Prudhoe Bay field, potentially the largest oil discovery on the North American continent, is on the flat-lying arctic coastal plain of Alaska.

The structure of the field resulted from 2 periods of deformation. The Lisburne and Sadlerochit (Carboniferous-Triassic) productive structure is a westward-plunging anticline superimposed on a stable ancestral shelf. The anticline broadens eastward until it is progressively truncated by a major unconformity. The geologic structure is complicated by 2 fault patterns cutting rocks of pre-Cretaceous age: (1) normal, NW-SE-trending faults on the south flank, and (2) high angle stair-step faults parallel with the axial plane of the fold on the north flank. The structure of the field, contoured with the top of the Kuparuk River (Cretaceous) as datum, is a pronounced eastward-plunging structural nose.

Two major depositional sequences have been recognized in northern Alaska: (1) an upper sequence composed of orogenic rocks with poor reservoir characteristics, and (2) a lower sequence ranging in age from Mississippian to Early Cretaceous which contains most of the Lisburne Group. Accumulation was controlled by structure and porosity-permeability variations. The middle and main producing zones are sandstones of Permian to Triassic age which compose a gigantic stratigraphic trap abruptly truncated and overlapped by impervious Cretaceous shales. The upper producing zones are confined to the western part of the field and consist of discontinuous lenticular sandstones of Late Jurassic and Early Cretaceous ages.

A model concept of field-wide unitization will be in effect before production commences in the mid-1970s.

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