Journal of Sedimentary Research, 2009, v. 79, 584–592 Research Article DOI: 10.2110/jsr.2009.063
MAPPING AND QUANTIFYING THE CLAY AGGREGATE MICROPOROSITY IN MEDIUM- TO COARSE-GRAINED SANDSTONES USING THE 14C-PMMA METHOD PAUL SARDINI,1 ABDERRAZAK EL ALBANI,1 DIMITRI PRET,1 STE´PHANE GABOREAU,1 MARJA SIITARI-KAUPPI,2 AND DANIEL BEAUFORT1 1
Universite´ de Poitiers, HydrASA, FRE 3114 CNRS/INSU, 40 Avenue du Recteur Pineau, 86022 Poitiers Cedex, France e-mail:
[email protected] 2 University of Helsinki, HYRL, Department of Chemistry, Helsinki, Finland
ABSTRACT: Microporous aggregates of kaolin minerals and illite are common products filling the intergranular volume (IGV) of deeply buried sandstones. Using BSEi (backscattered electron imaging), a full analysis of IGV is difficult and time consuming since it necessitates a two-step process, at the scales of (1) macroporosity between the detrital grains, and (2) microporosity within the clay aggregates. The 14C-PMMA method permits the mapping and quantification of the connected porosity in sandstone core samples across scales. This method is based on the complete impregnation of the connected porosity by 14C-labeled MethylMethAcrylate (MMA) monomer, and its subsequent in situ polymerization. Imaging the spatial distribution of the radioactive tracer within a section of an impregnated sample allows an integrated analysis of IGV from the core scale to the nanometer scale. This technique is illustrated through analysis of the clay cement of a medium- to coarsegrained sandstone from the Athabasca Basin (Canada). The measured connected porosity distributions of kaolin mineral aggregates (average porosity 42%) and illite fillings (average porosity 70%) are in accordance with Gaussian distributions. This method also permits quantification of the relative volumetric amounts of the both illite and kaolin aggregate.
INTRODUCTION
Over the last few decades, several laboratory methods have been developed for determination of the spatial distribution of porosity in sandstone petroleum reservoirs. Since the 1980s, petrographic image analysis (PIA) has permitted characterization of pores in terms of contents, size distribution, and morphological parameters (e.g., Cooper 1997; Ehrlich et al. 1984; Francus 1998; Williams et al. 1998; Butler et al. 2001; Sime and Ferguson 2003; van den Berg et al. 2003; Rubin 2004; among others). Using optical microscopy (applied to samples saturated with a colored-dye resin), SEM using backscattered electron imaging (BSEi) or X-ray computerized tomography (XRCT), many studies have demonstrated that PIA is well adapted to quantify the intergranular volume (IGV) when it is free of mineral fillings. However, IGV is commonly cemented by aggregates of small mineral grains (Worden and Morad 2003). Clays, carbonates, and oxides are the most common minerals forming microporous aggregates (with pore sizes less than 1 mm). Imaging both the macroporosity and the microporosity of samples using BSEi is difficult because porosity has to be observed and quantified at different magnifications (Mowers and Budd 1996; Cerepi et al. 2002) and even the 0.1–0.5 mm resolution of this method represents a limitation for imaging small micropores. This study documents an alternative method of imaging rock porosity called the 14C-PMMA method, which allows characterization of the connected pores of rocks, for pore sizes ranging from centimeter to nanometer. It is based on autoradiographs of the surface of core sections after total impregnation of the connected pore spaces with radioactive resins such as 14C or 3H labeled MethylMethAcrylate (MMA) (Hellmuth et al. 1993; Siitari-Kauppi 2002; Sammartino et al. 2002; Sardini et al.
Copyright E 2009, SEPM (Society for Sedimentary Geology)
2006). In the present study, we used the 14C-PMMA method to map and quantify the porosity in a sandstone sample that suffered deep burial conditions with cementation of the residual pore space by clay aggregates composed of mesodiagenetic kaolin minerals and illite. This paper addresses the two following questions:
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What are the advantages of the 14C-PMMA method compare to other, more conventional, techniques in imaging and measuring porosity in clay-cemented sandstones? To what extent can the 14C-PMMA method discriminate and quantify the microporosity of coexisting kaolin minerals and illite aggregates in a sample?
The results of this study illustrate how this method permits a complete characterization of the distribution of connected porosity in samples with complex pore systems. These results should be broadly applicable and useful for analysis of other complicated sandstone (and carbonate) reservoirs. GEOLOGICAL BACKGROUND AND SAMPLE PETROGRAPHY
The core sample analyzed in this study (diameter 6.5 cm, length 13.3 cm) was collected in a drill hole intersecting the Athabasca Group sandstones in the surroundings of the Shea Creek unconformity-type uranium deposits (West Athabasca Basin, Canada). All details on the geological setting and the clay mineralogy of these sandstones can be found in Laverret (2002) and Laverret et al. (2006). The ca. 1700 Ma Athabasca Basin is composed of several sequences of moderate-grain-size quartz arenite and has been intensely drilled because it hosts most of the world-class unconformity–type uranium deposits presently known
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FIG. 1.—Macrophotograph of A) the half core surface and B) related autoradiograph. Grain-size variations are controlled by cross stratification. Autoradiograph of a large area that allowed analysis of thousands of pore elements and detrital grains in the same image with a spatial resolution of 20 mm.
worldwide (Pagel 1975; Hoeve and Quirt 1984; Rameakers et al. 2001, among many others). The sample exhibits medium to coarse, subangular to rounded grains, showing cross stratification with normal grading (Fig. 1). This sandstone is composed of quartz grains (, 90%) and minor amounts (, 10%) of phyllosilicates including diagenetic clay minerals and detrital muscovite, hematite, and zircon. The size of the quartz grains ranges from 0.1 to
2 mm. Petrographic analysis indicates a drastic occlusion of the primary porosity of the sandstone. This occlusion consists of (i) pressure solution, (ii) crystallization of quartz overgrowths, and (iii) precipitation of multistage clay cements within the residual IGV. IGV was partially or completely filled by a mixture of coarse-grained kaolinite and dickite (hereafter referred to as kaolin minerals) cogenetic to quartz overgrowths, which were subsequently altered and partly replaced by thin platy and
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TABLE 1.—Comparison of bulk porosities determined on different materials using different methods. Ww, Wd, WHg, and W14C-PMMA are porosities measured from water saturation, pycnometry, mercury porosimetry, and 14 C-PMMA method, respectively. Ref. [1] [1] [2] [3] [4] [4] [2] [2] [1] [1] [4] [1]: [2]: [3]: [4]:
Material Type Location
Ww or Wd (%)
WHg (%)
W14C-PMMA (%)
Basalt Not classified altered tonalite Sievi (Finland) Packstone Bure (France) Mudstone Bure (France) Cement Paste unaltered Mortar unaltered Wackestone Bure (France) Mudstone Bure (France) Geltech silica 2.5 nm pore size Geltech silica 5 nm pore size Cement Paste altered
2.2 8 12.1 14.4 17.6 18.4 21.4 22 43 51 59.4
0.97 6.2 14 13.9 23.3 17.7 -
2 5 14.4 15.4 20.5 20.4 21.4 24.3 44 59 73.6
Siitari-Kauppi (2002) Sammartino (2004) Sammartino et al. (2002) Sardini et al. (2007)
fibrous particles of illite (see Fig. A in JSR’s Data Archive, URL in Acknowledgments section). The bulk connected porosity of the sandstone sample was measured at 10.0% by the triple-weight method using water saturation (Franklin et al. 1981). 14
C-PMMA METHOD
The 14C-PMMA method involves sample drying, 14C-MethylMethAcrylate (14C-MMA) resin impregnation and polymerization, sawing, polishing, autoradiograph acquisition, and finally image analysis (Hellmuth et al. 1993; Siitari-Kauppi 2002). The total duration of the procedure varies from 2–3 weeks to 2–3 months depending on the permeability of the rock samples. For small samples (volume lower than 1 cm3), the cost of 14C-PMMA method is comparable to that of the mercury porosimetry technique; however, it varies as a function of the amount of radioactive resin used for the sample impregnation, and as a function of tracer activity. Impregnation with radioactive MMA was performed at the radiochemistry laboratory at University of Helsinki, Finland, the only laboratory in the world that specializes in this technique. The six-step method of analysis includes:
(1) (2) (3) (4) (5) (6)
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sample drying sample impregnation with 14C-labeled MMA 14 C-MMA polymerization sample sawing and polishing autoradiographs of polished surfaces digitization and image analysis of autoradiographs
The sample was completely dried to remove any free water from the connected void space. This step is essential because the MMA monomer used for impregnation is strongly hydrophobic. However, drying can induce damage by changing the volume of swelling minerals such as smectite, although this drawback is also encountered for SEM sample preparation or mercury intrusion porosimetry. In this study, the core sample was dried at 120uC for eight days. We assume that these drying conditions provided a good compromise with a short drying time and negligible damage of the material, since the rock sample used here contains no swelling clays, and, after drying, no changes on the spatial arrangement of clay particles were observed using SEM. The core was impregnated with 14C-labeled MMA under vacuum (10 Pa) for four days. Hellmuth et al. (1993) emphasized the need to determine empirically the impregnation time, which depends on rock permeability and sample size. Compared to crystalline rocks, sandstones require quite low impregnation times because of their much higher permeability. The maximum sample size is limited by the dimensions of the cylindrical impregnation cell; the inner diameter of the employed cell is 7 cm and the height is 18 cm. Conversely, small samples (grains of millimeter size or cuttings) can be impregnated with this technique. Very small pores are successfully impregnated by the MMA monomer, which is less viscous than water (at 20uC, dynamic viscosity of MMA and water are 0.584 mPas and 1.005 mPas respectively), and which have a small contact angle with silicates. For example, porous silica glass with nominally 2.5 nm pore diameter was successfully impregnated by MMA (table 1). Furthermore, Blumstein and Watterson (1968) and Preˆt (2003) illustrated that MMA works at diameters of less than one nanometer. Because water and MMA molecules are polar, they are both able to penetrate within the interlayer space of smectite (dipolar moment of MMA and water are 1.7 Debye and 1.85 Debye respectively). The 14CMMA is then radiolytically polymerized within the sample using a 60Co gamma-ray source for two days. During polymerization, the sample was immersed in MMA-saturated water to avoid excessive heating. The core sample was axially sawed. The first sawed surface was polished using corundum and silicon carbide powders (20 mm and 5 mm size, respectively). The final stage of this half-core surface preparation
FIG. 2.—Theoretical ranges of b particles emitted by A) 14C and B) 3H calculated using the Kanaya-Okayama (1972) relationship (Kanaya and Okayama 1972), as a function of their specific porosity filled by PMMA in various sandstone-forming minerals.
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FIG. 3.—Autoradiograph of the Athabasca sandstone at the thin section scale. Bright areas correspond to impervious grains (mainly quartz). Dark areas are related to intergranular porosity filled by authigenic clay minerals. The rectangle is the area magnified in Figure 4.
does not require an extra-fine grade of polishing because such a large sample cannot be observed under SEM. Different subsamples were cut from the second half-core. In order to examine these surfaces using SEM, the planar surface of these small samples were highly polished using
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diamond suspensions of 1 mm and 0.25 mm size. It should be emphasized that only the connected porosities calculated from a sample thicker than 300 mm can be considered as quantitative, because this distance is equal to the maximal range of beta emission from 14C within PMMA (Fig. 2). In thinner samples even the grains would be penetrated by the beta particles and tend to show up as porosity. Therefore, an autoradiograph obtained from a thin section (30 mm thick) cannot be used to obtain a quantitative porosity map, but it is suitable for a qualitative comparison with optical microscope observations (Sardini et al. 2006). Autoradiographs of polished surfaces were prepared. This step corresponds to detection of beta radiation emitted from the doped resin using a nuclear emulsion (film). Autoradiographs were performed using Kodak Biomax MR films for , 3 days, with a resolution of 20 mm. However, note that a microcrack of 0.1 mm aperture separating two impervious grains is visible in the autoradiograph, since the film sensitivity is very high. Conversely, a population of interconnected micropores of micrometric size which are close to each other (, 20 mm) are detected as a ‘‘background’’ porosity level because individual pores are not resolved by the film (Sardini et al. 2006). Autoradiographs were digitized using a desktop scanner (UMAX PowerLook III) in 8-bit graylevel and transmission mode with a 2400 dot-per-inch resolution (pixel size: 10.6 mm). Quantification of the connected porosity is based on the
FIG. 4.—A) Mosaic from BSEi and B) corresponding connected porosity map from a ROI of the thin section (Fig. 3). In this map, contrary to an autoradiograph, the darker gray level is associated with nonporous silicate grains, and white corresponds to pure resin. White arrow in Part B indicates a pore in the porosity map which is not visible on the SEM mosaic Part A. White rectangles located in Part A represent ROI analyzed in Figure 6.
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The mean connected porosity W of each pixel of the analyzed image is calculated from the optical density by the combination of Equations 1, 2, and 3. The calculation of porosity for all pixels images the frequency histogram of the connected porosity (or connected porosity distribution, CPD) (Hellmuth et al. 1993; Oila et al. 2005). CPD of regions of interest (ROI) were constructed pixel by pixel. The average porosity of a ROI is the mean value of porosity of all pixels of the analyzed ROI. Finally, connected-porosity maps were computed from the autoradiograph (Sammartino et al. 2002). Results of the 14C-PMMA method can be compared to other techniques also aimed at quantifying rock porosity (Table 1), including bulk porosity measurements obtained from water saturation (Ww, Franklin et al. 1981), pycnometric measurements (Wd), mercury porosimetry (WHg) and 14C-PMMA porosity. Since all these techniques quantify the connected porosity of rocks, it is not surprising that 14C-PMMA porosity is comparable to Ww, Wd, or WHg. However, note that 14CPMMA porosity is, on average, slightly higher than porosities obtained from conventional methods. This difference can be explained in terms of molecule properties of the fluid used to probe the rock: the low viscosity of MMA liquid, the low contact angle, and the dipole moment of MMA facilitates its penetration through nanometer-scale pore throats. MICROPOROSITY OF CLAY AGGREGATES
FIG. 5.— Connected-porosity distribution (CPD) from the thin section illustrated in Figure 3. See text for discussion.
relationship which exists between the optical density measured on autoradiograph and the local quantity of radioactive resin. The optical density of each pixel of the autoradiograph D 5 2log10(I/I0) is calculated from its gray level (I) and from the background (I0), which corresponds to the average gray level of an unexposed zone of the film. Optical density (D) is converted to specific activity (A) using an empirical calibration function A 5 f(D) proposed by Treutler and Freyer (1988): A ~ ð{1=kÞ ln ð1 { ðD { D0 Þ=Dmax Þ
ð1Þ
where the three parameters (k, Dmax, D0) are determined from the adjustment of standards of known specific activities exposed concomitantly with the sample. The connected-porosity calculation is applicable when the major fraction of the emitted electrons is attenuated by a homogeneous mixture of rock and resin (Siitari-Kauppi 2002). This assumption is valid for pores between clay-mineral aggregates because the size of such pores is under the resolution of the autoradiographic film (about 20 mm) and under the range of beta particles emitted by 14C (Fig. 2). For each pixel, the connected porosity W [0–100%] is readily obtained from specific activity A: W ~ 100 | b | ðA=A0 Þ
ð2Þ
where A0 is the activity of the pure resin used for impregnation (here 55 kBq/ml) and b 5 (ra/r0) represents a correction factor related to the difference of absorption of emitted electrons between the impregnated rock and the standards (ra is the bulk rock density and r0 is the density of the PMMA polymer, 1.18. Thus, ra is calculated (Equation 3) as ra ~ ð1 { WÞrg z Wr0
ð3Þ
where W is the connected porosity and rg is the grain density (here rg is set to 2.6 from Helium pycnometer measurements).
Observations and Bulk Measurements Several aspects of the heterogeneous distribution of 14C-MMA impregnating the pore space are evident on the autoradiographic film (Figs. 1, 3). First, the whole sample was fully impregnated with the tracer, because no porosity gradient is observed between the boundary (in contact with the external resin) and the inner part of the half-core sample (Fig. 1). Second, the autoradiograph provides a macroscopic image of the connected porosity of the sandstone compared to a macroscopic photograph of the half-core sawed surface. The variations in grain and pore sizes occurring parallel to the length of the core are the result of depositional sedimentary processes. Third, at the scale of the thin section (Fig. 3), the higher gray levels of the film (nonporous zones) are related to the detrital silicate grains. Intergranular volume seems to be fully impregnated with the tracer; however, the connected porosity map of a ROI of the thin section reveals important variations of porosity within this volume (Fig. 4B). Investigations presented in the next section analyze how these variations are correlated to the type of clay cements filling the intergranular volume. The comparison between the connected-porosity map and the SEM backscattered view shows that only rarely are pores (estimated to be , 0.01% of the total intersected pore area) observed under SEM not identified in the autoradiograph (Fig. 4). This omission is most common for voids within quartz grains which were not impregnated because they are disconnected pores. Conversely, some rare bright spots (estimated to be , 0.01% of the total intersected pore area) are visible in the connected-porosity map that do not correspond to any pores identified by SEM (see white arrow in Fig. 4B). Presumably these spots are related to cavities filled with 14C-PMMA that are located below the observation surface, at a very shallow depth lower than the maximal b particle range within silicates, which is close to 120 mm (Fig. 2). Calculation of connected-porosity distribution and average connected porosity were first performed at the scale of the whole thin section (Fig. 3). For the entire thin section, the average connected porosity is 12%. This value is slightly higher than the porosity obtained by gravimetric measurements (10%), confirming the trend (Table 1). The CPD of the
R FIG. 6.—Comparison of connected porosity maps (B, D, F, H) and BSEi (A, C, E, G) for four types of intergranular fillings (KAO 5 kaolinite, ILL 5 illite, KAO/ ILL 5 mixed kaolinite and illite, MUS 5 muscovite). A, B) ROI with KAO fillings (Wmean 5 20.7%). C, D) ROI with KAO + ILL fillings (Wmean 5 19.2%). E, F) ROI with KAO fillings and MUS (Wmean 5 21.6%). G, H) ROI with KAO + ILL + KAO/ILL fillings (Wmean 5 12.8%).
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FIG. 7.— Connected-porosity distribution related to porosity maps provided in Figure 6. See text for discussion.
entire section (Fig. 5), using only those pixels related to porosities ? 0%, reveals several relations. First, the porosity bin [99%–100%] is mainly related to volumes that are totally filled with the radioactive resin. Second, the low-porosity bins (close to 0%) are mainly associated with the margins of detrital grains. These pixels obscure other modes in the intermediary porosity bins, which are related to clay-cement microporosity.
FIG. 8.— BSEi view of A) a pore element, and B) its corresponding porosity map, where the set of nonporous pixels is represented with hachures. C, D) Hatched zones in porosity maps are pixels of dilation size 4 and 8, respectively, from the set of nonporous pixels given in Part B. Border effects are progressively masked by the dilation. However, small intercepted grains are not systematically masked by the dilation (see arrow in Part A).
Characterization of Intergranular Volume (IGV) Microporosity The connected porosity was quantified within the sample containing a variable proportion of each of the clay-mineral fillings visible under BSEi. Four regions of interest (ROI) are given here as an example (Fig. 6, ROI located Fig. 4A): (1) kaolin minerals, (2) illite, (3) detrital muscovite and kaolin minerals, and (4) kaolin minerals replaced to varying extents by illite. Connected-porosity distributions were computed for each ROI. The relationship between the type of mineral fillings and the associated CPD (Fig. 7) illustrate that the Gaussian distributions on connected-porosity distributions centered near 40%, 60–70%, and 80% correspond to kaolin minerals, illite, and muscovite, respectively. These trends were systematically observed for any pore element analyzed, but they remain inadequate to provide a statistically representative quantification of the CPDs of the different mineral fillings and to estimate the relative amount of these fillings within the thin section. The low-porosity bins of the CPD determined on the thin section obscure porosity bins related to clay-aggregate microporosity. The detailed connected-porosity maps reveal these low-porosity pixels and show that they are mainly associated with the boundaries of silicate grains (Fig. 8, and also see Fig. B in Data Archive). A progressive transition of the film blackening occurs systematically at the interface between two solids having different tracer content (i.e., different porosities), the thickness of this transition depending on the path length of a beta particle laterally emitted by the 14C disintegration (Fig. 2A, and also see Fig. C in data archive). Therefore, we chose to ignore these boundary effects using a simple imaging operator, i.e., the dilation operator (Serra 1982). Elementary dilations of the ‘‘nonporous pixels’’ (corresponding to the nonporous quartz grains) were performed using a six-neighbor configuration (Fig. 8). The connected-porosity distribution of the remaining
pixels in the intergranular volume was successively computed according to the dilation size. This size is equal to the number of elementary dilations performed from the nonporous grains. From 8 to 10 dilations were necessary to reach stability of the CPD (and also see Fig. D in Data Archive). This result is realistic from a physical point of view, because 10 dilations represent here about 100 mm, a distance that is close to the maximal trajectory length of beta particles emitted by 14C disintegration within silicates (Fig. 2). It must be emphasized that the lateral range of 14 C-emitted b particles would prevent the applicability of the method to fine-grained sandstone. To generalize the CPD calculation it would be useful to employ 3H-MMA (tritium-labeled MMA) as a tracer, because the range of b particles emitted by tritium is less than 8 mm whatever the surrounding material (Fig. 2B). Decomposition of the connected-porosity distributions at each step of the dilation process was performed using two normal distributions, one for kaolin fillings and one for illite fillings (Fig. 9). This operation allowed determining the CPD of each type of clay filling and their respective amounts. For a dilation size larger than 10, the modes found for the normal distributions of kaolin minerals and illite remain stable around porosity values of 42% (s.d. 13.5%) and 70% (s.d. 15.5%), respectively (Fig. 10A). The average connected-porosity values we found for these different types of clay fillings are in line with those by Hurst and Nadeau (1995), which are used as a reference for characterization of aggregate microporosity (Worden and Morad 2003; Milliken 2003). Hurst and Nadeau (1995) found average porosities of kaolin minerals and illite aggregates equal to 43% and 63% respectively. An improvement would be to estimate the impact of dickitization on the microporosity of kaolin mineral aggregates by quantifying the relative amount of these
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FIG. 9.—Connected-porosity distribution of the remnant IGV are decomposed using two normal distributions. From left to right, decomposition is illustrated for the connected-porosity distribution related to dilation size 2, 6, and 10 from the set of nonporous pixels. Low and high porosity peaks characterize the CPDs of kaolinite and illite aggregates, respectively.
minerals using IR spectrometry (Lanson et al. 2002). The average porosity of illite aggregates is slightly higher than the value given by Hurst and Nadeau but is close to the value given by Worden and Burley (2003) for a depositional mud (, 70%). The standard deviation of the connected-porosity distribution of illite aggregates (15.5%) is slightly higher than that obtained for kaolin aggregates (13.5%); this is probably due to the fact that mixed kaolin minerals and illite aggregates also present an average porosity of 70%, increasing the standard deviation of the related CPD. Multi-mineral aggregates (containing kaolin minerals and illite) present the same microporosity as illite fillings (, 70%). In addition, the area ratio of these two normal distributions (i.e., the ratio of modal contents) also converges to 1:1 for dilation sizes higher than 10 (Fig. 10B). This area ratio should represent the volumetric ratio of the two clay filling types (considering the total volume of fillings, including pores and crystals). CONCLUSION 14
The C-PMMA method was applied at the scale of the core as a microimaging tool in order to map and quantify the microporosity of a medium- to coarse-grained sandstone cemented with a clay matrix composed of mixed kaolinite, dickite, and illite. The approach provides a new method to perform quantitative investigations of the microporosity at a scale for which data are still scarce in the literature: the scale of the pore-filling clay-mineral aggregates within cemented sandstones. It is possible to apply the method to any medium- to coarse-grained sandstone presenting microcrystalline cementation. Further developments of the method would be of interest to investigate in more detail the pore cementation by more complex mineral mixtures including clay minerals and other associated minerals. The 14C-PMMA method could also be an alternative tool in the evaluation of the ‘‘clay-bound water volume’’ which is an important parameter for estimation of reservoir quality in hydrocarbon sandstones. ACKNOWLEDGMENTS
FIG. 10.— Evolution of A) modes and B) normalized areas of the two normal distributions according to the dilation size. The stability of these parameters is reached after 8–10 dilations.
The studied samples were provided by the AREVA NC Company. The authors wish to thank R. Worden, S. Morad, A. Hurst, and an anonymous reviewer for their constructive suggestions having considerably improved the quality of the manuscript. The authors are also grateful to Alain Meunier for his careful reading of the manuscript, and we wish to thank Karl-Heinz Hellmuth for sample impregnations. We also thank Denis Paquet for the preparation of thin sections and thick sections used for this work. Figures A– D are available from JSR’s Data Archive at: http://www.sepm.org/jsr/ jsr_data_archive.asp.
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