In less than ten years the Cook Inlet basin of south-central Alaska has become one of this nation's most promising sources of oil and gas. From 1957 to 1966, significant discoveries include 5 major oil fields, 1 gas-condensate field, and 10 dry-gas fields with total proved reserves in excess of 500 million bbls of oil and 3 trillion cu ft of gas.

Rocks ranging in age from Late Triassic to Recent are present in the basin; however, commercial oil and gas accumulations have been found only in Tertiary rocks assigned to the Kenai Formation, a thick heterogeneous deposit composed of sandstone, siltstone, conglomerate, shale, and abundant coal beds. Lateral variation of individual beds within the Kenai Formation makes correlation difficult; however, three arbitrarily defined stratigraphic units comprising at least five distinct lithologic zones are recognizable on the Kenai Peninsula where adequate well control is available. The United States Geological Survey, as a result of recent paleobotanical studies, has subdivided the Kenai Formation into three provincial stages which correspond to time-stratigraphic units. Although subdivisi ns of the Kenai Formation used in this paper may be roughly correlative with the provincial stages on the basis of distribution, spatial occurrence of coal within the section, and degree of lithification, they were chosen arbitrarily.

The Cook Inlet basin is an elongate, deep, moderately asymmetrical structural depression which overlies much of the Matanuska geosyncline, a Mesozoic sedimentary trough. The structure of the basin is mainly a result of Late Cretaceous-early Tertiary deformation; however, the major tectonic features are a result of middle Jurassic orogeny. The principal structural features within the basin are narrow, elongate, complexly faulted anticlinal belts which trend north-northeast parallel with the grain of the major tectonic features which flank the basin. All of the oil and gas fields are located near the crest of local structural closures on the anticlinal belts.

Accumulation of oil in the Hemlock Zone, the principal oil reservoir in the lower part of the Kenai Formation, probably is related to old fold belts which were uplifted before Late Cretaceous time and remained positive until Tertiary time. Gas accumulations in the middle and upper parts of the Kenai Formation are indigenous, and were generated from coal and lignite in the section. Time of structural uplift, diagenesis of carbonaceous source rocks, and stratigraphic relations between coal and reservoir host rocks control the accumulation of gas in commercial quantities.

Most of the proved gas reserves of the basin are shut in because there is no market. Planned facilities for methane liquefaction, anhydrous ammonia production, and pressure maintenance of oil reservoirs soon will greatly increase the use of natural gas and the demand may exceed the supply.