U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2010–3102

October 2010

National Oil and Gas Assessment Project

2010 Updated Assessment of Undiscovered Oil and Gas 

Resources of the National Petroleum Reserve in Alaska 

(NPRA)

Printed on recycled paper

Introduction

The National Petroleum Reserve in Alaska (NPRA) 

has been the focus of oil exploration during the past decade, 

stimulated by the mid-1990s discovery of the adjacent Alpine 

field—the largest onshore oil discovery in the United States 

during the past 25 years. Recent activities in NPRA, including 

extensive 3–D seismic surveys, six Federal lease sales totaling 

more than $250 million in bonus bids, and completion of more 

than 30 exploration wells on Federal and Native lands, indicate 

in key formations more gas than oil and poorer reservoir quality 

than anticipated. In the absence of a gas pipeline from northern 

Alaska, exploration has waned and several petroleum companies have relinquished assets in the NPRA.

This fact sheet updates U.S. Geological Survey (USGS) 

estimates of undiscovered oil and gas in NPRA, based on 

publicly released information from exploration wells completed 

during the past decade and on the results of research that documents significant Cenozoic uplift and erosion in NPRA. The 

results included in this fact sheet—released in October 2010—

supersede those of a previous assessment completed by the 

USGS in 2002.

Recent Exploration Drilling

Most wells drilled during recent NPRA exploration, whose 

initial drilling season occurred in 2000, are within 50 miles 

of the Alpine oil field and targeted the Alpine sandstone, the 

main reservoir in Alpine field (fig. 1). The stratigraphic interval 

including the Alpine sandstone was assessed as the Beaufortian 

Upper Jurassic play in the USGS 2002 assessment of NPRA.

Five discoveries of oil have been reported in the Alpine 

sandstone in northeastern NPRA (fig. 2). Alpine West, Lookout, 

and Pioneer are oil accumulations with little or no free gas. A 

fourth discovery, Mitre, appears to be predominantly a gas accumulation with an oil leg in the south (fig. 2). The fifth discovery, 

the Spark-Rendezvous accumulation, is a much larger reservoir 

system that includes gas plus condensate at shallower depths in 

the north and oil at greater depths in the south (fig. 2). 

Using a geology-based assessment methodology, the U.S. Geological Survey estimated mean volumes of 896 million barrels of oil 

(MMBO) and about 53 trillion cubic feet (TCFG) of nonassociated natural gas in conventional, undiscovered accumulations within the 

National Petroleum Reserve in Alaska and adjacent State waters. The estimated volume of undiscovered oil is significantly lower than 

estimates released in 2002, owing primarily to recent exploration drilling that revealed an abrupt transition from oil to gas and reduced 

reservoir quality in the Alpine sandstone 15–20 miles west of the giant Alpine oil field.

Significantly, the Spark-Rendezvous accumulation represents an abrupt transition of hydrocarbon phase within the 

Alpine sandstone—from oil on the east to gas on the west. Most 

known or inferred hydrocarbon accumulations west of SparkRendezvous are gas. This transition occurs just 15 to 20 miles 

west of the Alpine oil field and is not related to the structural 

position of the reservoir (fig. 2). Tests of gas flow rates and 

volumes are not known to have been conducted west of the 

Spark-Rendezvous accumulation.

Although the data released are insufficient to make a 

precise calculation of the volume of oil and gas discovered, we 

estimate that 120 to 200 MMBO (including oil and condensate) 

and 1.9 to 3.0 TCFG may be technically recoverable from these 

accumulations. The Spark-Rendezvous accumulation is so large 

that if it were entirely oil, it would rival or exceed the Alpine 

field in recoverable resources.

Several recently drilled wells, most located west and 

northwest from the Spark-Rendezvous accumulation, evaluated 

other stratigraphic intervals, including (from oldest to youngest) the Triassic Ivishak Sandstone (Ellesmerian Ivishak play

in the USGS 2002 assessment of NPRA), lower Kingak Shale 

(Beaufortian Lower Jurassic Topset play), uppermost Kingak 

Shale (Beaufortian Cretaceous Topset play), and the Torok Formation (Brookian Clinoform play). One well in the Ellesmerian 

Ivishak play and two wells in the Beaufortian Cretaceous Topset 

play are dry holes. At least four wells in the Beaufortian Lower 

Jurassic Topset play encountered gas shows but no indications 

of recoverable oil. Two wells in the Brookian Clinoform play 

encountered oil-saturated sandstone with poor reservoir quality 

and were not tested (fig. 2; Kokoda). Also, two wells (Spark 4 

and Hunter; fig. 2) that targeted the Alpine sandstone encountered thin intervals of oil-saturated sandstone in the Brookian 

Topset play. Seven wells drilled west of Spark-Rendezvous 

remain proprietary (figs. 1 and 2); we infer that these are either 

gas shows or dry holes based on the predominance of gas in 

wells lower on structure, relinquished leases, and company 

divestiture of assets.

In southeastern NPRA, one recently drilled well at the 

Wolf Creek gas accumulation (discovered in 1951) remains 
proprietary. Just outside the southeastern boundary of NPRA, 

two wells were drilled recently at the Gubik gas accumulation 

(discovered in 1963) and another well, located near the East 

Umiat gas accumulation (discovered in 1964), was drilled to 

a deeper target. One of the recent Gubik wells indicates producible gas. The other recent Gubik well and the deeper test 

at East Umiat remain proprietary. These wells drilled in the 

Brooks Range foothills add no information to indicate the need 

for updating the 2002 USGS assessment of plays in southern 

NPRA.

Cenozoic Uplift in NPRA

Cenozoic uplift and erosion (from about 60 to 15 million 

years ago) of the Brooks Range foothills in southern NPRA 

and of a broad area in northern NPRA (fig. 1) may provide an 

explanation for the unanticipated predominance of gas in the 

Alpine sandstone and other strata. The magnitude and timing of 

uplift and erosion have been estimated by using sonic logs from 

Figure 1. National Petroleum Reserve in Alaska (NPRA; boundary shown by white line) with locations of exploration wells, 

oil and gas accumulations known prior to 2000 (names in yellow; all too small to be economically viable except Alpine), and 

Native lands. Total area assessed includes NPRA and adjacent State waters. White contours indicate amount of Cenozoic 

exhumation (in feet) estimated by analysis of sonic logs from exploration wells (source: http://pubs.usgs.gov/pp/pp1732/

pp1732d/). Yellow rectangle is area shown in figure 2. Map base is a false-color composite Landsat image.

exploration wells (fig. 1) and additional geological evidence. 

The uplift and erosion caused a decrease in confining pressure 

on subsurface fluids, resulting in expansion of free gas in reservoirs, degassing of oil in reservoirs, and degassing of formation 

water.

Oil generation and most gas generation were complete 

across NPRA by about 90 million years ago. Thus, at the time 

of Cenozoic uplift and erosion, it is likely that oil was present in 

various reservoirs in northern NPRA (lower thermal maturity) 

and that gas was present in various reservoirs in southern NPRA 

(higher thermal maturity). In areas of modest uplift in northeastern NPRA, degassing of oil accumulations likely generated gas 

caps and displaced oil downward into poorer quality reservoir 

rocks (for example, Spark-Rendezvous). In areas of moderate 

uplift in northwestern NPRA and significant uplift in southern 

NPRA, degassing of oil combined with significant gas expansion likely caused a gas “flush” outward, away from the uplifted 

areas. Oil degassing and gas expansion likely explain the abrupt 

oil-to-gas transition in the Alpine sandstone in northeastern 

AK

0 50 MILES

0 50 KILOMETERS

Native land

NPRA boundary

Federal - State

7500

5000

2500

2500

Contours are estimated amount

of Cenozoic erosion (feet)

Alpine

Gubik

E. Umiat

Umiat

Walakpa Sikulik

Square

Lake

Wolf

Creek

E. Kurupa

Meade

Oil Gas

Accumulations known

prior to 2000

Simpson

Fish

Creek

S. Barrow E. Barrow

Exploration wells

 NPRA and selected

 others 2000–2010

 Proprietary July 2010

 Others 68°

69°

70°

71°

160°

° 5 5 1

Fig. 2
NPRA. Significantly, evidence of oil saturation in Brookian 

stratigraphic plays west of the oil-to-gas transition (fig. 2) suggests that Brookian rocks may have been shielded from these 

processes and may retain potential for undiscovered oil across 

northern NPRA.

Results of Updated Assessment

The updated 2010 assessment of NPRA used the same 24 

plays and methodology as the 2002 assessment. Quantitative 

assessments were revised for 11 plays, and 9 of the 2002 plays 

are unchanged (table 1). Four plays were eliminated from quantitative assessment because results of recent drilling indicate 

that reservoir quality is insufficient for conventional petroleum 

accumulations (table 1); these have potential for unconventional 

resources—probably as “tight gas sandstone” plays.

Results of this 2010 updated assessment include total mean 

values for fully risked, undiscovered, technically recoverable 

resources in conventional accumulations of 896 MMBO and 

7200

7100

7400

7600

7800

8000

8200

8400

8600

Contours are structure

at approximate top

of Alpine sandstone

(feet below sea level)

153°W 152°W 151°W

70°30’N

70°N

Native

NPRA

boundary

land

Alpine sandstone truncation

Colville

River

delta

Federal

State

Oil Gas

Announced:

Inferred:

Discovered accumulations

in Alpine sandstone

Alpine:

Other:

Producing oil pools

in Alpine field

Main reservoir

Exploration wells

 NPRA and selected

 others 2000–2010

 Proprietary July 2010

 Others

Results outside of accumulations

Oil show Gas show Dry hole

Alpine

Nanuk

Alpine

Lookout West

Mitre

Pioneer

Spark -

Rendezvous

Scout

Hunter

Kokoda 5

10 MILES

0

0

10 KILOMETERS

Fish

Creek

Spark 4

Kokoda 1

Figure 2. Northeastern NPRA and adjacent Colville River delta with locations of exploration wells within NPRA, selected wells 

outside NPRA, inferred oil and gas accumulations discovered in NPRA during 2000–2010 (yellow labels), and Native lands (white 

shading). Accumulations confirmed by public announcements of production tests are indicated by yellow dashed outlines; 

accumulations inferred from our analysis of public data are indicated by yellow dotted outlines. Pioneer has been confirmed 

by announcement of a production test even though no well data have been released. Specific exploration wells discussed in 

text are labeled in white. Alpine field oil pools that extend into NPRA and that produce from the Alpine sandstone and from 

other reservoirs are labeled “Alpine” and “Nanuk,” respectively; these pools have been developed with numerous vertical 

and horizontal wells, which are not shown. The Alpine West accumulation has been confirmed by horizontal wells drilled from 

outside NPRA. No information is publicly available for the wells labeled by symbols indicating “proprietary.” Fish Creek is a small 

oil accumulation in Brookian topsets discovered in 1949 and is considered subeconomic.

52,839 BCFG (nonassociated gas only). The sum of undiscovered oil resources (896 MMBO) plus liquids (oil and condensate) estimated to have been discovered during 2000–2010 (120 

to 200 MMBO) is only about 10 percent of the total undiscovered oil estimated by the 2002 assessment (10,560 MMBO; 

table 1). This striking reduction reflects the results of exploration drilling, specifically (1) the unanticipated and abrupt transition from oil to gas just 15–20 miles west of the Alpine oil field 

and (2) poor reservoir quality in key formations.

The sum of undiscovered nonassociated gas resources 

(52,839 BCFG) plus gas resources estimated to have been discovered during 2000–2010 (1,900 to 3,000 BCFG) is approximately 90 percent of the total undiscovered nonassociated gas 

estimated by the 2002 assessment (61,352 BCFG; table 1). 

This slight reduction mostly reflects (1) the elimination of four 

gas-prone plays from quantitative assessment as conventional 

resources, (2) a substantial shift of resources from oil to gas 

in Beaufortian stratigraphic plays, and (3) a moderate shift 
of resources from oil to gas in Brookian stratigraphic plays 

(table 1).

The results of the 2010 updated assessment indicate that 

the largest potential for undiscovered oil resources occurs in 

Brookian stratigraphic plays (table 1), which are widely distributed across NPRA. Beaufortian stratigraphic plays, which previously were considered to hold the greatest oil potential, are now 

thought to have limited undiscovered oil potential. The largest 

potential for undiscovered oil lies in northeastern NPRA, generally in the area estimated to have undergone less than 2,500 feet 

of Cenozoic uplift (fig. 1).

The largest potential for undiscovered nonassociated gas 

resources is in structural plays in southern NPRA. Also, significant potential for nonassociated gas resources is now shifted to 

Beaufortian and Brookian stratigraphic plays in northern NPRA, 

which previously were considered primarily oil-prone (table 1).

F95 F50 F05 Mean F95 F50 F05 Mean Oil* Gas**

 Brookian Topset 21 107 249 117 0 300 1,079 386 239 192

 Brookian Clinoform North 49 241 557 265 0 870 2,502 1,004 1,306 674

 Brookian Clinoform Central 0 104 352 129 0 1,926 5,235 2,215 973 5,405

 Brookian Clinoform South-Shallow 0 102 346 127 0 1,988 5,201 2,253 508 2,405

 Brookian Clinoform South-Deep 0 0 0 0 0 3,707 8,796 3,788 0 3,788

 Beaufortian Cretaceous Topset North 0 0 48 8 0 592 1,685 670 103 405

 Beaufortian Upper Jurassic Topset NE 0 26 80 33 829 2,415 5,254 2,638 5,176 2,130

 Beaufortian Upper Jurassic Topset NW 0 0 0 0 251 967 2,211 1,047 1,859 0

 Beaufortian Lower Jurassic Topset 0 0 0 0 866 4,074 9,839 4,552 83 5,137

 Beaufortian Cretaceous Topset South 0 0

 Beaufortian Upper Jurassic Topset SE 0 5,220

 Beaufortian Upper Jurassic Topset SW 0 793

 Beaufortian Clinoform 12 822

 Ellesmerian Ivishak 0 0 91 21 0 319 1,237 416 84 106

 Ellesmerian Echooka North 0 0 0 7 0 0 0 7 7 7

 Ellesmerian Echooka South 0 0 0 0 0 0 2,087 505 0 505

 Ellesmerian Lisburne North 0 0 60 8 0 0 938 146 29 22

 Ellesmerian Lisburne South 0 0 0 0 0 0 2,732 646 0 646

 Ellesmerian Endicott North 0 0 0 3 0 0 0 1 3 1

 Ellesmerian Endicott South 0 0 0 0 0 0 2,627 544 0 1,072

 Brookian Topset Structural 0 0 654 137 4,801 10,119 18,012 10,606 137 10,606

 Torok Structural 0 0 222 35 0 17,769 35,012 17,905 35 17,905

 Ellesmerian Structural 0 0 0 0 0 1,613 5,675 1,990 0 1,990

 Thrust Belt 0 0 43 6 0 1,327 4,240 1,521 6 1,521

Total Conventional Resources 896 52,839 10,560 61,352

2010 Quantitative Assessment Update

Nonassociated Gas (BCFG)

Brookian Beaufortian Ellesmerian Structural

Brookian Beaufortian Ellesmerian Structural

The presence of reservoir quality sufficient for conventional accumulations is 

considered unlikely based on recent drilling results in northern NPRA.

2010 No Change from 2002 2010 No Quantitative Assessment

2002 Mean

*MMBO; **BCFG

STRUCTURAL PLAYS

ELLESMERIAN STRATIGRAPHIC PLAYS

BEAUFORTIAN STRATIGRAPHIC PLAYS

BROOKIAN STRATIGRAPHIC PLAYS

OIL (MMBO)

Table 1. NPRA 2010 updated assessment results, including estimated volumes of fully risked, undiscovered, technically recoverable oil 

and nonassociated gas. Play-level results are color coded as follows: yellow, quantitative assessment revised in current assessment; blue, 

quantitative assessment not revised from 2002 results; tan, eliminated from quantitative assessment because the presence of reservoir 

quality sufficient for conventional petroleum accumulations is considered unlikely. For comparison, mean resource estimates from 2002 

assessment are shown in gray columns at right. [MMBO, million barrels of oil; BCFG, billion cubic feet of gas; F95 represents a 95-percent 

chance of at least the amount tabulated; other fractiles are defined similarly]

For Further Information

Supporting geological reports on the 2010 updated assessment of NPRA will be released as completed at http://energy.

cr.usgs.gov/oilgas/noga.

Information regarding the 2002 USGS assessment of 

NPRA, including complete play descriptions and maps, is available at http://energy.usgs.gov/alaska/npra.html.

NPRA Assessment Team: 

David W. Houseknecht (Project Chief; dhouse@usgs.gov), 

Kenneth J. Bird, John H. Schuenemeyer, Emil D. Attanasi, 

Christopher P. Garrity, Christopher J. Schenk, Ronald R. 

Charpentier, Richard M. Pollastro, Troy A. Cook, and 

Timothy R. Klett.