US SG

U.S. Department of the Interior

U.S. Geological Survey

USGS Fact Sheet 045-02 

2002

U.S. Geological Survey 2002 Petroleum Resource Assessment 

of the National Petroleum Reserve in Alaska (NPRA)

“Whereas there are large seepages of petroleum along the Arctic Coast 

of Alaska and conditions favorable to the occurrence of valuable petroleum 

fields on the Arctic Coast . . .” President Warren G. Harding used those words 

in 1923 to describe the apparent petroleum potential of a tract of land on the 

western North Slope of Alaska, when he issued a one-page executive order 

establishing the 23-million acre (36,000 square miles) Naval Petroleum 

Reserve No. 4. During the following six decades, the U.S. Government conducted two petroleum exploration programs in the reserve, one in the wake 

of World War II and the second in the wake of the 1970’s oil embargo. These 

programs found only a handful of oil and gas fields, none of them commercial. The name of the reserve was changed to the National Petroleum Reserve 

in Alaska (NPRA) in 1976 (fig. 1). Four lease sales were held in the 1980’s, 

but only two exploration wells were drilled by industry within the NPRA 

boundary—one on a Federal lease and another on Native land—and neither 

resulted in development of petroleum resources.

Following a 10-year hiatus in exploration activity, NPRA again became a 

focus of interest with the 1996 announcement of the discovery of the Alpine 

oil field, located just outside NPRA (fig. 1). A Federal lease sale was held in 

northeastern NPRA in 1999, and a number of exploration wells in that lease 

sale area were completed by industry during the 2000 and 2001 winter drilling seasons. Five of those wells were announced in 2001 to have encountered 

oil and gas, and additional wells were drilled during 2002 to delineate those 

discoveries and to test additional prospects (fig. 2).

The U.S. Geological Survey (USGS) has reexamined the petroleum geology of NPRA and has prepared a new petroleum resource assessment. This 

new assessment was prompted by (1) the rapidly evolving exploration activity 

focused on the “Alpine trend,” (2) the growing interest in natural gas resources 

of the Arctic, and (3) the dated perspective of previous resource estimates (the 

last USGS assessment of NPRA was completed in 1980). Although none of the 

data from 3-D seismic surveys or new wells drilled in NPRA since the 1999 

lease sale was available for this study, the results nevertheless reflect a comprehensive examination of all public domain data and a consideration of new 

exploration and development strategies currently being applied on the Alaska 

North Slope. This new study complements the 1998 assessment of the Arctic 

National Wildlife Refuge (ANWR), 1002 Area (see USGS Fact Sheet 028-01). 

Assessment Project

The NPRA assessment involved nearly 4 years of study by a team of USGS 

scientists, most of whom also worked on the 1998 ANWR assessment. Research was coordinated with colleagues in other Federal agencies, Alaska State 

agencies, and several universities. New field studies were conducted, new well 

and sample data were analyzed, some new geophysical data were acquired, 

and public technical workshops examining core samples were held. Data and 

interpretations from previous U.S. Government exploration programs were 

incorporated. About one-third of the 14,000 line-miles of seismic data collected 

by the Government between 1974 and 1981 were reprocessed and reinterpreted. 

Special attention was focused on understanding the more recent oil discoveries immediately east of NPRA and the potential for those productive geologic 

trends to extend westward beneath NPRA. All information was integrated and 

used as basic input to the 2002 petroleum assessment.

In keeping with the USGS responsibility for assessing the petroleum potential of all onshore areas and State water areas of the United States, the total 

area considered in this assessment was extended offshore to the boundary 

between State and Federal jurisdiction. Thus, in addition to Federal lands of 

NPRA, this assessment includes resources beneath State waters offshore from 

NPRA and beneath Native lands within the NPRA boundary (fig. 2). The total assessment area consists of 24.2 million acres, of which 22.5 million acres 

are Federal and 1.7 million acres are non-Federal (State and Native). 

ALASKA

Area

of map

Northern margin

of Brooks Range

NPRA

Wilderness Area

ANWR

1002 Area

CANADA

UNITED

STATES

Prudhoe

Alpine Bay

Pt. Barrow

TAPS

0 100 MILES

0 100 KILOMETERS

Arctic Ocean

Native lands

Pt. Thomson

68°

69°

70°

71°

165°

160°

155°

150°

145°

140°

Figure 1. Map of northern Alaska showing locations and relative sizes of the National Petroleum 

Reserve in Alaska (NPRA) and the Arctic National Wildlife Refuge (ANWR). ANWR’s 1002 Area was 

evaluated for petroleum potential by the USGS in 1998. The Trans-Alaska Pipeline System (TAPS) and 

“feeder” pipelines extending east and west of Prudhoe Bay show the extent of existing petroleum 

infrastructure. Locations of the Alpine and Prudhoe Bay oil fields and the Point Thomson gas and oil 

accumulation also are shown. 

Anew USGS assessment concludes that 

NPRA holds significantly greater petroleum resources than previously estimated. Technically 

recoverable, undiscovered oil beneath the 

Federal part of NPRA likely ranges between 5.9 

and 13.2 billion barrels, with a mean (expected) value of 9.3 billion barrels. An estimated 

1.3 to 5.6 billion barrels of those technically 

recoverable oil resources are economically 

recoverable at market prices of $22 to $30 per 

barrel. Technically recoverable, undiscovered 

nonassociated natural gas for the same area 

likely ranges between 39.1 and 83.2 trillion cubic feet, with a mean (expected) value of 59.7 

trillion cubic feet. Economic viability of this 

gas will depend on the availability of a natural 

gas pipeline for transport to market. 
.

. .. . .

.. . ... .

. . .

.

.

. .

.

. . .

.

. .

. .

Gubik Fm.

pebble shale unit

AGE STRATIGRAPHY SW NE

Ma

2

50

65

96

144

208

JURASSIC

CRETACEOUS

CENOZOIC

Kingak

Shale

Shublik Fm.

Torok Formation

Nanushuk Group

Colville Group

Gamma-ray zone (Hue Sh.)

245

MISSISSIPPIAN

PENNSYLVANIAN

PERMIAN

TRIASSIC

Endicott

Group

Lisburne

Group

Sadlerochit

Group

Shallow

marine deposits Nonmarine

deposits

Hiatus or

erosion

Marine shale,

+ turbidites Condensed

marine shale

Metasedimentary

rocks Limestoneand dolomite

Granite

DEVONIAN

AND

OLDER

–

286

320

360

ELLESMERIAN SEQUENCE FRANKLINIAN

SEQUENCE

BEAUFORTIAN

SEQUENCE

BROOKIAN SEQUENCE

+ + +

+ + +

+

+

+ +

+ +

+ + +

+ +

+ +

PETROLEUM PLAYS

Mostly

Stratigraphic

Mostly

Structural

TOPSET

CLINOFORM+ CLINOFORM

TOPSET PLAYS

ELLESMERIAN STRUCTURAL

THRUST BELT

L.Jur. Cret.+ Ivishak

Echooka+

Lisburne+

Endicott

ELLESMERIAN PLAYS

Basins+

TOROK

TOPSET

BROOKIAN PLAYS BEAUFORTIAN

Plays that are further

subdivided by area.

+

U.Jur.+

<

Sagavanirktok Fm.

Assessment Methodology

The methodology used in this assessment is similar to that used in the earlier 

USGS assessments of NPRA (1978–80) and ANWR (1987 and 1998). As the 

initial step of the assessment, twenty-four petroleum plays were defined (fig. 3). 

A play is a volume of rock that contains similar geological parameters (such as 

petroleum charge, reservoir, and trap) that determine petroleum potential. The 

term “petroleum” is used to include crude oil, natural gas, and natural gas liquids.

For each play, distributions of the number and size of potential petroleum 

accumulations were estimated on the basis of a probabilistic range of values 

for certain geological attributes, such as reservoir thickness and porosity. 

These distributions were restricted to potential accumulations larger than 50 

million barrels of in-place oil (MMBO) or 250 billion cubic feet (BCF) of 

technically recoverable gas, so that the assessment would not be influenced 

by accumulations smaller than these minimum sizes, which are generally 

noneconomic on the North Slope.

The resulting distributions were subjected to a geologic risking procedure 

designed to weigh the likelihood that petroleum charge, reservoir, and trap 

conditions were sufficient to generate at least the minimum-size accumulation. In turn, a probabilistic estimate of in-place petroleum resources was calculated on the basis of the risked distributions of size and number of potential 

petroleum accumulations in each play. A recovery factor appropriate to each 

play was applied to the estimates of in-place petroleum resources to calculate 

technically recoverable petroleum resources. Typically, only 30% to 50% 

of in-place oil resources and only 60% to 70% of in-place gas resources are 

technically recoverable. Estimates for each play were aggregated to calculate 

total technically recoverable petroleum resources for the entire assessment 

area and for the Federal area (table 1).

This assessment methodology yields results that include probabilistic expressions of uncertainty. To stress the importance of this uncertainty, results reported 

Figure 2. Map of the National Petroleum Reserve in 

Alaska (NPRA; boundary shown by yellow line) and 

surrounding region with locations of seismic lines, 

wells, oil and gas accumulations, and Native lands. 

Red line shows the total area assessed in 2002 

by the U.S. Geological Survey (USGS), including 

Federal and Native lands and State offshore areas. 

USGS seismic grid shows location of those seismic 

lines reprocessed and reinterpreted for this assessment. The assessment incorporated information on 

all exploration wells except those in the northeast 

part of NPRA drilled during 2000 and 2001. Oil fi elds 

east of NPRA, including several recently discovered 

accumulations, are labeled as follows: A, Alpine; F, 

Fiord; K, Kuparuk River; M, Meltwater; N, Nanuq; 

and T, Tarn. The map base is a false-color composite Landsat image on which vegetation appears 

as shades of red, and soil and rock with little or no 

vegetation appear in shades of green and brown. 

Deep and clear water appears dark blue to black, 

and shallow or sediment-laden water appears in 

lighter shades of blue. Ice is white or bluish white. 

Figure 3. Summary of ages, names, and rock types present in NPRA. Colored 

bars at right show the stratigraphic position of the 24 petroleum plays evaluated in 

the 2002 assessment. Note that bars with a “+” symbol indicate multiple plays in 

different areas. Plays indicated by bold outlines include those with the greatest oil 

and (or) gas potential, as shown in fi gures 6 and 8. 

USGS 2002 Petroleum Resource Assessment of the NPRA
here include 95% and 5% probabilities, in addition to mean values. The 95% 

probability level means that there is a 19 in 20 chance that the amount present will 

be at least as large as the amount shown; the 5% probability level means that there 

is a 1 in 20 chance that the amount present will be at least as large as the amount 

shown. Volumes of petroleum associated with the 95% and 5% probabilities are 

considered reasonable estimates of minimum and maximum volumes that may be 

present, and the mean is the average or expected value (fig. 4).

large enough to be developed at a specific location will support the costs of 

constructing processing facilities and extending infrastructure into the area. 

Smaller accumulations then may become economically viable if they can be 

developed as satellites to the larger fields. Results of the economic analysis 

are presented in terms of oil volume as a function of market price.

No analysis of the commercial viability of undiscovered gas resources has 

been made by the USGS. Such analysis is deferred until plans for a natural 

gas transportation system are more firmly developed.

Assessment Results 

Oil.—The total quantity of undiscovered, technically recoverable oil 

within the entire assessment area is estimated to range between 6.7 and 15.0 

billion barrels (BBO) (95% and 5% probability), with a mean value of 10.6 

BBO. The quantity of undiscovered, technically recoverable oil beneath Federal lands in NPRA (excluding State and Native areas) is estimated to range 

between 5.9 and 13.2 BBO (95% and 5% probability), with a mean value of 

9.3 BBO (table 1). 

As illustrated in figure 5, most oil accumulations are expected to be of 

moderate size, on the order of 30 to 250 million barrels (MMBO) each, and 

large accumulations like Prudhoe Bay (ultimate recovery approximately 13 

BBO) are not expected to occur. This conclusion is consistent with the fact 

that numerous exploration wells previously drilled in NPRA and in adjacent 

State and Federal waters tested prospects that were geologically similar to 

Prudhoe Bay, without success. Significantly, figure 5 shows that NPRA is expected to contain many accumulations in the size range commonly developed 

as “stand alone” or satellite fields on the central Alaska North Slope in recent 

years. For comparison, announced estimates of ultimate recoveries from 

recently discovered fields shown in figure 2 include 429 MMBO for Alpine, 

70 MMBO for Tarn, 50 MMBO for Meltwater, 50 MMBO for Fiord, and 40 

MMBO for Nanuq.

Quantities of technically recoverable oil are not expected to be uniformly 

distributed throughout NPRA. This is illustrated by accumulation-size histograms and maps for the four plays estimated to hold the greatest oil potential 

in NPRA (fig. 6). Based on the mean estimate, about 80% of the technically 

recoverable oil resources are likely to occur in northern NPRA within plays that 

are westward continuations of the geologic trends that host Alpine, Fiord, Tarn, 

Meltwater, and Nanuq oil pools, just east of NPRA (fig. 2).

The economic analysis of undiscovered resources is particularly important in 

an area as large as NPRA, because some of the oil resources may be far from existing infrastructure. Figure 7 summarizes oil volume as a function of market price 

based on accumulation-size distributions associated with the mean, 95%, and 5% 

probability estimates of technically recoverable oil. Over a range of market prices 

between $22 and $30 per barrel, between 1.3 and 5.6 BBO are estimated to be 

economically recoverable from the Federal part of the study area on the basis of 

the mean estimate of technically recoverable oil volumes (fig. 7). 

Gas.—Significant volumes of natural gas also are estimated to occur in 

NPRA. Although North Slope gas is currently noncommercial for lack of a 

transportation system, it is of growing interest because of recent discussions 

and proposals of gas pipeline construction. The total quantity of undiscovered, technically recoverable nonassociated gas within the entire assessment 

area is estimated to range between 40.4 and 85.3 trillion cubic feet (TCF) 

(95% and 5% probability), with a mean value of 61.4 TCF. The quantity of 

Volume of oil, billions of barrels

Area F95 Mean F05

Entire assessment area1 6.7 10.6 15.0

Federal part of NPRA 5.9 9.3 13.2

OIL

Volume of gas, trillions of cubic feet

F95 Mean F05

Entire assessment area1 40.4 61.4 85.3

Federal part of NPRA 39.1 59.7 83.2

1 Includes NPRA, Native lands, and adjacent State offshore areas within 3-mile boundary (see figure 2).

NONASSOCIATED GAS

(gas in gas fields)

Table 1. U.S. Geological Survey estimates of volumes of technically recoverable 

oil and nonassociated gas in the NPRA study areas.

Commercial viability of undiscovered oil resources was estimated by considering costs associated with finding, developing, producing, and transporting to market (West Coast of the lower-48 States) the technically recoverable 

oil resources estimated to be present. The cost functions are calculated in constant 2001 dollars and are based on the expectation that production will repay 

all operating costs, including taxes and transport to market, and all investment 

expenditures and will provide an after-tax rate of return of at least 12% on the 

investment. The economic analysis simulates exploration by assuming that 

larger accumulations will be discovered early and that these accumulations 

may be developed depending on their size and location. Any accumulation 

F95

F05

V1 V2

In-place

Economically recoverable

Technically recoverable

Recovery

Factor Economics

100

75

50

25

0

PROBABILITY, IN PERCENT

VOLUME OF OIL INCREASING

Figure 4. Schematic graph illustrating petroleum volumes and probabilities. 

Curves represent categories of oil assessment. How one reads this graph is 

illustrated by the blue and orange lines projected to the curve for economically 

recoverable oil—in this example, there is a 95% chance (probability F95) of at least 

volume V1of economically recoverable oil, and there is a 5% chance (probability 

F05) of at least volume V2 of economically recoverable oil. 
Figure 5. Histograms showing the expected (mean) numbers of oil and nonassociated 

gas accumulations estimated to exist in various size categories of technically recoverable resources in the 2002 U.S. Geological Survey assessment of NPRA. Each histogram 

bar is divided into Federal and non-Federal portions. Left sides of histograms appear 

truncated because of minimum accumulation sizes considered in the assessments. 

A, Expected (mean) numbers of oil accumulations, read as follows: It is estimated that 

the assessment area contains approximately 10 accumulations containing between 

256 and 512 million barrels of technically recoverable oil; 8 of those accumulations 

are under Federal jurisdiction and 2 are non-Federal. B, Expected (mean) numbers of 

nonassociated gas accumulations, read as follows: It is estimated that the assessment 

area contains approximately 15 accumulations containing between 768 and 1,536 billion 

cubic feet of technically recoverable gas; 14 of those accumulations are under Federal 

jurisdiction and 1 is non-Federal. 

150 miles east of the U.S.–Canada border) is estimated to contain 200 to 300 BCF 

recoverable reserves. Quantities of technically recoverable gas are most abundant 

in central and southern NPRA, as illustrated by accumulation-size histograms and 

maps for the four plays estimated to hold the greatest gas potential in NPRA (fig. 8). 

Comparison With Previous NPRA Assessments

Of the prior assessments of NPRA oil and gas resources, only the USGS 

assessments of 1978–80 can be reliably compared to the current assessment 

because a common assessment methodology and well-documented assumptions and data were used. The current assessment shows an overall increase 

in estimated technically recoverable oil and gas resources when compared 

to the 1980 assessment (table 2). The increase in estimated oil resources has 

resulted in large part from incorporation of information available from recent, 

nearby oil discoveries, mostly in a play (fig. 6A) that was not previously 

known to exist in NPRA. The increase in estimated gas resources has resulted 

from a combination of factors, including a better understanding of regional 

thermal-maturity patterns and timing of trap formation relative to hydrocarbon generation and improved models of reservoir development.

Comparison with the ANWR 1002 Area Assessment

The volumes of undiscovered, technically recoverable oil estimated for 

NPRA in this assessment are similar to the volumes estimated for the ANWR 

study area in 1998 (table 3). The estimates for the Federal part of NPRA are: 

mean 9.3 BBO, range 5.9 to 13.2 BBO (95% and 5% probabilities). The 

estimates for the Federal part of ANWR are: mean 7.7 BBO, range 4.3 to 11.8 

BBO (95% and 5% probabilities). The similarities between these estimates 

may be misleading, however, because of differences in accumulation sizes (the 

ANWR study area is estimated to contain more accumulations in larger size 

classes) and differences in assessment area (the NPRA study area is more than 

12 times larger than the ANWR study area). Results of the economic analysis 

help clarify the comparisons between the estimates for NPRA and ANWR, because key components of the analysis include: (1) the largest accumulation sizes 

estimated to be present, (2) the number of accumulations in various size classes, 

 Figure 6. Maps of petroleum plays with histograms showing the expected 

(mean) numbers of undiscovered petroleum accumulations estimated to exist in 

various size categories of technically recoverable oil resources in the four plays 

estimated to hold the greatest oil potential in NPRA. About 80% of the technically 

recoverable oil resources, on the basis of the mean estimate, are thought to occur 

in northern NPRA within these four plays. These plays are westward continuations of geologic trends that host the Alpine and Fiord oil pools (A, B) and the Tarn, 

Meltwater, and Nanuq oil pools (C, D), just east of NPRA (fi g. 2). 

Oil, billions of barrels Gas, trillions of ft3

F95 Mean F05 F95 Mean F05

1980 Assessment 0.3 2.1 5.4 1.8 8.5 20.4

2002 Assessment 5.9 9.3 13.2 39.1 59.7 83.2

Includes only the Federal part of the NPRA; Native lands and adjacent State offshore areas

are excluded (see figure 2). Gas resources are non-associated gas only.

2

2

1

1 Includes entire NPRA as Native selections had not been made. Does not include State offshore areas.

Reported gas resources are total gas (non-associated and associated).

 Table 2. Comparison of the 1980 and 2002 assessments of NPRA by the U.S. 

Geological Survey. Volumes are technically recoverable oil and gas. 

undiscovered, technically recoverable nonassociated gas beneath Federal 

lands in NPRA (excluding State and Native areas) is estimated to range 

between 39.1 and 83.2 TCF (95% and 5% probability), with a mean value of 

59.7 TCF (table 1).

As illustrated in figure 5, most gas accumulations are expected to range between about 200 and 1,500 billion cubic feet (BCF) each. For comparison, the gas 

cap at the Prudhoe Bay oil field (fig. 1) contains more than 23,000 BCF, the Point 

Thomson gas and oil accumulation (fig. 1) may contain more than 6,000 BCF, and 

a recently announced discovery in the Mackenzie River delta of Canada (about 

16 to 3232 to 64

64 to 128

128 to 256256 to 512

512 to 1,024

1,024 to 2,048

Oil Accumulation Size Class (million barrels)

Number of Oil Accumulations

10

8

6

4

2

0

NPRA

A

16 to 3232 to 64

64 to 128

128 to 256256 to 512

512 to 1,024

1,024 to 2,048

Oil Accumulation Size Class (million barrels)

Number of Oil Accumulations

10

8

6

4

2

0

NPRA

C

16 to 3232 to 64

64 to 128

128 to 256

256 to 512

512 to 1,024

1,024 to 2,048

Oil Accumulation Size Class (million barrels)

Number of Oil Accumulations

10

8

6

4

2

0

NPRA

D

16 to 3232 to 64

64 to 128

128 to 256256 to 512

512 to 1,024

1,024 to 2,048

Oil Accumulation Size Class (million barrels)

Number of Oil Accumulations

10

8

6

4

2

0

NPRA

B

Brookian Clinoform North Play

Beaufortian Upper Jurassic 

Topset Northeast Play

Beaufortian Upper Jurassic 

Topset Northwest Play

Brookian Clinoform Central Play

A

16 to 3232 to 64

64 to 128

128 to 256256 to 512

512 to 1,024

1,024 to 2,048

Oil Accumulation Size Class (million barrels)

Number of Oil Accumulations25

20

15

10

5

0

Aggregate Oil Resources (Mean Case)

Non-Federal

Federal

96 to 192

192 to 384

384 to 768

768 to 1,536

1,536 to 3,072

3,072 to 6,144

6,144 to 12,288

Gas Accumulation Size Class (billion cubic feet)

Number of Gas Accumulations 30

20

10

0

Aggregate Gas Resources (Mean Case) B

Non-Federal

Federal
Oil, billions of barrels Size of area

F95 Me F05 (Million acres)

ANWR 1002 Area 5.7 10.4 16.0 1.9

NPRA 6.7 10.6 15.0 24.2

ANWR 1002 Area 4.3 7.7 11.8 1.5

NPRA 5.9 9.3 13.2 22.5

1 Includes Federal and Native lands and State offshore areas.

1 ENTIRE AREA

FEDERAL AREA

and (3) distance from existing infrastructure. A direct comparison of results 

based on the mean estimates of technically recoverable oil (fig. 7) shows that 

a larger volume of oil is economically recoverable at market prices below $35 

per barrel in the ANWR study area. At market prices above $35 per barrel, 

estimates of economically recoverable oil for the two areas are similar. 

Summary

In anticipation of the need for scientific support for policy decisions 

and because the perspective of the 1980 USGS assessment of NPRA is two 

decades old, the USGS has completed a new assessment of undiscovered 

petroleum resources in NPRA. This comprehensive study was conducted 

by a team of USGS scientists with contributions on some technical issues 

from colleagues in other agencies and at universities. The study included 

evaluation of all available public data and previous interpretations, as well 

Table 3. Comparison of the 1998 ANWR and 2002 NPRA U.S. Geological Survey 

assessments. Volumes are technically recoverable oil. 

Figure 7. Summary of the 2002 U.S. Geological Survey (USGS) estimate of economically recoverable oil that may occur beneath the Federal part of NPRA. A, 

Graph relating market price to the volume of oil estimated to be profitably recoverable. The three curves are based on estimates of technically recoverable oil 

volumes at the mean (expected) value, and at the 95% (F95) and 5% (F05) probabilities. The 95% probability level means that there is a 19 in 20 chance that the 

amount economically recoverable will be at least as large as the amount shown; the 5% probability level means that there is a 1 in 20 chance that the amount 

economically recoverable will be at least as large as the amount shown. Included are the costs of fi nding, developing, producing, and transporting oil to market (West Coast of the lower-48 States) based on a 12% after-tax return on investment, all calculated in constant 2001 dollars. The chart is read as follows: 

At a market price of $25 per barrel, there is a 95% probability of at least 1.6 billion barrels of economically recoverable oil and a 5% probability of at least 6.2 

billion barrels. The mean or expected value is at least 3.7 billion barrels of economically recoverable oil. So that estimates of economically recoverable oil for 

the NPRA can be compared with those for the ANWR 1002 Area, the graph shows the mean curve (dashed line) from the 1998 USGS assessment of ANWR 

Federal lands, which was calculated in constant 1996 dollars. B, Histograms showing the expected (mean) numbers of oil accumulations estimated to exist 

in various size categories of technically recoverable oil resources in the 2002 USGS assessment of NPRA (dark green) and in the 1998 USGS assessment of 

ANWR (light green). Note that each size class is twice as big as the next smaller size class. The ANWR area is estimated to contain more accumulations in 

the largest two size classes shown, whereas the NPRA area is estimated to contain more accumulations in all the smaller size classes. C, Table of economically recoverable oil resources estimated to occur in the Federal parts of NPRA and ANWR assessment areas at various market prices. Values are based on 

the mean curves shown in A. NPRA results are calculated in constant 2001 dollars, whereas ANWR results are calculated in constant 1996 dollars. 

Market Price ($ per barrel)

10

20

30

40

95%

Probability

Mean

5%

Probability

Oil Volume (billions of barrels)

0 2 4 6 8 10 12

A

Estimates of Economically Recoverable Oil in Federal Study Areas

Table of mean values

Market

Price

($/barrel)

15

-

20

21

22

23

24

25

26

27

28

29

30

-

35

40

Economically

Recoverable Oil

(billion barrels)

0

-

3.2

4.0

4.4

5.0

5.2

5.6

5.8

6.0

6.2

6.3

6.3

-

6.6

6.8

ANWR

0

-

0

0.4

1.3

2.8

3.1

3.7

4.0

4.8

5.1

5.4

5.6

-

6.4

6.9

NPRA

C

Dashed line - ANWR 1002 Area Mean Federal

Resources (USGS Fact Sheet 028-01)

Solid lines - NPRA Federal Resources 16-32

32-64

64-128

128-256256-512

512-1,024

1,024-2,048

Oil Accumulation Size Class (million barrels) Number of Oil Accumulations25

20

15

10

5

0

B ANWR 1002

Area Federal

NPRA Federal
For further information: 

send email to gd-energyprogram@usgs.gov 

or contact

Kenneth J. Bird: kbird@usgs.gov (650) 329-4907

David W. Houseknecht: dhouse@usgs.gov (703) 648-6466

See also Arctic National Wildlife Refuge, 1002 Area, Petroleum Assessment, 1998,

Including Economic Analysis (USGS Fact Sheet 028-01).

This fact sheet and any updates to it are available online at:

http://geopubs.wr.usgs.gov/fact-sheet/fs045-02/

Explanation of Some Terms

Charge. Conditions of petroleum generation and migration adequate for an

accumulation of the minimum size considered in the assessment.

Reservoir. Occurrence of reservoir rocks of suffi cient quantity and quality to 

permit containment of petroleum in volumes suffi cient for an accumulation of the 

minimum size.

Trap. Occurrence of those structures, pinch-outs, permeability changes, and

similar features necessary for the entrapment and sealing of petroleum in an

accumulation of the minimum size. 

In-place resources. Volume of petroleum contained in accumulations of at least 50 

MMBOE, without regard to recoverability.

Technically recoverable resources. Volume of petroleum representing that 

proportion of assessed in-place resources that may be recoverable using current 

recovery technology, without regard to cost.

Economically recoverable resources. Volume of technically recoverable 

petroleum for which the costs of discovery, development, production, and transport, 

including a return to capital, can be recovered at a given market price.

Associated gas. Natural gas in oil accumulations.

Nonassociated gas. Natural gas in gas accumulations.

95% probability level. Refers to a 19 in 20 chance that the amount present will be

at least as large as the amount shown.

5% probability level. Refers to a 1 in 20 chance that the amount present will be at 

least as large as the amount shown.

as new data stemming from fi eld and laboratory analyses. The assessment

was conducted using a methodology similar to that used in previous USGS

assessments in NPRA and ANWR.

This new assessment concludes that the volume of technically recoverable, undiscovered oil beneath Federal lands in NPRA ranges between

5.9 and 13.2 billion barrels (95% and 5% probabilities), with a mean

(expected) value of 9.3 billion barrels. Most of the oil is estimated to occur

in the northern third of NPRA, to be distributed among several plays, and

to occur in accumulations of moderate size. It is unlikely that a Prudhoe

Bay-size accumulation occurs in NPRA. Over a range of market prices

between $22 and $30 per barrel, between 1.3 and 5.6 billion barrels of oil

are estimated to be economically recoverable, on the basis of the mean

estimate of technically recoverable oil volumes.

Estimates of technically recoverable, undiscovered nonassociated natural

gas resources for the same area range between 39.1 and 83.2 trillion cubic

feet (95% and 5% probabilities), with a mean (expected) value of 59.7 trillion

cubic feet. The economic viability of these natural gas resources will depend

on the availability of a pipeline to transport the gas to market.

The amounts of oil and gas estimated for the NPRA area are signifi cantly

greater than USGS estimates made in 1980. The increase in estimated oil

resources is largely the result of the recognition of new plays based on oil

accumulations recently discovered just east of NPRA. Increased gas estimates

result from improved understanding of thermal maturity, reservoir development, and timing of trap development relative to hydrocarbon generation.

The amount of technically recoverable oil estimated for NPRA is similar

to that estimated for theANWR study area. However, economic analysis—

which takes into consideration differences in accumulation sizes, the number

of accumulations of various size, and proximity of those accumulations to

infrastructure—shows that for market prices below $35 per barrel, a larger

volume of oil will be economic in theANWR area.

The USGS is solely responsible for the input and results of the 2002

petroleum assessment of NPRA. The USGS acknowledges the cooperation

of the U.S. Fish and Wildlife Service, Bureau of Land Management, Minerals

Management Service,Alaska Department of Natural Resources (Geological

and Geophysical Surveys and Division of Oil and Gas), and Alaska Oil and

Gas Conservation Commission. The USGS provides the Federal Government

with scientifi c information and analyses related to natural resource issues.

USGS research provides a scientifi c basis for policy discussions regarding

land, water, biological, energy, and mineral resources management, environmental quality, and economic and strategic policy. 

Kenneth J. Bird and David W. Houseknecht 

Edited by Peter H. Stauffer and James W. Hendley II 

Layout and design by Sara Boore and Susan Mayfi eld 

96 to 192 Number of Gas Accumulations

10

8

6

4

2

0

96 to 192

192 to 384384 to 768

7 ,1ot86 536

1,536 to 3,0723,072 to 6,144

6,144 82,21ot 8

Gas Accumulation Size Class (billion cubic feet)

NPRA

A

96 to 192 Number of Gas Accumulations

10

8

6

4

2

0

291ot69

192 to 384384 to 768

76 ot8 1,536

1,536 to 3,072

441,6ot270,3

6,144 to 12,288

Gas Accumulation Size Class (billion cubic feet)

NPRA

C

Number of Gas Accumulations

10

8

6

4

2

0

3ot291 84

867ot483

768 to 1,536

1,536 to 3,0723,072 to 6,144

6,144 to 12,288

Gas Accumulation Size Class (billion cubic feet)

NPRA

B

Number of Gas Accumulations

10

8

6

4

2

0

19 3ot2 84

384 to 768

768 to 1,536

1,536 to 3,072

441,6ot270,3

6,144 to 12,288

Gas Accumulation Size Class (billion cubic feet)

NPRA

D

Brookian Clinoform Central Play

Torok Structural Play Brookian Topset Structural Play

Brookian Clinoform South-Deep Play

Figure 8. Maps of petroleum plays with histograms showing the expected (mean) 

numbers of undiscovered petroleum accumulations estimated to exist in various size categories of technically recoverable gas resources in the four plays 

estimated to hold the greatest gas potential in NPRA. About 60% of the technically 

recoverable gas resources, on the basis of the mean estimate, are thought to occur in central and southern NPRA within these four plays.