Western Oregon and Washington and the adjacent continental margin occupy the site of a Tertiary geosyncline. The oldest rocks in most of the geosyncline are early Eocene volcanic rocks, which are considered as "economic basement" with respect to oil production. Marine sandstone and siltstone of Eocene to Pliocene age reach a maximum thickness of 25,000 ft (7,620 m) and in places contain interbedded volcanic and nonmarine sedimentary rocks. A total of approximately 100,000 cu mi (416,700 cu km) of Tertiary marine and nonmarine sedimentary rocks is present within the trough, of which 70,000 cu mi (291,700 cu km) is considered to have potential as petroleum source rock.

Interest in the oil and gas possibilities of this region is reflected in the drilling of about 560 wells since the turn of the century. Only within the last 10 years has exploration been directed toward the continental margin, where 14 wells now have been drilled. Although commercial production has not been obtained, a favorable geologic environment, including source rocks, reservoir rocks, and structure, is an incentive to future petroleum exploration.