U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2012–3013

February 2012

National Oil and Gas Assessment Project

Assessment of Potential Oil and Gas Resources in Source 

Rocks of the Alaska North Slope, 2012

Introduction

The U.S. Geological Survey (USGS) in 2012 completed 

an assessment of undiscovered, technically recoverable oil and 

gas resources in three source rocks of the Alaska North Slope. 

Although these source rocks are known to have generated oil and 

gas that migrated into conventional accumulations, including the 

super-giant Prudhoe Bay field, no attempt has been made to produce oil or gas directly from the source rocks. Thus, they represent 

an unexplored frontier for shale-oil and shale-gas resources.

Geology of Alaska North Slope Source Rocks

Three source rocks were assessed: (1) The Triassic Shublik 

Formation, (2) the lower part of the Jurassic–Lower Cretaceous 

Kingak Shale, and (3) the Cretaceous pebble shale unit and Hue 

Shale, together called Brookian shale in this fact sheet. The Shublik 

Formation mostly contains a mixture of Type I and IIS kerogen, 

and oil in conventional accumulations sourced from the Shublik is 

of relatively low gravity (23–39° API) and high sulfur (more than 

1.5 percent). In contrast, the Kingak and Brookian shales mostly 

contain a mixture of Type II and III kerogen, and oil in conventional accumulations sourced from those rocks is of relatively high 

gravity (35–42° API) and low sulfur (less than 0.3 percent). These 

source rocks occur at depths that range from less than 3,000 feet 

along the northern coast to more than 20,000 feet in the Brooks 

Range foothills. Over this range of depth, thermal maturity of the 

source rocks grades from the onset of oil generation in the north, 

through the oil window, and well into the dry gas window in the 

south. Vitrinite reflectance data plus the results of time-temperature 

and petroleum-generation modeling were used to estimate a boundary between oil (north) and gas (south) assessment units for each 

source rock. Southern boundaries of the gas assessment units were 

based on either the surface expression of the northern limit of tectonic structures formed during the Early Cretaceous or the modeled 

location of the 4 percent vitrinite reflectance contour, whichever is 

farther north.

Considering that no attempt has been made to produce oil or 

gas from the source rocks (limited oil production and gas tests from 

the Shublik are considered local occurrences of adequate reservoir 

quality in conventional accumulations), the presence of brittle reservoir lithologies is an important consideration. Both the Shublik 

and Brookian source rocks include rock types that are brittle and 

in which natural fractures are common. 

Brittle lithologies include limestone, phosphatic limestone, and chert in the Shublik 

and very fine grained sandstone, siltstone, 

concretionary carbonate, and silicified tuff 

in the Brookian. In contrast, the Kingak is 

mostly clay shale that deforms plastically, 

and brittle lithologies generally are absent.

Additional geological information 

was used to define assessment units and 

to evaluate the potential for oil and gas 

to occur in the source rocks. Maps of 

gamma-ray response in geophysical logs 

from exploration wells, regional distribution of overpressure, and reconstructed 

history of burial, uplift, and cooling of 

North Slope source rocks were among the 

most important considerations.

Shale-oil and shale-gas formations in 

the lower 48 States were used as geologic 

and engineering analogs in the assessment. Analog data include “estimated 

ultimate recovery” (EUR) from shale-oil 

and shale-gas wells, mean drainage areas 

of wells (cell sizes), and ranges of well 

success ratios. Key assessment input data 

are listed in table 1.

The U.S. Geological Survey estimated potential, technically recoverable 

oil and gas resources for source rocks of the Alaska North Slope. Estimates (95-percent to 5-percent probability) range from zero to 2 billion 

barrels of oil and from zero to nearly 80 trillion cubic feet of gas.

National Petroleum Reserve - Alaska

Arctic National Wildlife Refuge 

1002 Area

State-Federal

offshore boundary

National Oil and Gas Assessment 

region boundary

Trans Alaska

Pipeline System

EXPLANATION

Brookian Shale Oil AU

Brookian Shale Gas AU

Kingak Shale Oil AU

Kingak Shale Gas AU

Shublik Shale Oil AU

Shublik Shale Gas AU

AK

MAP LOCATION

BROOKS

RANGE

CENTRAL ALASKA

NORTHERN ALASKA 0 25 50 100 MILES

0 25 50 100 KILOMETERS

168°W 164°W 160°W 156°W 152°W 148°W 144°W 140°W

72°N

70°N

68°N

Figure 1. Northern Alaska province showing boundaries of assessment units (AU) discussed in text. 

Assessment units extend offshore to State-Federal boundary except where strata are absent because 

of erosion or nondeposition. Small gray dots are exploration wells drilled in search of conventional 

petroleum accumulations.
Resource Summary

USGS estimates of potential, technically recoverable shale-oil 

resources in northern Alaska (95- to 5-percent probability) range 

from zero to 2 billion barrels of oil (table 2). Oil resources are 

about equally distributed between the Shublik and Brookian source 

rocks, with significantly less potential in the Kingak (table 2). 

Estimates of potential, technically recoverable shale-gas 

resources in northern Alaska range from zero to nearly 80 trillion 

cubic feet of gas (table 2). Gas resources are heavily concentrated 

in Shublik source rocks, with significantly less gas potential in the 

Brookian and small gas potential in the Kingak (table 2). These 

estimates include “nonassociated” gas in the Shublik and Brookian 

shale-gas assessment units, as well as “associated” gas in all three 

shale-oil assessment units.

Estimates of technically recoverable natural gas liquids 

(NGL), from both nonassociated and associated gas, range from 

zero to more than 500 million barrels (table 2). The Shublik is 

estimated to contain most of the NGL.

Although mean resource values are included in table 2, 

they must be considered within the context of the large range of 

uncertainty associated with estimation of undiscovered, continuous 

resources in source rocks from which no attempt has been made to 

produce either oil or gas. Therefore, mean values are not emphasized in this fact sheet.

For Further Information

Supporting reports on geology and methodology used in the 

Alaska North Slope assessment are in progress. Assessment results 

and related reports are available at the USGS Energy Web site: 

http://energy.usgs.gov

North Slope Source-Rock Resource Assessment 

Team

David W. Houseknecht (dhouse@usgs.gov), William A. 

Rouse, Christopher P. Garrity, Katherine J. Whidden, Julie A. 

Dumoulin, Christopher J. Schenk, Ronald R. Charpentier, Troy A. 

Cook, Stephanie B. Gaswirth, Mark A. Kirschbaum, and Richard 

M. Pollastro.

Acknowledgments

We thank Mike Whalen (University of Alaska-Fairbanks), 

the Alaska Department of Natural Resources (Paul Decker, Robert 

Swenson, and Marwan Wartes), and participants in a public workshop held in Anchorage in October 2011 for input regarding the 

geology of petroleum source rocks of Arctic Alaska prior to the 

USGS assessment.

Assessment input data Shublik Shale Oil AU Shublik Shale Gas AU Brookian Shale Oil AU Brookian Shale Gas AU Kingak Shale Oil AU

Min. Mode Max. Mean Min. Mode Max. Mean Min. Mode Max. Mean Min. Mode Max. Mean Min. Mode Max. Mean

Potential production area (million acres) 5.0 7.3 7.5 6.6 20.0 25.6 30.0 25.2 14.0 18.2 20.0 17.4 14.0 17.3 20.0 17.1 5.0 7.4 8.0 6.8

Average drainage area of wells (acres) 100 160 400 220 75 150 200 142 100 200 400 233 120 150 180 150 100 160 400 220

Percentage of AU in sweet spots (%) 0.5 15.0 40.0 18.5 2.0 20.0 50.0 24.0 0.2 10.0 40.0 16.8 0.5 5.0 10.0 5.2 0.0 0.0 10.0 3.3

Input for inside sweet spots

 Average EUR (MMBO, oil; BCFG, gas) 0.020 0.050 0.250 0.061 0.200 0.800 2.500 0.887 0.010 0.040 0.100 0.043 0.040 0.400 1.000 0.426 0.010 0.040 0.100 0.043

 Success ratio (%) 70 85 95 83 50 80 95 75 50 70 95 72 40 65 95 67 50 70 95 72

Input for outside sweet spots

 Average EUR (MMBO, oil; BCFG, gas) 0.010 0.025 0.125 0.031 0.100 0.400 1.250 0.444 0.005 0.010 0.060 0.013 0.040 0.200 0.500 0.214 0.005 0.010 0.060 0.013

 Success ratio (%) 0 10 70 27 1 10 50 20 0 10 30 13 0 0 10 3 0 5 25 10

Table 1. Key assessment input data for shale-oil and shale-gas assessment units of the Alaska North Slope.

[EUR (estimated ultimate recovery per well), cell size, and success ratios are based on U.S. lower-48 shale-oil and shale-gas analogs. MMBO, million barrels of oil; 

BCFG, billion cubic feet of gas; AU, assessment unit; %, percent, Min., minimum; Max., maximum. For the average EUR variables, the mode columns contain the 

medians of the EUR distributions.]

Total petroleum systems (TPS) 

and assessment units (AU)

AU

probability

Field 

type

Total undiscovered resources

Oil (MMBO) Gas (BCFG) NGL (MMBNGL)

F95 F50 F5 Mean F95 F50 F5 Mean F95 F50 F5 Mean

Shublik TPS

Shublik Shale Oil AU 0.95 Oil 0 428 928 463 0 418 981 462 0 10 26 12

Shublik Shale Gas AU 0.95 Gas 0 36,612 72,195 38,405 0 184 442 205

Brookian TPS

Brookian Shale Oil AU 0.90 Oil 0 421 955 449 0 818 1,996 898 0 20 51 22

Brookian Shale Gas AU 0.90 Gas 0 2,124 4,375 2,184 0 21 46 22

Kingak TPS

Kingak Shale Oil AU 0.40 Oil 0 0 117 28 0 0 238 57 0 0 6 1

Kingak Shale Gas AU Gas Not quantitatively assessed

Total continuous resources 0 849 2,000 940 0 39,972 79,785 42,006 0 235 571 262

Table 2. Alaska North Slope shale-oil and shale-gas assessment results.

[MMBO, million barrels of oil; BCFG, billion cubic feet of gas; MMBNGL, million barrels of natural gas liquids. Results shown are fully risked estimates. For gas 

accumulations, all liquids are included as NGL (natural gas liquids). Undiscovered gas resources are the sum of nonassociated and associated gas. F95 represents a 

95-percent chance of at least the amount tabulated; other fractiles are defined similarly. Fractiles are additive under the assumption of perfect positive correlation. AU, 

assessment unit. AU probability is the chance of at least one well within the AU having a production capacity of the minimum estimated ultimate recovery. TPS, total 

petroleum system. Gray shading indicates not applicable.]