ABSTRACT: This Paper is based on detailed mineralogical, structural, petrophysical and geochemical studies of sandstone core samples, using routine methods. These include Magnetic Resonance Imaging (MRI), Nuclear Magnetic Resonance (NMR), X-ray Computer Tomography (CT) Scanning, 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 of measurements in the characterization 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 more complete characterization of the rock samples than each method used on its own. This study has shown that rock heterogeneity at the sub-cm scale may have a significant effect on reservoir petrophysical characterization

B T Ngwenya

I G Main

S Baraka-Lokmane

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SCA2007-57 1/6
 
BENEFIT OF COMPLEMENTARY METHODS FOR 
CHARACTERIZING SANDSTONE CORES 
 
S. Baraka-Lokmane 1,2, B.T. Ngwenya 1, I.G. Main 1 
 
1 School of Geosciences, University of Edinburgh, UK 
2 Present address: Institute of Petroleum Engineering, Heriot-Watt University, 
Edinburgh, UK 
 
 This paper was prepared for presentation at the International Symposium of the 
Society of Core Analysts held in Calgary, Canada, 10-12 September, 2007 
 
ABSTRACT: 
This Paper is based on detailed mineralogical, structural, petrophysical and 
geochemical studies of sandstone core samples, using routine methods. These include 
Magnetic Resonance Imaging (MRI), Nuclear Magnetic Resonance (NMR), X-ray 
Computer Tomography (CT) Scanning, 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 of measurements in the 
characterization 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 more complete characterization of the rock samples than each method 
used on its own. This study has shown that rock heterogeneity at the sub-cm scale may 
have a significant effect on reservoir petrophysical characterization. 
 
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). Field development methods that rely on core flooding 
are particularly sensitive to small-scale changes in reservoir quality. Meanwhile, most 
waterfloods in sandstone cores are carried out either in almost homogeneous samples 
or else in core samples of uncertain heterogeneity. Several experimental techniques 
exist to identify, characterize and take into account the small scale heterogeneity in 
SCAL program. It is current practice to combine CT-scanner, NMR, Tracer tests, and 
Hg porosimetry to select the best appropriate samples. Even with a certain degree of 
heterogeneity, dedicated interpretation techniques based on the fluctuations observed 
SCA2007-57 2/6
 
on the repeated saturation profiles as a function of time during the core flooding 
enable to account for the heterogeneity in the final results (Egermann and Lenormand, 
2005). Arns et al. (2001) have shown that direct calculations on high-resolution X-ray 
computed microtomography images of Fontainebleau sandstone produce permeability 
and conductivity values which are in very good agreement with laboratory 
measurements. Ioannidis et al. (1996) have analyzed back-scatter SEM images of 
cross-sections of several porous rocks in order to determine the statistical properties of 
the porous microstructure. This study has the aim to demonstrate that by using a suite 
of techniques in combination; subtle variations in petrophysical characteristics emerge 
that adds significant value towards a better description of reservoir properties.  
 
METHODS OF MEASUREMENTS AND RESULTS 
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. The four different type of sandstone used in this 
study present very variable microstructural properties. The preliminary MRI 
measurements performed on sample 8 of the Slick Rock Aeolian sandstone showed 
that no images were obtainable using the conventional MRI at 4.7T, because the 
transverse relaxation T2* was reduced dramatically by the presence of iron. The 
transverse relaxation T2* used for this experiment was equal to 100us. In general, a 
transverse relaxation T2* of 10ms or greater is necessary for conventional MRI. We 
have repeated the experiment using a lower magnetic field of 1.5T unfortunately it 
was not possible to obtain any image. The reason is the heterogeneity of local 
magnetic susceptibility. We conclude that 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 
(Baraka-Lokmane et al., 2001). The results using the continuous-wave magnetic 
resonance imaging (CW-MRI) technique clearly show the development of water front 
(Fig.1). X-ray Computer Tomography (CT) scanning images (Fig.2) show the 
different mineral phases: calcite in white, microcline in light grey, and quartz in dark 
grey. The white areas seen on the images indicate the presence of calcite cements 
(Figs 2a and 2d). The iron banding is seen in the near-white and white layers (Figs 2a 
and 2b). The XRD, XRF analyses and the petrographical study have shown that the 
highly attenuating mineral contains iron. For the Berea sandstone samples, the 
different CT images have shown a very homogeneous and isotropic material. Figure 
2c shows the presence of white areas, indicating the development of nodules, which 
could be calcite or iron. We conclude that the method is particularly useful for 
imaging calcite and hematite or iron. 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. This study showed that fresh surfaces of quartz and 
feldspar are both water wet (Figs 3 and 4). Interstitial illite particles or quartz and 
feldspar minerals covered by clay particles (illite, kaolinite, and smectite) are all very 
water wet (Figs 5 and 6). The Slick Rock Aeolian sandstones present the lowest 
SCA2007-57 3/6
 
porosity (21.6%). For the Locharbriggs, Fife and Berea sandstones the porosity is in 
the 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 with an amount 
of 83%. These samples present the biggest mean grain size (varying between 
294.1 mµ  and 368.4 mµ ). They present the lowest percentage of cement, varying 
between 4.7% and 6.3% constituted mainly by kaolinite; with these microstrutural 
characterizations, we expect the highest values of permeability for these samples. 
Indeed the permeability measurements showed that the Fife sandstones have the 
highest liquid permeability, 1980.50mD±388.20 and gas permeability equal to 
1366.50mD±62.93.  The Slick Rock Aeolian sandstones present the lowest porosity 
(21.6%). The cement varies between 5.5% and 13.5%; it is in the form of clay 
minerals (mainly illite) varying between 4% and 8%. The Locharbriggs sandstone 
samples present the smallest mean grain size (varying between 193.7 mµ and 
206.1 mµ ) and a relatively high percentage of cement (5.7% and 10.2%), mainly in the 
form of clay minerals (illite, kaolinite and smectite). These swelling clays reduced the 
water permeability. 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 parameters, 
we expect lower values of permeability compared to the other types of sandstone 
because of the high level of cement and a relatively high level of clay minerals. In fact 
the Berea sandstones have the lowest permeability. The liquid permeability is in the 
range of 172.50mD±3.53, the gas permeability, 306.50mD±12.02. 
 
DISCUSSION 
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 
1). 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 and 
ESEM measurements). The point counting method has shown that 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, which was not detected by the XRD measurements. The ESEM measurements 
carried out on the Berea sandstone samples have also shown the presence of iron 
SCA2007-57 4/6
 
nodules. This indicates the importance of using complementary method of 
measurements for characterizing a rock sample. The point counting has shown the 
finite presence of hematite, which was not detected with the XRD measurement 
because of its small concentration (less than 1% of the sample). This is also the case of 
the pyrite cement for the Locharbriggs sandstone samples. A structural method (point 
counting technique) is therefore complementary to a quantitative method (bulk XRD 
analysis) in order to identify the different mineral species and quantify their 
abundance. The use of X-ray diffraction (XRD) techniques for quantifying clay 
content can be very time consuming, requiring significant sample preparation, and 
generally examine only a relatively small sample volume. Potter et al. (2004) have 
used magnetic susceptibility for quantifying illite content in sedimentary rocks. The 
magnetic method is also capable of estimating other minerals in the core samples.  
 
CONCLUSIONS 
Although accurate description of complex oil reservoirs requires significant effort on 
the part of a number of technical disciplines, the systematic analysis of the routine 
core data is necessary to gain insight into the complexity of permeability distribution 
and to focus reservoir description effort. Good core coverage and full use of the 
resulting core data are required to manage complex oil reservoirs. Core scale data can 
play a vital role in development drilling programs and recovery planning operations. 
Laboratory testing procedures can strengthen log interpretation criteria, aid in 
completion/stimulation operations, provide a sound basis for reserves estimates and 
reservoir modeling, and supply much needed guidance in secondary and tertiary 
recovery programs. The results of this study conducted on four sandstone cores 
indicate the added value of combining several techniques. The 3-D X-ray computer 
tomography technique provides information about the rock heterogeneity, the point 
counting using thin sections is necessary for obtaining the mineralogy composition of 
the rock as well as the estimation of the porosity. The XRD measurements will refine 
the results of the point counting using thin sections by providing the different types of 
clays. The ESEM measurements are essential for the wettability determination of the 
different minerals, and the grain size analysis is useful to supplement porosity for 
empirical permeability estimation.  
 
ACKNOWLEDGEMENTS 
This work was funded by Engineering and Physical Sciences Research Council, grant: GR/M6 
2150, with support from Exxon-Mobil, BP Amoco, DTS and RML.  
 
REFERENCES 
Arns, C.H., Knackstedt, M.A., Pinczewski, W.V., Lindquist, W.B., 2001. Accurate estimation 
of transport properties from microtomographic images. Geophys. Res. Lett. 28 (17), 3361-
3364. 
  
Baraka-Lokmane, S., Main, I., Elphick, S., and Ngwenya, B., 2001. Complementary methods 
for characterising Slick Rock Aeolian Sandstone. Presented at the International Symposium of 
the Society of Core Analysts (SCA), 17-19 September, 42, 1-5. 
 
SCA2007-57 5/6
 
Cade, C. A., Evans, I. J., and Bryant, S. L., 1994. Analysis of permeability controls-a new 
approach. Clay Minerals, 29, 491 – 501. 
 
Egermann, P. And Lenormand, R., 2005. A new methodology to evaluate the impact of 
localized heterogeneity on petrophysical parameters (k[r], P[c]) applied to carbonate rocks. 
Petrophysics. 46, 5, 335-345.  
 
Ioannidis, M.A., Kwiecien, M.J. Chatziz, I., 1996. Statistical analysis of the porous 
microstructure as a method for estimating reservoir permeability. Journal of Petroleum 
Science & Engineering, 16, 251-261. 
 
Potter, D.K., Corbett, P.W.M., Barclay, S.A. and Haszeldine, R.S., 2004. Quantification of 
Illite Content in Sedimentary Rocks Using Magnetic Susceptibility – A Rapid Complement or 
Alternative to X-Ray Diffraction. Journal of Sedimentary Research, September, 74, 5, 730-
735.  
 
  Table 1: 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 
grey scale is 
challenging 
• Previous petrography 
analysis  is necessary 
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 
• CW- 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/ 
EDX 
• 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  
2-D Point 
counting using 
thin sections 
• Percentages of the 
different mineral 
constituents and 
porosity 
• Identification of the 
dominant cement 
• 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 
1ppm 
• time consuming 
SCA2007-57 6/6
 
 
 
 
 
 
 
 
 
 
          Start 11:59 end 12:17   Start 13:46 end 14:03           Start 17:06 end 17:23 
Figure 1: Development of the waterfront inside sample 8 of the Slick Rock Aeolian sandstone 
 
 
 
 
 
 
 
 
      
          (a)    (b)     (c)                          (d) 
Figure 2: X-ray Computer Tomography (CT-Scanning) images of Slice 2 of Slick Rock Aeolian 
sandstone (a), Locharbriggs sandstone (b), Berea sandstone (c) and Fife sandstone (d)  
               
 
       
 
 
      
 
 
       Figure 3: ESEM photo, showing                 Figure 4:  ESEM photo, showing  
     water wet feldspar                                water wet quartz 
 
 
 
 
 
    
 
                       
                  
                 Figure 5: ESEM photo of a cement                   Figure 6: ESEM photo, showing the very  
             containing kaolinite, smectite and illite    water wet property of the clays (kaolinite,  
             (Locharbriggs sandstone)                          smectite and illite) (Locharbriggs sandstone)  
Calcite layer
Iron banding
Calcite patch
Iron 
banding
Iron
Calcite


BENEFIT OF COMPLEMENTARY METHODS FOR CHARACTERIZING SANDSTONE CORES

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BENEFIT OF COMPLEMENTARY METHODS FOR CHARACTERIZING SANDSTONE CORES

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