The Grays Harbor Basin (GHB) is one of a series of Tertiary basins comprising a regional forearc basin extending from Washington to Oregon west of the Cascade Mountains. The GHB contains up to 15,000’ of Tertiary (M. Eocene – Pliocene) sedimentary rocks. Eocene and Oligocene sediments consist predominantly of deep-water siliciclastics. Arkosic sandstones and interbedded volcaniclastic sandstones are contained within thick marine shale sequences. Sandstone petrography, paleocurrent measurements and regional depositional models indicate an eastern provenance for the arkosic sandstones in the GHB. Probable sources for these sandstones were Mesozoic granitic and metamophic rocks located in eastern Washington and Idaho. Coarse-grained volcaniclastic sandstones and conglomerates were derived locally and deposited proximal to intrabasinal oceanic highlands (e.g. islands) formed by exposed basement volcanic rocks.

A depositional model for Eocene and Oligocene sedimentation in the GHB has been developed from outcrop studies, well and seismic data. Deltaic sandstones, eroded during sea level lowstands, were reworked and carried westward by fluvial systems originating in the Puget and Chehalis Basins. Fluvial systems fed submarine canyons located between basement highs which, in turn, fed submarine fan systems in the GHB. At least four distinct episodes of submarine fan sedimentation are recognized in the GHB. Two submarine fan sandstones, the Basal Cowlitz and Basal Lincoln Creek sandstones, appear to be correlative the productive Clark & Wilson Sandstone at Mist Field and the Zone 2 sandstone reservoir at Jackson Prairie Gas Storage Field.