COALS OF THE LOWER OLMOS FORMATION (UPPER CRETACEOUS) IN SOUTH TEXAS, AND COAHUILA, MEXICO: PRELIMINARY CSM (COAL-SYSTEM MODEL) DATA

A coal-system model, or CSM, incorporates data on coal formation, burial, and diagenesis in order to predict or assess the potential energy resources of a given coal deposit. This approach is illustrated by an ongoing study of the Olmos Formation (Taylor Group, Upper Cretaceous) in the Maverick Basin of South Texas and the adjacent Rio Escondido Basin of northern Coahuila, Mexico, where a CSM is being developed to refine estimates of coal and coalbed methane resources in the Rio Grande Embayment. Data include geophysical logs, borehole cuttings, continuous cores, gas samples, and limited outcrop observations. Coals of the Olmos Formation are thin (< 2 m), laterally discontinuous, host to numerous rock and volcanic ash partings, and most abundant in the lowermost part of the Olmos Formation, where multiple coalbeds occur within an approximately 10-m-thick interval. Abrupt and repeated splitting, as well as minor faulting, account for most coal-seam discontinuities, which generally increase down dip or eastwardly. Published coal-quality data from near-surface Olmos coals record high ash yields (about 20% as-received basis), medium sulfur content (1.2%, as-received basis), mean calorific values of about 6,670 kcal/kg (moist, mineral-matter-free basis), and reflectance values (Ro) as high as 0.58%, indicating an apparent rank of high-volatile C bituminous. Combined with palynological, sedimentological, and structural data in a CSM, these findings show that the Olmos coals of the Rio Grande Embayment represent a significant coal and coalbed methane resource.