The Alaskan Pacific-margin Tertiary basin includes an onshore and offshore area of approximately 40,000 sq mi (103,600 sq km) underlain by a thick sequence of continental and marine strata ranging in age from Paleocene through Pliocene. The Tertiary sequence is broadly divisible into (1) a thick lower unit of well-indurated, intensely deformed rocks, mainly of Paleocene and Eocene age, and (2) an upper unit, largely of Oligocene through Pliocene age, that is notably less deformed and indurated. Most of the known indications of petroleum in the basin are in rocks of the younger sequence, which has a composite thickness on the order of 20,000-25,000 ft (6,096-7,620 m). The petroleum possibilities of the younger sequence appear to be good if adequate reservoir sandstone can e found in favorable structural positions. The early Tertiary sequence is too indurated and too intensely deformed to have more than modest potential for accumulation of petroleum in commercial quantities, and pre-Tertiary rocks are considered to be an effective basement for petroleum.