Recent discovery and successful development of a gas field near Mist, Oregon, have conclusively demonstrated the presence of commercial quantities of hydrocarbons in western Oregon and Washington. Understanding of the source and reservoir facies of this discovery may help develop additional exploration targets.

Reservoir sandstones of the Mist area occur in the Cowlitz Formation of middle Eocene age (Narizian Foraminifera Stage). The Cowlitz Formation of northwest Oregon and southwest Washington consists of nearshore-marine and brackish-water deposits of massive arkosic sandstone, shale, subbituminous coal, and interbedded basalt. It was deposited as part of a broad marine-deltaic system that extended from southern Oregon to northern Washington during middle Eocene time.

The feldspathic-quartzose sandstone of the Cowlitz Formation has porosities ranging from 14 to 41%, averaging approximately 25%. Permeabilities range from 46 to 8,500 md, averaging approximately 200 md.

Marine shale outcrops of the Cowlitz Formation in Washington contain dominantly terrestrially derived organic matter and have total organic carbon content averaging approximately 0.75%. Vitrinite reflectance values average approximately 0.45% for rocks inferred to have been buried to a depth of approximately 10,000 ft (3,000 m).

Traps, including the Mist gas field, appear to be structural-stratigraphic with reservoir sandstones thinning against plunging basement noses. These traps are complex and masked beneath several unconformities, underscoring the importance of geophysical exploration.

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