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Marine and Petroleum Geology 26 (2009) 39–56

Application of complementary methods for more robustcharacterization of sandstone cores

S. Baraka-Lokmanea,b,�, I.G. Mainc, B.T. Ngwenyac, S.C. Elphickc

aInstitute of Petroleum Engineering, Heriot-Watt University, Edinburgh, UKbSchool of Environment and Technology, University of Brighton, UK

cSchool 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 isimportant in developing an understanding of geologic com-plexity that impacts field development. Reservoir quality,primarily determined by the porosity and permeability ofthe relevant formations, is controlled by many parameters.These include the nature of the constituent minerals andcement, the degree of rock cementation; the degree ofsorting or equivalently the particle size distribution of thegrains; and the pore-size distribution (Cade et al., 1994;

Baraka-Lokmane et al., 2001b). Field development meth-ods that rely on core flooding are particularly sensitive tosmall-scale changes in reservoir quality. Meanwhile, mostwater floods in sandstone cores are carried out either inalmost homogeneous samples or else in core samples ofuncertain heterogeneity. As a result, the interaction ofsmall-scale sedimentary heterogeneity with the fluid me-chanics of water–oil displacement cannot be adequatelyunderstood or quantified. Since most clastic sedimentsshow some degree of lamination, we might expect this tohave a significant influence on oil displacement efficiencyand residual/remaining oil saturation (Huang et al., 1994).In enhanced oil recovery processes, where the displace-ment mechanism may be particularly complex, the struc-ture of the heterogeneity may have a vital bearing on thesuccess of the scheme (Sorbie et al., 1992; Corbett et al.,1992).

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www.elsevier.com/locate/marpetgeo

0264-8172/$ - see front matter r 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.marpetgeo.2007.11.003

�Corresponding author at: School of Environment and Technology,

University of Brighton, Lewes Road, Brighton BN2 4GJ, UK.

Tel.: +441273 642306; fax: +44 1273 642285.

E-mail address: [email protected]

(S. Baraka-Lokmane).


This paper presents a method of characterizing andquantifying petrographic and petrophysical properties insandstone cores using a combination of several techniquesused in conventional routine core analysis. These includeX-ray computer tomography (CT) scanning, magneticresonance imaging (MRI), particle size analysis, pointcounting based on petrographic thin sections, environ-mental scanning microscopy (ESEM), X-ray diffraction(XRD), and X-ray fluorescence (XRF). The aim is todemonstrate that by using a suite of techniques incombination; subtle variations in petrophysical character-istics emerge that add significant value towards a betterdescription of reservoir properties. It also becomesapparent that the use of magnetic resonance methods tomeasure fluid saturation and the distribution of waterwithin the core samples is inherently problematic in caseswhere samples contain iron.

2. Summary of materials and methods

Four types of sandstones that differ in grain size,porosity, and mineralogy were studied. The materialschosen for the study were Fife and Locharbriggs sand-stones—both from southwest Scotland, Slick Rock Aeoliansandstone from Utah, United States, and Berea sandstonefrom Ohio, United States.

MRI analysis was carried out on one sample. CT scanningwas performed on 24 samples. About 48 thin sections werestudied using the petrographic microscope and modalmineral and pore percentages, based on 400-point counts,were determined for 24 samples. The porosity was estimatedusing the commercially available imaging software Scionimage. Twenty-four samples have been studied using ESEMwith an energy dispersive X-ray (EDX) spectrometer and forXRF analysis. A total of 37 samples were analyzed forparticle size analysis, bulk mineral composition, andclay–mineral composition of the 2-mm size fractions usingXRD. Helium porosity as well as gas and liquid permeabilityhave been measured on 18 samples.

In this study, we have carried out MRI measurements inorder to determine the distribution of water within thesamples and the water saturations. CT scans have beenused to characterise core level heterogenieties, surfacefeatures, and internal structure. The particle size analyseswere used for characterizing and classifying the samples,for determining their heterogeneity, the percentages of thedifferent particles (clays, silt, and sand), the degree ofsorting, and the mean grain size of the samples.

The thin sections were used to characterize mineralogicaland textural features. Modal analysis was done by pointcounting. Pore space characteristics were determined byexamining areas of the thin sections that were impregnatedwith the blue epoxy. The ESEM was used for studying thewettability and the fluid distribution. Clay size (o2 mm)fractions analyzed by XRD were separated using sedimen-tation techniques. XRD analysis of bulk rock samples wasused to verify significant variations in mineralogy and to

determine the major mineralogical composition of samplestoo fine grained for thin section analysis. XRF analysis wascarried out for determining the geochemical composition ofthe samples.

3. X-ray computer tomography (CT-scanner)

Over the past several decades, X-ray CT has gainedacceptance as a routine core analysis tool. Generally,medical CT scanners have been employed because of theiravailability and relative ease of use. This technique is usedto characterise core level heterogenieties and to explaintheir effect on horizontal and vertical permeabilities(Honarpour et al., 2003). This scanning technique is anon-destructive imaging technique that allows visualizationof surface features and internal structure within the coresamples. The following features can be characterized:bedding features and sedimentary structures, natural andcoring-induced fractures, cement distribution, small-scalegrain size variation, and density (mineralogical andchemical composition) variation (Coles et al., 1998). SixCT-scanning images were taken for each sample positionedat an equal distance of 5.42mm. The resolution of CTscanning image is 60 mm� 60 mm� 1mm. These differentfeatures are distinguished by their different penetrabilitiesto X-rays. Higher luminance values represent greaterpenetration of the rock by the X-rays and, therefore, lowerdensity. Conversely, lower luminance represents areas ofrock with higher density and therefore greater X-ray‘‘stopping ability’’. The luminance values are, therefore,inversely related to the rock density. Variations incomposition can produce differences in absorption ofradiation if there is a significant contrast in the electrondensity of the materials. This allows precise identificationof mineral constituents via their grayscale representation ina CT image.

Very dark

gray:kaolinite, it has a very low physical density(r ¼ 2.60 g/cm3).

Dark gray: quartz and feldspar, they have a physicaldensity of 2.65 g/cm3. The quartz has a lowradiological density varying between �120and �160H.M.U.

Light gray: muscovite and illite, they have a mediumphysical density (2.80 g/cm3) and therefore amedium radiological density due to thepresence in their chemical formula ofpotassium.

Light gray: potassium feldspars (microcline) have amedium radiological density (�50H.M.U.).This density could be low if Na and Careplace partially K. Indeed, the attenuation ofNa2O and CaO is lower than the attenuationof K2O.

White: calcite is a very attenuating mineral; it has aphysical density of 2.94 g/cm3 and a highradiological density (241H.M.U.).

ARTICLE IN PRESSS. Baraka-Lokmane et al. / Marine and Petroleum Geology 26 (2009) 39–5640


White: hematite has a physical density higher thanthe other minerals (5.26 g/cm3) and thereforea high radiological density due to the presencein its chemical formula of iron.

In sandstone cores, calcite and ferromagnesian mineralswould likely be registered as an anomaly in X-ray scan.

X-ray CT scanning images of Slick Rock Aeoliansandstone samples show the different mineral phases:calcite in white, microcline in light gray, and quartz indark gray (Fig. 1). The white areas seen on the imagesindicate the presence of calcite cements. The distribution ofcalcite cement is very heterogeneous, being distributed inwhite patches or layers, and iron banding is seen in thenear-white layers. The XRD, XRF analyses, and thepetrographical study have shown that the highly attenuat-ing mineral contains iron; which constitute around 0.32%of the samples (see Sections 8 and 9).

For the Locharbriggs sandstone samples, the X-ray CTscanning images have shown that the samples arecharacterized by white colored layers, which suggest ahighly attenuating mineral; indeed the bulk samples arecharacterized by a red tint, and the highly attenuatingmineral probably contains more concentrated iron bands(Fig. 2). The XRD, XRF analyses, and the petrographicalstudy have shown that these samples contain 0.7% ofhematite present as cement. Hematite gives the red pigmen-tation to the samples.

For the Berea sandstone samples, the different CTimages have shown a very homogeneous and isotropicmaterial. Fig. 3 shows the presence of white areas, indi-cating the development of nodules, which could be calciteor iron. Indeed both calcite and iron are very attenuatingminerals to X-rays, characterized by high values of physicaland radiological densities.

In the case of Fife sandstone samples, the different X-rayscanning images have shown the presence of traces ofcalcite cements, which are locally nodular (Fig. 4), and the

presence of areas with a very dark gray color, indicatingthe presence of kaolinite nodules (Fig. 5). Indeed kaoliniteis characterized by a low physical density (r ¼ 2.60 g/cm3)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

The NMR technique is a very powerful tool forquantitative, qualitative, and structural analysis. Theprincipal advantage of this method of measurement is thatit is not a destructive method, and no absorbent compoundis used. NMR has long been used in the oil industry as aresearch tool for the measurement of petrophysical rockproperties such as bulk porosity (Cowgill et al., 1981),permeability (Kenyon et al., 1986), wettability (Williams

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Fig. 1. Slice 2 of Sample 4 showing the banding and location the calcite

cement (Slick Rock Aeolian sandstone).

Fig. 2. Slice 4 of Sample 5 of the of iron (Locharbriggs Sandstones).

Fig. 3. Slice 5 of Sample 7 of the Berea sandstone.

S. Baraka-Lokmane et al. / Marine and Petroleum Geology 26 (2009) 39–56 41


and Fung, 1982), the determination of the distribution ofpore sizes (Kozlov and Ivanchuk, 1982; Gallegos andSmith, 1988), the degree of water saturations in sample(Saraf and Fatt, 1967), the determination of the fracturegeometry, and the distribution of water saturation in awater flood experiment in a porous fractured sandstone(Baraka-Lokmane et al., 2001a).

In this study, we have carried out MRI measurements inorder to determine the distribution of water within thesamples and the water saturations.

The principle behind magnetic resonance methods is thatwhen nuclei with magnetic moments are put in a statichomogeneous magnetic field, populations are distributedon discrete energy levels. A suitable radio-frequency (RF)field is then applied to induce transitions between theseenergy levels, thus changing the magnetization. At the endof radio-frequency excitation, nuclei return to equilibriumand the magnetization decays as a function of time. Thefrequency of this decay depends on the strength of themagnetic field and is characteristic of both the type ofnuclei species and its chemical environment.

Preliminary MRI measurements have been carried outon Sample 8 of the Slick Rock Aeolian sandstone. Thedimensions of the sample are 45mm in length and37.22mm in diameter. The preliminary MRI measurementsshowed that no images were obtainable for the Slick RockAeolian sandstone sample using the conventional MRIwith a magnetic field strength of 4.7 T, because thetransverse relaxation T2* was reduced dramatically bythe presence of small quantities of iron. The transverserelaxation T2* used for this experiment was equal to100 ms. In general, a transverse relaxation T2* of 10ms orgreater is necessary for conventional MRI. The problemsof ‘‘noise’’ due to the presence of iron may be related to theuse of relatively high magnetic fields. According to theliterature (Baraka-Lokmane et al., 2001a; Britton et al.,2001), this type of measurement should be done at low fieldor even in the fringe field of magnets to avoid suchproblems. The principal difficulty presented by fluid/rocksystems in terms of NMR imaging is that, for many suchmaterials, the transverse relaxation of the contained fluidscan be strongly affected by magnetic susceptibility varia-tions on the scale of the pore dimensions (De Panfilis andPacker, 1999). The choice of magnetic field strength forimaging studies of fluid-saturated rocks is a compromisebetween the sensitivity and resolution required and theperformance needed to overcome, as much as possible, thefield-dependent T2 effects. Many imaging studies of rockshave used field strength of 2T, corresponding to 1Hresonance frequency strength of 85MHz, and excellentquality images are achieved at this frequency (Britton et al.,2001; Pape et al., 2005). We therefore repeated theexperiment using a lower magnetic field of 1.5 T, butunfortunately it was not possible to obtain any image. Thereason could be the screening effect of the spatialheterogeneity of local magnetic susceptibility. Some rocksamples are described to be not ‘‘NMR friendly’’; the SlickRock Aeolian samples belong to this category of samples.The problem of ‘‘noise’’ due to the presence of iron occursprincipally when the iron is not homogeneously distrib-uted; indeed the rock samples present red layers, which areprobably more concentrated in iron than the rest of thesample. In summary, the Slick Rock Aeolian samples couldnot be characterized by NMR at high field or at low fieldsdue to total iron content and to its large spatialheterogeneity.As a consequence, the continuous-wave magnetic

resonance imaging (CW-MRI) technique was used for thevisualization of the sample. In continuous-wave (CW)magnetic resonance, a radio-frequency is applied continu-ously at a fixed frequency (f0) at low power. To detect theresonance, the magnetic field (B0) is increased slowly andwhen the resonance condition is satisfied (o0 ¼ 2pf0 ¼ gB0)the electronic properties of the radio-frequency resonatorare altered and the signal is detected. To improve thesignal-to-noise ratio a technique called magnetic fieldmodulation with lock-in detection is used. Here, the valueof B0 is modulated at low frequency (about 1 kHz in this

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Fig. 4. Slice 3 of Sample 4 of the Fife sandstone.

Fig. 5. Slice 4 of Sample 9 of the Fife sandstone.

S. Baraka-Lokmane et al. / Marine and Petroleum Geology 26 (2009) 39–5642


case). This means that when the swept field comes close toresonance the sample will actually go in and out ofresonance at the modulation frequency, and so the signalfrom the resonator has a strong component at themodulation frequency. A ‘‘lock-in amplifier’’ is used topick out this signal from within a noisy background (Lurieet al., 1996). The bore of the magnet is not large enough tostand the core sample on end. The sample was verticallyimbibed with water, and then was transferred horizontallyinto the magnet for imaging. The imbibition took place at11:45 with the sample immersed in 10mm of water for 15 s.The results using the CW-MRI technique clearly show thedevelopment of water front (Fig. 6). Initially the waterfrontis concave (first image). This waterfront spreads out slowlyon subsequent images. (Second-order local fluctuations inthe amplitude of the signal are due to amplification ofbackground noise during digital data processing.) Thequality 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 donot represent a thin enough slice through the core; theyshow a more diffuse projection of the spin densitythroughout its whole volume.

We conclude that the finite presence of heterogeneouslydistributed hematite for the Slick Rock Aeolian sandstonesimplied by the null MRI results is confirmed.

5. Particle size analysis

Mean grain size (dmean) is defined as the average value ofthe diameters of all grains in a reference area. The grainsize is often a major control of permeability. The LS 100Grain Size Analyzer is used for these measurements.Because of the relatively low cement content in the rocksamples a simple finger manipulation allowed disaggrega-tion of the grains of the sandstones cores.

The 3-D bulk particle size analyses were used forcharacterizing and classifying the samples, for determiningtheir heterogeneity, the percentages of the differentparticles (clays, silt, and sand), the degree of sorting (So),and the mean grain size of the samples (dmean) (Table 1).However, this information does not describe the spatialrelationships of the minerals and cements in the rockmicrostructure. This study has shown that the four groups

of samples are very well sorted (the degree of sorting (So) isless than 1.4) (Fig. 7), but only the Fife sandstone samplesare homogeneous, in the sense that they are composedmainly of fine sand, which comprises between 96.42% and100% of the sample by volume. The Fife sandstonespresent the lowest percentage of clay and silt (about 0.8%together) and it is the most uniform material (thecoefficient of uniformity lower than 2).The Berea sandstones present the highest percentage of

clay and silt (about 5.6% together) and it is the mostheterogeneous material (the coefficient of uniformitygreater than 2). The Locharbriggs sandstone samples pre-sent the smallest mean grain size (varying between 193.7and 206.1 mm) and the Fife sandstone samples present thebiggest mean grain size (varying between 294.1 and368.4 mm) (Table 1).

6. Environmental scanning electron microscopy (ESEM)

measurements

Wettability is the term used to describe the relativeadhesion of fluids to a solid surface, and is a measure of thetendency of one of the fluids to wet (closely adhere to) asurface (Tiab and Donaldson, 1996). The wettability stateof any reservoir rock is of great importance; as this willhelp determine how the fluids (oil and water) aredistributed within pore spaces and how they interact withthe mineral phases (grains) present within the rock.Wettability (relative hydrophobic/hydrophilic nature ofthe mineral phases/reservoir) is therefore an importantfactor in the prediction of oil or water flow, retention andlikely oil yield. Wettability may also be significantly alteredduring the injection of remedial chemicals such assurfactants introduce prior to scale inhibitor treatments.Wettability data is therefore a highly important parameterto be considered within oilfield economics (Buckman et al.,2000). The wettability characteristics of a porous mediumplay a major role in a diverse range of measurementsincluding: capillary pressure data, relative permeabilitycurves, electrical conductivity, and water flood recoveryefficiency. The advantage of this technique is that it is muchfaster than the majority of techniques used to quantifywettability (e.g. Armott and United States Bureau of Mineswettability indexes), which also only produce, results at the

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Fig. 6. Development of the waterfront inside Sample 8 of the Slick Rock Aeolian sandstone.



whole oilfield reservoir. Moreover, the low-vacuum ESEMenables the examination at the micrometer scale of non-vacuum compatible samples, in their natural state, withoutany form of special preparation. In this study, we haveused the ESEM technique in order to observe the dynamicsof wetting and any changing in wettability property of thedifferent minerals.

The wettability measurements were carried out using aPhilips XL30 Environmental SEM equipped with a LaB6gun, and an EDAX SUTW EDX detector. Our 3-D ESEMmeasurements have enabled the direct measurement of thewettability of the different minerals, and the identificationof the mineralogy of the samples. Through manipulation oftemperature and pressure, it is possible to control thesamples relative humidity. This feature makes it possible tocondense water droplets on the specimen’s surface and toobserve the contact angle between mineral phases andwater droplet. An indication of wettability can be obtained

through examination of the contact angle formed betweena solid surface and a droplet of fluid on that surface, withthe angle being recorded through the denser fluid (Gauchetet al., 1993; Robin and Cuiec, 1998). High contact angles(greater than 901) are associated with surfaces that arehydrophobic (Fig. 8a), consequently oil wet, while lowcontact angles (lower than 901) occur in association withhydrophilic water wet surfaces (Fig. 8b) and intermediatein wetness are characterized by an intermediate contactangle (E901) (Fig. 8c). The ability to use EDX analysis(with light element capabilities) during such procedures isalso important for the indication of the mineral phasesbeing 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 thecase of Berea sandstones, show that this mineral is clearly

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Table 1

Particle size analysis of the four groups of samples

Sample % of

clay

% of

silt

% of

sand

d10(mm)

d25(mm)

d30(mm)

d50(mm)

d60(mm)

d75(mm)

So Uc Cc dmean

(mm)

Median

(mm)

Mode

(mm)

S1 1.49 8.11 90.40 0.07 0.12 0.13 0.16 0.18 0.22 0.26 2.57 1.35 180.1 174.0 190.2

S2 1.77 8.94 91.06 0.07 0.10 0.12 0.14 0.16 0.20 0.26 2.36 1.24 163.4 159.8 170.8

S3 1.32 5.75 94.25 0.08 0.14 0.14 0.20 0.25 0.28 0.26 2.92 1.00 227.0 218.8 292.0

S4 0.83 6.29 93.71 0.08 0.14 0.16 0.22 0.25 0.28 0.26 2.92 1.24 225.3 229.9 262.3

S5 1.46 6.17 93.83 0.08 0.16 0.18 0.25 0.28 0.31 0.26 3.25 1.38 243.1 254.9 325.0

S6 1.33 5.25 94.75 0.08 0.16 0.16 0.32 0.25 0.28 0.29 2.92 1.24 233.9 239.9 292.0

S7 0.97 6.08 93.92 0.08 0.13 0.14 0.18 0.20 0.22 0.29 2.36 1.24 191.2 187.9 190.2

S8 1.97 12.06 87.94 0.05 0.01 0.10 0.14 0.16 0.20 0.24 3.25 1.38 170.2 157.7 170.8

S9 1.08 5.87 94.13 0.08 0.14 0.16 0.20 0.22 0.28 0.26 2.62 1.38 221.6 224.6 262.3

S10 1.51 6.78 93.22 0.08 0.13 0.13 0.16 0.18 0.22 0.29 2.36 1.24 183.5 181.3 190.2

S11 0.89 6.96 93.04 0.08 0.14 0.16 0.20 0.22 0.25 0.29 2.62 1.38 213.8 217.0 262.3

S12 1.02 7.09 92.91 0.07 0.14 0.16 0.22 0.24 0.28 0.26 3.25 1.38 230.5 238.4 292.0

S13 0.99 7.69 92.31 0.07 0.14 0.18 0.25 0.28 0.31 0.23 3.62 1.53 248.1 257.5 325.0

S14 1.11 6.00 94.00 0.08 0.13 0.14 0.18 0.20 0.25 0.26 2.36 1.24 202.0 192.8 190.2

S15 1.12 6.78 93.22 0.08 0.13 0.13 0.18 0.20 0.22 0.29 2.36 1.00 191.6 188.3 211.7

S16 1.00 6.90 93.10 0.08 0.13 0.14 0.18 0.20 0.22 0.29 2.36 1.24 137.7 140.0 170.8

L1 1.6 7.68 92.32 0.07 0.14 0.16 0.18 0.20 0.25 0.29 2.92 1.90 197.2 205.3 235.6

L3 1.37 13.85 86.15 0.02 0.14 0.14 0.20 0.22 0.25 0.29 10.58 4.48 193.7 205.8 235.6

L4 1.16 11.78 88.22 0.03 0.14 0.16 0.20 0.22 0.25 0.29 6.89 3.62 206.1 215.9 235.6

L5 1.11 15.68 84.32 0.02 0.13 0.14 0.20 0.22 0.25 0.263 10.58 4.48 198.4 203.1 235.6

F2 0.00 0.00 100.0 0.20 0.25 0.25 0.31 0.31 0.38 0.32 1.53 1.00 328.4 317.0 292.0

F3 0.52 1.85 97.63 0.20 0.25 0.25 0.31 0.34 0.38 0.32 1.71 0.90 326.9 321.0 325.0

F4 0.00 0.72 99.28 0.20 0.25 0.25 0.31 0.34 0.38 0.32 1.71 0.90 342.0 328.0 325.0

F5 0.84 2.43 96.73 0.18 0.25 0.25 0.31 0.31 0.38 0.32 1.71 1.11 321.9 317.1 325.0

F6 0.50 1.72 97.78 0.18 0.22 0.22 0.27 0.31 0.34 0.32 1.71 0.90 294.1 290.2 292.0

F7 0.44 1.41 98.15 0.20 0.25 0.27 0.31 0.34 0.38 0.32 1.71 1.11 343.1 335.9 325.0

F8 0.00 0.00 100.0 0.22 0.27 0.27 0.34 0.38 0.38 0.36 1.71 0.90 368.4 355.7 402.8

F9 1.15 2.43 96.42 0.20 0.25 0.27 0.34 0.34 0.47 0.26 1.71 1.11 353.9 349.9 361.8

B1 3.01 13.32 83.67 0.02 0.12 0.13 0.18 0.20 0.22 0.26 10.57 4.49 212.3 207.1 235.6

B2 2.38 10.96 86.66 0.03 0.15 0.16 0.20 0.22 0.28 0.26 7.67 4.03 240.7 223.9 235.6

B3 1.94 9.26 88.80 0.05 0.16 0.16 0.22 0.25 0.28 0.29 4.99 2.12 255.2 232.2 262.3

B4 2.07 9.51 88.42 0.04 0.16 0.18 0.22 0.25 0.31 0.26 5.56 2.92 274.1 240.3 262.3

B5 2.15 9.27 88.58 0.04 0.16 0.18 0.22 0.25 0.31 0.26 5.56 2.92 271.5 235.8 262.3

B6 1.92 9.45 88.63 0.05 0.16 0.18 0.22 0.25 0.28 0.29 4.99 2.62 266.9 234.5 262.3

B7 2.19 9.29 88.52 0.04 0.16 0.18 0.22 0.25 0.31 0.26 5.56 2.92 273.1 237.6 262.3

B8 1.40 8.68 89.92 0.07 0.16 0.18 0.22 0.25 0.31 0.26 3.62 1.90 267.7 234.1 235.6

B9 1.57 8.18 90.25 0.07 0.16 0.18 0.22 0.25 0.31 0.26 3.62 1.90 285.8 246.5 262.3

S, Slick Rock Aeolian sandstone; L, Locharbriggs sandstone; F, Fife sandstone; and B, Berea sandstone. d10: grain size (mm) of 10% (cumulative wt%);

So: degree of sorting; Uc: coefficient of uniformity; Cc: coefficient of curvature, and dmean: mean grain size.



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). TheESEM measurements have shown that illite is thedominant cement for the Slick Rock Aeolian sandstonesand it covers the rounded grains of quartz and the feldspar

minerals creating irregular surfaces. Interstitial illiteparticles, quartz, and feldspar minerals covered by illite,are all very water wet (Fig. 13). Fig. 14 shows threedifferent states of wettability for the Slick Rock Aeoliansandstone. Quartz covered by illite shows the forming of adistinctive sheet of water. This shows clearly that quartz

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110

100

90

80

70

60

50

40

50

20

10

0

0.0001 0.001 0.01 0.1 1

Grain size [mm]

Sum

mation p

erc

enta

ge [%

]Clay Fine Silt Medium Silt Coarse Silt Fine Sand

S

LFB

Particle Size Distribution

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).



covered by illite is very water wet, calcite crystal, with lowdroplets of water forming, this indicates that calcite iswater wet, and a surface, which is intermediate water wet.Indeed, the contact angle formed between a solid surfaceand a droplet of fluid on that surface is exactly equal to 901.This surface is the aluminium plate of the ESEMapparatus. Locharbriggs samples show the presence ofsmectite, illite and kaolinite cements (Figs. 15–18). Figs. 15and 16 show a wettability test performed on smectite forLocharbriggs samples. Fig. 16 shows the very water wetproperty of the semectite. Figs. 17 and 18 show awettability test performed on kaolinite, smectite, and illitefor Locharbriggs samples. Fig. 18 shows the very waterwet property of the clays. For the Fife sandstones, thewettability measurements carried out on kaolinite cover-ing feldspar minerals (Fig. 19) show that kaolinite iswater wet, showing low droplets of water forming (Fig. 20).

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Fig. 11. ESEM photo, showing water wet feldspar.

Fig. 12. ESEM photo, showing water wet quartz.

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).

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. 15. ESEM photo of a cement smectite (Locharbriggs sandstone).



The ESEM technique is a relatively rapid method ofmeasurement, which is useful for the identification of thedifferent minerals of the rocks as well as studying theirwettability characteristics.

7. Petrography and porosity estimation

The study has been extended to the 2-D thin sectionsanalysis, using the point counting technique. The resultsare reported as percentages of the different mineralconstituents and the dominant cement, and the percentageporosity (Table 2).During the liquid permeability measurements, clays

may absorb brine and increase in volume. Kaolinite[Al4Si4O10(OH)8], having a stable, ‘‘rigid’’ structure (dueto the strong book bond), reacts with water to a minimumextent, by contrast with minerals of the smectite group

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Fig. 17. ESEM photo of a cement containing kaolinite, smectite, and illite

(Locharbriggs sandstone).

Fig. 16. ESEM photo, showing very water wet smectite (Locharbriggs

sandstone).

Fig. 18. ESEM photo, showing the very water wet property of the clays

(kaolinite, smectite, and illite) (Locharbriggs sandstone).

Fig. 19. ESEM photo, showing kaolinite cement covering a feldspar

crystal (Fife sandstone).

Fig. 20. ESEM photo, showing the low dome shaped of water on the

kaolinite clays (Fife sandstone).



[((1/2)Ca,Na)0.7(Al,Mg,Fe)4(Si,Al)8O20(OH)47nH2O], whichare regarded as very hydrophilic due to their labile, ‘‘mobile’’structure. Illite [K1–1.5Al4(Si7–6.5Al1–1.5O20)(OH)4], in turn,is a mineral that reacts with water to a limited extent(Pajak-Komorowska, 2003). Such swelling clays may thenmove in the flow field and block pore throats, thus reducingpermeability (Baraka-Lokmane, 2002). We therefore char-acterized the clay content of the samples as closely aspossible, both in the bulk sample, and also the separatefraction of mobile fine particles or ‘fines’.

In this study, 48 thin sections were point counted using400-point density. Analysis of thin sections remains animportant technique for identifying mineral compositionand texture, and point counting provides the fastest andmost accurate method of grain selection that is in commonuse. It requires using an ocular with cross hairs for apetrography microscope. A grid system is set up so theentire slide is traversed. Modern petrography allows pointcounting of 200–500 grains per slide for estimates ofcomposition.

This study has shown that for the Slick Rock Aeoliansandstone samples, the grains are rounded, well sorted,and are friable. The permeability is controlled by theporosity, varying between 20% and 23% rather than the

cement, which varies between 5% and 14%. Quartz(71.5–88.6%), K-feldspar (2.9–14.5%), various rockfragments (0.6–6.2%), and detrital mica (muscovite),which is only a minor component (0–0.3%), are the maindetrital components of the Slick Rock Aeolian sandstones(Fig. 21). The cements are in the form of clay minerals,hematite interspersed with clays, carbonate (calcite), andquartz and feldspar in the form of overgrowths. Most ofthe microclines are largely replaced by calcite. The samplesare characterized by a high amount of clay minerals,varying between 4.2% and 7.8%. Early stages of claycementation are visible with thin, authigenic coatings ofclays having formed around most of the detrital grains.Even such thin coatings may serve to isolate the grainsfrom the pore fluids and thus inhibit alteration orcementation processes. In certain areas the clays cementforms a thick grain coating as well as completely bridgingpores in numerous places. Fluid movements would begreatly retarded by such cementation. Hematite cementa-tion, when it is present, is interspersed with clays aroundthe detrital grains (Fig. 21).The Locharbriggs sandstone samples present a red

pigmentation. The grains are sub-rounded to angular andare very well sorted. The porosity varies between 20% and

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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

(%)

Porosity 2

(%)

Quartz

(%)

Feldspar

(%)

Mica

(%)

Rock

fragments

(%)

Clay+hematite

(%)

Calcite

or pyrite

(%)

Feldspar

overgrowths

(%)

Quartz

overgrowths

(%)

Point

counting

2-D (%)

Imaging

software 2-

D (%)

S1 71.50 14.50 0.25 3.14 7.80 1.95 0.25 0.61 23.00 23.33

S2 77.36 11.03 0.24 3.02 5.81 1.94 0.25 0.35 22.60 22.19

S3 76.48 11.50 0.32 3.19 6.50 0.62 0.77 0.62 20.00 19.34

S4 86.76 3.17 0.25 1.73 4.18 3.66 0.00 0.25 20.00 20.29

S5 88.63 3.30 0.00 1.25 4.57 1.23 0.25 0.77 20.70 22.83

S6 78.59 7.86 0.35 1.53 7.24 3.80 0.00 0.63 21.00 20.63

S7 82.32 6.85 0.25 1.62 4.98 3.73 0.00 0.25 20.70 20.22

S8 81.94 3.96 0.00 0.62 5.81 7.05 0.00 0.62 20.70 22.84

S9 79.47 8.50 0.25 1.48 4.95 4.33 0.25 0.77 20.20 21.30

S10 82.84 3.90 0.00 1.65 5.96 4.63 0.77 0.25 23.00 23.07

S11 82.22 2.94 0.00 2.47 4.30 6.30 0.00 1.77 21.00 21.75

S12 82.35 6.01 0.00 1.69 5.32 3.13 0.00 1.50 20.20 22.36

S13 78.86 7.01 0.32 2.57 6.36 4.11 0.00 0.77 22.20 22.34

S14 79.93 6.92 0.25 5.09 5.41 0.62 0.00 1.78 21.50 21.52

S15 75.92 2.91 0.32 2.55 4.78 10.98 0.77 1.77 21.70 21.16

S16 79.39 8.89 0.00 6.23 4.19 0.00 0.00 1.30 23.60 23.54

L1 81.22 11.41 0.00 1.61 4.51 0.72 0.33 0.20 23.00 24.63

L2 78.81 13.46 0.00 1.61 5.10 0.68 0.00 0.34 22.60 23.22

L4 83.47 8.87 0.00 0.41 5.98 0.72 0.28 0.27 20.00 23.45

L5 72.23 16.87 0.00 0.68 8.63 0.78 0.34 0.47 20.00 23.16

F2 82.85 10.16 0.00 0.63 6.35 0.00 0.00 0.00 27.00 20.87

F8 83.19 8.07 0.00 4.03 4.03 0.00 0.67 0.00 25.00 24.86

B1 71.76 4.39 0.00 5.02 10.04 8.16 0.00 0.00 25.50 23.74

B7 67.14 8.06 0.00 6.82 9.30 8.06 0.62 0.00 24.00 24.02

S, Slick Rock Aeolian sandstone; L, Locharbriggs sandstone; F, Fife sandstone; and B, Berea sandstone.



23%, similar to values reported by Ngwenya et al. (2000).Monomineralic and polycrystalline quartz (72.2–83.4%),plagioclase and K-feldspar (8.8–16.8%), and various rockfragments (0.4–1.6%) are the main detrital components ofthe 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 thesample. Authigenic clays coat most of the detrital grains.The hematite cementation is in most cases interspersed withclays, lines quartz, and other detrital grains. Hematite isapparently mainly derived from the breakdown of unstableiron-bearing heavy minerals and even small amounts ofsuch cement can yield strongly pigmented red sandstone.The pyrite crystals have been formed as a replacement indetrital grains (quartz or feldspar).

The Fife sandstones present a porosity varying between25% and 27%. Cement content between 4.7% and 6.3%mainly by clay minerals (Fig. 23). The main detritalcomponents of the Fife sandstones are represented byquartz with an amount of 83%, microcline (8–10%), andvarious rock fragments (0.6–4%) (Fig. 23). The study ofthe thin sections belonging to the Berea sandstones hasshown 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 rocksamples. The main components are quartz (67.1–71.7%),microcline (4.4–8%), and rock fragments (5–6.8%).

7.1. Porosity estimation

The imaging software Scion Image (Beta 4.02 forWindows) developed by Scion Corporation was used asan additional mechanism of estimating the porosity in 2-Dsection. Ten images were taken for each sample, and an

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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. 22. Thin section photo of a Locharbriggs sandstone showing cement

containing clay minerals (Cl), hematite (H), and pyrite (P).

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).



arithmetic mean was calculated from the 10 different valuesof porosity. For the Slick Rock Aeolian sandstone samples,the porosity inferred from this method varies between19.3% and 23.5%. The porosity varies between 23.1% and24.6% for the Locharbriggs. The Fife sandstones have aporosity varying between 20.9% and 24.8% and the Bereasandstones present a porosity varying between 23.7% and24% (Table 2).

We can see that the values obtained from the pointcounting method and the imaging software Scion Image,using the 2-D thin sections, are in the same order. Theporosity estimation from thin sections can be rapidlyobtained by using imaging software like Scion Image,comparing to the point counting method, which is timeconsuming. However, the point counting method can inaddition to the estimation of the porosity give thepercentages of the different mineral constituents and thedominant cement of the rock samples.

8. X-ray diffraction (XRD) analysis

The most definitive basis for mineralogical analysisis XRD, which distinguishes minerals from each otherbased on their atomic or crystal structure rather than theiroptical properties or chemical composition. This is parti-cularly important for components such as clay minerals,which are difficult to identify by any other means (Wardet al., 2005).XRD analyses were carried out both on whole rock

(Table 3) and the fine particle fraction (Table 4) in order toquantify the mineralogy independently from the pointcounting exercise and to estimate clay type and itsabundance. The equipment used for this type of measure-ment is Philips PW1800 X-ray diffractometer with Cutarget tube run at 40 kV and 50mA.The results of the XRD analysis of the Locharbriggs

sandstone samples show that two mineral compounds

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Table 3

Mineralogy composition of the four groups of samples (from 3-D XRD analysis)

Sample number Quartz (%) Microcline (%) Calcite (%) Muscovite (%) Kaolinite (%)

S1 76 18 2 2 0

S2 80 15 2 2 0

S3 80 16 0.6 2 0

S4 91 5 1 2 0

S5 93 5 1 0 0

S6 86 9 2 2 0

S7 87 8 1 2 0

S8 93 4 1 0 0

S9 83 11 3 2 0

S10 92 5 2 0 0

S11 93 4 1 0 0

S12 91 7 1 0 0

S13 86 10 1 2 0

S14 86 10 1 1 0

S15 90 4 2 2 0

S16 90 9 0 0 0

L1 85 14 0 0 0

L3 82 17 0 0 0

L4 88 11 0 0 0

L5 79 20 0 0 0

F2 100 Trace 0 0 0

F3 97 3 0 0 0

F4 97 3 0 0 Trace

F5 97 3 0 0 Trace

F6 97 3 Trace 0 Trace

F7 97 3 0 0 Trace

F8 97 3 Trace 0 Trace

F9 97 3 0 0 Trace

B1 83 5 9 0 3

B2 84 5 8 0 3

B3 85 5 7 0 3

B4 84 5 8 0 3

B5 79 10 9 0 2

B6 87 3 7 0 3

B7 86 3 8 0 3

B8 83 5 9 0 3

B9 84 5 8 0 3




(quartz and microcline) are identified. In addition to thesetwo minerals, the Berea sandstones have shown thepresence of kaolinite, the Fife sandstones have shownthe presence of traces of kaolinite and calcite, and theSlick Rock Aeolian sandstones have shown the pre-sence of calcite and muscovite. The analysis of the fineparticles of the Fife sandstone samples shows the pre-sence of kaolinite for all the samples and illite forsample number 7. Illite is the only clay mineral for theSlick Rock Aeolian sandstones. Two clay minerals(kaolinite and illite) have been identified for the Lochar-briggs and Berea sandstones. This study has shown thatthe results of the 3-D XRD analysis are comparablewith those of the 2-D point counting method. How-ever, the XRD analysis can identify the different typesof clays; this is not possible with the point countingmethod.

9. X-ray fluorescence (XRF) analysis

XRF spectrometry is one of the most widely used andversatile of instrumental analytical techniques. An XRFspectrometer uses primary radiation from an X-ray tube toexcite secondary X-ray emission from a sample. Theradiation emerging from the sample includes the char-acteristic X-ray peaks of major elements present in thesample. The height of each characteristic X-ray peakrelates to the concentration of the corresponding elementin the sample, allowing quantitative analysis of the samplefor most elements with concentrations as low as 1 ppm.Approximately 50 g of material was cut from each rocksample. The cut pieces were crushed in a tungsten carbidebarrel for 2min. Samples were analyzed for major elementsusing Philips PW 1480 automatic XRF spectrometer withRh-anode X-ray tube.

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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 (%)

S1 55 15 5 25 0 0

S2 45 15 5 35 0 0

S3 60 15 3 22 0 0

S4 85 10 5 0 0 0

S5 75 10 5 10 0 0

S6 70 10 5 15 0 0

S7 40 15 5 40 0 0

S8 90 5 5 0 0 0

S9 35 10 5 50 0 0

S10 80 10 5 5 0 0

S11 91 5 4 0 0 0

S12 60 10 5 25 0 0

S13 50 15 5 30 0 0

S14 60 6 4 30 0 0

S15 70 10 10 10 0 0

S16 65 25 0 10 0 0

L1 40 35 0 12 5 8

L3 55 25 0 10 5 4

L4 55 20 0 12 5 7

L5 50 25 0 11 5 9

F2 80 7 0 0 0 13

F3 83 7 0 0 0 10

F4 73 7 0 0 0 20

F5 68 7 0 0 0 25

F6 75 7 0 0 0 18

F7 73 10 0 2 0 15

F8 60 10 0 0 0 30

F9 60 5 0 0 0 35

B1 45 7 17 14 0 17

B2 55 7 18 10 0 10

B3 54 7 17 11 0 11

B4 35 10 18 16 0 21

B5 42 7 18 13 0 20

B6 52 10 17 10 0 11

B7 42 10 19 19 0 10

B8 43 7 17 15 0 18

B9 55 5 15 15 0 10




XRF analyses have shown that Fife sandstone presentthe highest percentage of SiO2 (mainly quartz) with anaverage of around 97.48% (Table 5). XRF analysesconfirmed the presence of iron oxide in the samples. Theresults of the XRF analysis carried out for the four groupsof samples show that the Fife sandstones contain the lowestamount of Fe2O3, which is in the range of 0.04% (Table 5).The Berea sandstones contain the highest amount ofFe2O3, which is equal to approximately 1%. This wasconfirmed by the 2-D point counting method and the 3-DESEM measurements, which have showed the presence ofiron. The Locharbriggs sandstone samples present thedouble amount of Fe2O3 than for the Slick Rock Aeoliansandstone samples. This explains the red pigmentation ofthe Locharbriggs sandstone.

10. Permeability and porosity measurements

The porosity was measured using a conventional BoylesLaw helium porosimeter. Both gas and liquid permeabilitywere measured (Table 6). The gas permeability tests weremeasured using a nitrogen gas permeameter. For the liquidpermeability, the cores were mounted vertically in a coreholder normally used in rock deformation experiments,known as a Hoek cell. This cell provided a means ofapplying confining pressure to the sample. It consisted of asteel body (rated to 10,000 psi) within which was located a

polyurethane sleeve. Expelled volume measurements ofbrine were used to determine average saturation. Brine waspumped in the inlet using a Pharmacia pump (1000 psi,1–499ml/h). Fluid flowed through the core and wascollected in a reservoir mounted on a balance. The weightmeasurements were used to calculate flow rates. Thebalance was also used in this configuration to measurethe expelled pore fluid. Before permeability testing, thesamples were vacuum saturated with brine, which gave90–95% brine saturation. The samples were then saturatedin a high-pressure cell at 2000 psi pore pressure for twodays. 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, witha density of 1.067 g/cm3 and a viscosity at 20 1C of 1.31 cp.These are measured values. Table 6 shows the petrophy-sical properties of the four rock types. The Slick RockAeolian sandstones have porosity equal to 22.2670.50%.The liquid permeability is in the range of 285.40770.80mD, the gas permeability, 499.50749.29mD. TheFife sandstones have the highest liquid permeability,1980.507388.20mD, gas permeability equal to 1366.50762.93mD and porosity is in the range of 22.8673.04%.The Locharbriggs sandstones have the highest porosity,26.0570.10%, a liquid permeability of 707.757177.41mDand the highest gas permeability, 1424.00735.29mD. TheBerea sandstones have the lowest permeability. The liquid

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Table 5

X-ray fluorescence results of the four groups of samples (from 3-D XRF analysis)

Sample SiO2 Al2O3 Fe2O3 MgO CaO K2O TiO2 MnO P2O5 LOI Total

S1 90.130 3.140 0.290 0.190 1.870 1.729 0.065 0.023 0.024 2.020 99.440

S2 91.610 2.970 0.260 0.170 1.730 1.622 0.063 0.028 0.020 1.890 100.220

S3 92.540 3.030 0.330 0.140 1.190 1.681 0.086 0.045 0.023 0.800 99.730

S4 91.060 2.900 0.350 0.160 1.200 1.640 0.082 0.040 0.024 2.190 99.650

S5 91.580 3.270 0.340 0.180 1.340 1.744 0.068 0.030 0.024 1.770 100.250

S6 91.390 2.950 0.290 0.140 1.170 1.628 0.081 0.037 0.024 2.000 99.710

S7 91.920 3.080 0.310 0.150 1.220 1.751 0.075 0.034 0.026 1.450 99.920

S8 91.100 3.040 0.320 0.180 1.770 1.731 0.080 0.038 0.024 2.000 100.150

S9 91.710 2.880 0.340 0.160 1.580 1.592 0.080 0.048 0.023 1.730 100.150

S10 90.640 2.920 0.300 0.210 2.080 1.664 0.073 0.041 0.024 2.210 100.060

S11 91.770 3.030 0.310 0.170 1.440 1.645 0.078 0.039 0.023 1.840 100.230

S12 92.130 3.040 0.320 0.140 1.190 1.707 0.087 0.034 0.024 1.400 100.280

S13 92.690 3.130 0.320 0.160 0.870 1.797 0.079 0.030 0.024 1.270 100.260

S14 92.390 3.170 0.310 0.180 0.700 1.814 0.064 0.025 0.023 1.140 99.790

S15 89.780 2.720 0.380 0.200 2.650 1.391 0.078 0.057 0.022 2.370 99.680

S16 89.540 3.870 0.390 0.200 0.242 2.256 0.087 0.015 0.030 2.820 99.450

L1 93.400 3.410 0.660 0.130 0.060 2.032 0.086 0.022 0.014 0.590 99.770

L3 93.050 3.540 0.640 0.120 0.050 2.116 0.084 0.013 0.014 0.350 99.977

L4 92.690 3.810 0.650 0.130 0.060 2.112 0.084 0.014 0.014 0.590 99.520

L5 92.630 3.890 0.640 0.130 0.060 2.101 0.083 0.012 0.017 0.580 99.550

F2 97.750 0.950 0.040 0.180 0.010 0.367 0.077 0.001 0.006 0.120 99.500

F8 97.560 1.050 0.040 0.200 0.010 0.389 0.069 0.001 0.005 0.280 99.604

B1 91.020 3.100 1.110 0.500 0.860 0.871 0.239 0.026 0.014 1.860 99.600

B7 89.900 2.970 1.330 0.630 1.130 0.821 0.211 0.039 0.015 2.410 99.460




permeability is in the range of 172.5073.53mD, the gaspermeability, 306.50712.02mD. The helium porosity(Table 6) results are comparable with those obtainedwith the image analyses and the point counting method(Table 2). These petrophysical results can be reasonablyexplained by the microstructural properties, essentially bythe type and the percentage of cement and the mean grainsize.

11. Discussion

A variety of complementary and overlapping techniqueshave been employed in this study to characterize fourgroups of samples (Slick Rock Aeolian, Locharbriggs, Fife,and Berea sandstones). This strategy allows a full range ofcharacteristics to be determined. In this section we discussthe internal checking between measurements, and concludethat the results of the different methods used were generallyfound to be consistent with each other, although noindividual technique gave the full picture. Each of thesemethods of measurement has its own particular advantagesand disadvantage (Table 7). The finite and heterogeneousnature of iron in the sample is confirmed independently bythe CT scans and by the absence of a resolvable signalin the MRI measurements. Likewise, the determinationof the mineralogical composition of the samples, the typeand the abundance of the cements, and particularly the

location and the abundance of the calcite and hematite, areall 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 onthe images with a white color. According to the pointcounting method, the Slick Rock Aeolian sandstonessamples present around 4% of calcite cement. These areascolored in white correspond to the areas where the calcitecement is more abundant. The CT scanning images havealso shown the Slick Rock Aeolian sandstone samples withattenuating (white) layers, corresponding to the reddishones observed optically on the samples. The point countingmethod has shown the presence of hematite cement, whichis interspersed with clays around the detrital grains.The reddish layers of the samples coincide with thehematite cement, which has a very high density andappears on the CT scanning images with a white color.This explains the problems encountered in the MRImeasurements; predominantly due to magnetic suscept-ibility variations within the sample, which distort the fieldlines is due to the presence of iron (hematite), especiallywhen it is not homogeneously distributed, as here.The ESEM measurements carried out on the Lochar-

briggs samples show the presence of smectite cement,

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Table 6

Petrophysical parameters of the four groups of samples

Sample He porosity, F (%) Brine permeability, kl (mD) Gas permeability, kg (mD) Ratio kg/kl

S2 22.2 290 447 1.54

S4 21.9 306 558 1.82

S5 22.2 280 425 1.52

S6 21.8 292 521 1.78

S7 23.1 289 470 1.63

S8 22.6 210 530 2.52

S9 21.6 280 473 1.69

S10 23.0 177 577 3.26

S11 22.0 280 521 1.86

S12 22.2 450 473 1.05

Average for S 22.2670.50% 285.40770.80mD 499.50749.29mD 1.8770.61

L1 26.0 894 1392 1.56

L3 26.2 664 1446 2.18

L4 26.0 784 1396 1.78

L5 26.0 480 1462 3.05

Average for L 26.0570.10% 705.757177.41mD 1424.00735.29mD 2.1470.66

F2 20.7 1706 1411 0.83

F8 25.0 2255 1322 0.59

Average for F 22.8573.04% 1980.507388.20% 1366.50762.93mD 0.7170.17

B1 24.0 170 298 1.75

B7 24.5 175 315 1.80

Average for B 24.2570.35% 172.5073.53mD 306.50712.02mD 1.7870.03




which was not detected by the XRD measurements. TheESEM measurements carried out on the Berea sandstonesamples have also shown the presence of iron nodules. Thisindicates the importance of using complementary methodof measurements for characterizing a rock sample.

The characterization of the samples has shown that theresults of the 2-D point counting method are comparablewith those of the 3-D XRD analysis. However, pointcounting has shown the finite presence of hematite in thecase of the Slick Rock Aeolian sandstone samples. Thiswas not detected with the XRD measurement because of itssmall concentration (less than 1% of the sample). This isalso the case of the pyrite cement for the Locharbriggssandstone samples. A structural method (point countingtechnique) is therefore complementary to a quantitativemethod (bulk XRD analysis) in order to identify the

different mineral species and quantify their abundance.XRF analyses of the samples were used to determine thechemical composition of the samples. This confirmed thepresence of iron oxide in the samples. The effects of small-scale structure are frequently ignored in reservoir simula-tion, although they may have a significant effect onhydrocarbon recovery. Many sandstones exhibit lamina-tion, and in such rock structures, permeability may vary byan order of magnitude over distances of a centimeter orless. In these cases, CT scanning is a useful method ofmeasurement for detecting laminations. Field-scale reser-voir models must take account of these small-scale effectsin order to lay claim to reasonable accuracy in productionforecasts (Pickup et al., 1994). Flow simulations for modelsincluding and ignoring the geological heterogeneity showthat the repetitive features in sandstone reservoirs can

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Table 7

Benefit of the integrated approach used in this study

Method of

measurement

Measured parameters Comments on the advantages Comments on the disadvantages

3-D X-ray computer

tomography

� Rock heterogeneity

� Presence of iron and

calcite

� Relatively rapid method of

measurement

� Not destructive method

� Interpretation of the gray scale is challenging

� Can only be used if the petrography is previously

performed with point counting using thin sections or

XRD

3-D MRI � Structure of the core

� Distribution of water

inside the core

� Fracture geometry

� Powerful tool for qualitative and

structural analysis

� Not destructive method

� Continous-wave MRI: quality of

images need to be improved

� Presence of iron

� Heterogeneity distribution of iron

3-D ESEM � Wettability of the

different minerals

� Relatively rapid method of

measurement

� No disadvantages

3-D SEM/DX � Mineral identification � Relatively rapid method of

measurement

� Qualitative and not quantitative method of measurement

3-D particle size � Rock heterogeneity

� Mean grain size

� Useful to supplement porosity for

empirical permeability estimation

� Not always easy to obtain grains from a sandstone core.

This depends on the degree of cementation of the rock

2-D point counting

using thin sections

� Percentages of the

different mineral

constituents

� Identification of the

dominant cement

� Percentage of porosity

� Structural analysis

� Identification of rare minerals

� Time consuming

2-D analysis: imaging

software Scion Image

� Porosity estimation � Relatively rapid method of

measurement

� No disadvantages

3-D XRD � Mineralogy

composition

� Quantitative analysis

� Independent method from point

counting

� Identification of the different

types of clays

� Minerals present with less than 1% of the rock are not

identified

3-D XRF � Chemical composition � Quantitative analysis for most

elements with concentrations as

low as 1 ppm

� Time consuming



significantly change the performance (Jensen et al., 1994).The combination of the different method of measurementsused in this study has shown that for obtaining the fullcharacterization for the majority of reservoir cores, it isessential to use the X-ray CT technique, the point countingusing thin sections, the XRD measurements, the ESEMmeasurements, and the grain size analysis. These methodsof measurements allow obtaining the information aboutthe rock heterogeneity, mineralogy composition, identifica-tion of the different types of clays, estimation of theporosity, the wettability characteristics of the differentminerals, and the mean grain size of the rocks.

12. Summary and conclusions

The four different type of sandstone used in this studypresent very variable microstructural properties. Theseinclude: the nature of the constituent minerals and cement,the degree of rock cementation, the degree of sorting, orequivalently the particle size distribution of the grains.These rocks are however all strongly water wet.

The preliminary MRI measurements performed onSample 8 of the Slick Rock Aeolian sandstone showedthat no images were obtainable using the conventionalMRI at 4.7 T, because the transverse relaxation T2* wasreduced dramatically by the presence of iron. Thetransverse relaxation T2* used for this experiment wasequal to 100ms. In general, a transverse relaxation T2* of10ms or greater is necessary for conventional MRI. Wehave repeated the experiment using a lower magnetic fieldof 1.5 T unfortunately it was not possible to obtain anyimage. The reason is the heterogeneity of local magneticsusceptibility. We conclude that the Slick Rock Aeoliansamples could not be characterized by NMR at high fieldor at low fields due to total iron content and to its largespatial heterogeneity. As a consequence, the CW-MRItechnique was used for the visualization of the sample. Theresults using the CW-MRI technique clearly show thedevelopment of water front.

The CT scanning images have shown that the Slick RockAeolian sandstones, Berea sandstones, and some Fifesandstones samples present areas containing very attenuat-ing elements, appearing on the images with a white color.According to the point counting method, these samplespresent either high percentage of calcite cement (Slick RockAeolian and Berea sandstones) or calcite nodules (Fife).The ESEM measurements carried out on Berea sandstoneshave shown the presence of iron, the very attenuatingelements, appeared on the CT scan images are interpretedas iron nodules.

Our 3-D ESEM measurements have enabled the directmeasurement of the wettability of the different minerals,and the identification of the mineralogy of the samples.With the help of EDX analysis, the chemical compositionof the constituent minerals was determined. This studyshowed that fresh surfaces of quartz and feldspar are bothwater wet. Interstitial illite particles or quartz and feldspar

minerals covered by illite are all very water wet. The ESEMmeasurements have shown that the clay particles (illite,kaolinite, and smectite) are all very water wet, except forthe kaolinite cement of the Fife sandstone samples, whichis water wet. Calcite minerals present on the Slick RockAeolian sandstone samples are water wet.For the petrophysical properties the Slick Rock Aeolian

sandstones present the lowest porosity (21.6%). For theLocharbriggs, Fife, and Berea sandstones the porosity is inthe range of 22.5%, 24.4%, and 24.3%, respectively.The Fife sandstones are the most homogeneous; the

main detrital components are represented by quartz withan amount of 83%. These samples present the biggest meangrain size (varying between 294.1 and 368.4 mm). Theypresent the lowest percentage of cement, varying between4.7% and 6.3% constituted mainly by kaolinite; with thesemicrostructural characterizations, we expect the highestvalues of permeability for these samples. Indeed the perme-ability measurements showed that the Fife sandstones havethe highest liquid permeability, 1980.507388.20mD andgas permeability equal to 1366.50762.93mD.The Slick Rock Aeolian sandstones present the lowest

porosity (21.6%). The cement varies between 5.5% and13.5%, it is in the form of clay minerals (mainly illite)varying between 4% and 8%, hematite interspersed withillite and carbonate (calcite), which is about 3.6%.The Locharbriggs sandstone samples present the smal-

lest mean grain size (varying between 193.7 and 206.1 mm)and a relatively high percentage of cement (5.7% and10.2%), mainly in the form of clay minerals (illite,kaolinite, and smectite). These swelling clays reduced thewater permeability. The measurements of gas and liquidpermeabilities showed that the ratio between gas and liquidpermeability is in the average of 2.14.Berea sandstone presents porosity in the range of 24.3%.

The cement (around 18%) is represented by clay minerals(illite and kaolinite), varying between 9.3% and 10%,calcite (about 8%), and iron. According to these para-meters, we expect lower values of permeability compared tothe other types of sandstone because of the high level ofcement and a relatively high level of clay and calciteminerals. In fact, the Berea sandstones have the lowestpermeability. The liquid permeability is in the range of172.5073.53mD, the gas permeability, 306.50712.02mD.In summary, although accurate description of complex

oil reservoirs requires significant effort on the part of anumber of technical disciplines, the systematic analysis ofthe routine core data is necessary to gain insight into thecomplexity of permeability distribution and to focusreservoir description effort. Good core coverage and fulluse of the resulting core data are required to managecomplex oil reservoirs. Core scale data can play a vital rolein development drilling programs and recovery planningoperations. Laboratory testing procedures can strengthenlog interpretation criteria, aid in completion/stimulationoperations, provide a sound basis for reserves estimatesand reservoir modeling, and supply much needed guidance

ARTICLE IN PRESSS. Baraka-Lokmane et al. / Marine and Petroleum Geology 26 (2009) 39–56 55


in secondary and tertiary recovery programs. Pickup et al.,1994 have investigated the effect of small-scale geometrystructure on single-phase flow. In addition, a range of othertypes of bedding is considered, including models withstochastic variation. It has been found that, in general, thegeometry of the sedimentary structure has a significanteffect on flow. The cross-flow is greater (and thereforetensors are most likely to be necessary) when the angle ofthe laminae is large relative to the pressure gradient (up to45 1C), when the permeability contrast between laminae islarge, or when the structure is asymmetric. In order toobtain the full characterization for the majority of reservoircores with a minimum of combination of techniques, it isessential to use the 3-D X-ray CT technique, whichprovides information about the rock heterogeneity, thepoint counting using thin sections is necessary forobtaining the mineralogy composition of the rock as wellas the estimation of the porosity. The XRD measurementswill refine the results of the point counting using thinsections by providing the different types of clays. TheESEM measurements are essential for the wettabilitydetermination of the different minerals, and the grain sizeanalysis is useful to supplement porosity for empiricalpermeability estimation.

Acknowledgments

This work was funded by Engineering and PhysicalSciences Research Council, Grant: GR/M6 2150, withsupport from Exxon-Mobil, BP Amoco, DTS, and RML.

The authors wish to thank Dr. D.J. Lurie from thedepartment of Bio-Medical Physics at the University ofAberdeen, Scotland for the MRI measurements.

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