Some Notes on Petroleum Geology of Offshore Washington State

William S. Lingley, Jr., Stephen P. Palmer

Offshore Washington contains several kilometers of middle Eocene to Holocene bathyal siliciclastic strata with minor basaltic and paralic rocks. Porous sandstones are uncommon in offshore wells. However, feldspatholithic sandstones ranging from a few millimeters to greater than 130-m thick are common in coastal onshore wells. Analyses in wireline logs and 279 core samples show that porosities range from nil to greater than 35% and generally increase in younger rocks. Of note are uppermost Miocene to Pliocene paralic sandstone units with measured permeabilities in excess of 1.5 darcys. These units are interpreted to lie along paleodrainages issuing from the incipient Olympic Mountains. Only impoverished levels of type III kerogen have been identified to date, and these are thermally immature to depths of 2 km in offshore wells. Paradoxically, sweet paraffinic oil seeps and shows are common on the tidal western Olympic Peninsula.

The pre-upper Miocene shelfal section is complexly folded and faulted in response to pelagic mud intrusion and accretionary thrust tectonics. Deformation of upper Pliocene and younger shelfal rocks appears to be limited to mud intrusion structures and east-northeast-trending strike-slip faults. Diapiric mud domes range from 1 to 4 km in profile. Anticlinal folds on the shelf not associated with diapirs range from 2 to 12 km in profile. Anticlinal folds on the rise and slope are limited to the leading edges of west-dipping obduction thrust planes and are greater than 4 km in profile.

AAPG Search and Discovery Article #91024©1989 AAPG Pacific Section, May 10-12, 1989, Palm Springs, California.