Exploration for petroleum in the Cook Inlet subprovince started more than 70 years ago, but little petroleum was found until 1957, when Richfield Oil Corporation discovered oil at Swanson River field.

About 60,000 ft (18,290 m) of Mesozoic and Cenozoic sedimentary rock is present in the Cook Inlet basin. The Mesozoic rocks of marine origin crop out around the rim of the basin and do not appear to be favorable as reservoirs. Recent subsurface data indicate that they may have reservoir characteristics in the deeper part of the basin.

All Tertiary strata younger than the Chickaloon Formation are included in the Kenai Group. They are divided into the West Foreland, Hemlock, Tyonek, Beluga River, end Sterling Formations. These apparently nonmarine rocks include interbedded sandstone, claystone, siltstone, and coal. Reservoir rocks are present throughout the sedimentary section.

In structural and tectonic history, the subprovince is related closely to the Alaskan orocline, which dominates the southern half of Alaska. Four north-trending anticlines containing oil and gas have been mapped in the upper Cook Inlet.

As of December 1968, the Cook Inlet had produced 160 million bbl of oil at an average rate of 1,213 bbl per day per well from five fields, and 162 billion cu ft of gas from 15 fields. Estimated proved in-place reserves are 2.6 billion bbl of oil and 5 trillion cu ft of gas. The industry expects to recover a little more then 1 billion bbl.

Additional reserves should be found by drilling in unexplored areas, by deeper exploratory drilling in the Mesozoic section, and by searching for accumulations in stratigraphic traps. Exploratory activity has proceeded more slowly than elsewhere because of the high cost of operation, severe weather, and logistic problems.

The oil industry ultimately can be expected to discover additional estimated potential in-place reserves of 7.9 billion bbl of oil and 14.6 trillion cu ft of gas in the Cook Inlet subprovince.