Abstract
During the Eocene, the west coast of North America experienced significant tectonic activity as the Farallon Plate was subducted eastward beneath the North America Plate, resulting in thick successions of sediment (≤ 8000 m; 26,247 ft) being deposited within inland basins, specifically the Swauk Basin, located in what is now central Washington State (Atwater, 1970; Frizzell, 1979; Dickinson, 1976; Taylor et al., 1988; Mustoe and Gannaway, 1997; Breedlovestrout, 2011; Eddy et. al., 2016). A subsurface sequence stratigraphic analysis has been applied to deposits of the Eocene Swauk and Roslyn Formations and used for fluvial correlation. Thickness maps generated from fluvial sequence correlation horizons and net-to-gross sand ratios illustrate the amount and location of accommodation available and how it shifted through the Eocene and into the Oligocene. Depositional fairways illustrate how deposition migrated from a southeast direction in the lower portion of the Swauk Formation to a northeastern direction in the middle and upper Swauk Formation and back to a southeastern pattern for the Roslyn Formation. Net-to-gross sand ratios were calculated to determine reservoir quality and to highlight promising target. Seven stratigraphic horizons were identified in the subsurface of the Swauk and Roslyn Formations; the most promising reservoirs were identified between horizons four through seven within the Roslyn Formation. Target areas for petroleum exploration were identified between horizons four through seven in the thick, blocky (lacking interbeds) packages of sandstone within the middle and lower Roslyn Formation in the northwestern portion of the area, indicating that central Washington contains key aspects of a valuable petroleum prospect.