The Potential for Petroleum Systems in Fore-Arc of Western Washington and Oregon
Eocene sediments deposited in a fore-arc located west of an ancestral Cascade Range include a coal-bearing sequence covering much of the Puget-Willamette Lowland. To the west, these terrestrial deposits pass into marine deposits beneath the Pacific Ocean . Syndepositional normal faulting and strike-slip faulting are evident in several onshore 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. Onshore exploration has discovered small oil and gas accumulations. The most intriguing indication of an active petroleum system are oil seeps found on the coast of the Olympic Peninsula. To date, no source rock has been found that might generate these oils.
Locally preserved sediments indicate that subsidence continued into Miocene and Pliocene time, and was followed in the Puget Lowland by extensive Pleistocene glaciation.
Models of vitrinite reflectance data from wells located onshore 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, and was subjected to advective heat flow.
At present, active faulting affects the region in several discrete zones. East-west striking thrust faults are present in the Puget Sound region. Northwest-trending strike-slip faults are present in western Oregon, where they continue offshore. North-south trending thrust faults also occur offshore both Oregon and Washington. 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.
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.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009