OVERVIEW OF THE OIL AND GAS POTENTIAL OF SW WASHINGTON AND WESTERN OREGON

Western Washington and Oregon is composed of a number of Tertiary forearc basins. This paper focuses on the Puget, Chehalis, Grays Harbor, Astoria-Nehalem, Willamette and Tyee-Coos Bay Basins. The majority of these basins have a basement of accreted Early Eocene oceanic volcanics and are filled with 7,000 to 20,000+ feet of Tertiary sands, shales, coals and localized volcanics.

Conventional reservoir objectives include widespread arkosic Middle to Late Eocene sands and sub-regional sands of Oligocene and Miocene age. Proven reservoirs exist in two gas storage fields: 1) Cowlitz (Clark and Wilson) sands which produce and store gas at Mist Field in NW Oregon and have porosities up to 36% and average permeabilities of 1.5 darcies; and 2) Cowlitz and basal Lincoln Creek sands which store gas at Jackson Prairie Field in the Chehalis Basin and have porosities up to 34% and perms to 3.2 darcies. Other sands exhibit reservoir qualities in a number of areas.

A large coal basin of Middle to Late Eocene extended from the Puget Basin south to the Chehalis Basin and into the Nehalem Basin. Coals are also present in the Bellingham and Coos Basins. These coals range from bituminous to lignite and are the source of gas and perhaps oil in a variety of settings. No widespread marine source rock has been identified to date, but several formations contain significant amounts of humic kerogen, and the underplated Hoh Melange may be a potential oil source in western Grays Harbor Basin. The region has a low geothermal gradient, but areas flanking the Cascades and near intrusions are mature and burial depths are adequate in the center of some of the basins. Gas shows are common and both thermogenic and biogenic gas has been recognized. Oil shows are documented in several areas including Grays Harbor and Puget Basins.

Mist Field (65B) is the only commercial production to date, but the region is sparsely drilled and most previous exploration did not utilize modern seismic. A wide variety of conventional play concepts remain untested and coalbed methane exploration is underway in several areas.