout sample CH-42 which would not be expected if dissolution occurred.
3.5. 450° C Step-Down Temperature Experiments Experiments CH-26, CH-23, and CH-30, which were run at 450°C and 150 MPa for one, two, and four weeks, respectively, showed consistent zones of cementation at the sample base above the synthetic quartz and amorphous silica powder interface. Synthetic quartz step-down experiment CH-32 (Appendix B13 (A)) and Pocono Sandstone CH-33 (Appendix B13(B)) were conducted to determine if similar cement volumes would precipitate at reduced time with rapid drops in temperature. Experiments were drop from 450° to 200° C after two-day equilibration, and two-day growth time yielding 1.8% cement for CH-32, measuring 0.7% less than one week synthetic quartz experiment CH-26’s 2.5%, illustrating little effect on the percent cement generated. The Pocono step down experiment CH-33 shows no new measurable cement growth, and the sample porosity is 1.0% less than the sample CH-41. Cement within the synthetic quartz occurs as fracture infill and limited grain overgrowth over a 1.0 mm region 0.6 mm above the powder/sample interface. Both CH-32 and 33 show evidence of dissolution; however, within the synthetic quartz system, dissolution appears restricted to the top half of the sample, while in the Pocono Sandstone grain dissolution is distributed throughout, although not on all grains.
32 3.6. 450° C Goethite Experiments Experiments CH-44, CH-34, CH-35, and CH-45 were run at 450°C and 150 MPa for one, two and four weeks, respectively, to determine the mobility and effect of iron oxides on grain dissolution, silica precipitation, and cementation in quartz-rich systems. Sample CH-44 (Appendix B14), a one week experiment, shows large grains with microfracture infilling and partial overgrowth at the base just above blue granular to colloidal amorphous silica and dissolution of grains at the top of the sample. The porosity is similar to that measured for the starting material (sample CH-37) and the one week synthetic quartz system experiment CH-26 as shown in Table 1. However, the 4.6% cement in CH-44 is notably greater than the 2.5% in CH26. Two week experiments CH-34 and CH-35 (Appendix B15 and B16) show cementation at the base up to 1.6 mm from the sample/powder interface as overgrowth and microfracture infill and dissolution of grains at the top with porosities. The porosities of 20.9% and 26.3%, respectively, are much higher than the sample run without goethite under similar conditions (CH23). Samples CH-34 and CH-35 differ in the manners in which the goethite was added to the charge. In sample CH-34 the goethite was isolated to the amorphous silica powder, whereas in CH-35, it was loaded in both amorphous silica and amongst synthetic quartz grains. Cement in the two samples differs. Sample CH-34 is 6.2%, similar to that in goethite free experiment CH23, whereas CH-35 has 8.3%. Figure 6 shows CL and PPL images to illustrate the proximity and relation of goethite to the formation of overgrowths, as goethite is largely contrasted to synthetic quartz in PL, and quartz overgrowths are easily discerned in CL. Thick goethite rims at the base of CH-35 (Fig. 6) indicates cement formation occurred in the presence of goethite; however, cement occurs at different thicknesses depending upon the extent and thickness of the goethite surrounding the synthetic quartz grains. Areas of very thick goethite rims show thinner quartz
33
A B
C
a
A B
b Figure 6. Synthetic quartz and SiO2 powder interface using PPL (a) and CL (b) of CH-35 showing Fe-oxides surrounding synthetic quartz grains (A), and infilling pore space (B). Fe-Oxides appear dark brown to black in contrast to the clear synthetic quartz grains shown in PPL. (C). The tan to brown color observed in PPL image (a) represents the burnt epoxy surrounding synthetic quartz and goethite grains.
34 overgrowths compared to areas of little to no observable goethite, which appear to show thicker quartz overgrowths. In addition, grains under PL show goethite dusted rims by new overgrowth of quartz. Amorphous silica powder with intermixed goethite crystals in both CH-34 and CH-35 has a dark purple-red CL. Sample CH-45 run at 450 C and 150 MPa for four weeks (Appendix B17) has 13.3% cement as pervasive overgrowth and infill cement around large grains at the base 2.2 mm from the sample powder interface. This is 1.4% greater than the goethite-free system equivalent, sample CH-30; however, the porosity is 13.9%, which is a 3.4% increase compared to 10.5% measured in CH-30 (Table 1). Amorphous silica at the base of the sample shows the same purple-red CL color and a colloidal texture similar to that observed in the two week long goethite experiments. Fe-oxide experiments loaded with goethite mixed with the amorphous silica powder and synthetic quartz clasts consistently yielded a greater cement volume compared to equivalent time and temperature experiments run without goethite as shown in Figure 2. A porosity comparison of the experiments with and without goethite is shown in Figure 3, where goethite experiments have equivalent or higher porosities than pure system equivalents.
3.7. Precipitation and Transport Rates Differences in precipitation rates between pure and goethite system experiments are apparent over one, two, and four week experiments. Figure 7 shows goethite system experiments at consistent rates of 2.77 x 10-8 and 2.76 x 10-8 mol/hr for one and two-week experiments, which then drop to 1.93 x 10-8 mol/hr for four-week experiments. Goethite free experiments show precipitation climbs 1.44 x 10-8 to 1.86 x 10-8 mol/hr from one to two weeks then drops to 1.72 x 10-8 mol/hr. Transport rates for systems loaded with and without goethite
35
Precipitation Rate (mol/sec)
8.00E-12 Go e thite S ys te m
7.00E-12 6.00E-12 5.00E-12
No n Go e thite S ys te m
4.00E-12 3.00E-12 2.00E-12 1.00E-12 0.00E+00 0
0.5
1
1.5
Unc e rta nty = 0.9677E -12
2 2.5 Duration (Weeks)
3
3.5
4
4.5
Figure 7. The calculated precipitation rates, in mol/sec, for one, two, and four-week experiments for systems with (blue) and without (pink) the addition of goethite. Boxes represent single experiment values.
6.00E-09 Transport Rates (m/sec)
No n Go e thite S ys te m
5.00E-09 4.00E-09 Go e thite S ys te m
3.00E-09 2.00E-09 1.00E-09 0.00E+00 0
Unc e rta nty = 0.0
0.5
1
1.5
2 2.5 Duration (Weeks)
3
3.5
4
4.5
Figure 8. The calculated transport rates, in m/sec, for one, two, and four week experiments for systems with (blue) and without (pink) the addition of goethite. Boxes represent single experiment values.
36 illustrate initial differences in rate, which appear to converge after two weeks and are then similar (Fig. 8). Goethite free experiments show transport rates for one week experiment at 1.85 x 10-3 cm/hr which sharply drop to 5.74 x 10-4 and 3.41 x 10-4 cm/hr for two and four-week experiments. This behavior is mimicked in goethite systems as transport rates 1.24 x 10-3 cm/hr for the one week experiment quickly drops to 6.10 x 10-4 and 3.84 x 10-4 cm/hr for the two and four week experiments. Figure 8 shows this initial difference; however, rates and slopes for two and four-week experiments are virtually identical and decrease for both systems.
3.8. Base and Top Grain Long Axis Measures in Goethite and Non-goethite Experiments Observable differences in the size of grains at the base and top of samples between experiments with and without goethite prompted long axis grain measurements within a given charge. Comparisons of median long axis length for areas of base and top grains suggested differences existed; however, confirmation could not be achieve though standard deviation measures or mean averages, as their values were respectively too great and too varied. The median values were chosen to support these observable differences as outlying values would not skew this data. Histograms were constructed to verify normal distribution of the data to support use of median values. Figures 9 and 10 illustrate the differences in median long axis grain measure between system experiments with and without goethite. In both systems, base grains are longer axis, and top grains appear smaller. The differences between base and top grains in both systems appears to diverge from the starting standards to one week’s time. From one to four weeks time, the lengths of base and top grains in each system become less different and measured values trend towards convergence or to differences similar to the initial starting material CH-37.
37
160 140
Base Grains
Long Axis (μm)
120 100 80 Top Grains
60 40 20 0 0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
Duration (Weeks)
Figure 9. Median long axis lengths of top and base grains in system experiments without goethite. .
200 180 Base Grains
Long Axis (μm)
160 140 120 100
Top Grains
80 60 40 20 0 0
0.5
1
1.5
2
2.5
3
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4
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Figure 10. Median long axis lengths of top and base grains in experiments loaded with goethite.
4.5
38 3.9 450° C Sandwich Experiment Experiment CH-39 run at 450° C and 150 MPa for two weeks was conducted to determine the means by which silica is transported from the amorphous silica powder within experiments. Sample CH-39 (Appendix B18) shows a scanned plain light image of the amorphous silica powder at the sample base, and “sandwiched” half the distance from the base between equal amounts of synthetic quartz grains. Adjacent to the amorphous silica at the bottom and mid-section of the sample are zones of cementation that appear white, as the encasing blue dyed epoxy did not surround those cemented grains during vacuum impregnation. This shows cementation occurring both above and below the amorphous silica where soluble silica was transported. The three zones of cementation within this sample appear to be of equal amounts and distances formed away from the amorphous silica.
39 4. DISCUSSION 4.1. Effects of Time and Temperature on Cementation The effect of time on the amount of cementation is progressive and can be seen by comparing samples CH-26, CH-23, and CH-30 all run at similar conditions (450° C, 150 MPa) for one, two, and four weeks, respectively. The cement percentages are listed in Table 1. The blue luminescent cement formed along the grain boundaries occurs as both microfracture infill and overgrowth and increases linearly from 2.5% in one week to 11.9% in four weeks with a reproducibility of
+
1.2%. This confirms that with longer time durations, the amount of cement
will increase; however, differences in temperature, 300° C verses 450° C, also show there is a temperature dependency for cement growth. Sample CH-29 (five weeks at 300° C and 150 MPa) yielded much less cement than CH30 (4.3% vs 11.9) which was run for four weeks at 450° C. This is attributed to temperaturedriven solubility differences, and reduced reaction kinetics at lower temperatures. In addition, cement around base grains appears different within CH-29 as it does not luminescence blue as does the cement in experiments run at 450°C. Grains within CH-29 show little to no evidence for dissolution supporting that lower temperature experiments have slower reaction rates which aid in preserving original grain boundaries. These differences will be discussed in greater detail below.
4.2. Effect of AlCl3 on Cement Growth The AlCl3 was added to the synthetic quartz experiments to provide a CL tracer and also to increase silica solubility for grain growth. Charge CH-42 loaded without AlCl3 shows 1.5 % cement formed at the sample base indicating the amount of cementation is 4x greater with
40 addition of AlCl3. In samples that did not contain AlCl3, run under similar temperatures and durations, the cement luminesced white-pink. Additionally, the cement was found at the sample/powder interface, or a very short distance from the amorphous silica powder, which is a much shorter distance compared to experiments loaded with AlCl3. Silica powder in CH-42 luminesces pink; however, when viewed under cross polarized light, the silica powder clearly shows grains with extinction angles which have not been observed in any experimental charge with AlCl3. Amorphous silica powder after experimental run conditions, with the addition of AlCl3, show non-birefringent grains or material with no extinction angles under cross polarized light suggesting the amorphous structure is maintained. Sample CH-40, an amorphous silica powder sample run two weeks at 450°C and 150 MPa, was analyzed using an x-ray diffractometer (XRD) and determined that amorphous silica maintains it’s structure after twoweek experiments, which would also explain the lack of extinction under cross polarized light. Increased grain dissolution at sample tops and concentrated cementation at sample bases of the 450° C experiments containing AlCl3 occurs due to increase silica solubility, and differences in saturation at either end of the experimental charges. Synthetic quartz experiments CH-30, CH-23, and CH-26 loaded without goethite, (Appendix B7, B8, and B11) each show this base cement and top dissolution suggesting soluble material from sample tops is transported towards the base, resulting in increased cement formation in addition to transported and precipitated silica from the silica powder at the base of the sample. Despite increased silica solubility due to the addition of AlCl3 and amorphous silica at the base of the sample suggest that solubility remains the rate-limiting factor. To attain this persistent dissolution suggests that brine fluid surrounding the top grains is marginally undersaturated in silica as it is transported and precipitated at the base. If precipitation were rate-limiting, the amount of dissolution at the top
41 would largely be reduced as saturation of silica would be persistent though out the charge. Cement formation would also be reduced, and may not be limited to high concentrations at the sample base. High transport rates explain silica undersaturation and extensive dissolution of synthetic quartz grains at charge tops suggesting direction of transport from top to base and along microfractures, indicating it is not rate-limiting.
4.3. Evidence for cement within the Pocono Sandstone The amount of cement precipitated within the Pocono Sandstones cannot be determined with a high degree of confidence as all cement in the samples had the same CL color as that in the control sample CH-41 (Appendix B (2)). Suspected new cement found at the powder sample interface does have the CL colors expected; however, it is not extensive and uniformly distributed throughout. As a result, we must rely on the porosity to serve as a proxy for new cement formation within the Pocono Sandstone, although there could be redistribution of preexisting cement that would not be reflected in porosity. Measured porosities (Table 1) in experimental charges indicate cement formation. Both CH-21, a five week experiment at 300°C, and CH-33, 2 days at 450°C then 2 days at 200°C experiment, have porosities less than the control porosity of 20.6 % from CH-41. CL observations support newly-formed cement, which is generally darker blue luminescent, is found in these two experiments compared to starting material. The majority of the Pocono samples, however, have a greater amount of porosity in comparison to starting material which is partially attributed to grain dissolution, but more so, to sample heterogeneity. Notably, samples run with goethite and at 600°C temperatures, have higher porosities than the starting material, which may be attributed to greater dissolution due to increased
42 temperatures or the presence of goethite, which is suggested by O’Kane et al. (2007), and Morris and Fletcher (1987) to increase the solubility of quartz resulting in increased synthetic quartz grain dissolution. It is known from dissolving precipitate formed along the Au-tube after puncture that silica is not lost through fluid release, so if extensive dissolution has occurred it would be found as precipitated cement within the sample. It is suspected that if dissolution of large volumes of silica occurred from the center of the sample, then perhaps the reason it is not found in CL images is due to distributed along the outer margins of the sample, which would cause this apparent increase in sample porosity. However observed sample margins show no apparent cementation, or differences in porosity which suggests large volumes of silica are not being dissolved. Differences in porosity are suspected to be the result of sample heterogeneity at these scales which would result in porosity differences influencing the measured values despite the formation of new cement.
4.4. Evidence for Sources of Cement Sources of cement within samples are both internal and external as illustrated by sample CH-31 and CH-39. Charge CH-31 was run for two weeks at 450°C without silica powder and shows only one location of cement formation, but does show microfractures healing as fractures close and silica bridges these boundaries. A dark red-pink luminescence of cement and healed microfractures in CH-31 suggests silica is derived internally from local dissolution of fractured synthetic quartz grains. The high surface energy of fractured grains would predict some dissolution and local redistribution of silica. Absence of the anticipated blue-purple CL colors in both samples is attributed to a lack of amorphous silica powder interacting with AlCl3 at 450° C as both CH-31 and CH-42 lack amorphous silica powder and AlCl3 respectively. The presence
43 of pink luminescent cement in sample CH-29 run with both amorphous silica powder and AlCl3 suggests that changes in solubility and luminescence are also a function of temperature, as this sample was run at 300° C rather than 450° C. Cement in experiments without goethite, CH-23, CH-26, and CH-30 occurs as blue-purple luminescent material and red-pink material depending upon the source. Soluble silica from the external powder infiltrates base synthetic quartz grains along grain boundaries precipitating as blue-purple cement while cementation and healing of microfractures in sample tops occurs as red-pink luminescence reflecting an internal source of silica. This suggests formation of cement comes from internal and external sources that transport silica in solution from the top towards the base and from the powder at the base into the sample as shown in sample CH-39 (Appendix B18). Sample CH-39, a sandwich experiment, was run by loading an Au-tube with two sequences of materials loaded atop each other, showing the equivalence of two separate samples into the same tube to determine direction and precipitation of transported silica from the powder. The image (Appendix B18) shows extensive cement formation in the lower synthetic quartz section as silica is migrating from base section of powder upward, and the midsection of powder downward. The upper synthetic quartz section shows silica from the mid-section powder migrating upwards in to the sample, with dissolution occurring at the top of the upper section of the synthetic quartz sample. This illustrates migration of silica into the synthetic quartz above and below the midsection powder, supporting a bi-directional transport of silica within the charge.
4.5. Amorphous Silica Alteration and Trace Elements Incorporation The general observations of silica powder including color, grain size and texture indicate alteration and consumption of reagent material changes over varied experimental conditions.
44 Initial luminescence of the amorphous silica prior to the experiments appears bright blue (Appendix B1); however, this changes under different P/T conditions. The amorphous silica powder in system experiments without goethite has a granular appearance after 1-2 weeks; however, alteration within the goethite system is rapid, showing colloidal amorphous silica powder within the one, two, and four-week experiments. This suggests alterations and solubilities of amorphous silica is increased in the presence of goethite. Within the Pocono Sandstone samples that utilized Al2O3 as the source of CL tracer, the silica powder is altered to a dark indiscernible material, except in 600° C experiments, such as CH-14, where alteration yields a dark purple-blue color of the silica powder in contrast to the Al2O3 which generally appears white. The behavior of AlCl3 is not the same as Al2O3, as salts and oxides differ in reactivity due to their solubilities in water. Experiments such as CH-29 run at lower temperatures (300°C) and CH-32 and CH-33 run for shorter time durations (4 days), show silica powder as granular deep purple-blue CL colors, indicating that alteration and solubility rates are inhibited with shorter time constraints and lower temperatures. Charge CH-42, which contained no aluminum salt or oxide tracer, shows bight pink CL amorphous silica, with colloidal textures similar to that seen in CH-30, CH-27, and CH-23, but lacking the deep blue CL observed in these. This suggests incorporation of Al+3 ion into amorphous silica and cement resulting in the CL colors observed for cement within the synthetic quartz and Pocono Sandstone experiment. Unique, however, are the textural differences, colloidal verses granular, observed in these different experiments without goethite, which indicate material changes from granular to colloidal at 450° C temperatures or above, as well as longer time durations in excess of 2 weeks for experimental systems without goethite, and 1 week for goethite system experiments. Faster alteration within the goethite experiments indicates increased solubilities of amorphous silica
45 which would account for accelerated alteration for the same time, pressure, and temperature conditions. In summary, the alteration of amorphous silica from granular to colloidal is independent of Al+3 tracer salt or oxide; however, elevated temperatures and longer time durations allow further incorporation of Al+3 ions during amorphous silica powder alteration which explains CL transitions from dark non-luminescent to lighter purple-blue luminescent colors.
4.6. Nature of Goethite on Silica Mobility, Solubility, and Cementation Experiments CH-34 and CH-21 (Appendix (B15) and (B3 )) show goethite both in and outside of the synthetic quartz and Pocono Sandstone samples, demonstrating migration of goethite in both the synthetic quartz systems as well as the Pocono Sandstone. The mobility of goethite is driven by system disequilibrium, as Fe-oxides in CH-34 loaded into amorphous silica powder were driven into the synthetic quartz pore spaces and along grain boundaries, and in CH21 Fe-oxides were driven out of the Pocono Sandstone into the amorphous silica, as a result of this disequilibrium. As O’Kane et al. (2007) and Morris and Fletcher (1987) noted, goethite increases silica solubility, which suggests that understanding of both mobility and solubility are critical for understanding temporal variations of quartz dissolution and precipitation within reservoir and vein formations. Mobile vein or reservoir fluids containing soluble or suspended particulate goethite may be responsible for extensive dissolution of quartz grains along fault boundaries, as well as the rapid precipitation elsewhere. Samples CH-44, CH-35, and CH-45 shown in Appendix (B14), (B16), and (B17), respectively, contain goethite throughout the entire sample to minimize goethite mobility, and illustrate saturation effects on quartz systems. Mean grain long axis lengths in Figure 10 show
46 changes in CH-44, CH-35, and CH-45 indicating a larger grain bias at sample bases compared to tops. Figures 9 and 10 show the reduced grain mean long axis lengths is a function of smaller grain sizes caused by the dissolution and crushing of grains within the upper core of the sample. Compared to experiments without goethite, grains from sample tops in goethite experiments show greater dissolution of synthetic quartz grains, which are supported by apparent increases in sample top porosity. Evidence for goethite enhanced dissolution of quartz is supportive of observations made by O’Kane et al. (2007), and Morris and Fletcher (1987). Inherently, increases in synthetic quartz solubility and dissolution are followed by precipitation at a local or distant location; however, while goethite increases synthetic quartz grain dissolution, it is also shown that thick concentrations along grain boundaries may act to inhibit quartz precipitation. Figure 6 of sample CH-35 shows partial coating of base grains by goethite resulting in a higher silica flux creating greater porosity while partially armored grains inhibits growth adjacent to cement overgrowths on the same grain. Other base grains show goethite dusted grain boundaries surrounded by overgrowth material. This suggests the concentration of goethite coatings along most synthetic quartz grain boundaries is permissive of grain growth, and does not act to poison boundaries. Localized inhibited precipitation is due to the mobile nature of goethite in the sample, as during sample preparation, goethite is loaded as a powder not adhered to the synthetic quartz grains. This may explain why this occurrence is not more widespread within samples loaded with goethite.
4.7. Comparison of Goethite vs. Non-goethite System Experiments Compared to non-goethite system experiments, goethite experiments show differences in porosity, cement, and grain size indicating goethite alters system kinetics under the same
47 experimental conditions. The amount of cement (Table 1) in experiments run for the same experimental temperatures, pressures, and durations, show that samples loaded with goethite have 1.4 to 2.1% greater cement than non-goethite system experiments. In addition, porosity is 1.4 to 13.6% higher for goethite experiments than those without goethite, indicating greater dissolution of material within upper cores of charges. Figures 9 and 10 show these differences between charge bases and tops between these two systems. One reason for these differences is cementing and annealing of smaller synthetic quartz grains from synthetic quartz derived silica forming apparently lager grains with indistinguishable primary grain boundaries. Angular synthetic quartz grains are more susceptible to dissolution or precipitation to minimize free energy available from open ended bonds and broken surfaces after grain crushing. Larger base grains observed in goethite systems may occur due to the increased synthetic quartz grain dissolution prompted by goethite; however, this would seem insignificant or unlikely as differences in amorphous verses synthetic quartz solubilities cannot account for dissolution and precipitation of silica derived dominantly from synthetic quartz grains that precipitates free of the Al+3 tracer. This would result in cement without blue luminescence; however, due to synthetic quartz solubilities being almost an order of magnitude less than amorphous silica, and the unlikely partitioning of Al+3 ion from cemented sites, this suggests grains annealing along connected surfaces accounts for the appearances of larger grain sizes found at sample bases (Dove and Rimstidt 1994). The absence of larger grains found in charge tops of both systems is due to grain crushing at charge tops and the extensive dissolution of smaller synthetic quartz grains which dominates up until 4 weeks as indicated by experiment CH-30 which first shows evidence of microfracture healing and formation of red-pink luminescent cement bridges. Dissolution is largely
48 responsible for the absence of fine synthetic quartz shards within the upper core which also explains increases in sample porosity towards sample tops. Comparison of both systems shows similar extents of grain dissolution; however, four week experiments CH-30 and CH-45 differ as CH-45 appears to have less dissolution of grains at sample top by comparison. This may result, however, from sample preservation biases, where loss of some top grains typically occurs when removing samples from the Au tube, before they are sealed in epoxy.
4.8. Rates of Precipitation and Transport Rates of silica precipitation, transport, and flux vary with time and goethite content in experimental charges. Precipitation rates (Figure 7) show generally higher values for goethite than non-goethite systems, which is attributed to greater measured cement volumes caused by increased silica saturations from the addition of goethite for similar durration non-goehtite system experiments. Goethite addition is believed to increase ionic strength, in turn increasing solubilities and overall silica saturations forcing greater volumes of precipitation. Notably these precipitation rates have decreased at week four and precipitation curves appear to move towards convergence. Reduced cement formation over longer times is attributed to this trend resulting from loss of available precipitation sites as porosity and fracture networks are cemented or healed. In addition, loss of pore-fluid network and fluid connectivity translates to reduced transport of amorphous silica through highly cemented sample bases to available precipitation sites at sample tops. Once total external amorphous silica has been transported and precipitated on synthetic quartz grains, precipitation rates are expected to diminish rapidly. As cementation of fluid networks continues precipitation and transport rates diminish.
49 Transport rates (Fig. 8) show different values at one week for both goethite and nongoethite systems where rate curves converge after two and four weeks time. Transport rates calculated by furthest observable distance over time show one week pure system of 1.85 x 10-3 cm/hr much larger than one week goethite 1.24 x 10-3 cm/hr indicating precipitation of silica from saturated conditions occurred much quicker and closer to the charge base thereby preventing transport of silica saturated fluids further up into synthetic quartz sample. Convergence of rate curves between these systems suggests goethite may initially influence transport rates; however, overtime its influence is diminished or negligible. Silica saturation gradients established by amorphous silica powder result in limited distances silica is transported; however, as equilibrium conditions are approached, the distance silica infiltrates and cements along synthetic quartz increases. Cementation largely influences transport as fluid pathways are reduced or obstructed by newly formed cement resulting in the diminished flow of material from source to precipitation sites.
4.9. Rate Limiting and Driving forces in Goethite and Non-goethite Experiments Examination of pure and goethite system experiments show consistent differences in the amount of cementation and dissolution over any given time interval. This prompts consideration of two important concepts that must be accounted for when determining rate-limiting steps. First, amorphous silica has almost an order of magnitude higher solubility than synthetic quartz (Dove and Rimstidt 1994): therefore, it would be the prevalent source of silica in solution. Second, goethite and non-goethite systems show differences in dissolution and precipitation from one end of the sample to the other, which brings into question whether the rate-limiting steps are uniform throughout the sample, and whether they vary over time. Closer inspection of the
50 bottom of sample CH-30 shows grains surrounded by blue luminescent cement with no apparent dissolution, preserving shards or fragments of synthetic quartz that otherwise would have undergone dissolution. This is important because it suggests the fluid near the base of the sample rapidly equilibrated with the amorphous silica powder producing a solution oversaturated in silica with respect to quartz. This isolated base region of the sample, approximately 1mm thick, shows extensive cementation, which decreases upward towards the top of the sample. This cement gradient indicates rapid silica precipitation from oversaturated solution due to higher amorphous silica solubilities. Precipitation along grain boundaries within this saturated region are expected from minimizing high energy surfaces generated during crushing through the sample preparation process. The rapid precipitation observed at base grains is responsible for under-saturated conditions found at sample tops, and while precipitation rates are shown to change for one, two and four week experiments, (Fig. 7), precipitation is still considered the fast reaction step. Silica transport observed in CH-26 show migration of blue-purple luminescent cement approximately 2mm into synthetic quartz samples after one week suggesting precipitation is fast and transport is not rate-limiting. Observations of healed microfractures in the one-week experiment CH-26 show blue luminescent cement transported very close to distances measured for one, two, and four-week experiments, suggesting that silica is precipitated out before reaching top grains. Extensive dissolution observed in top grains of all pure and goethite system experiments shows dissolution dominated and the only observable cementation is found as annealing of grains or microfractures with pink luminescent cement derived from top synthetic quartz grains. These observations coupled with high initial porosities suggests that transport of material was not rate-limiting, or fluid saturation conditions would have existed preventing
51 extensive synthetic quartz grain dissolution at the top of the sample and the migration of silica from amorphous powder into the sample. Transport rates were found to decrease in four-week experiments (Fig. 8) as fluid flow decreases in response to increased cementation reducing fluid networks and connectivity allowing fluid saturation at the charge top where microfracture healing and grain annealing occurred amongst top grains of CH-30. Despite this change, transport rates are not considered rate limiting. Dissolution of synthetic quartz grains is considered rate-limiting within these experiments. Sample base and top show gradational differences in cement and porosity which initially leads to the conclusion that transport was rate-limiting. If that were true, however, silica saturation conditions would have existed at the sample top, and dissolution would not be observed. As mentioned above, the higher solubility of amorphous silica accounts for oversaturation conditions and cementation observed at sample bases. It may be anticipated that saturation conditions would exist throughout if dissolution was rate-limiting; however, transport of silica undersaturated fluids after precipitation at sample base and mid-levels causes a consistent influx of undersaturated fluid causing dissolution of top synthetic quartz grains. Dissolution is not isolated to the entire sample top, but rather an upper core, as mosaics CH-30 side cut illustrates, very fine synthetic quartz grains are found throughout the side margins at sample base and top suggesting a centralized core of dissolution. Fluids in this core are then transported out along Au tube side walls maintaining undersaturated conditions within the upper core of the charge. This accounts for little observable dissolution of synthetic quartz grains and the preservation of synthetic quartz shards, needles, or fragments found along side wall transects of sample CH-30. It is believed that fluid convection does occur within Au-tubes, which would
52 account for reduction in measured cement between CH-30 side and center cuts, and differences in cementation are not generated through changing temperatures or pressures. Dissolution and precipitation of quartz can be driven by changes in pressure, temperature, or fluid concentration. Pressures generated in the cold seal reaction vessels are hydrostatic and are held constant over the duration of the experiment, as were temperatures. Spatial variations in temperature within the sample vessel have been determined to be 6° C or less over the length of the Au tube. This suggests thermal gradients as the driving force in these experiments can be ruled out. In addition, if thermal gradients did exist, it is anticipated that precipitation and cementation would occur primarily towards the top of the Au tube as migrating fluids would encounter cooler conditions, thereby driving precipitation. Contrary to this, cementation in these experiments is found at sample bases, indicating precipitation is occurring in the hotter region of the reaction vessel, validating temperature is not responsible. Ruling out changes in temperature and pressure as driving forces for dissolution and precipitation leaves concentration gradients and diffusion as the source of changes observed in the experiments. The greater solubility of amorphous silica relative to quartz created a concentration gradient from the amorphous silica to the quartz grains. This can shown by the results of CH-39, which had two amorphous silica layers: one at the sample base and one sandwiched between synthetic quartz grains at the charge midpoint. The zones of cement both above and below the central amorphous layer show that cementation is related to proximity to amorphous silica. If precipitation in this experiment was driven by a thermal gradient, the observed bi-directional precipitation and cementation would not occur. In addition, experiment CH-39 indicates the main mechanism of transport within these samples is diffusion as the zones of cementation are found adjacent both above and below the
53 amorphous silica powder. Advective processes would not facilitate cementation above and below the amorphous silica powder.
54 5. CONCLUSIONS: This study has shown that variations in cementation rates and amounts are dependent upon time, temperature, and ionic strength of the fluid. The additions of goethite and AlCl3 to experiments run at 450°C yielded much greater amounts of cement than experiments at lower ionic strength and/or temperature run for the same durations. The presence of goethite also increased grain dissolution. In addition, the occurrence of thick goethite rims in sample CH-35 (Fig. 6) illustrate that differences in quartz overgrowths can be attributed to the extent of goethite-coated synthetic quartz grains. Goethite dusted synthetic quartz grains surrounded by newly formed cement also suggest that as rim thicknesses of goethite increase, so does the probability that overgrowth formation will be reduced due to grain boundary poisoning. Extensive cementation adjacent to the amorphous silica layer in both pure and goethite system experiments indicates that amorphous silica is the primary source of cement in the quartz grains. Cementation volumes, rate-limiting steps, and rates of precipitation and transport all vary as a function of time, temperature, and reagents loaded into charges. Rate-limiting is dissolutions as observed base grains show no apparent dissolution and extensive cementation, while top grains exhibit extensive grain dissolution due to differences in saturation conditions controlled by the influx of undersaturated fluids into sample tops inhibiting equilibrium conditions. The addition of NaCl, AlCl3, and goethite facilitate mechanisms responsible for increased silica solubility resulting in the gradation of cement observable in pure and goethite system experiments. In addition, AlCl3 is responsible for the blue-purple CL colors representing newly formed silica cement due to Al+3 incorporation. Variations in CL cement colors are attributed to different concentrations of Al+3 ions incorporated. While CL colors could not definitively indicate new cement within Pocono Sandstone samples, use of porosity as a proxy
55 does show experiments undergoing step-down in P/T conditions and run at longer durations of time show overall reduction in porosity indicating cementation. Similarly, it is the porosity reduction observed at the base of the sample that is responsible for the changes in precipitation and transport rates as loss of porosity leads to reduced precipitation sites and constriction or obstruction of fluid pathways thereby reducing transport of silica in saturated fluids.
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62
Appendix A. Illustration of Au-tube showing the location of amorphous silica, cementation, and synthetic quartz grains. Sections of the Au-tube are designated and referred to in the text.
63
A
B
Appendix B (1) Synthetic quartz starting experiments CH-37 (A) and CH-38 (B) cathodoluminescence (CL) transect images.
64 Appendix B (2) Cathodoluminescence (CL) transect of CH-41 Pocono Sandstone starting experiment.
65
A
B
Appendix B (3) Cathodoluminescence (CL) transect of Pocono experiments CH-11 (A) and CH-21 (B) Pocono Sandstone experiments run at 300º C for 4 and 35 days, respectively.
66
A
Appendix B (4) Cathodoluminescence (CL) transect of Pocono experiments CH-15 (A) and CH-18 (B) Pocono Sandstone experiments run at 450º C for 7 and 28 days, respectively.
B
67
A
Appendix B (5) Cathodoluminescence (CL) transect of CH-14 (A), CH-20 (B), and CH-22 (C) Pocono Sandstone experiments run at 600º C for 7 days, 1 hour, and 7 days, respectively. Note vug found within sample.
B
68
C
69 Appendix B (6) Cathodoluminescence (CL) transect of CH-29 synthetic quartz experiment run at 300º C for 34.1 days.
70
A
B
Appendix B (7) Cathodoluminescence (CL) transect under 5X (A) and 10X (B) of synthetic quartz experiment CH-26 run one week at 450º C.
71
A
Appendix B (8) Cathodoluminescence (CL) transect under 5X (A) and 10X (B) of synthetic quartz experiment CH-23 run two weeks at 450º C.
B
72 Appendix B (9) Cathodoluminescence (CL) transect under 5X of synthetic quartz experiment CH-27, a duplicate experiment of CH-23, run two weeks at 450º C.
73
A
Appendix B (10) Cathodoluminescence (CL) transect under 5X (A) and 10X (B) of synthetic quartz experiment CH-30 side cut run four weeks at 450º C.
B
74
A
Appendix B (11) Cathodoluminescence (CL) transect under 5X (A) and 10X (B) of synthetic quartz experiment CH-30 center cut run four weeks at 450º C.
B
75
A
Appendix B (12) Cathodoluminescence (CL) transect of CH-31 (A) and CH-42 (B) synthetic quartz experiments run for two weeks at 450º C.
B
76
AA
B
Appendix B (13) Cathodoluminescence (CL) transect of CH-32 (A) and CH-33 (B) synthetic quartz and Pocono Sandstone temperature drop experiments run two days at 450º C and two days at 200º C.
77
A
Appendix B (14) Cathodoluminescence (CL) transect under 5X (A) and 10X (B) of synthetic quartz experiment CH-44 run with goethite for one week at 450º C.
B
78
A
Appendix B (15) Cathodoluminescence (CL) transect under 5X (A) and 10X (B) of synthetic quartz experiment CH-34 run with goethite for two weeks at 450º C.
B
79
A
Appendix B (16) Cathodoluminescence (CL) transect under 5X (A) and 10X (B) of synthetic quartz experiment CH-35 run with goethite for two weeks at 450º C.
B
80
A
Appendix B (17) Cathodoluminescence (CL) transect under 5X (A) and 10X (B) of synthetic quartz experiment CH-45 run with goethite for two weeks at 450º C.
B
81 Appendix B (18) Scanned transect image under plain light of synthetic quartz sandwich experiment CH-39 run for two weeks at 450º C.
