Shale Hydrocarbon Reservoirs: Some Influences of ...

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Eoff, Jennifer D., 2013, Shale Hydrocarbon Reservoirs: Some Influences of Tectonics and Paleogeography During Deposition, in U. Hammes and J. Gale, eds., Geology of the Haynesville Gas Shale in East Texas and West Louisiana, U.S.A.: AAPG Memoir 105, p. 5–24.

Shale Hydrocarbon Reservoirs: Some Influences of Tectonics and Paleogeography During Deposition Jennifer D. Eoff U.S. Geological Survey, Box 25046, Denver Federal Center, MS 939, Denver, Colorado 80225 (e-mail: [email protected]).

ABSTRACT Continuous hydrocarbon accumulations in shale reservoirs appear to be characterized by common paleotectonic and paleogeographic histories and are limited to specific intervals of geologic time. In addition, most North American self-sourced shale correlates with geologic time periods of calcitic seas and greenhouse conditions and with evolutionary turnover of marine metazoans. More knowledge about the relations among these controls on deposition is needed, but conceptual modeling suggests that integrating tectonic histories, paleogeographic reconstructions, and eustatic curves may be a useful means by which to better understand shale plays already in development stages and potentially identify new organic-carbon-rich shale targets suitable for continuous resource development. Upwelling and anoxic waters are commonly cited to explain the accumulation and preservation, respectively, of marine organic carbon. In addition, and perhaps alternatively, the broad correlation of self-sourced shale with macroevolutionary trends in land plants and marine metazoans suggests that reduced consumption of organic matter by benthos during periods of high terrestrial and marine organic productivity was responsible. Fundamental to any of the processes that acted during deposition, however, was active tectonism. Basin type can often distinguish self-sourced shale plays from other types of hydrocarbon source rocks. The deposition of North American self-sourced shale was associated with the assembly and subsequent fragmentation of Pangea. Flooded foreland basins along collisional margins were the predominant depositional settings during the Paleozoic, whereas deposition in semirestricted basins was responsible along the rifted passive margin of the U.S. Gulf Coast during the Mesozoic. Tectonism during deposition of self-sourced shale, such as the Upper Jurassic Haynesville Formation, confined (re)cycling of organic materials to relatively closed systems, which promoted uncommonly thick accumulations of organic matter.

Copyright ©2013 by The American Association of Petroleum Geologists. DOI:10.1306/13441842M1053597

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6  Jennifer D. Eoff

INTRODUCTION Not all mature hydrocarbon source rocks are effective as self-sourcing hydrocarbon reservoirs. Because self-sourced shale appears to be limited to discrete time periods in the stratigraphic column (Ettensohn, 1998), atypical combinations of geological and paleoenvironmental processes must have characterized their deposition. As such, the stratigraphic distribution of continuous hydrocarbon accumulations in self-sourced shale is what warrants development of a conceptual model that then encourages focused empirical studies and supports global exploration of these important resources. Many processes contributed to the deposition and preservation of thick accumulations of Type II (marine) kerogen, such that discrimination among various causes and feedback mechanisms is challenging. Eustasy has been proposed as a strong control (Arthur and Sageman, 2005; Hannisdal and Peters, 2011), and Ettensohn (1994, 1998, 2008) argued effectively on the importance of underlying tectonic drivers on sourcerock deposition. The sedimentary record documents the unique coincidence of several geological, paleoenvironmental, and paleobiological events that were necessary for the development of settings conducive to uncommonly thick and rich accumulations of organic carbon (e.g., Ulmishek and Klemme, 1990; Ettensohn, 1994, 1998; Hannisdal and Peters, 2011). With few exceptions, similar paleotectonic histories and paleogeographic settings, orders of magnitude of eustatic events, paleoclimatic conditions, and seawater geochemistry characterized the deposition of North American shale reservoirs (Ettensohn, 1998; summarized in Eoff, 2012, her Figure 1 and references therein). In general, marine self-sourced shale was deposited in lowlatitude foreland basins or in semirestricted settings along rifted passive margins during periods of rapid tectonic-plate reconfiguration and concomitantly high sea level, during “greenhouse” or transitional climates, and in calcitic seas (e.g., Sandberg, 1983; Ulmishek and Klemme, 1990; Frakes et al., 1992; Arthur and Sageman, 1994, 2005; Huc et al., 2005; Stanley et al., 2010; Hammes and Frébourg, 2012; Slatt and Rodriguez, 2012; Eoff, 2012; also see Ettensohn, 1998). Debate continues, however, whether oxygenminimum to anoxic bottom waters were needed to preserve organic carbon (Demaison and Moore, 1980; Demaison, 1981; Parrish and Curtis, 1982; Pedersen and Calvert, 1990; Schieber, 1998; Bohacs et al., 2005; van Buchem et al., 2005; Huc et al., 2005). The residence time of organic carbon at the sediment-water

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interface and exposure time to oxidative processes needed to be minimized (Demaison and Moore, 1980; Demaison, 1981; Sinton and Duncan, 1997; Bohacs et al., 2005). From sedimentological and paleoecological perspectives, explanations simpler than anoxia, however, have been suggested for the preservation of organic matter (Schieber, 2003; Macquaker et al., 2010; Eoff, 2012), although these alternatives still need to withstand scrutiny by empirical studies. The co-occurrence of enhanced terrestrial and marine productivity (Algeo et al., 1995; Bambach, 1999; Huc et al., 2005), unconsolidated, muddy substrate (Schieber, 2003; Dashtgard et al., 2008), and decline of rapidly bioturbating metazoan benthos (Bush and Bambach, 2011) established a unique paleoecosystem in which rates of genesis of organic carbon exceeded rates of its consumption. The same tectonism that generated cratonic or marginal-marine accommodation by initiating subsidence and inducing eustatic flooding also impacted the transport and preservation potential of organic-carbon compounds. Conceptually, and at the scale of regional to global geological evaluation, the history of organic materials in shallow-marine water is regulated by a number of factors. Among them, the type of continental margin controls drainage basin size, the supply of terrestrial nutrients, alteration of terrestrial organic matter, and the mixing of terrestrial and marine components (Blair and Aller, 2012), which ultimately affect marine food chains (Bambach, 1993, 1999). Paleolatitude and basin orientation with respect to prevailing winds (Gorsline, 1981; Sinclair, 2012) may have prejudiced the distribution of organic matter by defining areas of rain-shadow effects (Dickinson, 1974; Ettensohn, 1994) and governing the relative strength and directionality of local winds and ocean currents (Moore et al., 1995). Subadjacent topographic highs provided basin restriction needed to confine (re)cycling of nutrients to relatively closed systems, which further encouraged the production of marine organic carbon.

AIM Data from several specialties within the geosciences, including sea-level studies, paleoclimatology, and geochemistry, compiled against a single geologic timescale address questions of scientific and economic interest regarding shale resource plays (see Eoff, 2012, her Figure 1, for published studies integrated as part of the model expanded here). Integration of the current understandings of these processes with

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evolving knowledge of biodiversity trends throughout parts of the Phanerozoic was initiated recently (Eoff, 2012, her Figures 2–3). The agenda of this paper is to evaluate the relevant processes in their paleotectonic and paleogeographic frameworks. Many suggestions of this model remain conjectural, but the goal of the author is to draw attention to geologic questions needing renewed attention in light of current interest in shale hydrocarbon reservoirs. Model components are not meant to be treated as absolutes, but rather, the model is to help understand broad correlations that may prove to be significant tools for exploration. Although the Mississippian Barnett Shale gas play in Texas has more than 16,000 producing wells, in March 2011, the Jurassic Haynesville Formation of northwest Louisiana and east Texas became the largest producing gas area in the United States, albeit with only about 2000 wells drilled in the play to date (IHS Energy Group, 2011). The Barnett has served as the primary analog for understanding continuous gas accumulations in shale reservoirs. As newer shale plays emerged from exploration to development stages, it became clear that each shale play requires individualized attention. Studies of

the Haynesville–Bossier Formations are the next to contribute insight into the shared and dissimilar geological characteristics of self-sourced shale plays, and the depositional setting of the Haynesville Formation is reviewed herein.

ORGANIC MATTER TYPE Kerogen in self-sourced shale is predominantly Type II, or mixed Type II-III (Arthur and Sageman, 2005; van Buchem et al, 2005; Slatt and Rodriguez, 2012). In addition to marine organic matter (OMmar) generated in situ, marine basins receive three types of terrestrial organic matter (OMterr) (Herbin et al., 1986; Hedges et al.,1997; Burdige, 2005, 2007; Blair and Aller, 2012): (1) “new” material from recent terrestrial production; (2) variably aged and altered organic matter from soils; and (3) fossil kerogen from weathered sedimentary rocks. Both new terrestrial and new marine types of organic matter (OM) were essential in promoting the deposition of potential self-sourced shale in marine basins because increased availability of terrestrially derived nutrients enhanced generation of the latter in

Key for Figures 1–5 General Sea level Source of terrestrial nutrients (OM terr) OM terr terflux (Re)cycling of nutrients and OM in water column

Deposition of organic carbon

Clastic transport

Fault (arrow indicates relative displacement)

Z

Z’

Organic Matter

Figure 4

Adopted from Blair and Aller (2012)

Latest Jurassic deposition, Baltimore Canyon Trough, U.S. Atlantic Margin

Upper Jurassic Haynesville Formation, U.S. Gulf Coast Region

Modern terrestrial

Coastal plain

Cotton Valley Group (CV) clastic units

Variably aged, altered terrestrial

Fluvial deltaic

Sabine Island Complex

Ancient / fossil (kerogen)

Transitional marine shoreface

Haynesville Fm. (HSVL) shale deposition

Modern marine

Backreef / lagoonal

Haynesville Fm. (transitional)

Uncertainty

Prograding carbonate margin

Haynesville Lime (Gilmer/Overton areas)

Foreslope talus

Buckner Anhydrite

Slope facies

Smackover Formation (SMKV) limestone

(exposed ?)

Smackover Formation basinal

Prevailing wind direction

Basinal abyssal plain

Calcareous phytoplankton

Basin-floor fan

Louann Salt

Siliceous microorganisms

Reef / bioherm

Basement uplift (?)

Line-of-section

Prevailing current direction

(with possible halokinesis)

Optimal play area

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8  Jennifer D. Eoff

russian) foreland-basin setting during supercontinent assembly favorable for deposition of thick marine shale containing abundant Type II (marine) kerogen. (A) Paleogeographic setting (Blakey, 2011) of the Barnett Shale in the Fort Worth Basin, Texas, during the Late Mississippian (325 Ma). Latitude of 30o S estimated from Middle Mississippian paleogeography (340 Ma; Blakey, 2011). USGS Province Boundary for 2004 hydrocarbon resource assessment in red (USGS Fact Sheet 2004-3022, Bend Arch-Fort Worth Basin Province Assess­ ment Team, 2004). (400 km = 248.5 mi). (B) Schematic foreland basin model (no scale). Rain shadow effects across the building orogen limited the flux of siliciclastic material to the basin. Eustatic flooding pooled coarse sediment inboard near sediment sources. Shallow water was loaded with terrestrial organic matter (OMterr) and nutrients (heavy arrows), which enhanced productivity of marine microorganisms. Barrier opposite the orogen, such as denser saline water over forebulge carbonate platform, helped confine (re)cycling of organic nutrients within the basin (arrows), prompting positive feedback on productivity within this relatively “closed” system. Unstable substrate of fluid-rich mud, poor in silt-sized and coarser fractions, discouraged inhabitation by effective metazoan consumers, permitting preservation of accumulating organic material. (C) Distribution of types of accumulating organic matter after Blair and Aller (2012) [Corg/SA = ratio of organic carbon to surface area of particle]. Accumulation of modern OMterr (green), new marine organic matter (OMmar) (blue), and fossil kerogen (black) characterized active margins with small drainage basins and short shelfal areas.

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A. Paleogeographic Setting, Barnett Shale (Upper Mississippian) 130°0'0"W

120°0'0"W

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Ron Blakey CPGS, 2011

45°0'0"N

45°0'0"N

Laurussian collision with Gondwana 40°0'0"N

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A

30°0'0"N

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t

ua

Eq

or

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A’

325 Ma Mississippian

25°0'0"N

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o

30 15°0'0"N 120°0'0"W

110°0'0"W

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0

400

800

90°0'0"W

1,200

1,600

S USGS Province Boundary

15°0'0"N

80°0'0"W

2,000

kilometers 1 : 20,000,000 KILOMETERS

B. Collisional Margin, Foreland Basin A

A’

bathymetric barrier and dense saline water

OM terr

carbonate shelf

OM mar

Type II-III Kerogen

Type II Kerogen

orogen thixotropic mud

C. Corg/ org SA

Figure 1. North American (Lau-

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shallow-marine water (Algeo et al., 1995; Ettensohn, 1998; Bambach, 1999; Huc et al., 2005). Higher sedimentation rates enhance the burial efficiency of organic carbon in general (Canfield, 1994; Burdige, 2007), and although OM mar is more reactive than OM terr, much more OM mar accumulates in shallowmarine sediment because of the greater volume of OMmar produced in the photic zone (Burdige, 2005). Blair and Aller (2012) contrasted the relative abundances of each kerogen type at active margins and across passive margins. Although numerous processes affect decomposition rates and the net decomposition of OM, Blair and Aller (2012) were able to differentiate active and passive margins into depositional systems distinct in their cycling of OMterr+mar. The residence time of OMterr is shorter at active margins, which are characterized by small drainage basins and narrower shelves without large fluvial systems (Garrison, 1981; Gorsline, 1981), and the transport of OM to low-energy settings is more rapid than along passive margins (Blair and Aller, 2012). In contrast, cycles of erosion and deposition along broad drainage basins and extensive lowlands of passive margins increase the dilution and diagenetic alteration of terrestrial kerogens (Blair and Aller, 2012; see also Burdige, 2005). In open-marine water of passive margins, OMmar cycles rapidly within the system, whereas OMterr undergoes slower cycles of deposition and resuspension (Blair and Aller, 2012). For deposition of self-sourced shale, basin restriction was necessary to confine (re)cycling of nutrients and OM to the depocenter (e.g., Arthur and Sageman, 2005; Hammes and Frébourg, 2012). Nearly enclosed geometries of intracratonic or marginal basins may have doubled organic productivity compared to open-marine settings because of proximity to sources of terrestrial nutrients and the ability of the basin to concentrate the nutrients (Ettensohn, 1998).

Nordstrom, 1971) and orbitally forced climate changes (Sinclair, 2012) (adopted from van Buchem et al., 2005). Deposition of potential self-sourced shale corresponded to rapid, high-frequency eustatic rise superimposed on low-frequency rise during the assembly and fragmentation of Pangea (summarized in Eoff, 2012, her Figure 1 column A; original sealevel curves in Haq et al., 1988; Haq and Schutter, 2008). Flooding of cratonic or marginal-marine basins was not only needed to develop accommodation for deposition of this shale type, but rapid transgression may have pooled coarser siliciclastic sediment closer to source areas along basin margins, slowing the dilution of organic material in the water column and marine sediment (Howell and von Huene, 1981; Ettensohn, 1994; Murphy et al., 2000; Arthur and Sageman, 2005; van Buchem et al., 2005; Hammes et al., 2011). Loading of terrestrial nutrients in the upper water column during periods of maximum flooding also supported productivity of marine phytoplankton (Murphy et al., 2000). Conversely, extensive coal swamps during eustatic lowstand at the end of global tectonic phases, such as during parts of the Pennsylvanian and Permian, sequestered terrestrial nutrients in nearshore settings so that organic productivity in the oceans declined (Tappan and Loeblich, 1988; Huc et al., 2005). In addition, rapid transgression and reduced influx of coarser detritus culminated in fluid, muddy substrate (“thixotropic,” sensu Dashtgard et al., 2008). Substrate instability limited the diversity of benthos and benthonic feeding strategies (Dashtgard et al., 2008). Rates of consumption of accumulating OM were reduced as infaunal and epifaunal taxa faltered in response to rapidly changing environmental conditions during flooding events (see summary of diversity studies in Eoff, 2012).

EUSTASY

Tectonics (and associated volcanism during rapid seafloor spreading) was the initial driver behind many of the other processes that controlled sedimentation and potentially augmented the accumulation of OM (Inman and Nordstrom, 1971; Howell and von Huene, 1981; Mackenzie and Pigott, 1981; Waples, 1983; Ettensohn, 1994, 1998; Huc et al., 2005). Ettensohn (1994) recognized the association of source rocks with supercontinent organization prior to focused exploitation of continuous hydrocarbon accumulations in self-sourced shale reservoirs as a common play type. Whereas the deposition of thick, organic-rich shale in rapidly subsiding foreland basins was archetypal of the Paleozoic

Many organic-rich shales were deposited during 2nd-order transgressions that included stacked 3rdorder and 4th-order transgressive-regressive cycles (Bohacs et al., 2005; van Buchem et al., 2005; Huc et al., 2005; Loucks and Ruppel, 2007; Cicero et al., 2010; Hammes et al., 2011; Lash and Engelder, 2011; Slatt and Rodriguez, 2012). Second-order and 3rd-order sequences likely developed in response to global tectonics and long-term tectonics- and climate-driven eustasy, whereas higher-order sequences reflect the controls of drainage-basin size and dispersal of sediment by associated river systems (Inman and

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TECTONICS

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10  Jennifer D. Eoff

A. Late Jurassic (~150 Ma) Paleogeography of North America 110°0'0"W

100°0'0"W

90°0'0"W

80°0'0"W

70°0'0"W 45°0'0"N

Ron Blakey CPGS, 2011

North America, United States of America

45°0'0"N

40°0'0"N

Figure 2. North American pas-

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4A.

B

40°0'0"N

B’ 35°0'0"N

Atlantic Ocean

o

30

35°0'0"N

N

150 Ma Jurassic

4B.

30°0'0"N

30°0'0"N

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Gulf of Mexico USGS Province Boundary

25°0'0"N

100°0'0"W

0

90°0'0"W

400

800

1,200

80°0'0"W

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2,000

kilometers 1 : 15,000,000

B. Passive Margin, Atlantic-Type Basin B

OM terr terr : cycles of erosion and deposition

OM terr+mar terr : cycles of deposition and resuspension Type III Kerogen

B’

X Type II-III Kerogen

OM terr+mar

X

Type II(S) Kerogen

transport into and out of plane of view

Type I Kerogen

C.

Corg/ org SA

sive margins. (A) Late Jurassic (150 Ma) paleogeography (Blakey, 2011) of North America with approximate locations of block diagrams in Figure 4AB. Latitude of 30o N estimated from Blakey (2011). USGS Province Boundary for 2011 hydrocarbon resource assessment of Haynesville Formation in red (USGS Fact Sheet 2011-3020, Gulf Coast Assessment Team, 2011). (400 km = 248.5 mi). (B) Schematic sub-basin similar to the Baltimore Canyon Trough of Atlantic margin (e.g., Prather, 1991; Miall et al., 2008). Cycles of erosion and deposition degrade OMterr (Blair and Aller, 2012). On the shelf, marine (OMmar) and terrestrial (OMterr) types of organic matter undergo cycles of deposition and resuspension (Blair and Aller, 2012). Remaining organic material is transported laterally or offshore. (C) Distribution of types of accumulating organic matter after Blair and Aller (2012) [Corg/ SA = ratio of organic carbon to surface area of particle]. Fossil (black) and mixed modern and aged (gray) OMterr derived from broad coastal drainage basin with soils. Newly generated OMmar increases and is dominant basinward. Unconfined (“open”) transport of OM in all directions, and dilution by siliciclastic or carbonate material, precluded the deposition of potential self-sourced shale. Source rocks in early to late rift-fill or earliest passive margin sections may have included lacustrine Type I and carbonate Type II(S) kerogens. Coastal plains and deltas supply Type III kerogen that may generate ­gas-prone hydrocarbons.

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Figure 3. North American passive-

A. Paleogeographic Setting, Haynesville Formation (Upper Jurassic) 100°0'0"W 35°0'0"N

90°0'0"W 35°0'0"N

Haynesville Sabine Platform Continuous AU 1 / Approx. Haynesville basin 2

United States Gulf Coast Region

Sabine Island Complex 3 Paleobathymetric High (?) 2

C

Gilmer/Overton areas

Pinnacle 2-3 La Salle Arch Eagle Ford Shale (Upper Cretaceous) Production 4 Natchitoches island (?)

C’

Pinnacles (E. TX Salt Basin)

Sabine Island Complex

Angelina Island

30°0'0"N

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150 Ma Jurassic

Ron Blakey CPGS, 2011 100°0'0"W

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200

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B. Rifted Passive Margin, Semi-Restricted Basin C

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OM mar

terr : *OM terr cycles of erosion and deposition

Type II Kerogen

bathymetric barrier and dense saline water

thixotropic mud Type II(S) Kerogen (e.g., Smackover Fm. source rock) structural inversion (basement), halokinesis, or carbonate platform / buildups

Corg/ org SA

C.

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margin setting, subsequent to supercontinent fragmentation, favorable for deposition of thick shale containing abundant Type II (marine) kerogen. Dashing indicates uncertainty. (A) Paleogeographic setting (Blakey, 2011) of the Haynesville Formation in northwest Louisiana and east Texas during the Late Jurassic. USGS Province Boundary for 2011 hydrocarbon resource assessment of Haynesville Formation in red (USGS Fact Sheet 2011-3020, Gulf Coast Assessment Team, 2011). Modified from: 1—Gulf Coast Assessment Team, 2011; 2—Cicero et al., 2010; 3—Hammes et al., 2011; and 4—Harbor, 2011. Accommodation developed between the Gilmer/Haynesville Platform (HSVL Lime, see Figure 4B) to the northwest-west and the basement- or salt-cored uplifts to the south and southeast. (100 km = 62.1 mi). (B) Schematic U.S. Gulf Coast basin favorable for deposition of self-sourced shale (no scale). The craton provided OMterr (heavy arrows) needed to enhance marine primary productivity. Uplifts, salt-tectonic structures, and carbonate buildups helped trap nutrient (re)cycling to shallow water over the basin (arrows), and they may have also diverted clastic detritus away from the site of organic-rich shale deposition. Muddy substrate could not support diverse benthos, so consumption of organic carbon declined. (*) The richness of shale in schematic transects across basins of this type will vary according to proximity to sources of clastic sediment and OMterr, transitional settings, and carbonate-dominated environments. (C) Distribution of types of accumulating organic matter after Blair and Aller (2012) [Corg/SA = ratio of organic carbon to surface area of particle]. Fossil (black) and mixed modern-aged (gray) OMterr derived from broad coastal drainage basin with soils. Newly generated OMmar increases and is dominant basinward. If ­exposed subaerially, uplifts along periphery of basin may have ­supplied additional OMterr.

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(e.g., Ettensohn, 1994, 1998, 2008), Mesozoic organicrich shales of the U.S. Gulf Coast were deposited primarily in semirestricted basins along an overall ramp-type setting of the rifted margin (Goldhammer, 1998; Goldhammer and Johnson, 2001; Mancini et al., 2008). Inherited lithospheric strength was critical in development of accommodation (Stockmal et al., 1986; Tankard, 1986; DeCelles, 2012; GarciaCastellanos and Cloetingh, 2012) in basins discussed below. Ingersoll (2012) recently provided a review of the mechanisms of subsidence. Attenuation of continental crust (Dickinson, 1974) during rifting of tectonic plates enabled marked future subsidence when a margin experienced renewed tectonic activity and sedimentary loading (Tankard, 1986; Ingersoll, 2012). The foreland-basin examples recorded advances of orogenic fronts and impingement on thermally older margins (Stockmal et al., 1986). In contrast, the semirestricted, passive-margin setting during deposition of the Haynesville–Bossier Formations was situated on thermally younger lithosphere by comparison. Any combinations of surface and subsurface mechanisms permitting these basins to remain “underfilled” (Cant and Stockmal, 1993; DeCelles and Giles, 1996; DeCelles, 2012; Garcia-Castellanos and Cloetingh, 2012) with respect to sedimentation would have encouraged deposition of organic-rich sediment when in low latitudes characterized by high bioproductivity.

Active Margins: Foreland Basins Contrasting schools of thought would have tectonic subsidence and accommodation develop fastest at the initiation of orogenesis (Heller et al., 1988; Cant and Stockmal, 1993; Ettensohn, 1994, 1998; Plint et al., 2012) or accelerate throughout foreland-basin development (Tankard, 1986; Garcia-Castellanos and Cloetingh, 2012; Sinclair, 2012). Because deposition of organic-rich shale is generally accepted to represent comparatively deeper-water deposition than other types of foreland-basin fill, the first model is currently favored. Debate regarding elastic or viscoelastic behavior of foreland lithosphere (Stockmal et al., 1986; Tankard, 1986; DeCelles, 2012; Sinclair, 2012) is beyond the scope of this paper. The destruction of interior ocean basins and formerly rifted passive margins between approaching landmasses during the assembly of Pangea was characterized by foreland-basin development and voluminous postorogenic sedimentation related to isostatic rebound (Ettensohn, 1994, his Figure 3;

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Nance and Murphy, 1994; Miall, 2008). Development of rapidly subsiding, peripheral foreland basins (Dickinson, 1974; DeCelles and Giles, 1996; GarciaCastellanos and Cloetingh, 2012) along the suture between Laurussia and Gondwana established basin geometries favorable for deposition of thick, organic-rich shale units in the United States, such as the Devonian Marcellus Shale (Lash and Engelder, 2011) and the Mississippian Barnett Shale (Pollastro et al., 2007; Loucks and Ruppel, 2007; Slatt and Rodriguez, 2012) (Figure 1A-B). Because there was a time lag between initial flexure-induced subsidence and later topographic expression and erosion of the thrust belt (Cant and Stockmal, 1993), early foreland-basin development in the Appalachian and Fort Worth basins, respectively, was predisposed to “basinal” conditions and shale deposition. Diachronous, progressive closure of the former ocean (Dickinson, 1974) provided an element of restriction to developing basins. The subsurface loading driving flexural cratonic downwarping (Burgess, 2008; Sinclair, 2012) was matched by sediment starvation of the incipient (marine) proforeland basin (Heller et al., 1988; Cant and Stockmal, 1993; DeCelles and Giles, 1996; Ettensohn, 1994, 1998, his Figure 9, 2008). Sedimentation was dominated by biogenic material during the early, most active phases of tectonism (Cant and Stockmal, 1993; Ettensohn, 1994, 1998, 2008), as coarser clastic material was captured by proximal foredeep accommodation (Heller et al., 1988; Meckel et al., 1992; DeCelles and Giles, 1996; Murphy et al., 2000; Poole et al., 2005; DeCelles, 2012; Plint et al., 2012) and the greater foreland-basin system remained underfilled. Any clay or silt entered basins at active margins primarily as suspended load (Gorsline, 1981). Progradation of coarser siliciclastic sediment eroded from the exposed orogen into the marine foreland basin postdated earliest loading phases (Heller et al., 1988; Cant and Stockmal, 1993; Ettensohn, 1994, 1998, 2008). The distribution of OM types at active, convergent margins adopted from Blair and Aller (2012) is shown on Figure 1C. New OM of terrestrial and marine origin dominated accumulating sediment (Loucks and Ruppel, 2007; Blair and Aller, 2012). Sedimentation in foreland basins is predominantly a function of erosion in mountain catchments and dispersal by river systems (Sinclair, 2012). Small catchment basins of growing orogenic topography of active margins restricted coarse sediment to small fans along narrow thrust fronts (DeCelles and Giles, 1996), which prevented dilution of OM in the adjacent marine basin (Blair and Aller, 2012).

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Passive Margins Creation of new, expanding oceans characterized the breakup of Pangea (Nance and Murphy, 1994), but developing passive margins were less likely to encourage source-rock deposition than active margins (Ettensohn, 1994). Basins along rifted passive margins must have satisfied additional conditions to promote deposition of self-sourced shale. Partial basin restriction resulted from inversion of basement features during later phases of extension or from far-field compression, various halokinetic processes, or extensive development of carbonate platforms or buildups on inherited topographic highs or highs resulting from any of the former types of deformation (see following text for Haynesville Formation as an example). Flow of denser marine water over sills may have established estuarine-style circulation in which nutrients were trapped and (re)cycled within the basin (Arthur and Sageman, 2005). In addition, as with basins at colliding margins, mitigation of the diluting effects of clastic influx was necessary. This part of the model presented herein is evaluated by contrasting Late Jurassic–Early Cretaceous depositional settings along the central North American Atlantic margin (e.g., Libby-French, 1984; Grow et al., 1988; Grow and Sheridan, 1988; Klitgord et al., 1988; Prather, 1991; Miall et al., 2008) (Figures 2, 4A) with deposition along parts of the U.S. Gulf Coast region (e.g., Ewing, 2001; Cicero et al., 2010; Hammes et al., 2011; Hentz and Ruppel, 2011; Hammes and Frébourg, 2012) (Figures 3, 4B). Paleogeographic reconstructions (Blakey, 2011) and depositional models are invaluable for testing the following conceptual reasoning. Baltimore Canyon Trough, U.S. Atlantic Margin During the Late Jurassic and Early Cretaceous, OM of terrestrial and marine origin in shallow water along the central U.S. Atlantic margin would have been diluted by siliciclastic and carbonate detritus and by connection to the open ocean, which permitted unconfined transport of organic and inorganic materials along the shelf and offshore (Figure 2AB). Broad, energetic margins are associated with rapid destruction of OM because cycling of OM is favored over burial in this relatively open system (Blair and Aller, 2012). The Baltimore Canyon Trough, a northeast-southwest trending sub-basin of the North American Atlantic margin, is bounded by the Long Island Platform to the north and the Carolina Platform to the south (Prather, 1991; Miall et al., 2008),

13859_ch02_ptg01_005-024.indd 13

but segmentation of the margin was not adequate to enrich shelf sediment in OMmar during the middle to late Mesozoic (Figure 4A). In fact, no wells drilled along the U.S. Atlantic coast have encountered an economically viable source rock to date (Miall et al., 2008). The aggradation-progradation of carbonate rim facies, on the upthrown edges of rotated fault blocks along the shelf margin (Prather, 1991), created a linear trend of topography not sufficient to enclose nutrients during shelfal sedimentation. The distribution of OM types would be as modeled by Blair and Aller (2012) for passive margins in open-ocean stages (of Dickinson, 1974) (Figure 2C). Haynesville Formation, U.S. Gulf Coast In contrast, bathymetric barriers of some sub-basins of the U.S. Gulf Coast region restricted nutrient (re)cycling to comparatively closed systems, pro­ voking a positive feedback on marine productivity in a way similar to that discussed for foreland basins. If marine substrate was unfavorable for inhabitation by effective consumers, then much of the additional organic material collected in such basins would have had higher preservation potential. The Gulf Coast margin is characterized by a series of structural highs and lows, often overlying thicker continental crust or attenuated transitional crust, respectively (Galloway, 2008; Ewing, 2009; Hammes et al., 2011). The greater Gulf of Mexico Basin was dominated by passive-margin development during the Mesozoic but may have also experienced farfield effects during Laramide orogenesis (Jackson and Laubach, 1988; Goldhammer, 1998; Goldhammer and Johnson, 2001; Galloway, 2008; Ewing, 2009). The origin of the Sabine area of northwestern Louisiana and eastern Texas, for example, remains problematic (Hammes et al., 2011), but it is known to have experienced several episodes of movement (Ewing, 2009). Deposition of the Upper Jurassic Haynesville Formation (Ewing, 2001; Cicero et al., 2010; Hammes et al., 2011; Steinhoff et al., 2011; Hammes and Frébourg, 2012; Cicero and Steinhoff, this volume) occurred at low latitudes in the Western Tethys, which was dominated by westward-flowing paleocurrents (Moore et al., 1995; Martin and Ewing, 2009). Uplifts in the Gilmer/Overton, Angelina Island, and Sabine Island Complex areas (Figure 3A-B, 4B) mark the reactivation of deep structures or halokinesis (Jackson and Seni, 1983; Ewing et al., 2008; Martin and Ewing, 2009; Cicero et al., 2010; Hammes et al., 2011; Hammes and Frébourg, 2012; Cicero and

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13859_ch02_ptg01_005-024.indd 14

ent

Post-rift unconformity

Transitional marine

Ba s & r eme ift-f nt ill s edim

SW

ent

SMKV

HSVL Lm

B. Haynesville Basin, Gulf Coast Region

Ba s & r eme ift-f nt ill s edim

S

Type II(S) Kerogen

Re tr r og ad at

?

?

Type II(S) Kerogen

?

?

Angelina Island

HSVL Organic-Rich-Shale Deposition (Type II)

“Closed” System: Recycling of organic nutrients (terrestrial and marine primary) in water column

Lower – Middle Jurassic sequences

Onlapping basin fill

(argillaceous limestone and marl)

“Open” System: Cycling of organic nutrients (terrestrial and marine primary) in water column

Upper Jurassic sequences

OM mar

A. Baltimore Canyon Trough, Atlantic Margin

SMKV

OM mar

Shelf

n

margi

OM terr terr

Sabine Complex

Type II-III Kerogen

Type III Kerogen

OM terr terr

Type II-III Kerogen

Type III Kerogen

Shelf margin

Along-shore and basinward transport of siliciclastic and organic detritus

Projected subsurface basement- or salt-cored features

HSVL Organic-Lean Shale

Dilution of organic matter

Coarse siliciclastic material diverted away from Haynesville basin by subaqueous basement uplifts or incipient halokinesis

Bathyal-to-abyssal plain sediment over transitional or oceanic crust

CV clastics

NE

Bypass (?)

N

14  Jennifer D. Eoff

ion al

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Shale Hydrocarbon Reservoirs  15

Steinhoff, this volume), some of which helped to confine (re)cycling of nutrients to the Haynesville basin (sensu Cicero et al., 2010) in a relatively closed system during deposition of the transgressive shale unit (Cicero et al., 2010; Steinhoff et al., 2011) (Figures 3B, 4B). In addition, subtle to well-developed relief resulting from subaqueous topography or halokinesis to the east near Natchitoches Island and the proto-La Salle Arch (Cicero et al., 2010; Steinhoff et al., 2011) either captured clastic sediment of the Bossier Formation and other units of the Cotton Valley Group in peripheral structural lows (Lobao and Pilger, 1985; Hammes and Frébourg, 2012; Frébourg and Ruppel, this volume) or at least directed clastic material from the early Mississippi River in a southerly direction away from sites of carbonate and shale deposition (Cicero et al., 2010, their Figure 9; Steinhoff et al., 2011). Dilution of accumulating organic material by siliciclastic detritus was minimized in the early development of the Haynesville basin; however, dilution became more prevalent upsection as deltas prograded basinward from the north and northeast (Hammes et al., 2011). Similarly, the Upper Cretaceous (Cenomanian– Turonian) Eagle Ford Shale in southeast Texas records deposition of organic-rich sediment in a semirestricted basin of the U.S. Gulf Coast region (Hentz and Ruppel, 2011) (Figure 3A). The south Texas area (of Goldhammer and Johnson, 2001) was situated at relatively low latitudes during deposition of the Eagle Ford Shale (Harbor, 2011; Hentz and Ruppel, 2011). Exposed parts of the Llano area (Miall, 2008) and Sabine (Rusk) uplift (Ewing, 2009) may have been sources of terrestrial nutrients to Gulf waters. The San Marcos arch may have limited dispersal of nutrients out of the Maverick

Basin area situated to its southwest. The arch may have also pooled clastic material from the Harris delta in the nearby Houston embayment to its northeasteast or diverted it offshore, preventing dilution of organic material accumulating in the south Texas and Maverick Basin areas (Hentz, T.F., person. comm., 2012). The distribution of types of OM may have been as shown on Figure 3C, adopted from Blair and Aller (2012). The richness of shale in schematic transects across basins of this type will vary according to paleoproximity to fluvial-deltaic sources of clastic sediment and OM terr, transitional settings of mixed Type II-III kerogens and variable lithologic composition, and carbonate-dominated environments with Type II(S) kerogen (compare siliciclastic-dominated environments and carbonate settings on Figure 4B). Although simple, this model, summarized in Figure 5, can be combined with thorough knowledge of regional geologic histories as an exploration guide.

PALEOGEOGRAPHY AND CLIMATE The paleogeographic locations of basins were important for several reasons (Dickinson, 1974), the most obvious of which was the need for low latitudes. Higher productivity of land plants and marine autotrophs, and, therefore, production of organiccarbon compounds, characterizes low latitudes. Tropical forests export about twice as much organic carbon than temperate or boreal forests (Hedges et al., 1997). Furthermore, basin placement within trade wind belts was important (Ettensohn, 1994;

Figure 4. Modified block diagrams (refer to Figure 2A for locations) depicting the three-dimensionality of complex relation­ ships among processes that contributed to source-rock deposition, passive-margin settings. Basin margins curve away from the viewer toward the background of the display. Scaling has not been applied, and dashing indicates uncertainty. (A) Passive-margin setting of the Baltimore Canyon Trough, U.S. Atlantic margin (see Figure 2). Modified from multiple sources (e.g., Libby-French, 1984; Klitgord et al., 1988; Prather, 1991; Miall et al., 2008). During the Late Jurassic, a carbonate margin prograded above rotated fault blocks of down-to-the-basin normal faults. OMmar (Type II(S) kerogen) would have been one of the dominant constituents in accumulating sediment. Toward the north, deltaic supply of OMterr (heavy arrows) may have permitted deposition of Type III, gas-prone kerogen. Transitional and offshore areas may preserve mixed kerogens, depending on proximity to various sources. Deposition of potential self-sourced shale, however, was unlikely in this setting. The large, broad drainage basin would have altered OMterr while in transit, and the broad, “open” shelf encouraged dilution and destruction of OMterr+mar rather than burial (Blair and Aller, 2012). (B) Semi-restricted basin of shale deposition of Haynesville (HSVL) and Bossier Formations (see Figure 3). Modified from Hammes et al. (2011) and Hammes and Frébourg (2012). During Late Jurassic deposition of the Haynesville Formation, carbonate facies developed on the western basin margin and over basementor salt-cored highs to the south and southeast. OMterr (heavy arrows) was supplied to the basin from a deltaic system to the northeast. Bathymetric barriers helped prevent dilution of organic matter by diverting siliciclastic material away from the depocenter. Tectonism or halokinesis drove uplifts that caused basin restriction, prompting positive feedback on marine productivity. Cycling of OMterr and OMmar (arrows) in this “closed” system encouraged the deposition of self-sourced shale.

13859_ch02_ptg01_005-024.indd 15

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13859_ch02_ptg01_005-024.indd 16

topographic barrier

PASSIVE MARGIN : * exposed

forebulge

ACTIVE MARGIN :

FSST - LST

carbonate shelf

bathymetric barrier and inflow of dense saline water

TST

SB / CC

depocenter

Nutrient (re)cycling

OM mar

LST (?)

TST

craton

orogen

HST FSST

OM terr terr

Decomposition of OM mar(+terr)

Dilution by carbonate detritus

Dilution by siliciclastic detritus

Generation OMmar

OM terr flux

TS SB

TST: Coarse clastic sediment trapped behind estuarine marshes

16  Jennifer D. Eoff

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Shale Hydrocarbon Reservoirs  17

Moore et al., 1995) for deposition of North American self-sourced shale along the eastern margin(s) of Laurentia / Laurussia during the Paleozoic and along the U.S. Gulf Coast during the Mesozoic. During the assembly of Pangea, westerly flow of terrestrial nutrients (e.g., OM terr), from uplifting terranes toward foreland basins of the Laurussian landmass, promoted marine productivity. Rainshadow effects across incipient, building orogens also explain sediment starvation on the leeward, foreland sides (Ettensohn, 1994). Return of nutrients may have occurred with dense marine flow over the (forebulge) carbonate shelf opposite the orogen (Figure 1B). Southwesterly directed net windflow (Moore et al., 1995) toward uplifts and carbonate buildups along the periphery of semirestricted basins of the Gulf Coast region during middle Mesozoic breakup of Pangea may have acted to slow the migration of OMterr and OMmar material into lateral and offshore areas, creating relatively closed systems of nutrient (re)cycling (Figure 3B). Both scenarios generated positive feedback on marine productivity and the accumulation of organic carbon (Figure 5). Siliciclastic material of probable eolian origin in the Haynesville Formation (Hammes and Frébourg, 2012) confirms the transport of fine-grained terrestrial detritus to the Haynesville basin during organic-rich shale deposition. For comparison, the central Atlantic margin of North America was located within the prevailing westerlies (near or higher than 30oN latitude) during the Late Jurassic–Early Cretaceous (Miall et al., 2008; Blakey, 2011). Windflow from the craton toward the open ocean would have promoted mixing, dilution, and ultimate decimation of nutrients and OM in shallow water, thereby decreasing the preservation potential of OMmar. Similarly, deposition of Cretaceous strata in the greater Rocky Mountain region of the United States likely occurred in the lower westerlies (Robinson Roberts and Kirschbaum,

1995; Sonnenberg, 2011). In contrast to the Paleozoic foreland basins characterized by thick shale deposition discussed earlier, orogenesis was along the western margins of inundated basins of western North America (Robinson Roberts and Kirschbaum, 1995; Sonnenberg, 2011). The subtropical climate during deposition of the widespread Upper Cretaceous Mancos Shale (Robinson Roberts and Kirschbaum, 1995), for example, may have resulted from maritime air blowing from the southwest-west and connection with warm Gulf currents from the southeast (Sonnenberg, 2011), which encouraged high bioproductivity in an otherwise higher-latitude setting. Periodic reactivation of paleotectonic structural elements on the Cretaceous seafloor (Weimer, 1984) could have provided basin restriction needed to promote the generation and preservation of OM.

ORGANIC MATTER PRESERVATION Dilution In the foreland basins, dilution of accumulating OM by clastic detritus was minimized through a combination of the following: (1) pooling of sediment near proximal basin margins during eustatic flooding (Howell and von Huene, 1981); (2) rain shadow effects that reduced the supply of coarse sediment from the adjacent orogeny (Dickinson, 1974; Ettensohn, 1994); and (3) capture of sediment in foredeep accommodation and/or transport of detritus parallel to the orogen in a relatively narrow system along tectonic strike (Dickinson, 1974; Cant and Stockmal, 1993; Miall, 2008; Garcia-Castellanos and Cloetingh, 2012; Ingersoll, 2012). The first process reduced dilution along passive margins as well, and topographic relief was also needed to direct flow of coarse sediment away from sites of shale deposition.

Figure 5. Simplified model showing controls on the accumulation and preservation of marine organic material during deposition of potential self-sourced shale reservoirs. No scaling, and basin symmetry is assumed for simplicity. Sequencestratigraphic nomenclature: HST—highstand systems tract; FSST—falling-stage systems tract; SB—sequence boundary (or CC—correlative conformity); LST—lowstand systems tract; TS—transgressive surface; TST—transgressive systems tract. Deposition of organic-carbon-rich shale (black) occurred during transgression and maximum flooding when: (1) heightened availability of terrestrial matter (OMterr) and nutrients encouraged the productivity of marine phytoplankton and generation of marine organic matter (OMmar); (2) dilution by siliciclastic or carbonate material was minimized; and (3) the destruction or decomposition of organic material occurred at rates slower than formation. Adopted from Arthur and Sageman (2005). Basin restriction was inherent in foreland basins, but passive-margin settings needed some form of topographic relief opposite the craton to confine nutrients and organic matter to a closed system.

13859_ch02_ptg01_005-024.indd 17

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18  Jennifer D. Eoff

Organic carbon produced in shallow-marine basins is not only diluted by siliciclastic influx, but abundant inorganic and organic carbonate detritus dilutes OM in accumulating marine sediment as well. To accumulate thick deposits of OMmar in the geologic past, dilution by carbonate material may have been lessened by various limits on carbonate production (Schlager, 1981, 1992). Tectonic or volcanogenic events that caused detrimental environmental changes may have been responsible for reduced carbonate influx. For example, several researchers (Sinton and Duncan, 1997; Kerr, 1998; Turgeon and Creaser, 2008) linked marine extinctions and black-shale deposition during the Cenomanian–Turonian boundary interval to oceanicplateau volcanism. Drowning of carbonate platforms during transgressions may have also reduced dilution of OM by shutting down carbonate production. Rates of flooding resulting from seafloor spreading or other mechanisms of long-term subsidence, however, were too slow to drown carbonate platforms (Schlager, 1981; Philip and Airaud-Crumiere, 1991). For example, estimated rates of 3rd-order eustatic rises were an order of magnitude lower than average growth rates of Holocene carbonate platforms (Schlager, 1981, 1992). Shorter pulses of relative sea-level rise were needed to outpace carbonate growth (Schlager, 1981). Fault-block movements along segmented margins may have resulted in changes to the morphological features of platforms to cause local to regional flooding (Schlager, 1981; Philip and Airaud-Crumiere, 1991). Asymmetrical glacioeustatic rise that occurred at rates exceeding that of tectonoeustatic rise could have likewise contributed to platform drowning (Pitman, 1978, 1979; Howell and von Huene, 1981; Galloway, 1989). Environmental disturbance or nutrient poisoning, such as the influx of “inimical water” (Schlager, 1981, p. 202; Neuman and MacIntyre, 1985) or decline in the transparency of water over carbonate platforms during eutrophication (Philip and Airaud-Crumiere, 1991), could have caused demise of carbonate framework builders similar to drowning. Lastly, several factors either affected carbonate producers or caused an artificial absence of calcareous remains, with the latter impairing the ability to build interpretations without bias. Acidic oceans, resulting from submarine volcanism, dissolve carbonate (Kerr, 1998). Decay of OM releases carbon dioxide (CO 2), which also causes dissolution of carbonate (Garrison, 1981). Shallowing of the carbonate compensation depth (CCD) during globally warm temperatures and high atmospheric concentrations of CO2 (Waples, 1983) may have affected both rates of production and rates of preservation of carbonate material.

13859_ch02_ptg01_005-024.indd 18

Sedimentation Rates Sedimentation rate may be one of the most important factors influencing the preservation of OM (Waples, 1983; Arthur and Sageman, 1994, 2005; Canfield, 1994; Hannisdal and Peters, 2001). Work on the microstratigraphy of shale is yielding results that diverge from previous conclusions regarding the deposition of source rocks (Schieber, 2003; Macquaker and Bohacs, 2007; Schieber et al., 2007; Macquaker et al., 2010; Macquaker, Bentley, and Bohacs, 2010; Macquaker, Keller, and Davies, 2010; Alpin and Macquaker, 2011; Ghadeer and Macquaker, 2011). For example, mud floccules can migrate in ripples, and the laminated appearance of shale resulted from the compaction of these low-amplitude, long-wavelength ripple bedforms in mud-rich settings (Macquaker and Bohacs, 2007; Schieber et al., 2007; Macquacker et al., 2010; Ghadeer and Macquaker, 2011). Because flocculation of organic material and clays into aggregates increases sedimentation rates in settings dominated by very-fine grain sizes (Honjo, 1997; Stow et al., 2004; Schieber et al., 2007; Slatt and O’Brien, 2011; also see Garrison, 1981), one of the most curious observations regarding self-sourced shale deposition is satisfied. Self-sourced shale was typically deposited during transgression and maximum flooding, intervals of high accommodation typified by low clastic sedimentation rates and stratigraphic condensation (Galloway, 1989), yet rapid burial of organic carbon was needed to avoid decomposition and destruction (Arthur and Sageman, 2005; Bohacs et al., 2005). Higher sedimentation rates of claysized particles resulting from flocculation buried the OM and retarded its destruction by reducing its exposure time to (at least partially) oxygenated water. Conversely, pelagic deposition of dispersed clay and organic material from suspension in other settings produced shale units with smaller pore networks and lower permeability (Schieber et al., 2007), preserving organic-rich source rocks that are much less likely to perform as shale hydrocarbon reservoirs.

Bioproductivity and Substrate North American self-sourced shale reservoirs correspond to geologic time periods characterized by high terrestrial and marine primary productivity but declining marine metazoan biodiversity, especially of rapidly bioturbating infaunal and epifaunal taxa

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Shale Hydrocarbon Reservoirs  19

(Bambach, 1999; Bush and Bambach, 2011; Eoff, 2012, and references therein). During deposition, the accumulation and preservation of OM was augmented by enhanced production of biomass in the water column coupled with diminished consumption of organic material by benthos (see discussions in Eoff, 2012, and references therein). Scarcity of benthic fossils preserved in organic-rich shale (Schlanger et al., 1987; Bohacs et al., 2005; Loucks and Ruppel, 2007) does not imply that preservation of OM required stagnant, anoxic to dysoxic water but rather that unstable substrate of fluid-rich mud may have been limiting for infaunal and epifaunal consumers (Dashtgard et al., 2008) during transgression to maximum flooding and earliest highstand. Dashtgard et al. (2008) documented that grain size and substrate stability control the diversity, density, and preservation of bioturbation, and changes are suggestive of important changes in the paleoecological structure of the marine community. Phytoplankton and heterotrophic populations in the water column grew exponentially with unusually high nutrient supply, such as during land-plant radiations, but populations of benthic metazoans grew much more slowly under the same conditions and may have been overwhelmed by growth of the other forms (Schlanger, 1992; Bohacs et al., 2005). Production of marine organic carbon outpaced rates of its consumption. Thixotropic mud (sensu Dashtgard et al., 2008) hindered maintenance of structured dwelling and feeding burrows, and the lower-diversity feeding burrows created by remaining organisms (“sediment swimmers,” Schieber, 2003, p. 4) were not likely to be preserved. Pemberton et al. (2008) noted that cryptobioturbation records a major biogenic process, but in sediment with low variance in grain size and composition, such as organic-rich shale of the Haynesville–Bossier Formations (Hammes and Frébourg, 2012), this microfabric or subtle bioturbate texture is not readily apparent. Bioturbation, as well as bedforms and erosional features, only become evident in sections having higher supply of silt-sized and coarser grains (independent of composition), but the absence of coarser material did not prohibit similar processes active during deposition of the mud fraction at other times. For example, Bohacs et al. (2005) noted that both anoxic interstitial water and soupy substrate promoted preservation of OM in the Miocene Monterey Formation, but they did not consider that the latter explanation may eliminate the need for the first. Bioturbation indices of skeletonized metazoans should not be correlated with total organic carbon (TOC) in potential source rocks, as applied by Bohacs et al. (2005) for both the Monterey and the Cretaceous

13859_ch02_ptg01_005-024.indd 19

Mowry Shale, because this approach cannot account for subtler types of ichnofabrics.

Remarks Hammes and Frébourg (2012) postulated that anoxic conditions during the deposition of the organicrich but fossil-poor Haynesville Formation were punctuated by more oxygenated conditions in which benthos were able to colonize the seafloor. Substrate stability likely played an important role and may have biased interpretations using preserved fossil assemblages. Hammes and Frébourg (2012) agreed with others that flocculation and marine snow (e.g., Honjo, 1997; Stow et al., 2004) can parlay settling rates and preservation potential of OM. Pervasive bioturbation in the overlying Bossier Formation documents aerated habitats (Hammes and Frébourg, 2012; Frébourg and Ruppel, this volume), but the apparent reestablishment of benthonic communities at that time may have been in response also to coarser sediment that could—physically—support a higher diversity of infaunal and epifaunal taxa. Influx of additional siliciclastic material during deposition of the middle part of the Bossier Formation diluted the accumulating OM more than had occurred during deposition of the underlying Haynesville Formation, and it encouraged these detritus-feeding behaviors. This explains the discrepancy between average TOC, slightly higher in the Haynesville than the middle Bossier (Hammes and Frébourg, 2012).

CONCLUSIONS Many previous interpretations regarding sourcerock deposition operated under the assumption that anoxia was necessary for preservation of marine organic carbon. Bioproductivity, however, may have outweighed stagnation as a controlling factor (Parrish and Curtis, 1982; Pedersen and Calvert, 1990; Ghadeer and Macquaker, 2011). Stratification of the water column, presumed necessary for the development of widespread anoxia and consequent preservation of marine organic matter, may have limited upwelling of external nutrients to shallow water needed to increase rates of photosynthesis (Wilde and Berry, 1984). Anoxia should not be used to justify greater preservation potential of organic carbon by means of enhanced rates of burial efficiency when only upwelling and anoxia together are considered as contributing factors. Basin restriction, paleogeographic setting, and eustasy worked in concert and with paleoecological changes to

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20  Jennifer D. Eoff

ultimately permit the accumulation and preservation of Type II kerogen. Canfield (1994) pointed out that an end-member scenario in which only organic matter was deposited will lead to an organic-rich rock unit regardless of decomposition type. Exploration for new self-sourced gas or liquidsrich continuous hydrocarbon accumulations, therefore, should not be restricted to stratigraphic sections of inferred paleo-upwelling or anoxia but pursue sections with favorable paleotectonic, paleogeographic, and linked paleoenvironmentalpaleoecological histories. Advancing understanding of the deposition of shale in the Haynesville– Bossier Formations in particular has identified the importance of basin restriction, eustatic flooding, low latitudes, and high rates of organic productivity during restructuring paleoecosystems.

ACKNOWLEDGMENTS I would like to thank Drs. Ursula Hammes, University of Texas Bureau of Economic Geology; Jennifer Aschoff, Colorado School of Mines; Rob Forkner, Shell International Exploration and Production; and M.E. Brownfield, U.S. Geological Survey, for their insightful reviews of this paper.

REFERENCES CITED Algeo, T.J., R.A. Berner, J.B. Maynard, and S.E. Scheckler, 1995, Late Devonian oceanic anoxic events and biotic crises: “Rooted” in the evolution of vascular land plants?: Geological Society of America Today, v. 5, p. 45, 64–66. Alpin, C.A., and J.H.S. Macquaker, 2011, Mudstone diversity: Origin and implications for source, seal, and reservoir properties in petroleum systems: AAPG Bulletin, v. 95, p. 2031–2059. Arthur, M.A., and B.B. Sageman, 1994, Marine black shales: Depositional mechanisms and environment of ancient deposits: Annual Review of Earth and Planetary Sciences, v. 22, p. 499–551. Arthur, M.A., and B.B. Sageman, 2005, Sea-level control on source-rock development: Perspectives from the Holocene Black Sea, the mid-Cretaceous Western Interior Basin of North America, and the Late Devonian Appalachian Basin, in N.B. Harris, ed., The deposition of organic-carbon-rich sediments: Models, mechanisms, and consequences: Society for Sedimentary Geology Special Paper 82, p. 35–59. Bambach, R.K., 1993, Seafood through time: Changes in biomass, energetics, and productivity in the marine ecosystem: Paleobiology, v. 19, p. 372–397. Bambach, R.K., 1999, Energetics in the global marine fauna: A connection between terrestrial diversification

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and change in the marine biosphere: GEOBIOS, v. 32, p. 131–144. Bend Arch-Fort Worth Basin Province Assessment Team, 2004, Assessment of undiscovered oil and gas resources of the Bend Arch-Fort Worth Basin Province of northcentral Texas and southwestern Oklahoma: U.S. Geological Survey Fact Sheet 2004-3022, (accessed August 1, 2011). Blair, N.E., and R.C. Aller, 2012, The fate of terrestrial organic carbon in the marine environment: Annual Review of Marine Science, v. 4, p. 401–423. Blakey, R. 2011, Paleogeography of North America: Colorado Plateau Geosystems, Inc. Bohacs, K.M., G.J. Grabowski, A.R. Carroll, P.J. Mankiewicz, K.J. Miskell-Gerhardt, J.R. Schwalbach, M.B. Wegner, and J.A. Simo, 2005, Production, destruction and dilution—The many paths to source-rock development, in N.B. Harris, ed., The deposition of organic-carbon-rich sediments: Models, mechanisms, and consequences: Society for Sedimentary Geology Special Paper 82, p. 61–101. Burbank, D.W., R.A. Beck, R.G.H. Raynold, R. Hobbs, and R.A.K. Tahirkheli, 1988, Thrusting and gravel progradation in foreland basins: A test of post-thrusting gravel dispersal: Geology, v. 16, p. 1143–1146. Burdige, D.J., 2005, Burial of terrestrial organic matter i n m a r i n e s e d i m e n t s : A re - a s s e s s m e n t : G l o b a l Biogeochemical Cycles, v. 19, p. GB4011 (7 p.). Burdige, D.J., 2007, Preservation of organic matter in marine sediments: Controls, mechanisms, and an imbalance in sediment organic carbon budgets?: Chemical Reviews, v. 107, p. 467–485. Burgess, P.M., 2008, Phanerozoic evolution of the sedimentary cover of the North American craton, in K.J. Hsü, ed., Sedimentary basins of the world: The sedimentary basins of the United States and Canada, v. 5, p. 31–63. Bush, A.M., and R.K. Bambach, 2011, Paleoecologic megatrends in marine metazoan: Annual Review of Earth and Planetary Sciences, v. 39, p. 241–269. Canfield, D.E., 1994, Factors influencing organic carbon preservation in marine sediments: Chemical Geology, v. 114, p. 315–329. Cant, D.J., and G.S. Stockmal, 1993, Some controls on sedimentary sequences in foreland basins: Examples from the Alberta Basin, in L.E. Frostick and R.J. Steel, eds., Tectonic controls and signatures in sedimentary successions: International Association of Sedimentologists Special Publication 20, p. 49–65. Cicero, A.D., I. Steinhoff, T. McClain, K.A. Koepke, and J.D. Dezelle, 2010, Sequence stratigraphy of the Upper Jurassic mixed carbonate/siliciclastic Haynesville and Bossier shale depositional systems in east Texas and north Louisiana: Gulf Coast Association of Geological Societies Transactions, v. 60, p. 133–148. Dashtgard, S.E., M.K. Gingras, and S.G. Pemberton, 2008, Grain-size controls on the occurrence of bioturbation: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 257, p. 224–243.

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