THE ROLE OF VOLCANISM, INVERSION, AND NEOTECTONICS IN THE PUGET-WILLAMETTE LOWLAND PETROLEUM SYSTEM, WASHINGTON AND OREGON

Eocene sediments deposited west of an ancestral Cascade Range include a coal-bearing sequence covering much of the Puget-Willamette Lowland. The terrestrial deposits pass into marine deposits to the west. Syn-depositional normal faulting and strike-slip faulting are evident in several sub-basins, which are interpreted to represent a transtensional setting. Eocene volcanism locally effected sedimentation. Eocene fluvial sandstones overlain by intra-formation claystones are potential reservoir-seal couplets. Eocene coals and carbonaceous claystones represent a potential gas-prone source interval.

Locally preserved sediments indicate that subsidence continued into Miocene and Pliocene time, and was followed in the Puget Lowland by extensive Pleistocene glaciation. At present, active faulting affects the Lowland in several discrete zones.

Models of vitrinite reflectance data from wells located in the Lowland indicate that from the Eocene to the Present, the regional geothermal gradient ranged from 28 to 41°C/km. The eastern margin of the Lowland was bordered by the Cascade volcanic arc.

In the southern Lowland, normal faults were modified by episodes of late Eocene and Miocene transpression, which resulted in mild inversion of older normal faults. In the northern Lowland, Holocene faulting is recognized in outcrop and is interpreted on seismic data acquired in Puget Sound.

Structures formed by early Miocene or earlier events may have trapped migrating hydrocarbons. Structures formed or modified by Holocene faulting very probably post-date hydrocarbon generation and migration.