Abstract: Origin of Cook Inlet Oil

Leslie B. Magoon, George E. Claypool

The geologic evolution of the Cook Inlet basin through the Mesozoic and Tertiary has definite bearing on the origin of oil that is and probably will be produced from these rocks. Basin development commenced in Triassic time as a simple volcanic arc-trench system and evolved into a Tertiary nonmarine fore-arc basin. Tectonism has influenced sedimentation of source beds and reservoirs, structural style, unconformities, and regional-temperature history.

U.S. Bureau of Mines crude oil analyses indicate that oils from the major fields of the Cook Inlet region, most of which are produced from the Tertiary Hemlock Conglomerate, probably have a common source. More detailed work, including stable carbon isotope ratios and gasoline-range and heavy (C₁₅⁺) hydrocarbon distributions, confirms this genetic relation among the major fields. In addition, oils from Jurassic rocks under the Iniskin Peninsula and from the Hemlock Conglomerate at the North Fork gas field on the southwestern tip of the Kenai Peninsula are also members of the same, or a very similar oil family. The Middle Jurassic rocks of the Iniskin Peninsula are moderately rich in organic carbon (0.5 to 1.5%) and gave shows of oil or gas in wells and surface see s. Most Tertiary age reservoirs in the major producing fields unconformably overlie Middle Jurassic rocks, suggesting that the pre-Tertiary unconformity is an important factor in exploration for new oil reserves. This unconformable relation of reservoir rocks to probable source rocks also implies a delay in the generation and expulsion of oil from Jurassic until late Tertiary time, when thick sedimentary fill brought older, deeper rocks to the required temperatures for petroleum generation.

AAPG Search and Discovery Article #90966©1977 Alaska Geological Society 1977 Symposium, Anchorage, Alaska