A Depositional Setting for the Mist Gas Field Reservoir Sands

Abstract

The depositional origin of the upper Eocene Clark and Wilson (C and W) sandstone member (informal) of the Cowlitz Formation near Mist, Oregon, is controversial. Previous workers have suggested a deep-water setting for the C and W sands. However, recent interpretations of integrated surface and downhole data from Oregon's Mist gas field suggest a wave-dominated deltaic to shallow-marine depositional environment for the C and W sandstone. Sedimentary structures, lithofacies associations, mapped sand geometries, and petrographic examinations indicate that strong wave processes reworked a progradational delta-front environment during a transgression, winnowing fines and controlling C and W sandstone deposition. These well-sorted arkosic sands were deposited following an abrupt relative sea level regression; they onlap an underlying basaltic rock suite and were sourced largely from batholithic masses and Idaho and Washington. Outcrop mapping, studies of continuous cores, and electric-log interpretation show that the Clark and Wilson consists of a 400 to 800-ft-thick series of five stacked, massive-to-hummocky bedded, coarsening- and thickening-upward sandstones separated by minor bioturbated siltstones and lignites. Extensive subsurface mapping delineates individual sandstone bodies that exhibit sheet-like geometries and lateral continuity over areas as great as 60 mi{sup 2}. The authors interpret the siltstone and lignite deposits to represent restrictedmore » marine or brackish-water deposition behind a high-energy C and W shoreline.« less