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Michael H. Cox1,
Richard H. Groshong1
(1) The University of Alabama, Tuscaloosa, AL
Abstract: Relationship between fractures and the structure in coalbed-methane fields of eastern Black Warrior basin, Alabama
In the Black Warrior basin, methane is produced from the coals in the Pennsylvanian Pottsville Formation. Here we report the relationship between the northwest-trending normal faults that are abundant in the basin and the gas and water production in the Cedar Cove, Brookwood, Holt, Deerlick Creek, and Peterson coalbed methane fields. Because fractures provide the conduits for water and gas movement in coal, the production rates of both water and gas are measures of the fracture permeablity. The productive capability of a well is best measured by the amount produced during the month of peak production. Most wells are completed in the Mary Lee coals and so productivity has been compared to the structure on the top of the Mary Lee cycle.
Both peak gas and peak water productions have similar aerial distributions. The northern part of the area contains closely spaced normal faults and peak monthly productions of 4,507 Mcf gas and 7,614 bbl water. To the south is a wide unfaulted region, bordered by normal faults on the northeast and southwest. The unfaulted area has a considerably higher average peak monthly production of 7,028 Mcf gas and 14,117 bbl water. Regionally the production is greater in the structurally higher locations and the best production is in the unfaulted area. We suggest that the regional southwest-northeast extension is constant across the area and has been accommodated by both map-scale faults and bed-scale fractures. In areas without normal faults, the extension is accomplished entirely by bed-scale fracturing, creating more fracture permeability and thus increasing the peak monthly flow rates of both gas and water. The greater production from structural highs suggests some component of updip migration.
AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana