The Upper Eocene Cowlitz Formation is exposed in surface outcrops southwest of the town of Vernonia, in Columbia County, Oregon. The Cowlitz Formation also occurs in the subsurface of the Mist gas field where its Clark and Wilson (C and W) sandstone member (informal) acts as a natural gas reservoir, and its upper Cowlitz mudstone member (informal) acts as a cap rock. Surface exposures and continuous core were studied in order to determine Cowlitz Formation stratigraphy, and its depositional environment. Fresh core samples were also studied petrographically, and with a scanning electron microscope, in order to determine the effects of diagenesis in the gas producing C and W sandstone member.The distribution of other Tertiary rock units exposed within a designated field study area, located southwest of Vernonia, were also mapped and briefly described. The stratigraphy of the field study area, The strand plain/distributary mouth bar facies is the most laterally continuous of the five identified depositional facies. Its lateral continuity and consistently high porosity and permeability make it the best reservoir facies in the C and W sandstone.Diagenesis has produced a slight net decrease in porosity in the C and W sandstone member but has not had a large impact on its reservoir potential. Primary porosity reduction has occurred by compaction, cementation, the precipitation of authigenic minerals, and the alteration of micas. Secondary porosity has been produced in the C and W sandstone member by framework grain dissolution and possibly by decarbonatization of an early calcite cement.The C and W sandstone is derived from a predominantly plutonic/metamorphic source area during a period of volcanic quiescence.Possible sources include the Northern Cascades and the Idaho Batholith.The sediments may have been transported from the continent interior to the forearc basin by an ancestral Columbia River.