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.
Figures & Tables
Exploration for Heavy Crude Oil and Natural Bitumen
Gross volumes of oil, which must be kept in mind to address the volume/size framework, may be thought of in order from largest to probably smallest volumes as follows: (1) generated; (2) dissipated; (3) degraded/ partially preserved; and (4) trapped and conventionally producible. Basic knowledge of these volumes may be from greatest to least in essentially reverse order.
The 332 largest known accumulations (less than 1% of the total number) account for more than three-quarters of the known 7.6 trillion bbl of oil and heavy oil or tar in more than 40,000 accumulations in the world. About 2.4 trillion bbl of estimated undiscovered conventional oil added to the known volume of 7.6 trillion bbl yields a total of 10 trillion bbl known or reasonably estimated. World-wide cumulative production of about 500 billion bbl of oil accounts for only 5% of the gross.
Oil in place must be estimated for conventional oil fields before comparison with heavy oil and tar accumulations. The size range of accumulations considered in the size distribution of the 332 largest known accumulations is from 0.8 to 1850 billion bbl of oil. The smallest conventional fields in the distribution are about 1 billion bbl because the size cut-off is 0.5 billion bbl of oil recoverable. The size distribution of the 332 largest known accumulations approaches log normal and is overwhelmed by the largest three supergiant tar deposits that hold nearly half of the total 5495 billion bbl.
Globally, the largest three accumulations, all heavy oil or tar, are in South and North America; the two largest conventional oil fields are in the Middle East. Prudhoe Bay and East Texas fields rank 18 and 34, respectively, in descending size order.