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U.S. Department of the Interior

U.S. Geological Survey

USGS Fact Sheet 2005–3043

April 2005

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.

National Assessment of Oil and Gas Fact Sheet

Introduction

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 geologybased 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. 

Scientists with the U.S. Geological Survey (USGS) recently completed a new 

assessment of oil and gas resources of the central Alaska North Slope and the 

adjacent offshore belonging to the State of Alaska, a region of about 23,000 

square miles (59,500 km2). This assessment indicates the probable existence of 

additional undiscovered petroleum resources in the region.

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 
Printed on recycled paper

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,

 http://energy.usgs.gov/.

or contact

Kenneth J. Bird or David W. Houseknecht 

(kbird@usgs.gov; dhouse@usgs.gov)

This Fact Sheet and any updates to it are available 

online at

http://pubs.usgs.gov/fs/2005/3043/

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

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]

estimated to occur (see table for estimates 

over a range of probability). Nearly twothirds 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.