Hydrocarbon Exploration in Western Oregon and Washington

Abstract

Continuing development of the Mist gas field and stepout discovery wells affirm the hydrocarbon prospectivity of western Oregon and Washington. Understanding the factors controlling the Mist gas field accumulation helps define an exploration strategy for the Pacific Northwest. Reservoir sandstones in the Mist area are in the Cowlitz Formation of middle to late Eocene age. Reservoir-quality sandstones have average porosities of 25% and average permeabilities of 200 md. The reservoir sands are well-sorted feldspathic-quartzose sandstones and are less susceptible to diagenetically formed pore-filling authigenic minerals than are the more lithic sandstones of other horizons and less well-sorted depositional environments. Potential hydrocarbon source rocks consist of marine shale to coaly facies. Organic matter is predominately terrestrially derived. Mist gas field pools are small and have variable gas types, suggesting to some workers that the gas is generated from rocks immediately adjacent to the reservoir. Gas wetness and ^dgr13C values indicate that gas from the Bruer, Flora, and Newton pools is probably thermally generated. Shales encasing the Mist gas field sandstone reservoirs are thermally immature, having vitrinite reflectance values less than 0.4%. Thermal gas most likely would have been generated downdip within nearby depocenters and migrated into the reservoir. The integration of paleogeographic models for mineralogic provenance, well-sorted sand accumulation, and thermal maturation within Cenozoic depocenters provides an exploration strategy for defining areas of highest hydrocarbon potential in western Oregon and Washington. Each of the many Eocene formations in Oregon and Washington with relatively lithic-free, feldspathic-quartzose mineralogy should be evaluated as a potential target.