REVIEW OF LITERATUR.....

Society of Petroleum ErVneers

SPE 25609

Geochemical Study of Tar in the Uthmaniyah Reservoir M.H. Tobey, * H.1. Halpern, G.A. Cole, J.D. Lynn, * J.M. AI-Dubaisi, and P.C. Sese, Saudi Aramco

* SPE Members

Copyright 1993, Society of Petroleum Engineers, Inc.

This paper was prepared for presentation at the SPE Middle East Oil Technical Conference & Exhibition held in Bahrain, 3-6 April 1993.

This paper was selected for presentation by an SPE Program Committee following review of information contained in an abstract submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material, as presented, does not necesSarily reflect any position Of the Society of Petroleum Engineers, its officers, or members. Papers presented at SPE meetings are subject to publication review by Editorial Committees of the Society of Petroleum Engineers. Permission to copy is restricted to an abstract of not more than 300 words. Illustrations may not be copied. The abstract should contain conspicuous acknowledgment of where and by whom the paper is presented. Write Librarian, SPE, P.O. Box 633836, Richardson, TX 75083-3836, U.S.A., Telex, 163245 SPEUT.

ABSTRACT

Tar is believed to be the main factor impeding production in certain regions of the Uthmaniyah area, Ghawar field. The effect of extraction with several solvents on the permeability and porosity of core plugs from tar zones in the Arab-D Formation was determined in order to understand to what extent tars contribute to obstructing the rock pores. Thin section examinations of the extracted rock were conducted to discern where the tar was distributed microscopically and how that distribution corresponded with the permeability and porosity data. The data show that in general, while organic matter continued to be removed by increasingly polar solvents, the effective permeability, which is controlled by the macropore system, showed little improvement. While the major pore network and macroporosity can generally be improved in the initial extraction, the marginally accessible porosity is still largely occluded by tar. Elemental and pyrolytic analyses of core samples before and after extraction indicate that the tar is neither itself homogeneous, nor uniformly distributed through an individual well, or from well to well. Some components of the tars are not soluble to any of the organic solvent systems utilized, and evidence that some of the tar may result from thermochemical sulfate reduction (TSR) is presented.

INTRODUCTION

The Uthmaniyah area is located in the southern portion of the Ghawar field in Eastern Saudi Arabia (see Figure 1). Most of the wells in the field produce from the Arab-D Formation, an Upper Jurassic limestone sealed by anhydrite. In the eastern

131

portion of the reservoir, a tar mat up to 500 ft. thick is hindering both oil production and water injection. Earlier studies had examined the tar at Ghawar and concluded that the major mechanism of tar formation was gas de-asphaItening of the reservoired Uthmaniyah oil [1]. The gas was postulated to have been generated more recently in geological time from the same source rock as the oil. Migration of the gas up-dip from the east to Uthmaniyah would explain why the tar is observed only in the eastern portion of the reservoir. Other potential origins of tar such as thermochemical sulfate reduction (TSR) were just being postulated at the time of the Riley et aI. report [1].

Another early study found that extraction with benzene removed essentially all the tar from Uthmaniyah Well-C core samples [2]. A more recent study concluded that tars extracted from off-shore Abu Safah field, were more effectively extracted with toluene than benzene [3]. The studies presented here, however, show that the tar consists of different components, often with different solubilities. The fact that different tars can be identified and characterized has implications for the origins of the tar that have not yet been addressed fully.

The goals of this report are to: (1) determine to what extent the tar is affecting reservoir permeability and porosity, (2) understand the chemical and physical properties of the tars, and (3) gain an insight into the origin of the tar. The first objective was accomplished by testing the permeability and porosity of core plugs from Uthmaniyah Well-A and Uthmaniyah Well-B at each step in a series of solvent extractions. The plugs were first extracted with naphtha, then toluene, then twice with either methylene cWoride or tricWoroethane. Thin section microscopy

2 GEOCHEMICAL STUDY OF TARIN THE UTHMANIYAH RESERVOIR SPE 25609

of the extracted cores was used to evaluate the efficiency of the extractions and how residual tar was distributed throughout the microporosity. The second objective was accomplished through total organic carbon and sulfur and pyrolytic analyses of extracted and unextracted core samples. Additionally, organic petrographic assessment of core samples before extraction permitted the identification of discreet components of the tar. This work involved samples from Well-C in addition to samples from Well-A and Well-B. Finally, a mechanism consistent with the chemical properties of the insoluble tar is proposed.

PROCEDURES

Core plugs from Well-A and Well-B were extracted by reflux for at least 72 hours in a Soxhlet extractor separately with naphtha, toluene, and either methylene chloride or trichloroethane. After each extraction, the plugs were dried and tested for permeability and porosity. A period of several weeks followed the toluene extraction, so these plugs were retested for permeability and porosity prior to extraction with chlorinated solvents. The extraction with chlorinated solvents was repeated for each sample and colored extract continued to be obtained even after the second extraction. Following the final extraction, the plugs were cut in half lengthwise and rock from the center of the plugs and from the exterior of the plugs was drilled. This rock powder was analyzed for total organic carbon and was also pyrolyzed with a Rock-Eval II pyrolytic analyzer. Thin sections of the extracted rock were examined with a petrographic microscope.

Some core chips from Well-A and Well-B were acidized with concentrated HCI to dissolve the carbonate rock matrix, and recover the insoluble organic matter. The residue recovered after acidization was washed with de-ionized water and dried. It was then treated with MAC solvent (15% methanol, 15% acetone, and 70% chloroform) and the insoluble material was further treated with HF, water washed and dried. After a final MAC wash, elemental analysis was conducted on the remaining HF treated material.

Finally, core samples from Well-C, a well studied by Sobocinski in 1976 [2], were examined petrographically before and after thorough extraction with benzene and then 90% benzene/lO% methanol.

RESULTS AND DISCUSSION

Porosity and Permeability

The porosity and permeability data for the Well-A and Well-B cores is presented in Figures 2-5. Figure 2 shows the effects of the methylene chloride exft-actions on the core from Well-A. Changes in porosity and permeability as a function of the

extraction stage are illustrated. The pre-test data (i.e., the porosity and permeability data obtained for the core plugs before extraction with chlorinated solvents) is reasonably close to the data obtained after toluene extraction several weeks beforehand. All the samples show small increases in porosity after each of the methylene chloride extractions. The average porosity of all three samples increased 2.6% (15.9% relative to the pre-test porosity) after the second extraction. For the same samples, permeability shows only a minor change. Given the numerous extraction steps involved, and the degree of sample handling, the changes in measured permeability are not considered significant.

Figure 3 shows the data for the trichloroethane extraction from core plugs from the same well. These plugs were taken from positions adjacent to the core plugs used for the methylene chloride extractions. As before, the samples showed increases in porosity after both of the extractions, with the total increase after the two extractions averaging 2.7% (16.4% relative to the pre-test porosity) for the three samples. The changes in air permeability are again trivial.

Figure 4 presents the porosity and permeability data for three core plugs from Well-B subjected to methylene chloride extraction. In this case, the average increase in porosity for the three samples was 2.8% (24.6% relative to the pre-test porosity) while air permeability, as with the Well-A samples, was negligibly changed.

Core plugs taken from positions adjacent to those used for methylene chloride extraction were extracted with trichloroethane, and their permeability and porosity characteristics are shown in Figure 5. Two of the samples showed inconsistencies over the time of the toluene extraction and the pre-test measurement. Repeated testing at pre-test verified that the pre-test values were correct. Again, the porosity increase for the three samples averaged 3.1 % (21.3 % relative to the pre-test porosity) while the air permeabilities were basically unchanged.

Review of Figures 2-5 shows that for most of the samples, extraction with more polar solvents tended to remove organic material and increase porosity. However, air permeability did not increase as this organic matter was removed. This would imply that the heavier tar -- the material not removed by naphtha -- may be confined to pore walls and the more inaccessible portions of the pore spaces. It could also be that while the macropore network is initially cleaned by naphtha, the smaller pore throats remain plugged. Thin section examination of the core plugs was conducted to understand the phenomenon of increased porosity and unchanged permeability after extraction.

Thin Section Examination of The Extracted Cores

132

8PE 25609 M.H. TOBEY ET AL. 3

Thin section examination of the extracted cores showed that the samples fall into different lithological groups according to well: the Well-A samples are limestones while the samples from Well- B are dolomites. All the samples exhibited the same general behavior when extracted with the various solvents. The major difference attributed to the lithologic nature of the samples may be the fact that the increase in porosity after the extraction with chlorinated solvents averaged about 5 % higher, relative to the pre-test porosity, for the dolomitic samples. This may be due to the relatively simpler pore structure formed by the dolomitization of the original limestone matrix at the Well-B well. The pore complex seen in the Well-A samples was quite intricate, with numerous dead-end pores, many of which were filled with the tar material (after naphtha and toluene extraction). Well-B samples showed the simple polymetric pores often associated with recrystallization of a limestone. The pores were much cleaner, although there was still a minor amount of residual organic material lining parts of the pore despite two extractions with trichloroethane.

The most important insight into the permeability properties of the cores following extraction was provided by the microscopic examination after naphtha and toluene extraction. The controlling influence on permeability in a porous medium will be the largest pores. For nearly all the samples, the central, large scale pores were completely clear of organic material. The secondary pores were generally blocked by the "tar" material, and were not available to contribute to ~e overall flow of fluids. Even those samples which were extracted with trichloroethane and methylene chloride showed a large amount of residual material in the pore structure.

Additional extraction will continue to gradually increase the porosity by accessing more and more of the organic material lining large pores and the smaller pore structures, but overall permeability will be only marginally affected by the removal of the organic material since the pore structure controlling the flow has already been cleared. The fact that tar material remained in the pore system after the extractions with chlorinated solvents supports a case for different types of tar being present in the reservoir.

In some samples, most of the tar appeared to be confined to the pore walls and for others it was mainly at the more inaccessible portions of the pore space. While the pores were being cleaned of tar material, many of the smaller pore throats remained partially or completely plugged. This too would explain the noted increase in porosity while not providing any substantial gain in permeability.

Total Organic Carbon Content and Pyrolytic Analyses

The total organic carbon content measured at the interior and

133

exterior of the core plugs extracted with methylene chloride and trichloroethane is presented in Table 1 and illustrated graphically in Figure 6. As can be seen from the Table and Figure, organics are not being preferentially extracted from the exterior of the core plugs. For most samples, there is little difference in the organic carbon content at the exterior as opposed to the interior. The amount of unextracted tar distributed through the extracted plugs would appear to be dependent on both the pore network of the rock and the insolubility of some of the tars. As shown by the photomicrographs, most tar is extracted from the larger pores. Areas of the core where organic matter remains can be attributed either to restricted access to the solvent and/or to the insolubility of some of the tar even when access is provided. External areas of the core should provide the greatest access to the solvent. The fact that a substantial amount of organic matter in those external areas remains unextracted for some of the samples is evidence that some of the tar is largely insoluble and therefore chemically distinct from the material which is extractable.

Characterization of the unextracted organic matter by Rock-Eval II pyrolysis provides some indication of the chemical nature of this organic matter. Table 2 lists the 81, 82, and hydrogen index (HI) for the core plugs. Rock-Eval II pyrolysis .is commonly used to evaluate the source potential of hydrocarbon source rocks. The 8. pyrolytic yield is indicative of the distillable, or oil-like, hydrocarbons in a source unit. In this study, the S. values are all essentially negligible, as would be expected for rocks subjected to extraction, and these do not provide any useful information concerning the nature of the tar. The S2 pyrolytic yields are indicative of the non-distillable, heavier, hydrocarbons remaining in the rock. after the lighter components have been thermally desorbed. The 82 yield is obtained by thermally cracking (in an inert atmosphere) the heavy organic material into lighter components. The heavy Qlaterial may consist of asphaltenes, kerogen, or pyrobitumen. When related to the total organic carbon content (TOC), the hydrogen index can be calculated (HI= l00*S/TOC). The hydrogen index provides information about the hydrogen content of the organic matter. For petroleum source rock screening, gas- prone/condensate sources have HIs between 100 - 400. Oil prone rocks have HIs >400. For this study, the HI can be used to compare the unextractable organic matter from the two wells.

Figure 7 plots ~ vs. TOC for external and internal samples taken from the two wells after extraction with the chlorinated solvents. The slope of this line represents the hydrogen index for that group of samples. As can be seen, there is no distinction between the internal vs. external samples within one well, but there is a difference in the nature of the tars between the wells. Whereas the unextracted organic matter in the Well-A carbonates has a hydrogen index of approximately 163, the unextracted organic matter in the Well-B dolomites has a

4 GEOCHEMICAL STUDY OF TAR IN THE UTHMANIYAH RESERVOIR SPE 25609

hydrogen index of approximately 67. The R2 variance for the HI lines are 0.97 and 0.82 for Well-A and Well-B, respectively. The different trends are apparent. The unextracted matter in Well-B, in general, is not the same as that in Well-A. The greater hydrogen deficiency of the Well-B material could be due to the degree of aromatization, cross-linking, or incorporation of heteroatoms. It must also be considered that, as mentioned, the simpler pore system of the dolomite permitted more thorough extraction, and thus the residual organic matter in the dolomites may be more hydrogen deficient as a result of the greater extraction efficiency in these samples -- i.e., the Well-A samples may contain some extractable organic matter which the solvent could not access while nearly all of the extractable matter in the dolomite was removed leaving only the especially insoluble and more hydrogen deficient tar. However, because the quantity of organic matter in the dolomites is generally greater than in the carbonate samples, the extraction efficiency does not appear to be an important factor in the differences seen in the residual organic matter when the samples are thoroughly extracted with strong solvents.

Samples of unextracted core from Well-A and Well-B were acidized and the organic residue isolated. The same treatment was given a core sample from Well-A. Of the material recovered, 69 % was insoluble in MAC solvent for the Well-B sample while 32 % was insoluble in MAC for the Well-A sample. The insoluble material from Well-B was further treated with HF, washed and extracted with MAC again. Elemental analysis was conducted on the residual insoluble material. The data is presented in Table 2. This sample was from an interval 18 ft. deeper than that of any of the Well-B samples used for the multiple extraction experiments. The contrast in solubilities of the organic matter from each of the wells is compelling evidence that different types of tars are distributed through the reservoir. The sulfur content of the insoluble organic matter is an important parameter in pyrobitumens which result from TSR, as is discussed later below.

Organic Petrography of Well-C Cores

Core samples from Well-C, a well studied earlier by Sobocinski [2], were selected for organic petrographic study using reflected light methods. The study sought to evaluate the bitumens in the core samples before and after extraction, with the objective of identifying insoluble (probably high reflecting) solid bitumens and soluble (low reflecting) bitumens and bitumen staining. The extraction solvents used were benzene and 90% benzene/l0% methanol which further work showed to have nearly identical extraction efficiencies as MAC.

A survey of unextracted samples covering the depth interval 7194-7323 ft. showed that the basal portion of this interval (7318-7323 ft.) contained significant amounts of solid, high

134

reflecting bitumens similar to grahamite or the very early stages of epi-impsonite [4]. These bitumens were assumed to be mostly insoluble because little to no dissolution was observed using immersion oils. Most soluble bitumens will partially dissolve in immersion oils. These solid bitumens were found as interstitial pore fillings and as the fillings of large vugs in limestones. Moderate to heavy bitumen staining was also observed in this interval. The upper portion of the interval consisted of lesser amounts of solid, high reflecting bitumens. Most of the bitumens were low reflecting and partially dissolved in the immersion oil. This upper section was also very heavily bitumen stained.

A sample selected from the upper portion of the interval at 7204 ft. contained < 25 % high reflecting bitumen prior to and after extraction. The heavy bitumen stain and low reflecting bitumens, however, were removed through extraction. A sample taken from the basal portion of the interval at 7318.9 ft. showed similar results. Interestingly, differences were found in the bitumen reflectance of this grahamite/epi-impsonite. Pre- extraction bitumen reflectance values were 0.68 % for the sample at 7204 ft. and 0.60% for the sample at 7318.9 ft. Post extraction results for the sample bitumens were 0.75 % and 0.76%, respectively. The differences in reflectance before and after extraction suggest that the pre-extraction bitumen exhibited absorbed staining of the solid bitumen, which would decrease the pre-extraction reflectance value. The post-extraction reflectance value is probably more indicative of the true reflectance.

Thus, solid bitumens insoluble to organic solvents can easily be identified in the reservoir rock. Their distribution is not uniform throughout the well but their reflectance values are approximately 0.75%.

Thermochemical Sulfate Reduction· (TSR)

The large proportion of insoluble organic matter isolated from the acidized rock, coupled with its high sulfur content, indicate the possibility that some of the tars are actually pyrobitumens which were formed by TSR. TSR is thought to be a reaction which is initiated by high temperature (100 - 120° C, though there is some evidence that TSR can occur at lower temperatures). Required elements for TSR are a source of sulfate, water, and oil (or gas). Dissolved anhydrite provides a ready source ofsulfate. Because water is necessary, TSR occurs at the oil/water interface. One of the major products of TSR is pyrobitumen -- an essentially inert solid organic material which is heavily cross-linked with sulfide. Also produced is hydrogen sulfide gas which may be present as such or as metallic sulfides [5]. Because the sulfides are derived from sulfate (anhydrite), which has a different isotopic signature than organic sulfur found in petroleum, the pyrobitumen sulfur isotopic data can be used to determine whether TSR has occurred. Other signs of TSR

SPE 25609 M.H. TOBEY ET AL. 5

include the presence of primary and secondary anhydrite with secondary anhydrite often below the oil/water contact, hydrogen sulfide gas in the oil and/or the formation water, and the replacement of anhydrite with calcite [6]. TSR has been linked to hydrothermal dolomitization in northeastern British Columbia. Products formed included pyrobitumen, dolomite, methane, hydrogen sulfide, and water [5].

Because its sulfur content is not as high as might be expected for TSR pyrobitumen, the insoluble organic matter from Well-B may be indicative of incipient TSR. Isotopic data are still being collected in an investigation of TSR at the Uthmaniyah reservoir as the circumstantial evidence is not conclusive. Nevertheless, the tar mat occupies the eastern edge of the field near the oil/water contact. There is a plentiful source of sulfate in the form of anhydrite and Uthmaniyah oils contain dissolved hydrogen sulfide. Finally, the tar mat is composed, at least in part, of high sulfur, insoluble organic matter.

CONCLUSIONS

The macroporosity of core plugs was found to improve with extraction with more powerful solvents, but the permeability showed little change after the initial extraction with naphtha. Microscopic examination showed that residual tar remained even after extraction with chlorinated solvents, but the macropores which control the permeability were essentially clean after naphtha and toluene extractions. Bearing in mind that a relatively small interval of samples was examined, it does not appear that all the tar can be mitigated by either hot water flooding or solvent flooding, particularly if the tar distribution is extensive. Understanding the origin of the tar and how soluble and insoluble tars are distributed through the reservoir may allow well locations to be selected which avoid the impermeable tar zones.

The tars were found to consist of soluble (hydrogen rich) and insoluble (hydrogen deficient) components. The insoluble tars differ in their hydrogen indices for Well-A and Well-B, suggesting that the tar mat is composed of several classes of tar which may result from a variety of mechanisms. Some of the insoluble tar resembles bitumens with reflectance values of approximately 0.75%. Analyses of some insoluble tar showed at least one sample with a sulfur content of approximately 7.5 %.

The inert nature of some of the tar, coupled with its sulfur content, suggest that it may result from thermochemical sulfate reduction. Other, more hydrogen-rich, residual organic matter, however would not be due to TSR, but may instead be the result of gas de-asphaltening (soluble tars) or may be kerogen (insoluble).

ACKNOWLEDGEMENTS

The authors thank the Southern Area Reservoir Management Department of Saudi Aramco for their on-going support of this investigation. The assistance of Mr. Mohammad I. Khan for his contributions during the permeability and porosity studies are also gratefully acknowledged. We thank Mr. E.L. Colling for his critical review of this manuscript and his valuable suggestions and comments. Finally, we thank the Saudi Arabian Ministry of Petroleum and Mineral Resources and Saudi Aramco for permission to publish this paper.

REFERENCES

1. Riley, C.M., Rodgers, M.A., and Young, W.A.: "Physical- Chemical Characteristics of Tar at Uthmaniyah Area, Ghawar Field, Saudi Arabia -- Explanatory and Predictive Model for Such Occurrences," Report EPR.47ES.77 (1977), Exxon Production Research Company, Houston, Texas.

2. Sobocinski, D.P.: "An Evaluation of the Characteristics of Uthmaniyah Tar and the Potential for Stimulating Water Injectors Containing the Tar," Report EPR.112PS.76 (1976), Exxon Production Research Company, Houston, Texas.

3. Dabbagh, A.E.: "A Study of Tar Properties and Methods of Improving Injectivity in Tar Mats," Project Number PN 21061 (1989), The Research Institute, King Fahd University of Petroleum and Minerals, Dhahran, Saudi Arabia.

4. Robert, P.: Organic Metamorphism and Geothermal History: Microscopic Study of Organic Matter and Thermal Evolution of Sedimentary Basins, Elf-Aquitaine and D. Reidel Publishing Co., Dordrecht, Holland (1988) 311.

5. Teare, M.R. and Reimer, J.D.: "Thermochemical Sulfate Reduction and Hydrothermal Dolomitization (TSR-HTD): A Diagenetic Process That Created and Modified Middle Devonian Reservoirs in Northeastern British Columbia," presented at the 1992 AAPG mee!ing in Calgary, Canada, June 19-25.

6. Heydari, E. and Moore, C.H.: "Burial Diagenesis and Thermochemical Sulfate Reduction, Smackover Formation, Southeastern Mississippi Salt Basin," Geology Vol. 17 (1989) 1080-1084.

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6 GEOCHEMICAL STUDY OF TAR IN THE UTHMANIYAH RESERVOIR SPE 25609

TABLE 1:

TABLE 2:

Total organic carbon content and pyrolysis data for Well-A and Well-B core plugs after final methylene chloride (MC) or trichloroethane (TCE) extraction. Samples were taken at the interior (INT) and exterior (EXT) of the core plugs.

FINAl TOC S1 S2 HI

CORE SOLVENT (wt. 'MI) (mg HC/g rock) (mg HC/g rock)

WELL PLUG EXTRACTION INT. EXT. INT. EXT. INT. EXT. INT.

WELL-A 24 MC 2.4

WELL-A 52 MC 1.8

WELL-A 116 MC 0.4

WELL-A 23 TCE 1.6

WELL-A 51 TCE 1.0

WELL-A 115 TCE 0.7

WELL-B 182 MC 1.3

WELL-B 188 MC 1.8

WELL-B 201 MC 0.3

WELL-B 181 TCE 0.7

WELL-B 187 TCE 4.8

WELL-B 200 TCE 0.2

Elemental data for insoluble organic matter isolated from Well-B core sample at 6951 feet.

Carbon (wt. %) 81.2

Hydrogen (wt. %) 6.4

Sulfur (wt. %) 7.4

Oxygen (wt. %) 1.7

Nitrogen (wt. %) 1.4

Iron (wt. %) < 0.01

Loss on Ignition at 500 C (wt. %) 97.5

Loss on Ignition at 800 C (wt. %) 99.0

Loss on Ignition at 900 C (wt. %) 99.0

136

SPE 25609 M.H. TOBEY ET AL. 7

FIGURE 1: The Ghawar field showing the Uthmaniyah area, Saudi Arabia.

137

8 GEOCHEMICAL STUDY OF TAR IN 'THE UTHMANIYAH RESERVOIR SPE25609

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SPE25609 M. H. TOBEY ET AL. 9

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139

10 GEOCHEMICAL STUDY OF TAR IN THE UTHMANIYAHRESERVOIR SPE 25609

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External Toe (wt. 0/0)

. FIGURE 6: Total organic carbon cross-plot showing that solvent extraction of the core plugs did not preferentially remove organic matter from the exterior or interior of the plugs. TOC differences are more likely due to the heterogenous distribution of the organic matter through the rock.

1.0 1.5 2.0 2.5 3.0 3.5 4.0 Total Organic Carbon (wt. %)

4.0

~ 3.5 Co)

0 3.0-a-- 2.5(J

:J:

a 2.0 E- ~

1.5

.! 1.0>

N tn 0.5

0.0 0.0 0.5

....

..

Slope = HI of 67 r2 = 0.82

• WELL-A INTERIOR o WELL-A EXTERIOR

.. WELL-B INTERIOR

.. WELL-B EXTERIOR

4.5 5.0

FIGURE 7: ~ vs. TOC plot which permits group hydrogen indices to be cal~ulated. Data was obtained for the extracted core plugs listed in Table 1. Note that the Well-A unextracted organic matter has a HI of 163 while the Well-B unextracted organic matter has a HI of 67.

140