The Cook Inlet basin in south-central Alaska is approximately 200 mi long and 70 mi wide. It contains a thick section of predominantly nonmarine Tertiary sedimentary rocks with major reserves of oil and gas. Early geologists working in the area, hampered by scarcity of good outcrops, named these coal-bearing strata the Kenai Group or Series and assigned a probable thickness of 4,000-5,000 ft. Later workers renamed it the Kenai Formation. Oil and gas exploration, beginning in 1957, supplied previously unavailable subsurface control that greatly expanded geologic knowledge of the area.
Subsurface data have made it evident that the Kenai is at least 26,000 ft thick in the central part of the basin and that it is divisible into five major mappable lithologic units, each deserving formational rank. These units are distinguishable by lithology and mechanical-log characteristics. Therefore, the Kenai Formation is elevated to group status and renamed the Kenai Group. As herein redefined, the Kenai Group consists of five new formations, in ascending order, West Foreland Formation, Hemlock Conglomerate, Tyonek Formation, Beluga Formation, and Sterling Formation.