Analysis of Coal and Coal Bed Methane Resources of Warrior Basin, Alabama

Donald E. Wicks, Kathleen S. McFall, Philip Malone

The Warrior basin in Alabama is the most active area in the United States producing natural gas from coal beds. As of 1986, 300 coal-bed methane wells were producing from eight degasification fields, mainly from the Pennsylvanian coal seams along the eastern margin of the basin. Despite difficult market conditions, drilling and expansion are continuing.

A detailed geologic analysis of Warrior basin coal-bed methane targets the areas of the basin that show the most promise for future gas production. The geologic analysis is based on extensive well and core data and basin-wide correlations of the Pennsylvanian coal groups. Four detailed cross sections were constructed, correlating the target coal groups in the basin, namely the Cobb, Pratt, Mary Lee, and Black Creek. Data from 300 conventional oil and gas and coal-bed methane wells were used to construct net thickness and overburden maps. These maps established the vertical and lateral extent of the coals, and the in-place estimate of coal volume. Special vitrinite reflectance analyses, sponsored by the study, were used to develop coal rank maps, by coal group. Finally, newly derived c rrelations of coal rank and depth with gas content were used, along with coal volume, to estimate the in-place gas for the Warrior basin.

We estimate that the Warrior basin contains nearly 20 tcf of in-place coal-bed methane, mainly in three of the target coal groups--the Pratt, Mary Lee, and Black Creek coals, with 4, 7, and 8 tcf, respectively. The east-central area of the basin contains the greatest volume of natural gas resource due to its concentration of thicker, higher ranked coals with high gas content.

The geologic analysis also provided the underlying framework for the subsequent engineering analysis of economically recoverable gas reserves. For example, analysis of structure and tectonics showed the east-central area to be promising for gas recovery due to its proximity to the Appalachian structural front and consequent structural deformation and permeability enhancement.

AAPG Search and Discovery Article #91041©1987 AAPG Eastern Section Meeting, Columbus, Ohio, October 7-10, 1987.