DEACON, ROBERT J., Wright/Deacon Associates, Portland, OR, and M. W. DEACON, BP Exploration, Anchorage, AK
A 200-mi-long sand sequence of Late Eocene age extends from Eugene, Oregon, to Seattle, Washington. The sequence is partitioned into structural subbasins, which includes the central Willamette Valley basin (10-12K thickness), Tutatin Nehalem basin (12-16K thickness), and Astoria basin (12K thickness) of northwestern Oregon and the Chehalis Puget basin (12-20K thickness) and Grays Harbor basin (12K thickness) of western Washington. Geochemical and geothermal data indicate that basins with greater than 10,000 ft of sedimentary fill have the potential to generate hydrocarbons in commercial amounts.
The upper Eocene strata was deposited in an overall progradational sequence from offshore marine to alluvial plain. Coarsening-upward cycles reflect the progradation of easterly source fluvial/deltaic depositional systems within a framework of tectonic and volcanic controls. Reservoir facies consist of fine- to medium-grained, well-sorted, arkosic sandstones, which typically range from 20-30% porosity and 500-20,000 md permeability.
Oregon and Washington basins contain similar organic carbon content and maturation characteristics to the gas production region of the Sacramento Valley. Dry gas production in the Mist gas field area of northwestern Oregon indicates the high potential for further gas production throughout the 200-mi sand trend, although production probably will never compare to Sacramento Valley fields. Defining suitable traps for hydrocarbon accumulation is difficult in Oregon and Washington due to extensive vegetation, deep weathering, and volcanics that complicate geological and geophysical studies. Exploratory drilling in Oregon and Washington basins is sparse and the level of exploration is probably comparable to the Sacramento Valley in the 1960s.