Print this pageThe Application of Genetic Sequence Stratigraphy in Defining Coal Bed Gas Exploration Fairways: An Example from the Williams Fork Formation, Piceance Basin, Colorado
TYLER, ROGER, and R. G. McMURRY
In the Piceance Basin, the principal 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, the Williams Fork Formation was genetically subdivided into several coal-bearing sequences using marine flooding events and their correlative surfaces. Coal-bearing sequences extend up section above the Rollins-Trout Creek progradational shoreline sandstone and are genetically divided into three regionally correlatable sequences. Depositional systems recognized in each genetic sequence include (1) a shoreline (strandplain/delta) system, backed landward by (2) a coastal plain system, traversed by fluvial systems feeding the advancing shoreline, which in turn grade into (3) an alluvial plain with mixed-load fluvial systems. Shoreline systems prograded across the entire basin, extending coal-bearing coastal plain deposits beyond the present-day basin-margin outcrop.
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. However, coals that reach the eastern outcrop are reduced in number and total thickness; consequently, their ability to receive and transmit ground-water recharge basinward is inhibited. In the absence of dynamic ground-water flow and secondary biogenic gas generation, less dissolved gas is available for migration and resorption. Thus, exceptionally high coalbed gas production in the exploration fairway may be precluded; conventional traps may provide the only potential for high coalbed gas production.