Petrographic and Chemical Characterization of Selected Texas Teritary Coals




Mukhopadhyay, Prasanta K.

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Organic petrological and chemical analysis of Tertiary coals from the Wilcox, Claiborne, and Jackson Groups of Texas revealed characteristic properties. Wilcox coals can be differentiated from Jackson coals based on lower equilibrium moisture, ash, and sulfur content, higher calorific value (Btu/lb), and huminite reflectance. Claiborne coal (one sample) from the Rio Grande Valley differs from most Wilcox and Jackson coals based on the nature of liptinite or huminite macerals, calorific value, and huminite reflectance. Calorific value and mean random huminite reflectance suggest that Wilcox coals are borderline subbituminous C to lignite in rank, whereas Claiborne coals are subbituminous A and Jackson coals are lignite in rank. The decrease of calorific value (Btu/lb) and the increase of liptinite content (in volume percent) for most Wilcox and Jackson coals are directly related to an increase of ash content.

A maceral composition ternary diagram revealed organic facies indicative of peat-forming environments. Accordingly, the depositional environments of the sampled coals were reconstructed as follows: Wilcox coals were formed either at the junction of a lower alluvial plain and upper delta plain (northeast Texas) in the backswamp area or at the interdistributary swamp-marsh-complex basin in the upper delta plain (east-central Texas); Claiborne coal was formed in a lacustrine environment having mainly marsh and aquatic vegetation; Jackson coals were deposited either in the interdistributary basin on the lower delta plain or in a back-barrier/lagoonal basin in which conditions alternated between swamp and marsh.


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