Organic Petrography and Organic Geochemistry of Texas Tertiary Coals

RI0188. Organic Petrography and Organic Geochemistry of Texas Tertiary Coals in Relation to Depositional Environment and Hydrocarbon Generation, by P. K. Mukhopadhyay. 118 p., 36 figs., 14 tables, 1989. ISSN: 0082335X: Print.


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ABSTRACT
Organic petrological, organic geochemical, and chemical analyses of Tertiary (Paleocene to Eocene) coals (to a 2,000-ft [610-m] depth) from the Wilcox, Claiborne, and Jackson Groups of Texas reveal characteristic properties of these coals. Most of the Wilcox, Claiborne (one sample), and Jackson coals contain greater amounts of liptinite (especially fine-grained liptodetrinite) than do average humic coals. These liptinites can be identified only by blue-light excitation in reflected-light microscopy. Huminite reflectance and calorific value (moist, ash-free) indicate that the Wilcox coals lie between subbituminous C and lignite in rank, whereas the Claiborne coal is subbituminous A and the Jackson coals are lignite in rank. Huminite reflectance varies according to maceral types and postdepositional ground-water movement. A systematic increase in huminite/vitrinite reflectance with increased depth is characteristic of some of the Wilcox and Jackson deep-basin coals.

Ternary diagrams of maceral composition relationships between ratios of macerals of similar affinity, selected palynological data, and physicochemical properties record specific peat-forming environments. These data establish the variability of depositional environments, depositional relationships between maceral groups, and chemical properties of coals from the Wilcox (swamp and swamp-marsh complex in interdistributary or lacustrine basins on an upper delta plain), Claiborne (lacustrine basin on a delta plain), and Jackson (interdistributary basin on a lower delta plain or swamp, or marsh-dominated basin on a strandplain/barrier bar/lagoon) Groups. The Wilcox and Jackson coals are differentiated on the basis of equilibrium moisture, ash, calorific value, sulfur content, huminite reflectance, and palynomorph assemblages. The Claiborne coal is distinguishable from most of the Wilcox and Jackson coals on the basis of the nature of liptinite or huminite macerals, calorific value, and huminite reflectance.

Texas Tertiary coals are classified as humic, mixed, and sapropelic on the basis of hydrocarbon potential (hydrogen index and oxygen index determined from Rock-Eval pyrolysis or atomic H/C and O/C ratios determined from elemental composition analysis) and maceral composition, which reflects original floral components and depositional environment. All three of these coal types occur within the Type II and Type Ill kerogen maturation paths. The total yield of pyrolysates (percent of whole coal) recovered from hydrous and anhydrous pyrolysis (artificial maturation experiments) differs between humic (less than 8 percent), mixed (greater than 10 but less than 20 percent), and sapropelic (greater than 20 percent) coals. Aromatic and aliphatic ratios of low-molecular-weight compounds, pseudoactivation energies determined from anhydrous pyrolysis, and measured vitrinite reflectance after hydrous pyrolysis also vary between the three coal types. N-alkane distribution patterns (from hydrous and anhydrous pyrolysis) suggest that mixed and sapropelic coals are derived from alginite and cutinite macerals, whereas humic coals originate mainly from huminite and sporinite macerals. The n-alkane distributions (determined from hydrous pyrolysis) of the expelled oil and remaining coal extract show a fractionation effect. A modified model of hydrocarbon generation and primary migration within the coal network identifies mixed coal as the potential source rock for liquid hydrocarbons in deltaic and lacustrine basins.

Keywords: chemical composition, Claiborne Group, depositional environment, hydrocarbon generation, Jackson Group, lignite, maturation, organic geochemistry, organic petrography, primary migration, source rock, subbituminous coal, Texas, Wilcox Group

CONTENTS

Abstract

Introduction

Objectives

Samples

Lithotypes

Maceral terminology

Organic Petrography

Analytical methods

Results

Lithotypes

Maceral composition

Wilcox Group

Surface mines

Northeast Texas

East-central Texas

Sabine Uplift

Deep basin (200 to 2,000 ft [61 to 610 m])

East-central Texas

Sabine Uplift

Claiborne Group

Jackson Group

East Texas

South Texas

Early diagenesis

Maturation

Huminite/vitrinite reflectance

Spectral fluorescence

Palynology (selected samples)

Wilcox Group

Surface mines

Northeast Texas

East-central Texas

Sabine Uplift

Deep basin

East-central Texas

Sabine Uplift

Claiborne Group

Jackson Group

East Texas

South Texas

Physicochemical Properties

Experimental procedure and results

Depositional Environments

Regional facies analysis

Environmental analysis from organic petrology

Tertiary coal-bearing environments of Texas

Wilcox Group Northeast Texas

East-central Texas

Sabine Uplift

Claiborne Group

Jackson Group.

East Texas.

South Texas.

Organic Geochemistry (selected samples)

Experimental methods

Results

Rock-Eval pyrolysis

Wet chemistry

Pyrolysis- gas chromatography

Hydrous pyrolysis

Anhydrous pyrolysis

Correlation between Petrography and Chemical or

Geochemical Parameters

Maturation Hydrocarbon Generation Related to Coal Types

Coal as Potential Source Rock for Liquid Hydrocarbons

Primary Migration

Implications for Future Coal Utilization

Mine planning

Liquefaction of low-rank coal

Coal as a hydrocarbon source rock

Coalbed methane

Summary and Conclusions

Depositional environments and chemical properties

Maturity

Hydrocarbon potential and primary migration

Acknowledgments

References

Appendices

A. Macropetrographic (lithotype) classification of soft, brown coals

B. Lithotype classification of lignite and subbituminous coals.

C. Petrographic nomenclature of low-rank coals (reflected light)

D. Distribution of maceral ratios and percentages as plotted in figures 25 and 26

E. Hydrocarbon yield of sample 22 and total yield of six samples before and after hydrous pyrolysis

Figures

1. Outcrop of Tertiary coal-bearing units in Texas showing locations of mines, boreholes, and coal exposures from which samples were collected

2. Major structural features of the northwest Gulf Coast Basin

3. Lower Tertiary stratigraphic section and inferred sedimentary facies of the Texas Coastal Plain

4.-16. Photomicrographs of selected macerals

17. Reflectance histograms for different huminite maceral types

18. Mean huminite reflectance versus depth and projected maturity for various boreholes, Wilcox and Jackson Groups

19. Measurement of fluorescence alteration of globular resinite (positive alteration) and ground mass (negative alteration), sample 22

20. Position of near-surface Texas Tertiary coals relative to coalification rank parameters

21. Correlation of different chemical parameters from the Wilcox and Jackson Group coals

22. Schematic diagram showing major paleotransport directions and alluvial-plain/delta-plain transitions in the upper Wilcox Group

23. Schematic profile and lithologic and floral characteristics of peat-forming environments of the alluvial and delta plain

24. Areal map showing physiographic regions of the Barataria Basin (Mississippi River Delta

25. Graphic method of determining peat-forming environments from maceral assemblages

26. Graphic relation between ratios of a combination of macerals of similar affinity

27. Depositional environments inferred from maceral composition of Hooper and Calvert Bluff Formation coal seams in borehole FE-1-83

28. Relation between hydrogen index and oxygen index and atomic H/C ratio and atomic O/C ratio for all samples

29. Capillary gas chromatograms of saturated hydrocarbon fraction from the extract of selected coal samples before pyrolysis

30. Capillary gas chromatograms of saturated hydrocarbon fractions after hydrous pyrolysis

31. Capillary gas chromatograms of total hydrocarbons after anhydrous pyrolysis

32. Plot of T max in PYRAN system and vitrinite reflectance showing position of three analyzed samples and six standard humic coal samples

33. Relation between selected chemical and geochemical parameters and maceral composition of Texas low-rank coals

34. Relation between hydrogen index and percentage of macerals of similar type and atomic H/C ratio and percentage of macerals of similar type, vegetation pattern, and ground-water-table conditions

35. Classification of different types of coal according to atomic H/C ratio versus atomic O/C ratio and hydrogen index versus oxygen

36. Schematic diagram showing the location of exsudatinite and bitumen within the network of humic and mixed coal

Tables

1. Sample location, type, stratigraphy, lithotype, and huminite reflectance

2. Maceral composition in volume percent on a mineral-matter-free basis

3. Spectral fluorescence data from samples 22 and 143

4. Proximate analyses and calorific values on a dry basis

5. Elemental composition, or ultimate, analyses on a dry basis

6. Average proximate analyses and calorific values

7. Average sulfur content of Wilcox coals by formation

8. Rock-Eval pyrolysis data

9. Wet chemistry data and hydrocarbon ratios from gas chromatography of coal extracts before pyrolysis

10. Results of hydrous pyrolysis

11. Huminite/vitrinite reflectance before and after hydrous pyrolysis

12. Results of anhydrous pyrolysis

13. Coal pyrolysis kinetics from anhydrous pyrolysis

14. Calculated oil yield of Texas low-rank coals


Citation
Mukhopadhyay, P. K., 1989, Organic Petrography and Organic Geochemistry of Texas Tertiary Coals in Relation to Depositional Environment and Hydrocarbon Generation: The University of Texas at Austin, Bureau of Economic Geology, Report of Investigations No. 188, 118 p.