Roger Tyler¹, Douglas S. Hamilton¹
(1) University of Texas at Austin, Austin, TX

Abstract: Application of genetic sequence stratigraphy in defining coalbed-gas exploration fairways: Examples from Williams Fork Formation, Sand Wash and Piceance Basins, Colorado

In the Sand Wash and Piceance Basins, the main coal-bearing sequences and coalbed-gas exploration targets are associated with fluvial-deltaic reservoirs of the Upper Cretaceous Williams Fork Formation. To evaluate the genetic sequence stratigraphy and coal depositional systems related to coalbed-gas exploration, we genetically subdivided the formation into sequences by using marine flooding events and their correlative surfaces. Coal-bearing sequences, extending upsection above the progradational shoreline sandstone, are genetically divided into three regionally correlatable sequences. Depositional systems recognized in each of these include (1) a shoreline (strandplain/delta) system, backed landward by (2) a coastal-plain systems, traversed by fluvial systems feeding the advancing shoreline, which in turn grade into (3) an alluvial plain with mixed-load fluvial systems.

The western limit of the coalbed-gas exploration fairway is controlled by the net coal thickness of the genetic sequence and by the transition from coastal-plain to alluvial-plain deposition. to the east, coal beds pinch out against and/or override the shoreline sandstones; their downdip exploration potential is limited by the final progradational shoreline position, above which thickest coal deposition occurs. In the Sand Wash Basin, shoreline systems prograded across the entire basin, extending coal-bearing coastal-plain deposits beyond today’s basin margin. But in the Piceance Basin, coals that reach the eastern outcrop are fewer and less thick; they therefore cannot transmit ground-water recharge basinward. Without dynamic ground-water flow or secondary biogenic gas generation, less dissolved gas is available for migration and resorption. Thus, coalbed-gas production in the Piceance Basin may be precluded; conventional traps may provide the only potential for coalbed-gas production.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana