Conceptual model for origin of abnormally pressured gas accumulations in low-permeability reservoirs

The largest gas fields in the Rocky Mountain region occur in abnormally pressured reservoirs. These gas accumulations are different from more conventional gas accumulations in that they are commonly located in basin-center positions, they occur downdip from water-bearing rocks, and they are in overpressured or underpressured low-permeability reservoirs. We suggest that overpressured and underpressured gas accumulations of this type have a common origin. In basins containing overpressured gas accumulations, rates of thermogenic gas accumulation exceed gas loss, causing fluid (gas) pressure to rise above the regional hydrostatic pressure. Within the overpressured gas generation zone, free water in the larger pores is forced out of the gas generation zone into overlying and updip, normally pressured, water-bearing rocks. The remaining tightly bound water, at irreducible saturation levels, cannot remove dissolution products, precluding significant permeability and porosity enhancement. Thus, while other diagenetic processes continue, a pore network with very low permeability develops. As a result, gas accumulates in these low-permeability reservoirs at rates higher than it is lost. In basins containing underpressured gas accumulations, rates of gas generation and accumulation are less than gas loss. These basins have typically experienced significant uplift and erosion and/or temporal variations of paleotemperature. Despite these modifications, the basin-center gas accumulation persists, but because of changes in the basin dynamics, the overpressured accumulation evolves into an underpressured system.