Geological Assessment of Petroleum Potential of Tertiary Basin, Western Oregon and Washington: ABSTRACT

Abstract

Recent drilling and the subsequent discovery of gas near Mist, Oregon, have stirred new interest in the large Tertiary basin of western Oregon and Washington. Developments in the understanding of the regional tectonic framework of the western North America continental margin over the last 10 years has clarified some aspects of the geology of this area. Our new-found insight into the mobile nature of the crust provides little new indication, however, of the possible volumes of hydrocarbons in the basin. Our current work in the area indicates little that is End_Page 448------------------------------ dramatically new or different from other evaluations of this basin, and limitations due to vegetative cover, volcanic rocks, and scarce subsurface control still handicap any appraisal. Potential structural traps and many potentially good reservoir intervals are present. The upper Eocene section contains abundant coal, which provides a potential source for dry gas, either by bacterial or thermal generation. A good source for oil, however, has not been documented. Some fine-grained units within the Oligocene and Miocene section do indicate areas of abundant organic productivity and some approach to the conditions necessary to generate at least 2 to 3 gallons of oil per ton of rock. Also, analogy with the Eocene-Oligocene section of the Gulf of Alaska suggests that a Poul Creek-type oil shale could well exist in this basin, with perhaps 100 to 300 million bbl of undiscovered reserves a reasonable, although highly speculative, forecast. Assuming that a kerogen-rich, pot ntial oil source rock is present, it remains difficult to document areas where burial depths in this relatively low heat-flow basin have been sufficient for thermal maturation, especially for rocks younger than Eocene. We have little doubt, however, that areas of sufficient burial depth do exist and hope to document that in our current work. The abundant shows of gas in wells, including Mist, and active seeps, over the entire basin, and a corresponding scarcity of significant oil shows, active seeps, tar deposits, or kerogen-rich shales, suggest that coal-derived gas may well be the primary hydrocarbon resource. Based on a thermal maturation model and the potential volumes of coal buried below about 15,000 ft (4,572 m), this resource could total a trillion cubic ft of gas. Low temperature, bacterial generation of methane from the low-rank coals could double that total, although the probability of retaining this gas within the system is less than for the thermally derived gas. End_of_Article - Last_Page 449------------