The Ellesmerian(!) petroleum system covers an area of about 225,000 km² and is the source for about 98% of the in-place hydrocarbons (~77 10⁹ BOE, or bbl of oil equivalent) and 100% of the commercially recoverable hydrocarbons (~13 10⁹ bbl of oil) on the North Slope. The system developed when marine shale source rocks of the Triassic Shublik Formation and the Jurassic and Early Cretaceous Kingak Shale were buried sufficiently to begin generating hydrocarbons by the longitudinal filling of the Cretaceous and Tertiary Colville foreland basin. Distinct depocenters in the foreland basin suggest that hydrocarbon ge eration began in middle to Late Cretaceous time in the western part of the basin and was followed by a separate episode of generation in the east-central part of the basin in early Tertiary time. Reservoir rocks range in age from Mississippian to early Tertiary and are mostly sandstone. Carbonate reservoirs are of Mississippian-Pennsylvanian age. Traps are mainly broad, anticlinal structures with an important component of erosional truncation and thus are classified as combination structural-stratigraphic traps. All hydrocarbon accumulations in this system are located along the Barrow arch, a regional high derived from a buried rift margin. On the basis of average total organic carbon content and assumed hydrogen indices, the mass of hydrocarbons generated by the source rocks is ~8 10¹² bbl of oil. When compared to the amount of known in-place hydrocarbons, these calculations indicate that about 1% of the hydrocarbons generated actually accumulated in traps. If the amount of undiscovered oil and gas estimated for this region is added to the amount already discovered, the percentage increases to 2%.