Organic Petrography and Organic Geochemistry of Texas Tertiary Coals in Relation to Depositional Environment and Hydrocarbon Generation

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.
Prasanta K. Mukhopadhyay

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.

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The University of Texas at Austin, Bureau of Economic Geology
Report of Investigation