Vitrinite reflectance profiles from three drill holes in Oregon where sediments are mantled by thick sequences of volcanic rocks suggest that the sediments were subjected to an unusual diagenetic history. Anomalous, near-constant maturation profiles in sedimentary sequences more than 1300 m thick indicate that the levels of thermal maturity of organic matter in the sediments below the volcanic cover of the Pacific northwest were sufficiently high to have generated hydrocarbons. A hydrothermal mechanism appears to be the most effective form of thermal input, and supporting evidence can be found in the authigenic mineralization of the sediments. The occurrence of vertical maturation profiles in east-central Oregon may be associated with moderate-temperature geothermal systems resulting from a late Oligocene to early Miocene thermal event related to the incipient stages of basaltic volcanism of the Columbia River Group. The uniform maturation of the entire sedimentary column under the volcanic cover has broad implications for the exploration for oil and gas in the Pacific northwest because the presence of mature source rocks in these volcanically lidded basins provides some insight into the petroleum potential of the large volcanic-covered areas in Oregon, Washington, and California.