Abstract: Basin-Centered Gas Accumulations in the Pacific Northwest: A Potentially Large Source of Energy

B. E. Law, M. E. Tennyson, S. Y. Johnson

Several basins in the Pacific Northwest of Oregon and Washington have geologic attributes common to basin-centered, low-permeability (tight) gas reservoirs in other parts of the United States: thermally mature source rocks at depth interbedded with tight sandstone reservoirs. Previous exploration efforts in this region have focused on more conventional hydrocarbon accumulations in structural and stratigraphic traps. We suggest that the hydrocarbon potential of the Pacific Northwest might be more appropriately evaluated in the context of regionally extensive gas accumulations in tight reservoirs.

Potential hydrocarbon source rocks include coal, carbonaceous shale, and dispersed organic matter in argillaceous shale and siltstone in Tertiary and Cretaceous rocks. Excluding coal, the organic matter content in these rocks commonly ranges from 0.5 to 1.0% and, locally, as high as 20%. The organic matter is predominantly type III and is gas prone. At depth, source rocks are at levels of thermal maturity sufficiently high (>0.7% vitrinite reflectance) to have generated thermogenic gas. Interbedded with these source rocks are lithic or arkosic sandstone reservoirs containing large amounts of volcaniclastic and clay components that adversely affect reservoir quality. Although data on permeability are scarce, most studies indicate that it is low. The occurrence of interbedded source nd reservoir rocks imply short distances of hydrocarbon migration and favorable conditions for the development of overpressured gas-saturated reservoirs. Gas shows and seeps are widespread throughout the region. These observations, considered together, indicate that gas-saturated, basin-centered tight sandstone reservoirs are present in large areas of the Pacific Northwest, particularly in the Columbia basin of eastern Washington and Oregon, the Tyee basin of southwestern Oregon, and the Chehalis, Everett, and Seattle basins of western Washington.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994