Cretaceous and Tertiary Coal-bed Gas Resources in the Gulf of Mexico Coastal Plain
Warwick, P.D.
U.S. Geological Survey, 956 National Center, Reston, VA 20192
Coal-bed gas is currently produced from multiple coal zones (3-6 m, 10-20 ft thick) within the Olmos Formation (Upper Cretaceous) of the Maverick basin in south Texas, and from the Wilcox Group (Paleocene-Eocene) of north Louisiana. In addition, there are numerous untested coal beds that potentially contain coal-bed gas resources in Cretaceous and Tertiary strata across much of the Gulf of Mexico Coastal Plain. Although the Olmos Formation is limited in extent to south Texas and northeastern Mexico, the Wilcox Group is coal- and presumably gas-bearing across much of the Gulf Coastal Plain. Depths to the targeted Olmos and Wilcox coal beds range from 450 to 600 m (1,500-5,000 ft). Measured gas content of the Olmos coal beds is as much as 8.5 cm3/g (300 standard cubic ft per ton [scf/t] dry, ash-free basis, [daf]) and as much as 6 cm3/g (213 scf/t daf) for Louisiana Wilcox coal beds. Gas production from the Olmos coals was initiated in 2001 and has remained fairly low. In 2004, it averaged about 137,000 m3 (4,850 thousand cubic feet, MCF) per month from several dozen wells. Recently completed wells in Wilcox Group coal zones in north Louisiana have initial production ranging from 200 to 6,500 m3 (7-230 MCF) of gas per day, which indicates that Wilcox coal beds may be more productive than those of the Olmos Formation. However, optimum completion methods and excess water production from adjacent water-bearing sandstones remain as obstacles for Wilcox coal-gas development.
In addition to the Olmos and Wilcox coal intervals, coal-gas resources may occur in the Lower Cretaceous coal beds of Arkansas and Louisiana. Drill records indicate that there are coal beds greater than 3 m (10 ft) thick at depths of 450 to 1,830 m (1,500-6,000 ft) in south-central Arkansas and northern Louisiana. Coal zones in the Eocene Jackson and Claiborne Groups may also be gas bearing at depth across much of the Gulf region; more data are needed to evaluate the resource potential for the Lower Cretaceous and Eocene coal zones.