The occurrence of heavy oil in the Tertiary and Upper Cretaceous sands of the Kuparuk River area, North Slope of Alaska, has been known since 1969. It was not until 1981 that delineation and development drilling for the deeper Kuparuk Formation was sufficient to demonstrate the wide areal extent of the shallow heavy oil. An inventory of these sands, conducted during 1981 and 1982, indicated that individual accumulations extended over 520 km² (200 mi², and that the combined estimate of oil in place could be as great as 40 billion barrels.

The majority of the heavy oil occurs in two shallow intervals that are part of the Brookian marine and deltaic depositional system of the North Slope. The two informally named zones are the West Sak sands and the overlying Ugnu sands. These zones are oil-bearing primarily in the Kuparuk River and Milne Point units, where they occur at depths ranging from 610 to 1370 m (2000-4500 ft) subsea. The oil in the West Sak is a less heavy to intermediate crude with API gravities ranging from 16° to 22°. Most oil in the Ugnu sands is classified as bitumen at reservoir temperature, with API gravities between 8° and 12°.

ARCO Alaska and ARCO Oil and Gas Company are currently studying the technical and economic feasibility of producing the oil in these shallow reservoirs. The West Sak is the most likely target for near-term development, because of the higher gravity of its crude oil. A pilot waterflood project to study reservoir response and drilling technology was begun in the southeast Kuparuk River Unit in late 1983. In addition, exploratory drilling for other heavy oil accumulations is planned on existing leases peripheral to the Kuparuk River and Prudhoe Bay units.