Petroleum Potential of North-Central Oregon: an Overview

Lanny H. Fisk

Except for a few erosional inliers, north-central Oregon is covered by a blanket of Tertiary volcanic and volcaniclastic rocks which hide a thick sequence of Cretaceous and Tertiary sedimentary rocks that have led previous investigators to draw negative conclusions regarding the petroleum potential of the region. Present in the prevolcanic sequence are all the prerequisites for petroleum generation and accumulation: source rocks, thermal maturity, reservoir rocks, seals, and traps. Organic-rich source rocks include marine mudstones of the Lower Cretaceous "Mitchell Formation," black shales and coals of the Paleocene-Eocene "Herren Formation," and lacustrine shales and coals of the Eocene-Oligocene Clarno Formation. The Type II and III kerogens are thermally mature. Petrol um seeps and shows are further evidence that hydrocarbons have been generated. Potential reservoir rocks include proximal fan facies and turbidite sandstones of the "Mitchell Formation," fluvial channel sandstones of the "Herren Formation," volcaniclastic sandstones and fractured flows and ignimbrites in the Clarno and John Day Formations, and scoriaceous and brecciated flows in the Columbia River Basalt. Tuffaceous claystones form excellent seals. Traps of several kinds are present including large untested domes and anticlines, abundant fault traps, and facies traps in the interfingering sand-shale sequences. Thus, despite abundant volcanic activity and periodically rejuvenated tectonic activity, north-central Oregon has the potential to yield large quantities of both oil and gas.

AAPG Search and Discovery Article #91040©1987 AAPG Rocky Mountain Section Meeting, Boise, Idaho, September 13-16, 1987.