This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and

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1 This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier s archiving and manuscript policies are encouraged to visit:

2 Marine and Petroleum Geology 26 (2009) Application of complementary methods for more robust characterization of sandstone cores S. Baraka-Lokmane a,b,, I.G. Main c, B.T. Ngwenya c, S.C. Elphick c a Institute of Petroleum Engineering, Heriot-Watt University, Edinburgh, UK b School of Environment and Technology, University of Brighton, UK c School of Geosciences, University of Edinburgh, UK Received 19 January 2007; received in revised form 2 November 2007; accepted 4 November 2007 Abstract This paper is based on detailed mineralogical, structural, petrophysical, and geochemical studies of sandstone core samples, using routine core analysis methods. These include X-ray computer tomography (CT) scanning, magnetic resonance imaging (MRI), particle size analysis, point counting based on petrographic thin sections, environmental scanning microscopy (ESEM), X-ray diffraction (XRD), and X-ray fluorescence (XRF). In this study, we demonstrate the feasibility of combing these complementary methods in the characterization of reservoir properties of sandstone cores. Four types of sandstones (Slick Rock Aeolian, Fife, Locharbriggs, and Berea sandstones) that differ in grain size, porosity, and mineralogy have been characterized. The results of the different methods used were found to be consistent with each other, but the combination of a variety of methods has allowed a fuller characterization of the rock samples than each method used on its own, bringing out subtle variations in petrographic characteristics that add significant value towards a better description of reservoir properties. For example, it becomes apparent that some types of rocks like Berea sandstones thought to be homogeneous are in fact heterogeneous. The recognition that rock heterogeneity at the sub-centimeter scale may have a significant effect on hydrocarbon recovery requires that field-scale reservoir models take account of these small-scale effects in order to lay claim to reasonable accuracy in production forecasts. r 2007 Elsevier Ltd. All rights reserved. Keywords: Routine core analysis; Slick Rock Aeolian sandstone; Fife sandstone; Locharbriggs sandstone; Berea sandstone 1. Introduction The need for accurate reservoir characterization is important in developing an understanding of geologic complexity that impacts field development. Reservoir quality, primarily determined by the porosity and permeability of the relevant formations, is controlled by many parameters. These include the nature of the constituent minerals and cement, the degree of rock cementation; the degree of sorting or equivalently the particle size distribution of the grains; and the pore-size distribution (Cade et al., 1994; Corresponding author at: School of Environment and Technology, University of Brighton, Lewes Road, Brighton BN2 4GJ, UK. Tel.: ; fax: address: S.Baraka-Lokmane@brighton.ac.uk (S. Baraka-Lokmane). Baraka-Lokmane et al., 2001b). Field development methods that rely on core flooding are particularly sensitive to small-scale changes in reservoir quality. Meanwhile, most water floods in sandstone cores are carried out either in almost homogeneous samples or else in core samples of uncertain heterogeneity. As a result, the interaction of small-scale sedimentary heterogeneity with the fluid mechanics of water oil displacement cannot be adequately understood or quantified. Since most clastic sediments show some degree of lamination, we might expect this to have a significant influence on oil displacement efficiency and residual/remaining oil saturation (Huang et al., 1994). In enhanced oil recovery processes, where the displacement mechanism may be particularly complex, the structure of the heterogeneity may have a vital bearing on the success of the scheme (Sorbie et al., 1992; Corbett et al., 1992) /$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi: /j.marpetgeo

3 40 S. Baraka-Lokmane et al. / Marine and Petroleum Geology 26 (2009) This paper presents a method of characterizing and quantifying petrographic and petrophysical properties in sandstone cores using a combination of several techniques used in conventional routine core analysis. These include X-ray computer tomography (CT) scanning, magnetic resonance imaging (MRI), particle size analysis, point counting based on petrographic thin sections, environmental scanning microscopy (ESEM), X-ray diffraction (XRD), and X-ray fluorescence (XRF). The aim is to demonstrate that by using a suite of techniques in combination; subtle variations in petrophysical characteristics emerge that add significant value towards a better description of reservoir properties. It also becomes apparent that the use of magnetic resonance methods to measure fluid saturation and the distribution of water within the core samples is inherently problematic in cases where samples contain iron. 2. Summary of materials and methods Four types of sandstones that differ in grain size, porosity, and mineralogy were studied. The materials chosen for the study were Fife and Locharbriggs sandstones both from southwest Scotland, Slick Rock Aeolian sandstone from Utah, United States, and Berea sandstone from Ohio, United States. MRI analysis was carried out on one sample. CT scanning was performed on 24 samples. About 48 thin sections were studied using the petrographic microscope and modal mineral and pore percentages, based on 400-point counts, were determined for 24 samples. The porosity was estimated using the commercially available imaging software Scion image. Twenty-four samples have been studied using ESEM with an energy dispersive X-ray (EDX) spectrometer and for XRF analysis. A total of 37 samples were analyzed for particle size analysis, bulk mineral composition, and clay mineral composition of the 2-mm size fractions using XRD. Helium porosity as well as gas and liquid permeability have been measured on 18 samples. In this study, we have carried out MRI measurements in order to determine the distribution of water within the samples and the water saturations. CT scans have been used to characterise core level heterogenieties, surface features, and internal structure. The particle size analyses were used for characterizing and classifying the samples, for determining their heterogeneity, the percentages of the different particles (clays, silt, and sand), the degree of sorting, and the mean grain size of the samples. The thin sections were used to characterize mineralogical and textural features. Modal analysis was done by point counting. Pore space characteristics were determined by examining areas of the thin sections that were impregnated with the blue epoxy. The ESEM was used for studying the wettability and the fluid distribution. Clay size (o2 mm) fractions analyzed by XRD were separated using sedimentation techniques. XRD analysis of bulk rock samples was used to verify significant variations in mineralogy and to determine the major mineralogical composition of samples too fine grained for thin section analysis. XRF analysis was carried out for determining the geochemical composition of the samples. 3. X-ray computer tomography (CT-scanner) Over the past several decades, X-ray CT has gained acceptance as a routine core analysis tool. Generally, medical CT scanners have been employed because of their availability and relative ease of use. This technique is used to characterise core level heterogenieties and to explain their effect on horizontal and vertical permeabilities (Honarpour et al., 2003). This scanning technique is a non-destructive imaging technique that allows visualization of surface features and internal structure within the core samples. The following features can be characterized: bedding features and sedimentary structures, natural and coring-induced fractures, cement distribution, small-scale grain size variation, and density (mineralogical and chemical composition) variation (Coles et al., 1998). Six CT-scanning images were taken for each sample positioned at an equal distance of 5.42 mm. The resolution of CT scanning image is 60 mm 60 mm 1 mm. These different features are distinguished by their different penetrabilities to X-rays. Higher luminance values represent greater penetration of the rock by the X-rays and, therefore, lower density. Conversely, lower luminance represents areas of rock with higher density and therefore greater X-ray stopping ability. The luminance values are, therefore, inversely related to the rock density. Variations in composition can produce differences in absorption of radiation if there is a significant contrast in the electron density of the materials. This allows precise identification of mineral constituents via their grayscale representation in a CT image. Very dark kaolinite, it has a very low physical density gray: (r ¼ 2.60 g/cm 3 ). Dark gray: quartz and feldspar, they have a physical density of 2.65 g/cm 3. The quartz has a low radiological density varying between 120 and 160 H.M.U. Light gray: muscovite and illite, they have a medium physical density (2.80 g/cm 3 ) and therefore a medium radiological density due to the presence in their chemical formula of potassium. Light gray: potassium feldspars (microcline) have a medium radiological density ( 50 H.M.U.). This density could be low if Na and Ca replace partially K. Indeed, the attenuation of Na 2 O and CaO is lower than the attenuation of K 2 O. White: calcite is a very attenuating mineral; it has a physical density of 2.94 g/cm 3 and a high radiological density (241 H.M.U.).

4 S. Baraka-Lokmane et al. / Marine and Petroleum Geology 26 (2009) White: hematite has a physical density higher than the other minerals (5.26 g/cm 3 ) and therefore a high radiological density due to the presence in its chemical formula of iron. In sandstone cores, calcite and ferromagnesian minerals would likely be registered as an anomaly in X-ray scan. X-ray CT scanning images of Slick Rock Aeolian sandstone samples show the different mineral phases: calcite in white, microcline in light gray, and quartz in dark gray (Fig. 1). The white areas seen on the images indicate the presence of calcite cements. The distribution of calcite cement is very heterogeneous, being distributed in white patches or layers, and iron banding is seen in the near-white layers. The XRD, XRF analyses, and the petrographical study have shown that the highly attenuating mineral contains iron; which constitute around 0.32% of the samples (see Sections 8 and 9). For the Locharbriggs sandstone samples, the X-ray CT scanning images have shown that the samples are characterized by white colored layers, which suggest a highly attenuating mineral; indeed the bulk samples are characterized by a red tint, and the highly attenuating mineral probably contains more concentrated iron bands (Fig. 2). The XRD, XRF analyses, and the petrographical study have shown that these samples contain 0.7% of hematite present as cement. Hematite gives the red pigmentation to the samples. For the Berea sandstone samples, the different CT images have shown a very homogeneous and isotropic material. Fig. 3 shows the presence of white areas, indicating the development of nodules, which could be calcite or iron. Indeed both calcite and iron are very attenuating minerals to X-rays, characterized by high values of physical and radiological densities. In the case of Fife sandstone samples, the different X-ray scanning images have shown the presence of traces of calcite cements, which are locally nodular (Fig. 4), and the Fig. 2. Slice 4 of Sample 5 of the of iron (Locharbriggs Sandstones). Fig. 3. Slice 5 of Sample 7 of the Berea sandstone. presence of areas with a very dark gray color, indicating the presence of kaolinite nodules (Fig. 5). Indeed kaolinite is characterized by a low physical density (r ¼ 2.60 g/cm 3 ) and therefore a very low radiological density. We conclude that the method is particularly useful for imaging calcite and hematite or iron. 4. Magnetic resonance imaging (MRI) measurements Fig. 1. Slice 2 of Sample 4 showing the banding and location the calcite cement (Slick Rock Aeolian sandstone). The NMR technique is a very powerful tool for quantitative, qualitative, and structural analysis. The principal advantage of this method of measurement is that it is not a destructive method, and no absorbent compound is used. NMR has long been used in the oil industry as a research tool for the measurement of petrophysical rock properties such as bulk porosity (Cowgill et al., 1981), permeability (Kenyon et al., 1986), wettability (Williams

5 42 S. Baraka-Lokmane et al. / Marine and Petroleum Geology 26 (2009) Fig. 4. Slice 3 of Sample 4 of the Fife sandstone. Fig. 5. Slice 4 of Sample 9 of the Fife sandstone. and Fung, 1982), the determination of the distribution of pore sizes (Kozlov and Ivanchuk, 1982; Gallegos and Smith, 1988), the degree of water saturations in sample (Saraf and Fatt, 1967), the determination of the fracture geometry, and the distribution of water saturation in a water flood experiment in a porous fractured sandstone (Baraka-Lokmane et al., 2001a). In this study, we have carried out MRI measurements in order to determine the distribution of water within the samples and the water saturations. The principle behind magnetic resonance methods is that when nuclei with magnetic moments are put in a static homogeneous magnetic field, populations are distributed on discrete energy levels. A suitable radio-frequency (RF) field is then applied to induce transitions between these energy levels, thus changing the magnetization. At the end of radio-frequency excitation, nuclei return to equilibrium and the magnetization decays as a function of time. The frequency of this decay depends on the strength of the magnetic field and is characteristic of both the type of nuclei species and its chemical environment. Preliminary MRI measurements have been carried out on Sample 8 of the Slick Rock Aeolian sandstone. The dimensions of the sample are 45 mm in length and mm in diameter. The preliminary MRI measurements showed that no images were obtainable for the Slick Rock Aeolian sandstone sample using the conventional MRI with a magnetic field strength of 4.7 T, because the transverse relaxation T2* was reduced dramatically by the presence of small quantities of iron. The transverse relaxation T2* used for this experiment was equal to 100 ms. In general, a transverse relaxation T2* of 10 ms or greater is necessary for conventional MRI. The problems of noise due to the presence of iron may be related to the use of relatively high magnetic fields. According to the literature (Baraka-Lokmane et al., 2001a; Britton et al., 2001), this type of measurement should be done at low field or even in the fringe field of magnets to avoid such problems. The principal difficulty presented by fluid/rock systems in terms of NMR imaging is that, for many such materials, the transverse relaxation of the contained fluids can be strongly affected by magnetic susceptibility variations on the scale of the pore dimensions (De Panfilis and Packer, 1999). The choice of magnetic field strength for imaging studies of fluid-saturated rocks is a compromise between the sensitivity and resolution required and the performance needed to overcome, as much as possible, the field-dependent T 2 effects. Many imaging studies of rocks have used field strength of 2 T, corresponding to 1 H resonance frequency strength of 85 MHz, and excellent quality images are achieved at this frequency (Britton et al., 2001; Pape et al., 2005). We therefore repeated the experiment using a lower magnetic field of 1.5 T, but unfortunately it was not possible to obtain any image. The reason could be the screening effect of the spatial heterogeneity of local magnetic susceptibility. Some rock samples are described to be not NMR friendly ; the Slick Rock Aeolian samples belong to this category of samples. The problem of noise due to the presence of iron occurs principally when the iron is not homogeneously distributed; indeed the rock samples present red layers, which are probably more concentrated in iron than the rest of the sample. In summary, the Slick Rock Aeolian samples could not be characterized by NMR at high field or at low fields due to total iron content and to its large spatial heterogeneity. As a consequence, the continuous-wave magnetic resonance imaging (CW-MRI) technique was used for the visualization of the sample. In continuous-wave (CW) magnetic resonance, a radio-frequency is applied continuously at a fixed frequency (f 0 ) at low power. To detect the resonance, the magnetic field (B 0 ) is increased slowly and when the resonance condition is satisfied (o 0 ¼ 2pf 0 ¼ gb 0 ) the electronic properties of the radio-frequency resonator are altered and the signal is detected. To improve the signal-to-noise ratio a technique called magnetic field modulation with lock-in detection is used. Here, the value of B 0 is modulated at low frequency (about 1 khz in this

6 S. Baraka-Lokmane et al. / Marine and Petroleum Geology 26 (2009) Fig. 6. Development of the waterfront inside Sample 8 of the Slick Rock Aeolian sandstone. case). This means that when the swept field comes close to resonance the sample will actually go in and out of resonance at the modulation frequency, and so the signal from the resonator has a strong component at the modulation frequency. A lock-in amplifier is used to pick out this signal from within a noisy background (Lurie et al., 1996). The bore of the magnet is not large enough to stand the core sample on end. The sample was vertically imbibed with water, and then was transferred horizontally into the magnet for imaging. The imbibition took place at 11:45 with the sample immersed in 10 mm of water for 15 s. The results using the CW-MRI technique clearly show the development of water front (Fig. 6). Initially the waterfront is concave (first image). This waterfront spreads out slowly on subsequent images. (Second-order local fluctuations in the amplitude of the signal are due to amplification of background noise during digital data processing.) The quality of these preliminary images needs to be improved. The system used for the measurements is a prototype, which has only two gradient coils. As a result the images do not represent a thin enough slice through the core; they show a more diffuse projection of the spin density throughout its whole volume. We conclude that the finite presence of heterogeneously distributed hematite for the Slick Rock Aeolian sandstones implied by the null MRI results is confirmed. 5. Particle size analysis Mean grain size (d mean ) is defined as the average value of the diameters of all grains in a reference area. The grain size is often a major control of permeability. The LS 100 Grain Size Analyzer is used for these measurements. Because of the relatively low cement content in the rock samples a simple finger manipulation allowed disaggregation of the grains of the sandstones cores. The 3-D bulk particle size analyses were used for characterizing and classifying the samples, for determining their heterogeneity, the percentages of the different particles (clays, silt, and sand), the degree of sorting (So), and the mean grain size of the samples (d mean )(Table 1). However, this information does not describe the spatial relationships of the minerals and cements in the rock microstructure. This study has shown that the four groups of samples are very well sorted (the degree of sorting (So) is less than 1.4) (Fig. 7), but only the Fife sandstone samples are homogeneous, in the sense that they are composed mainly of fine sand, which comprises between 96.42% and 100% of the sample by volume. The Fife sandstones present the lowest percentage of clay and silt (about 0.8% together) and it is the most uniform material (the coefficient of uniformity lower than 2). The Berea sandstones present the highest percentage of clay and silt (about 5.6% together) and it is the most heterogeneous material (the coefficient of uniformity greater than 2). The Locharbriggs sandstone samples present the smallest mean grain size (varying between and mm) and the Fife sandstone samples present the biggest mean grain size (varying between and mm) (Table 1). 6. Environmental scanning electron microscopy (ESEM) measurements Wettability is the term used to describe the relative adhesion of fluids to a solid surface, and is a measure of the tendency of one of the fluids to wet (closely adhere to) a surface (Tiab and Donaldson, 1996). The wettability state of any reservoir rock is of great importance; as this will help determine how the fluids (oil and water) are distributed within pore spaces and how they interact with the mineral phases (grains) present within the rock. Wettability (relative hydrophobic/hydrophilic nature of the mineral phases/reservoir) is therefore an important factor in the prediction of oil or water flow, retention and likely oil yield. Wettability may also be significantly altered during the injection of remedial chemicals such as surfactants introduce prior to scale inhibitor treatments. Wettability data is therefore a highly important parameter to be considered within oilfield economics (Buckman et al., 2000). The wettability characteristics of a porous medium play a major role in a diverse range of measurements including: capillary pressure data, relative permeability curves, electrical conductivity, and water flood recovery efficiency. The advantage of this technique is that it is much faster than the majority of techniques used to quantify wettability (e.g. Armott and United States Bureau of Mines wettability indexes), which also only produce, results at the

7 44 S. Baraka-Lokmane et al. / Marine and Petroleum Geology 26 (2009) Table 1 Particle size analysis of the four groups of samples Sample % of clay %of silt %of sand d 10 (mm) d 25 (mm) d 30 (mm) d 50 (mm) d 60 (mm) d 75 (mm) So U c C c d mean (mm) Median (mm) Mode (mm) S S S S S S S S S S S S S S S S L L L L F F F F F F F F B B B B B B B B B S, Slick Rock Aeolian sandstone; L, Locharbriggs sandstone; F, Fife sandstone; and B, Berea sandstone. d 10 : grain size (mm) of 10% (cumulative wt%); So: degree of sorting; U c : coefficient of uniformity; C c : coefficient of curvature, and d mean : mean grain size. whole oilfield reservoir. Moreover, the low-vacuum ESEM enables the examination at the micrometer scale of nonvacuum compatible samples, in their natural state, without any form of special preparation. In this study, we have used the ESEM technique in order to observe the dynamics of wetting and any changing in wettability property of the different minerals. The wettability measurements were carried out using a Philips XL30 Environmental SEM equipped with a LaB6 gun, and an EDAX SUTW EDX detector. Our 3-D ESEM measurements have enabled the direct measurement of the wettability of the different minerals, and the identification of the mineralogy of the samples. Through manipulation of temperature and pressure, it is possible to control the samples relative humidity. This feature makes it possible to condense water droplets on the specimen s surface and to observe the contact angle between mineral phases and water droplet. An indication of wettability can be obtained through examination of the contact angle formed between a solid surface and a droplet of fluid on that surface, with the angle being recorded through the denser fluid (Gauchet et al., 1993; Robin and Cuiec, 1998). High contact angles (greater than 901) are associated with surfaces that are hydrophobic (Fig. 8a), consequently oil wet, while low contact angles (lower than 901) occur in association with hydrophilic water wet surfaces (Fig. 8b) and intermediate in wetness are characterized by an intermediate contact angle (E901) (Fig. 8c). The ability to use EDX analysis (with light element capabilities) during such procedures is also important for the indication of the mineral phases being examined. The ESEM measurements carried out on the Berea sandstone samples show the presence of iron (Fig. 9). Kaolinite is the dominant cement for Berea sandstones. The measurements of the wettability of kaolinite clay in the case of Berea sandstones, show that this mineral is clearly

8 S. Baraka-Lokmane et al. / Marine and Petroleum Geology 26 (2009) Particle Size Distribution Clay Fine Silt Medium Silt Coarse Silt Fine Sand Summation percentage [%] S L F B Grain size [mm] Fig. 7. Particle size distribution of the four groups of samples. Fig. 8. Schematic diagrams of three levels of wettability: (a) strongly oil wet condition; (b) strongly water wet condition; and (c) intermediate in wetness condition. Fig. 9. ESEM photo, showing the presence of iron (Berea sandstone). Fig. 10. ESEM photo, showing very water wet kaolinite (Berea sandstone). very water wet (Fig. 10). For the four groups of samples, fresh surfaces of feldspar and quartz are both water wet (low contact angles, lower than 901) (Figs. 11 and 12). The ESEM measurements have shown that illite is the dominant cement for the Slick Rock Aeolian sandstones and it covers the rounded grains of quartz and the feldspar minerals creating irregular surfaces. Interstitial illite particles, quartz, and feldspar minerals covered by illite, are all very water wet (Fig. 13). Fig. 14 shows three different states of wettability for the Slick Rock Aeolian sandstone. Quartz covered by illite shows the forming of a distinctive sheet of water. This shows clearly that quartz

9 46 S. Baraka-Lokmane et al. / Marine and Petroleum Geology 26 (2009) Fig. 11. ESEM photo, showing water wet feldspar. Fig. 14. ESEM photo, showing three different levels of wettability: water wet on clean quartz surface (Q) and calcite (C), very water on feldspar covered with illite (F), and oil wet on the aluminium plate (S) (Slick Rock Aeolian sandstone). Fig. 12. ESEM photo, showing water wet quartz. Fig. 15. ESEM photo of a cement smectite (Locharbriggs sandstone). Fig. 13. ESEM photo, showing the low dome shaped droplets of water on the feldspar, indicating the very water wet property of illite (Slick Rock Aeolian sandstone). covered by illite is very water wet, calcite crystal, with low droplets of water forming, this indicates that calcite is water wet, and a surface, which is intermediate water wet. Indeed, the contact angle formed between a solid surface and a droplet of fluid on that surface is exactly equal to 901. This surface is the aluminium plate of the ESEM apparatus. Locharbriggs samples show the presence of smectite, illite and kaolinite cements (Figs ). Figs. 15 and 16 show a wettability test performed on smectite for Locharbriggs samples. Fig. 16 shows the very water wet property of the semectite. Figs. 17 and 18 show a wettability test performed on kaolinite, smectite, and illite for Locharbriggs samples. Fig. 18 shows the very water wet property of the clays. For the Fife sandstones, the wettability measurements carried out on kaolinite covering feldspar minerals (Fig. 19) show that kaolinite is water wet, showing low droplets of water forming (Fig. 20).

10 S. Baraka-Lokmane et al. / Marine and Petroleum Geology 26 (2009) Fig. 16. ESEM photo, showing very water wet smectite (Locharbriggs sandstone). Fig. 19. ESEM photo, showing kaolinite cement covering a feldspar crystal (Fife sandstone). Fig. 17. ESEM photo of a cement containing kaolinite, smectite, and illite (Locharbriggs sandstone). Fig. 20. ESEM photo, showing the low dome shaped of water on the kaolinite clays (Fife sandstone). The ESEM technique is a relatively rapid method of measurement, which is useful for the identification of the different minerals of the rocks as well as studying their wettability characteristics. 7. Petrography and porosity estimation Fig. 18. ESEM photo, showing the very water wet property of the clays (kaolinite, smectite, and illite) (Locharbriggs sandstone). The study has been extended to the 2-D thin sections analysis, using the point counting technique. The results are reported as percentages of the different mineral constituents and the dominant cement, and the percentage porosity (Table 2). During the liquid permeability measurements, clays may absorb brine and increase in volume. Kaolinite [Al 4 Si 4 O 10 (OH) 8 ], having a stable, rigid structure (due to the strong book bond), reacts with water to a minimum extent, by contrast with minerals of the smectite group

11 48 S. Baraka-Lokmane et al. / Marine and Petroleum Geology 26 (2009) Table 2 Mineralogy compositions of the four groups of samples (from 2-D thin sections analysis using point counting technique) Sample number Framework Cement Porosity 1 (%) Quartz Feldspar Mica Clay+hematite (%) (%) (%) (%) Rock fragments (%) Calcite or pyrite (%) Feldspar overgrowths (%) Quartz overgrowths (%) Point counting 2-D (%) Porosity 2 (%) Imaging software 2- D(%) S S S S S S S S S S S S S S S S L L L L F F B B S, Slick Rock Aeolian sandstone; L, Locharbriggs sandstone; F, Fife sandstone; and B, Berea sandstone. [((1/2)Ca,Na) 0.7 (Al,Mg,Fe) 4 (Si,Al) 8 O 20 (OH) 4 7nH 2 O], which are regarded as very hydrophilic due to their labile, mobile structure. Illite [K Al 4 (Si Al O 20 )(OH) 4 ], in turn, is a mineral that reacts with water to a limited extent (Pajak-Komorowska, 2003). Such swelling clays may then move in the flow field and block pore throats, thus reducing permeability (Baraka-Lokmane, 2002). We therefore characterized the clay content of the samples as closely as possible, both in the bulk sample, and also the separate fraction of mobile fine particles or fines. In this study, 48 thin sections were point counted using 400-point density. Analysis of thin sections remains an important technique for identifying mineral composition and texture, and point counting provides the fastest and most accurate method of grain selection that is in common use. It requires using an ocular with cross hairs for a petrography microscope. A grid system is set up so the entire slide is traversed. Modern petrography allows point counting of grains per slide for estimates of composition. This study has shown that for the Slick Rock Aeolian sandstone samples, the grains are rounded, well sorted, and are friable. The permeability is controlled by the porosity, varying between 20% and 23% rather than the cement, which varies between 5% and 14%. Quartz ( %), K-feldspar ( %), various rock fragments ( %), and detrital mica (muscovite), which is only a minor component (0 0.3%), are the main detrital components of the Slick Rock Aeolian sandstones (Fig. 21). The cements are in the form of clay minerals, hematite interspersed with clays, carbonate (calcite), and quartz and feldspar in the form of overgrowths. Most of the microclines are largely replaced by calcite. The samples are characterized by a high amount of clay minerals, varying between 4.2% and 7.8%. Early stages of clay cementation are visible with thin, authigenic coatings of clays having formed around most of the detrital grains. Even such thin coatings may serve to isolate the grains from the pore fluids and thus inhibit alteration or cementation processes. In certain areas the clays cement forms a thick grain coating as well as completely bridging pores in numerous places. Fluid movements would be greatly retarded by such cementation. Hematite cementation, when it is present, is interspersed with clays around the detrital grains (Fig. 21). The Locharbriggs sandstone samples present a red pigmentation. The grains are sub-rounded to angular and are very well sorted. The porosity varies between 20% and

12 S. Baraka-Lokmane et al. / Marine and Petroleum Geology 26 (2009) Fig. 21. Thin section photo of a Slick Rock Aeolian. Cement containing clay minerals (Cl) and carbonate (Cal). Note clay minerals having formed around the quartz grains and also present in the pore space. Fig. 23. Thin section photo of a Fife sandstone. Note the homogeneity of the rock material (Q: quartz, Cl: clay). Fig. 24. Thin section photo of a Berea sandstone (Q: quartz, Ca: calcite). Fig. 22. Thin section photo of a Locharbriggs sandstone showing cement containing clay minerals (Cl), hematite (H), and pyrite (P). 23%, similar to values reported by Ngwenya et al. (2000). Monomineralic and polycrystalline quartz ( %), plagioclase and K-feldspar ( %), and various rock fragments ( %) are the main detrital components of the samples (Fig. 22). The cements make up between 5.7% and 10.2% of the sample, in the form of clay minerals, quartz and feldspar overgrowths, pyrite, and hematite (Fig. 22). Hematite gives the red pigmentation to the sample. Authigenic clays coat most of the detrital grains. The hematite cementation is in most cases interspersed with clays, lines quartz, and other detrital grains. Hematite is apparently mainly derived from the breakdown of unstable iron-bearing heavy minerals and even small amounts of such cement can yield strongly pigmented red sandstone. The pyrite crystals have been formed as a replacement in detrital grains (quartz or feldspar). The Fife sandstones present a porosity varying between 25% and 27%. Cement content between 4.7% and 6.3% mainly by clay minerals (Fig. 23). The main detrital components of the Fife sandstones are represented by quartz with an amount of 83%, microcline (8 10%), and various rock fragments (0.6 4%) (Fig. 23). The study of the thin sections belonging to the Berea sandstones has shown that the porosity varies between 24% and 25.5%. The cement is represented by clay minerals, calcite and iron (Fig. 24). The cement represents around 18% of the rock samples. The main components are quartz ( %), microcline (4.4 8%), and rock fragments (5 6.8%) Porosity estimation The imaging software Scion Image (Beta 4.02 for Windows) developed by Scion Corporation was used as an additional mechanism of estimating the porosity in 2-D section. Ten images were taken for each sample, and an

13 50 S. Baraka-Lokmane et al. / Marine and Petroleum Geology 26 (2009) arithmetic mean was calculated from the 10 different values of porosity. For the Slick Rock Aeolian sandstone samples, the porosity inferred from this method varies between 19.3% and 23.5%. The porosity varies between 23.1% and 24.6% for the Locharbriggs. The Fife sandstones have a porosity varying between 20.9% and 24.8% and the Berea sandstones present a porosity varying between 23.7% and 24% (Table 2). We can see that the values obtained from the point counting method and the imaging software Scion Image, using the 2-D thin sections, are in the same order. The porosity estimation from thin sections can be rapidly obtained by using imaging software like Scion Image, comparing to the point counting method, which is time consuming. However, the point counting method can in addition to the estimation of the porosity give the percentages of the different mineral constituents and the dominant cement of the rock samples. 8. X-ray diffraction (XRD) analysis The most definitive basis for mineralogical analysis is XRD, which distinguishes minerals from each other based on their atomic or crystal structure rather than their optical properties or chemical composition. This is particularly important for components such as clay minerals, which are difficult to identify by any other means (Ward et al., 2005). XRD analyses were carried out both on whole rock (Table 3) and the fine particle fraction (Table 4) in order to quantify the mineralogy independently from the point counting exercise and to estimate clay type and its abundance. The equipment used for this type of measurement is Philips PW1800 X-ray diffractometer with Cu target tube run at 40 kv and 50 ma. The results of the XRD analysis of the Locharbriggs sandstone samples show that two mineral compounds Table 3 Mineralogy composition of the four groups of samples (from 3-D XRD analysis) Sample number Quartz (%) Microcline (%) Calcite (%) Muscovite (%) Kaolinite (%) S S S S S S S S S S S S S S S S L L L L F2 100 Trace F F Trace F Trace F Trace 0 Trace F Trace F Trace 0 Trace F Trace B B B B B B B B B S, Slick Rock Aeolian sandstone; L, Locharbriggs sandstone; F, Fife sandstone; and B, Berea sandstone.

14 S. Baraka-Lokmane et al. / Marine and Petroleum Geology 26 (2009) Table 4 Mineralogy composition of the fine particles of the four groups of samples (from 3-D XRD analysis) Sample number Quartz (%) Microcline (%) Calcite (%) Illite (%) Hematite (%) Kaolinite (%) S S S S S S S S S S S S S S S S L L L L F F F F F F F F B B B B B B B B B S, Slick Rock Aeolian sandstone; L, Locharbriggs sandstone; F, Fife sandstone; and B, Berea sandstone. (quartz and microcline) are identified. In addition to these two minerals, the Berea sandstones have shown the presence of kaolinite, the Fife sandstones have shown the presence of traces of kaolinite and calcite, and the Slick Rock Aeolian sandstones have shown the presence of calcite and muscovite. The analysis of the fine particles of the Fife sandstone samples shows the presence of kaolinite for all the samples and illite for sample number 7. Illite is the only clay mineral for the Slick Rock Aeolian sandstones. Two clay minerals (kaolinite and illite) have been identified for the Locharbriggs and Berea sandstones. This study has shown that the results of the 3-D XRD analysis are comparable with those of the 2-D point counting method. However, the XRD analysis can identify the different types of clays; this is not possible with the point counting method. 9. X-ray fluorescence (XRF) analysis XRF spectrometry is one of the most widely used and versatile of instrumental analytical techniques. An XRF spectrometer uses primary radiation from an X-ray tube to excite secondary X-ray emission from a sample. The radiation emerging from the sample includes the characteristic X-ray peaks of major elements present in the sample. The height of each characteristic X-ray peak relates to the concentration of the corresponding element in the sample, allowing quantitative analysis of the sample for most elements with concentrations as low as 1 ppm. Approximately 50 g of material was cut from each rock sample. The cut pieces were crushed in a tungsten carbide barrel for 2 min. Samples were analyzed for major elements using Philips PW 1480 automatic XRF spectrometer with Rh-anode X-ray tube.

15 52 S. Baraka-Lokmane et al. / Marine and Petroleum Geology 26 (2009) Table 5 X-ray fluorescence results of the four groups of samples (from 3-D XRF analysis) Sample SiO 2 Al 2 O 3 Fe 2 O 3 MgO CaO K 2 O TiO 2 MnO P 2 O 5 LOI Total S S S S S S S S S S S S S S S S L L L L F F B B S, Slick Rock Aeolian sandstone; L, Locharbriggs sandstone; F, Fife sandstone; and B, Berea sandstone. XRF analyses have shown that Fife sandstone present the highest percentage of SiO 2 (mainly quartz) with an average of around 97.48% (Table 5). XRF analyses confirmed the presence of iron oxide in the samples. The results of the XRF analysis carried out for the four groups of samples show that the Fife sandstones contain the lowest amount of Fe 2 O 3, which is in the range of 0.04% (Table 5). The Berea sandstones contain the highest amount of Fe 2 O 3, which is equal to approximately 1%. This was confirmed by the 2-D point counting method and the 3-D ESEM measurements, which have showed the presence of iron. The Locharbriggs sandstone samples present the double amount of Fe 2 O 3 than for the Slick Rock Aeolian sandstone samples. This explains the red pigmentation of the Locharbriggs sandstone. 10. Permeability and porosity measurements The porosity was measured using a conventional Boyles Law helium porosimeter. Both gas and liquid permeability were measured (Table 6). The gas permeability tests were measured using a nitrogen gas permeameter. For the liquid permeability, the cores were mounted vertically in a core holder normally used in rock deformation experiments, known as a Hoek cell. This cell provided a means of applying confining pressure to the sample. It consisted of a steel body (rated to 10,000 psi) within which was located a polyurethane sleeve. Expelled volume measurements of brine were used to determine average saturation. Brine was pumped in the inlet using a Pharmacia pump (1000 psi, ml/h). Fluid flowed through the core and was collected in a reservoir mounted on a balance. The weight measurements were used to calculate flow rates. The balance was also used in this configuration to measure the expelled pore fluid. Before permeability testing, the samples were vacuum saturated with brine, which gave 90 95% brine saturation. The samples were then saturated in a high-pressure cell at 2000 psi pore pressure for two days. The brine saturation was then confirmed to be 100% by weight. Brine permeability was carried out at 400 psi. The brine used in the experiment was 9% NaCl brine, with a density of g/cm 3 and a viscosity at 20 1C of 1.31 cp. These are measured values. Table 6 shows the petrophysical properties of the four rock types. The Slick Rock Aeolian sandstones have porosity equal to %. The liquid permeability is in the range of md, the gas permeability, md. The Fife sandstones have the highest liquid permeability, md, gas permeability equal to md and porosity is in the range of %. The Locharbriggs sandstones have the highest porosity, %, a liquid permeability of md and the highest gas permeability, md. The Berea sandstones have the lowest permeability. The liquid

16 S. Baraka-Lokmane et al. / Marine and Petroleum Geology 26 (2009) Table 6 Petrophysical parameters of the four groups of samples Sample He porosity, F (%) Brine permeability, k l (md) Gas permeability, k g (md) Ratio k g /k l S S S S S S S S S S Average for S % md md L L L L Average for L % md md F F Average for F % % md B B Average for B % md md S, Slick Rock Aeolian sandstone; L, Locharbriggs sandstone; F, Fife sandstone; and B, Berea sandstone. permeability is in the range of md, the gas permeability, md. The helium porosity (Table 6) results are comparable with those obtained with the image analyses and the point counting method (Table 2). These petrophysical results can be reasonably explained by the microstructural properties, essentially by the type and the percentage of cement and the mean grain size. 11. Discussion A variety of complementary and overlapping techniques have been employed in this study to characterize four groups of samples (Slick Rock Aeolian, Locharbriggs, Fife, and Berea sandstones). This strategy allows a full range of characteristics to be determined. In this section we discuss the internal checking between measurements, and conclude that the results of the different methods used were generally found to be consistent with each other, although no individual technique gave the full picture. Each of these methods of measurement has its own particular advantages and disadvantage (Table 7). The finite and heterogeneous nature of iron in the sample is confirmed independently by the CT scans and by the absence of a resolvable signal in the MRI measurements. Likewise, the determination of the mineralogical composition of the samples, the type and the abundance of the cements, and particularly the location and the abundance of the calcite and hematite, are all well characterized by several independent methods. These include the 2-D point counting method, the 3-D (quantitative) XRD analysis, and 3-D (qualitative) methods (CT-scanning images, ESEM, and EDX measurements). The CT scanning images have shown some samples with areas containing very attenuating elements, appearing on the images with a white color. According to the point counting method, the Slick Rock Aeolian sandstones samples present around 4% of calcite cement. These areas colored in white correspond to the areas where the calcite cement is more abundant. The CT scanning images have also shown the Slick Rock Aeolian sandstone samples with attenuating (white) layers, corresponding to the reddish ones observed optically on the samples. The point counting method has shown the presence of hematite cement, which is interspersed with clays around the detrital grains. The reddish layers of the samples coincide with the hematite cement, which has a very high density and appears on the CT scanning images with a white color. This explains the problems encountered in the MRI measurements; predominantly due to magnetic susceptibility variations within the sample, which distort the field lines is due to the presence of iron (hematite), especially when it is not homogeneously distributed, as here. The ESEM measurements carried out on the Locharbriggs samples show the presence of smectite cement,