The North Slope is a prolific hydrocarbon province in which discoveries to date amount to some 60 billion bbl of oil in place and 50 tcf of gas in place.

Reservoirs and prolific source rocks occur throughout the stratigraphic column, which consists of a lower (or Ellesmerian) megasequence of Carboniferous to Jurassic age and an upper (or Brookian) megasequence of Early Cretaceous to Recent age. Discovered oil is almost equally divided between Ellesmerian and Brookian reservoirs.

Patterns of hydrocarbon generation and migration have been controlled by deposition of clastic sedimentary wedges derived from the Brooks Range orogen. In the Late Jurassic to Early Cretaceous, the main oil kitchen was located in the Western Colville trough. Clastic depocenters and associated kitchen areas migrated progressively eastward with time and are now located in the East Beaufort offshore. Important source rocks include the Jurassic Kingak and Late Triassic Shublik Formations of the Ellesmerian megasequence, and the Aptian-Cenomanian "HRZ" and Turonian-Paleocene Shale Wall formations of the Brookian megasequence.

In the Ellesmerian megasequence, productive reservoirs are known at several stratigraphic levels, with best reservoir properties associated with secondary porosity development in subcrop beneath a mid-Hauterivian unconformity.

The Kekiktuk Formation (Mississippian), oil- and gas-bearing in the Endicott field (ca. 1 billion STBOIP), is a fluvially dominated unit, locally deposited in fault-controlled basins. The Lisburne Group (Mississippian to Early Permian) contains oil in the Lisburne pool of the Prudhoe area (?2-3 billion STBOIP). The reservoir is primarily early diagenetic dolomites within a thick platform carbonate sequence.

The major reservoir on the North Slope is the Early Triassic Ivishak Formation, reservoir to the Prudhoe field (22 STBOIP), the Seal Island discovery (?1 billion STBOIP), and target in the recent Mukluk well. Sands and conglomerates were deposited by a series of alluvial fan deltas shed from a nearby northern landmass.

Arctic Ocean rift events culminated in mid-Hauterivian continental breakup, which generated the subcrop unconformity of the rift margin uplift. Post-unconformity sands associated with local erosion of Ellesmerian

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and basement strata occur in widespread sheets. These sands can be important reservoirs,such as the Kuparuk and Point Thomson formations, of late Hauterivian-Barremian age (?ca. 11 STBOIP total).

Finally, within the Brookian megasequence, large volumes (?ca. 20 billion STBOIP) of relatively heavy oils are trapped in the Late Cretaceous to early Tertiary West Sak and Ugnu formations. These sands are of marine-shelf to fluvial/deltaic depositional environments, topset strata of a Laramide prograding clastic wedge.

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