Depositional Environment of Tertiary Coals from Texas as Revealed by Organic Petrography and Geochemical Properties

Prasanta K. Mukhopadhyay

Organic petrological (maceral composition and palynomorph assemblages) and geochemical properties of selected coals from the Wilcox (R_o between 0.30 and 0.42%), Claiborne (R_o = 0.49%), and Jackson (R_o between 0.24 and 0.31%) Groups of Texas reveal characteristic features of these coals.

Maceral composition ternary diagrams, relationships between ratios of macerals of similar affinity, palynological data, and geochemical properties establish the variability of specific peat-forming environments, depositional relations between maceral groups, and the geochemistry of the peat-forming plant communities. Wilcox coals are deposited in swamp, swamp-marsh, and marsh-dominated interdistributary (palynomorphs are Nyssa, Taxodium, Betulaceae-Myricaceae, Calamuspollenites, Engelhardtia) or lacustrine (Botryococcus, Schizoporis, Pediastrum, etc) basins on an upper delta plain; Claiborne coals in a lacustrine (Botryococcus, Platycarya) basin on a delta plain; and Jackson coals in swamp-marsh complex (Nyssa) and marsh-dominated interdistributary (Arepites, Calamuspollenites, Liliac dites) areas on a lower delta plain or in a strand plain/barrier-bar/lagoonal environment.

Correlation of the percentage of macerals of similar affinity (humodetrinite + liptodetrinite + sapropelinite + alginite) and the atomic H/C ratio or hydrogen index (mg HC/g TOC) demonstrates the importance of the nature of the original plant community and the influence of relative ground-water elevation on peat-forming processes. Hydrocarbon potential of a coal depends on the type and amount of liptinite and humodetrinite content. The reed-marsh plant community has a higher hydrocarbon potential than the swamp-dominated community. On the basis of hydrocarbon potential and maceral composition, Tertiary coals from Texas are classified as humic, mixed, and sapropelic, representing the original floral components and depositional environment.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.