Subsurface Models of Coal Occurrence, Oak Grove field, Black Warrior Basin, Alabama

Abstract

Subsurface investigation of coal occurrence in the Black Creek-Cobb interval of Oak Grove field is based on cross sections and isopach maps made from more than 500 density logs. This study was designed to identify styles of coal occurrence in the Black Warrior basin to aid in coalbed-methane exploration and production. Coal occurrence in parts of the Black Warrior basin may be characterized in terms of end-member fluvial and structural control. Fluvial processes apparently were the major controls on coal occurrence in the Black Creek cycle, where sandstone and coal thickness are inversely related. Additionally, occurrence of thick sandstone sequences above the thickest coal beds suggests that peat compaction provided sites for channel avulsion. In the Mary Lee and Cobb cycles, more coal beds occur in a downthrown fault block than in an upthrown block, and in the Pratt and Cobb cycles, the thickest coal beds occur on the downthrown side of a fault. Only in the Mary Lee cycle, where thick peat accumulated in an abandoned tributary system, is coal thickest on the upthrown block. Most coal beds in Oak Grove field are thickest on the downthrown block because differential subsidence apparently promoted peat accumulation. Clastic influx favors bedsmore » splits in the downthrown block, but joining of beds in the Pratt cycle may reflect sheltering by the fault. In the Mary Lee cycle, in contrast, channel incision evidently provided local relief sufficient for thick peat to accumulate in lows on the upthrown block. Although fluvial and structural processes result in varied styles of coal occurrence, models of coal occurrence have resulted in a predictive framework that may aid in strategic well siting and completion.« less