Maturation Anomalies in Cretaceous Sediments Underlying Volcanic Plateaus in Oregon: ABSTRACT

Abstract

Vitrinite reflectance profiles from three drill holes in Oregon, supplemented by independent data from Oregon and northern California, suggest that a thick sequence of volcanics may lead to a significant thermal input into underlying sediments. Anomalous, nearly vertical maturation profiles, in sedimentary sequences over 4000 ft thick, imply that large volumes of mature source rocks may lie under the volcanic cover of the Pacific Northwest. Several factors may be involved in this vertical homogenization of the temperature field. Although a convective hydrothermal mechanism would be the most effective form of thermal input, this is not considered to be the dominant mechanism due to the low permeabilities of the volcanogenic sediments underlying the volcanics. However, the cooling of intrusive and extrusive volcanics, thermal slip at abrupt changes in lithology, and exothermic hydration reactions of the altering volcanics can all be considered viable mechanisms for the thermal input. These mechanisms can lead to the uniform maturation of the entire sedimentary column and may account for the gas fields in Mist, Oregon. There, poor and immature source rocks give no indication of the actual origin of the gas. However, a mature sedimentary column underlying volcanic cover to the north and east could providemore » a plausible source for the gas. Other settings in which potentially hydrocarbon-bearing sediments underlie volcanics occur elsewhere in the US, and in India, Argentina, Brazil, Russia, and eastern and southern Africa.« less