An interpretive geologic history for western Washington and Oregon based on recent plate-tectonic theories suggests that there is a significant potential for large petroleum accumulations in an area that is very sparsely drilled.

If, as many workers think, the early Tertiary edge of the continent was marked by a subduction zone in the vicinity of the present-day Cascade Mountains, then the trench associated with that subduction zone could have been the site of deposition of reservoir-quality turbidites as well as

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petroleum source beds. The present-day Coast Range basalts, which were probably an oceanic (spreading?) ridge, apparently were in place to provide the western barrier for a sediment trap and may also have encouraged an anoxic environment. A suite of coarse-grained, nonmarine to deltaic arkoses (the Puget Group of Washington) was available to be dumped into the trench as fan-type reservoirs. Thermal maturity may have been achieved by heat flow from the ridge and/or the Cascades, and depth of burial. Numerous, apparently large, structures have been mapped and several unconformities have been defined on the surface and in the subsurface.

The area of greatest potential poses some problems, such as glacial and volcanic cover and very sparse subsurface control, but there are enough oil and gas shows to suggest that a focused program, emphasizing the search for reservoir facies, may well prove successful in this classic frontier province.

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