Predicting the occurrence of methane reservoirs in the Piceance Basin is difficult due to complex structure and low conventional permeability. It requires an understanding of the fluid flow history and the timing and rate of methane generation. Important issues are: how intergranular porosity and permeability change with time; how overpressuring contributes to fracture development; methane generation timing relative to the development of conventional or fractured flow conduits; and how methane generation contributes to the development or destruction of flow paths. We answer these questions by modeling reaction-transport-mechanical (RTM) processes across the basin.

Piceance Basin productive gas fields are associated with major structures, linking tectonic stresses to fracturing. High present-day pore fluid pressures suggest that hydrofracturing may also contribute to the development of these fractured reservoir systems. Pore fluid pressures in excess of least compressive stress may result from the rapid compaction of the sediments and methanogenesis. Highest pore fluid pressures likely develop in the deepest part of the basin, correlating with coal thickness, thermal maturity, and overburden. These may result in hydrofracturing and hence greater potential future producibility.

We have modeled the Piceance Basin using the CIRFB RTM simulator which accounts for diagenetic reactions and related compaction, methane generation from coal, fracturing, and the effects of relative permeability and capillary pressures. Four key characteristics of viable methane reservoirs are predicted: sufficient fracture and intergranular permeability; elevated pore fluid pressures; and gas column height.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah