The Ivishak Formation, the main reservoir in the Prudhoe Bay field, averages 600 ft (180 m) within the field and consists of a lower progradational sequence (400 ft or 120 m) and an upper retrogradational sequence (200 ft or 60 m). Depositional environments interpreted from core examination and log correlation include delta-front, delta-plain, and coarse-grained coastal-plain facies within the lower sequence and a fine-grained coastal-plain facies in the upper sequence.

Coarsening-upward deposits of the delta-front facies overlie and interfinger with the marine to prodelta Kavik shale. Fluvially dominated deposition within a sand-rich delta resulted in overlapping and coalescing channel-mouth bars occasionally truncated by fining-upward distributary channel sequences averaging 7 ft (2 m) thick. Thicker fining-upward cycles (averaging 10.5 ft or 3.2 m) mark a shift from distributary to fluvial deposition on the delta plain. Some fluvial cycles grade vertically into flood-plain deposits of interbedded shale and siltstone that form the most extensive permeability barriers in the reservoir. A coastal-plain facies consisting of crossbedded pebbly sandstone and conglomerate deposited by braided rivers overlies the delta-plain deposits. Shale is uncommon an discontinuous within this facies. The upper retrogradational sequence rests with sharp contact upon the lower sequence and is interpreted as a coastal-plain facies composed predominantly of sandy braided-fluvial deposits.

Throughout the fluvial part of the reservoir, individual, complete, braided channel-fill deposits average 10 ft (3 m) thick. Less common meandering-stream deposits average 14 ft (4.2 m) thick. Channel dimension and depositional environment determine the geometry of silty and sandy shales that form the dominant permeability barriers in the reservoir. Continuous shales within the fluvial sequence were deposited as flood-plains. Discontinuous shale beds include abandoned channel-fill, bar-drape, and slough-fill deposits.

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