Alaska, comprising 3.6 × 10⁶ sq km (about 28%) of the land, shelf, and upper continental slope of the United States, has been estimated by the U.S. Geological Survey to contain about 20% of total petroleum liquids and natural gas resources of the nation. Some 15 billion bbl of petroleum liquids and about 31 trillion cu ft of natural gas have been discovered.

In northern Alaska, Paleozoic and Mesozoic shelf and slope deposits and some ophiolitic rocks of the Brooks Range orogen were thrust northward over the depressed south margin of the Paleozoic and Mesozoic Barrow platform, on which a foredeep (the Colville geosyncline) developed in Early Cretaceous time. Cretaceous and Tertiary sediments from the Brooks Range filled this foredeep and prograded northwest and northeast to form the Chukchi and Colville delta systems and to fill the Camden coastal basin.

A series of arc-trench systems developed on oceanic rocks in southern Alaska during the Jurassic and Cretaceous. These arcs were subparallel with the Mesozoic continental margin of southern Alaska. Between the arcs and the metamorphic (continental) terranes of east-central Alaska and the southern Brooks Range, a large marginal ocean basin received thick Jurassic and Cretaceous volcanic and detrital deposits. These deposits were extensively deformed and disrupted by widespread mid-Jurassic to Tertiary plutonism, Late Cretaceous and early Tertiary ("Laramide") oroclinal bending, wrench faulting, and arc-related compression.

The Laramide events "continentalized" the late Mesozoic marginal basin deposits and welded them to the older continental terranes. Subsequent sedimentation was more localized and nonmarine, except in basins along the Pacific, Arctic, and Bering coasts where thick mixed marine and nonmarine sections are present. The active Aleutian arc and associated Queen Charlotte transform-fault system were superimposed obliquely across the southern continental margin of Alaska in early Cenozoic time and have since dominated structural and depositional patterns in southern Alaska.

The largest petroleum reserves in Alaska and the best prospects for additional large discoveries are in northern Alaska, where an extensive terrane is underlain by upper Paleozoic to Tertiary shelf and slope clastic and carbonate deposits. The pre-Tertiary arc and marginal-sea deposits in southern and interior Alaska are either too intensely deformed or too low in porosity to offer more than modest local prospects. The Tertiary coastal onshore and offshore basins with

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thick marine and nonmarine clastic rocks, and locally many large folds, are attractive for exploration. These little explored basins are known to be petroliferous on Bristol Bay and the Gulf of Alaska and to contain major accumulations of oil and gas at Cook Inlet.

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