link to main US Geological Survey website
U.S. Geological Survey
Fact Sheet 2005-3043
version 1.1

National Assessment of Oil and Gas Fact Sheet

Oil and Gas Assessment of Central North Slope, Alaska, 2005

The U.S. Geological Survey (USGS) recently completed a new assessment of undiscovered oil and gas resources of the central part of the Alaska North Slope and the adjacent offshore area. Using a geology-based assessment methodology, the USGS estimates that there are undiscovered, technically recoverable mean resources of 4.0 billion barrels of oil, 37.5 trillion cubic feet of natural gas, and 478 million barrels of natural gas liquids.


The U.S. Geological Survey (USGS) recently completed an assessment of undiscovered oil and gas resources of the central Alaska North Slope and the adjacent offshore belonging to the State of Alaska. This area lies between the National Petroleum Reserve in Alaska (NPRA) and the Arctic National Wildlife Refuge (ANWR) and extends from the Brooks Range northward to the State-Federal offshore boundary. Most commercial oil fields and virtually all petroleum-producing infrastructure in northern Alaska are located within the assessment area. This area, which consists mostly of State and Native lands covering about 23,000 square miles (59,500 km2), is maturely explored in the north but only lightly explored in the south. Approximately 15 billion barrels of oil (including natural-gas liquids) have been produced from the assessment area (most from the giant Prudhoe Bay field), and remaining (discovered) reserves include about 7 billion barrels of oil and about 35 trillion cubic feet of natural gas. This assessment used the same geology-based methodology used in recent USGS assessments of NPRA and the ANWR 1002 area. The assessment is based on geologic elements, including hydrocarbon source rocks, reservoir rocks, and traps. The minimum accumulation sizes considered in this assessment are 5 million barrels of technically recoverable oil and 100 billion cubic feet of technically recoverable gas. These minimum accumulation sizes are smaller than those used in USGS assessments of NPRA and the ANWR 1002 area in recognition of the extensive infrastructure and recent development of relatively small oil accumulations in the assessment area. Resources assessed include technically recoverable conventional oil, natural gas, and natural-gas liquids.

Although six total petroleum systems were defined, geologic evidence suggests significant mixing of hydrocarbons among those systems. Therefore, the assessment was conducted under the assumption of a single, composite total petroleum system. Twenty-four plays (assessment units) were defined and assessed.

Resource Summary

The USGS estimated technically recoverable, undiscovered resources of oil, natural gas (nonassociated and associated), and natural-gas liquids (from nonassociated and associated gas) in the central North Slope assessment area. Oil resources range between 2.6 and 5.9 billion barrels of oil (BBO) (95% and 5% probabilities), with a mean of 4.0 BBO. Nonassociated gas resources range between 23.9 and 44.9 trillion cubic feet (TCF) (95% and 5% probabilities), with a mean of 33.3 TCF. In addition, means of 4.2 TCF of associated gas, 387 million barrels of natural-gas liquids (MMBNGL) from nonassociated gas, and 91 MMBNGL from associated gas are estimated to occur (see table for estimates over a range of probability). Nearly two-thirds of the mean undiscovered oil is estimated to occur in three plays in the northern part of the area—Brookian Clinoform, Brookian Topset, and Triassic Barrow Arch. About half of the mean undiscovered nonassociated natural gas is estimated to occur in four plays in the southern part of the area—Brookian Clinoform, Thrust Belt Triangle Zone, Thrust Belt Lisburne, and Basement Involved Structural.

It is estimated that 91% of undiscovered oil resources occurs in accumulations of less than 250 million barrels of oil (MMBO) recoverable and that 96% of undiscovered nonassociated gas resources occurs in accumulations smaller than 3.0 TCF recoverable. The occurrence of larger oil and gas accumulations is unlikely.

Central North Slope assessment results, showing technically recoverable, conventional, undiscovered oil and gas.

[MMBO, million barrels of oil; BCFG, billion cubic feet of gas; MMBNGL, million barrels of natural-gas liquids. Results shown are fully risked estimates. Gas volumes associated with oil fields are shown on the "Oil" row and nonassociated gas volumes on the "Gas" row. For gas fields, all liquids are included under NGL (natural-gas liquids) category. F95 denotes a 95% chance of at least the amount tabulated. Other fractiles are defined similarly. Fractiles are not additive. Gray shading indicates not applicable]

table showing data summarized above
(click table for a larger version)

Kenneth J. Bird, David W. Houseknecht, Emil D. Attanasi, Thomas E. Moore, Phillip H. Nelson, Christopher J. Potter, Christopher J. Schenk, John H. Schuenemeyer, Mahendra K. Verma, Richard W. Saltus, Jeffery D. Phillips, Ronald R. Charpentier, Troy A. Cook, Timothy R. Klett, and Richard M. Pollastro

Edited by Peter H. Stauffer and James W. Hendley II
Graphic design by Susan Mayfield and Sara Boore; Web design by Michael Diggles

For Further Information

Supporting geologic studies of total petroleum systems, assessment units, economic analysis, and the methodology used in the Central North Slope assessment are in progress. Assessment results are available at the USGS Energy Resources Program Web site,

or contact

Kenneth J. Bird or David W. Houseknecht

PDF version of this fact sheet (1.5 MB)

Press images. This folder contains press-quality images that were used in this publication (29.5 MB)

Version history

Download a copy of Acrobat Reader for free
| Help | PDF help | Western Publications main page | Western Fact Sheets |
| U.S. Geological Survey | Geologic Division | Western Energy Team |

| Department of the Interior | Privacy Statement | Disclaimer | Accessibility |
URL of this page:
Maintained by: Michael Diggles
Created: May 11, 2005
Last modified: May 11, 2005 (mfd)