Surface-subsurface facies and distribution of the Eocene Cowlitz and Hamlet formations, northwest Oregon

Graduation date: 1997Since 1979, over $114 million of natural gas has been produced at the Mist Gas Field, currently the only commercial gas field in the Pacific Northwest. In the Mist Gas Field, the sandstone-dominated Clark and Wilson member of the upper Eocene Cowlitz\ud Formation is the reservoir and the overlying upper mudstone member of the Cowlitz\ud Formation forms the seal. Surface field maps, lithostratigraphic facies and petrographic analyses of a core of the Cowlitz Formation from the gas field, along with isopach maps and correlation of well logs of the Cowlitz and Hamlet formations in and around the gas field were used to constrain the depositional and diagenetic history of the Cowlitz Formation and bounding volcanic and sedimentary units. The Cowlitz and Hamlet formations were deposited in a volcanically and tectonically active forearc basin during the late Eocene reorganization of the convergent margin of western North America. In outcrops, the Hamlet formation (informal) is a deepening-upwards (transgressive) sequence of a basal rocky coastline basaltic conglomerate, overlying shelfal mollusc-bearing micaceous arkosic sandstone and bathyal mudstone with minor thin turbidite sandstone beds which unconformably overlies "shield" volcanoes of the middle to upper Eocene Tillamook Volcanics. In the Mist Gas Field subsurface, the mudstone-dominated Hamlet formation thins to the south toward the Tillamook Volcanics paleohigh. The sequence boundary between the Cowlitz and Hamlet formations coincides with a series of basalt flows of Grays River volcanics mapped in the subsurface in the northern part of the gas field. These flows pinch out to the south. The Clark and Wilson member of the Cowlitz Formation onlaps this paleohigh at the Mist Gas Field. In southwest Washington, flows of the Grays River volcanics overlie arkosic\ud sandstone of the Cowlitz Formation. The basal facies of the Clark and Wilson member at Mist is a bioturbated mudstone-dominated offshore to lower-shoreface transition unit which grades upwards into hummocky-bedded arkosic micaceous reservoir-quality sandstone with minor lignite beds of a storm wave-influenced delta front facies. Both the lower-shoreface facies and the hummocky-bedded facies thin rapidly in the subsurface onto the Grays River Volcanics paleohigh in the north. The uppermost facies of the Clark and Wilson member, the parallel-bedded facies, was reworked by waves into a shore-parallel barrier bar. The northeast-southwest-trending distributary channel axis suggests paleodispersal from the southwest Washington type locality which is dominated by delta plain and estuarine facies. Following deposition of the Clark and Wilson member, an abrupt transgression (marine flooding surface) occurred and the thick parallel-laminated foraminifera-bearing\ud bioturbated mudstone of the upper member of the Cowlitz Formation was deposited at bathyal water depths. A soft sediment contorted siltstone facies of the upper mudstone member resulted from redeposition of the parallel-laminated mudstone facies of the upper mudstone member. This slumping and redeposition of the late Narizian strata during the early Refugian may be related to normal faulting and horst and graben formation during a regional extensional event that formed the structure at the Mist Gas Field. The Cowlitz Formation is unconformably overlain by the deep marine tuffaceous mudstone and minor graded basaltic conglomerate of the Keasey Formation. This heralded the development of\ud the western Cascade calc-alkaline arc and continuing forearc basaltic (Goble) volcanism