U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2017–3088

December 2017

National and Global Petroleum Assessment

Assessment of Undiscovered Oil and Gas Resources in the Cretaceous 

Nanushuk and Torok Formations, Alaska North Slope, and Summary of 

Resource Potential of the National Petroleum Reserve in Alaska, 2017

Figure 1. Map of Northern Alaska showing boundaries of the six assessment units (AUs) 

in the National Petroleum Reserve in Alaska (NPR-A) and adjacent areas. Assessment units 

include State waters. Older AUs assessed in 2010 are not shown. (ANWR, Arctic National 

Wildlife Refuge; H, Horseshoe; P, Pikka; S, Smith Bay; U, Umiat; W, Willow) 

Introduction

Recent exploration drilling has resulted 

in significant oil discoveries in the Cretaceous 

Nanushuk and Torok Formations in and near the 

National Petroleum Reserve in Alaska (NPR-A). 

These new discoveries have spurred exploration activity in the region and have increased 

demand for enhanced information regarding the 

oil-resource potential of these formations.

New U.S. Geological Survey (USGS) estimates of undiscovered, technically recoverable 

oil and gas resources in conventional accumulations across the entire onshore and State-waters 

extent of the Nanushuk and Torok Formations 

update previous assessments for these two 

formations (Bird and others, 2005; Houseknecht 

and others, 2010). This fact sheet also includes 

a summary of 2010 assessment results for older 

rocks in the NPR-A; no updated assessment 

was conducted as those rocks have not been 

penetrated by exploration drilling since 2010, 

and thus no new information is available about 

their oil and gas potential.

Recent Oil Discoveries

Prior to 2015, about 150 exploration 

wells had penetrated the Nanushuk and Torok 

Formations, yet oil production had been 

established in just one small oil pool (less than 

10 million barrels of recoverable oil [MMBO]) 

in each formation. The potential for undiscovered oil accumulations in both formations was 

recognized in previous USGS assessments, but 

the size of undiscovered pools was considered 

to be modest based on historical exploration 

results (Houseknecht, 2003).

Multiple oil discoveries announced during 

2015–2017 indicate that the potential for large oil 

accumulations in both formations is greater than 

previously thought. Discovery of the Pikka oil 

pool in the Nanushuk Formation was announced 

The U.S. Geological Survey estimated mean undiscovered, technically recoverable resources of 8.7 billion barrels of oil and 

25 trillion cubic feet of natural gas (associated and nonassociated) in conventional accumulations in the Cretaceous Nanushuk and 

Torok Formations in the National Petroleum Reserve in Alaska, adjacent State and Native lands, and State waters. The estimated 

undiscovered oil resources in the Nanushuk and Torok Formations are significantly higher than previous estimates, owing primarily to 

recent, larger than anticipated oil discoveries.

in 2015 and followed by the 2017 apparent confirmation of an extension of the same 

oil pool at Horseshoe, 21 miles south of the Pikka discovery (fig. 1). Industry estimates 

suggest that the Pikka-Horseshoe pool may hold more than 1,000 MMBO. Discovery 

of the Willow oil pool (fig. 1) in the Nanushuk Formation was announced in 2017, with 

estimated resources of more than 300 MMBO.

ALASKA

Prudhoe Bay

BROOKS RANGE

ARCTIC OCEAN

P

H

W

U

S

CENTRAL ALASKA PROVINCE

NORTHERN ALASKA PROVINCE

Base map from U.S. Department of the Interior National Park Service

–170°

72°

70°

68°

–166° –162° –158° –154° –150° –146°

EXPLANATION

H

Nanushuk South AU

Nanushuk Northwest AU

Nanushuk Northeast AU

Torok South AU

Torok Northwest AU

Torok Northeast AU

Oil discovery and identifier

ANWR boundary

Gates of the Arctic National

 Park and Preserve boundary

NPR-A boundary

Alaska Provinces

ALASKA

RUSSIA

CANADA

BERING

SEA

ARCTIC OCEAN

GULF

OF

ALASKA

Map

area

0 50 100 150 KILOMETERS

0 50 100 150 MILES
A significant oil discovery in 

the deeper Torok Formation was 

announced in 2016 at Smith Bay, less 

than 1 mile offshore from the NPR-A 

(fig. 1). The industry announcement

of the discovery suggests that the 

Smith Bay oil pool may contain more 

than 1,000 MMBO.

The ultimate volume of oil that 

may be produced from each of these 

discoveries will remain uncertain until 

development drilling and oil production 

are initiated. Nevertheless, a sufficient 

amount of data is available to confirm 

that the potential size of oil pools in 

the Nanushuk and Torok Formations is 

larger than previously thought. Moreover, 

availability to the USGS assessment team 

of three-dimensional (3D) seismic reflection 

data in the northeastern NPR-A and adjacent 

State lands and waters provided a capability 

to estimate the number and size of seismic 

amplitude anomalies, which are considered 

viable proxies for undrilled prospects.

Geological Framework and 

Definition of Assessment Units

The Nanushuk and Torok Formations together form a huge 

wedge of sediment deposited in a deep water basin and are preserved north of the Brooks Range beneath the Alaska North Slope 

and adjacent offshore. The Torok Formation was deposited on 

the floor of the deep basin and on a shallowing-upward slope and 

includes potential reservoirs in turbidite sandstones. The Nanushuk 

Formation was deposited in shallow water and includes potential 

reservoirs in deltaic, shoreface, and fluvial sandstones. The largescale geometry of these sedimentary deposits is preserved in the 

rock formations and imaged by seismic reflection data (fig. 2).

The new discoveries in both formations involve oil pools 

in stratigraphic traps concentrated along ancient shelf margins. 

Stratigraphic traps display subtle geometries that are challenging to recognize using seismic-reflection data. However, 

application of innovative 3D seismic techniques improves 

exploration success rates, as indicated by the recent Nanushuk 

and Torok discoveries.

The USGS used vintage two-dimensional (2D) and recent 

3D seismic data, where available, and data from exploration wells 

and outcrops to map the distribution and internal geometry of 

the Nanushuk and Torok Formations. This research resulted in 

the definition of three assessment units (AUs) in each formation 

(fig. 1). The Nanushuk Northeast and Torok Northeast AUs are 

defined by the geometry of a distinctive boundary between the 

two formations (shelf-margin trajectory) (Houseknecht and others, 

2009) that suggests favorable conditions for stratigraphic traps to 

have formed during deposition. In contrast, the geometry of the 

shelf-margin trajectory is considered less favorable for the development of stratigraphic traps in the western part of the study area, 

resulting in the Nanushuk Northwest and Torok Northwest AUs 

(fig. 1). The Nanushuk South and Torok South AUs are defined by 

the presence of large-scale folds and faults and by evidence that 

the rocks have been heated to temperatures at which oil is converted to natural gas. These geological contrasts between the two 

Figure 2. Segment of two-dimensional seismic reflection image showing the Nanushuk 

and Torok Formations. Seismic reflections in the lower part of the Torok Formation are nearly 

parallel to the base of the formation (strata deposited on floor of the basin) and in the upper 

part are inclined to the east (strata deposited on slope between deep and shallow water). 

Seismic reflections in the Nanushuk Formation are approximately horizontal (strata deposited 

in shallow water, including deltas and rivers). Stratigraphic traps mainly occur near the 

base of the Nanushuk Formation (bracket labeled N) and in the lower one-third of the Torok 

Formation (bracket labeled T). (AU, assessment unit)

Eastern boundary of

Nanushuk Northeast AU

0

APPROXIMATE

THICKNESS

FEET

1,000

2,000

3,000

4,000

5,000

6,000

7,000

8,000

0

APPROXIMATE

THICKNESS

FEET

1,000

2,000

3,000

4,000

5,000

6,000

7,000

8,000

WEST EAST

T

N

Approximate Scale

VERTICAL EXAGGERATION X7.7

DATUM IS FLATTENED HORIZON

IN UPPER CRETACEOUS STRATA

0 2 4 6 8 10 KILOMETERS

0 2 4 6 8 10 MILES

EXPLANATION

Nanushuk Formation

Torok Formation

formations and among the northeast, northwest, and south areas 

were used to define key assessment input data (table 1).

The four northern AUs are inferred to contain mainly stratigraphic traps, with fewer combination stratigraphic-structural 

traps. The two northeastern AUs are inferred to contain mainly 

oil-charged traps with fewer gas-charged traps, mainly near the 

southern boundary. The two northwestern AUs are inferred to 

contain oil-charged traps and a larger proportion of gas-charged 

traps because of source-rock character and evidence for a northward gas flush during Late Cretaceous and early Cenozoic uplift 

in the northwestern NPR-A (Houseknecht and others, 2010).

The two southern AUs are inferred to contain mainly structural traps, with fewer combination structural-stratigraphic traps. 

These two AUs are considered to contain mainly gas-charged 

traps because of elevated thermal maturity, with a small number 

of oil-charged traps. The potential for oil-charged structural traps 

is demonstrated by the Umiat oil pool (fig. 1), discovered in an 

anticlinal trap in 1946 and estimated to hold more than 70 MMBO 

(never produced).

Sandstone reservoirs in both the Nanushuk and Torok 

Formations are mainly fine grained and prone to reduction of porosity and permeability with burial. Deeper parts of the assessment 

units, therefore, may have reduced reservoir quality that jeopardizes 

the economic viability of oil accumulations. This issue is more 

likely to affect Torok reservoirs because they are buried 2,000–

4,000 feet deeper than Nanushuk reservoirs in the same area (fig. 2).

Undiscovered Resources Summary

Assessed mean resources for the six AUs in the Nanushuk 

and Torok Formations are 8,727 MMBO, or 8.7 billion barrels 

of oil, and 24,545 billion cubic feet of recoverable gas (BCFG), 

or 25 trillion cubic feet of recoverable gas, which includes 

6,992 BCFG of associated gas and 17,553 BCFG of nonassociated gas (table 2). All these estimates are associated with large 

ranges of uncertainty (table 2, range of results between F95 

and F5), a reflection of the immature stage of exploration for 

Eastern boundary of

Nanushuk Northeast AU

0

APPROXIMATE

THICKNESS

FEET

1,000

2,000

3,000

4,000

5,000

6,000

7,000

8,000

0

APPROXIMATE

THICKNESS

FEET

1,000

2,000

3,000

4,000

5,000

6,000

7,000

8,000

WEST EAST

T

N

Approximate Scale

VERTICAL EXAGGERATION X7.7

DATUM IS FLATTENED HORIZON

IN UPPER CRETACEOUS STRATA

0 2 4 6 8 10 KILOMETERS

0 2 4 6 8 10 MILES

EXPLANATION

Nanushuk Formation

Torok Formation
stratigraphic traps in these formations. The assessed resources 

are also not equally distributed, either spatially or by formation. 

The largest undiscovered oil resources are assessed in the 

two northeastern AUs. The Nanushuk Northeast AU is estimated 

to hold between 1,271 and 11,162 MMBO (F95–F5) with a 

mean of 4,862 MMBO. The Torok Northeast AU is estimated to 

hold between 466 and 5,417 MMBO (F95–F5) with a mean of 

2,177 MMBO. Assessed oil resources are smaller in the northwest 

AUs. The Nanushuk Northwest AU is estimated to hold between 

0 and 3,152 MMBO (F95–F5) with a mean of 1,146 MMBO. The 

Torok Northwest AU is estimated to hold between 0 and 1,379 

MMBO (F95–F5) with a mean of 421 MMBO. Assessed oil 

resources are smallest in the southern AUs. The Nanushuk South 

AU is estimated to hold between 6 and 266 MMBO (F95–F5) with 

a mean of 71 MMBO. The Torok South AU is estimated to hold 

between 11 and 161 MMBO (F95–F5) with a mean of 50 MMBO. 

Assessed gas resources are largest in the Nanushuk Northeast AU, with an estimated mean total for associated plus nonassociated gas of 8,689 BCFG. Assessed mean estimates for gas 

resources are substantially smaller in the other AUs (table 2). 

All six of the Nanushuk and Torok AUs are assessed to contain 

natural gas liquids (NGL) associated with both oil and gas accumulations, with the Nanushuk Northeast AU estimated to contain 

mean resources of 74 million barrels of NGL (table 2).

Table 2 includes a summary of results from the 2010 assessment of rocks older than the Torok Formation in the NPR-A, 

which were estimated to hold 86 MMBO and 14,732 BCFG 

(Houseknecht and others, 2010). When those assessed values are 

added to the new results from the Nanushuk and Torok AUs, the 

oil estimate ranges from 1,754 to 21,859 MMBO (F95–F5) with 

a mean of 8,813 MMBO, and the total gas (associated plus nonassociated) estimate ranges from 5,507 to 112,932 BCFG (F95–F5) 

with a mean of 39,277 BCFG (about 39 trillion cubic feet of gas).

Acknowledgments

The USGS acknowledges the Bureau of Land Management, 

Alaska Region, and the Bureau of Ocean Energy Management, 

Alaska Outer Continental Shelf Region, for outstanding collaboration in reviewing 3D seismic and other data and in discussing 

geological constraints for the Nanushuk and Torok AUs.

Table 1. Key input data for six conventional assessment units in the Nanushuk and Torok Formations in the National Petroleum Reserve in 

Alaska and adjacent areas. 

[AU, assessment unit; MMBO, million barrels of recoverable oil; BCFG, billion cubic feet of recoverable gas; km2, square kilometer. Shading indicates not applicable]

Assessment input data Nanushuk Northeast AU Nanushuk Northwest AU

Minimum Median Maximum Calculated mean Minimum Median Maximum Calculated mean

Number of oil fields 10 60 325 69.4 5 35 200 40.8

Number of gas fields 1 10 50 11.4 3 15 100 18.0

Sizes of oil fields (MMBO) 5 15 5,000 70.0 5 8 2,500 31.3

Sizes of gas fields (BCFG) 30 90 30,000 420.0 30 48 15,000 187.5

AU probability 1.0 0.9

AU area (km2) 29,874 23,803

Assessment input data Nanushuk South AU Torok Northeast AU

Minimum Median Maximum Calculated mean Minimum Median Maximum Calculated mean

Number of oil fields 1 2 10 2.3 4 40 200 45.7

Number of gas fields 5 20 100 22.8 1 8 40 9.1

Sizes of oil fields (MMBO) 5 8 2,500 31.3 5 10 4,000 47.7

Sizes of gas fields (BCFG) 30 48 15,000 187.5 30 60 24,000 286.4

AU probability 1.0 1.0

AU area (km2) 43,861 37,354

Assessment input data Torok Northwest AU Torok South AU

Minimum Median Maximum Calculated mean Minimum Median Maximum Calculated mean

Number of oil fields 2 15 75 17.1 1 3 15 3.4

Number of gas fields 1 5 25 5.7 5 20 100 22.8

Sizes of oil fields (MMBO) 5 7 2,500 27.4 5 6 1,000 14.8

Sizes of gas fields (BCFG) 30 47 15,000 164.7 30 36 6,000 88.5

AU probability 0.9 1.0

AU area (km2) 23,832 78,372

References Cited

Bird, K.J., Houseknecht, D.W., Attanasi, E.D., Moore, T.E., Nelson, 

P.H., Potter, C.J., Schenk, C.J., Schuenemeyer, J.H., Verma, 

M.K., Saltus, R.W., Phillips, J.D., Charpentier, R.R., Cook, T.A., 

Klett, T.R., and Pollastro, R.M., 2005, Oil and gas assessment 

of central North Slope, Alaska, 2005: U.S. Geological Survey 

Fact Sheet 2005–3043, 2 p., accessed December 7, 2017, at 

http://pubs.usgs.gov/fs/2005/3043/fs2005-3043.pdf.

Houseknecht, D.W., 2003, Brookian stratigraphic plays in the 

National Petroleum Reserve-Alaska (NPRA): U.S. Geological 

Survey Open-File Report 03–039, 34 p. plus tables and figures, 

accessed December 7, 2017, at http://pubs.usgs.gov/of/2003/

of03-039/.

Houseknecht, D.W., Bird, K.J., and Schenk, C.J., 2009, Seismic 

analysis of clinoform depositional sequences and shelf-margin 

trajectories in Lower Cretaceous (Albian) strata, Alaska North 

Slope: Basin Research, v. 21, no. 5, p. 644–654.

Houseknecht, D.W., Bird, K.J., Schuenemeyer, J.H., Attanasi, 

E.D., Garrity, C.P., Schenk, C.J., Charpentier, R.R., Pollastro, 

R.M., Cook, T.A., and Klett, T.R., 2010, 2010 updated assessment of undiscovered oil and gas resources of the National 

Petroleum Reserve in Alaska (NPRA): U.S. Geological Survey 

Fact Sheet 2010–3102, 4 p., accessed December 7, 2017, at 

https://pubs.usgs.gov/fs/2010/3102/pdf/FS10-3102.pdf.
ISSN 2327-6916 (print) ISSN 2327-6932 (online) https://doi.org/10.3133/fs20173088

Nanushuk–Torok Assessment Team

David W. Houseknecht, Richard O. Lease, Christopher J. Schenk, Tracey J. Mercier, William A. Rouse, Palma J. Jarboe, Katherine 

J. Whidden, Christopher P. Garrity, Kristen A. Lewis, Samuel J. Heller, William H. Craddock, Timothy R. Klett, Phuong A. Le, Rebecca

A. Smith, Marilyn E. Tennyson, Stephanie B. Gaswirth, Cheryl A. Woodall, Michael E. Brownfield, Heidi M. Leathers-Miller, and

Thomas M. Finn

For More Information

Assessment results are also available at the USGS Energy Resources Program website at https://energy.usgs.gov.

Table 2. New assessment results for six conventional assessment units in the Nanushuk and Torok Formations in the National Petroleum 

Reserve in Alaska and adjacent areas. Results of 2010 assessment for conventional oil and gas resources in older rocks in the NPR-A are 

also included for completeness (Houseknecht and others, 2010).

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

risked estimates. For gas accumulations, all liquids are included in the NGL category. 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. Shading indicates not applicable]

Assessment units (AUs)

AU

probability

Accumulation

type

Total undiscovered resources

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

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

2017 Nanushuk and Torok Assessment Units

Nanushuk Northeast AU 1.0 Oil 1,271 4,042 11,162 4,862 945 3,164 9,304 3,884 9 32 95 39

Gas 553 2,998 15,538 4,805 4 22 115 35

Nanushuk Northwest AU 0.9 Oil 0 878 3,152 1,146 0 688 2,574 917 0 7 25 9

Gas 0 1,940 9,923 3,029 0 14 74 22

Nanushuk South AU 1.0 Oil 6 26 266 71 5 20 212 57 0 0 2 1

Gas 873 3,098 11,901 4,264 6 23 93 33

Torok Northeast AU 1.0 Oil 466 1,720 5,417 2,177 348 1,342 4,479 1,738 5 20 71 27

Gas 277 1,446 8,887 2,590 3 15 100 29

Torok Northwest AU 0.9 Oil 0 267 1,379 421 0 210 1,121 336 0 3 18 5

Gas 0 388 3,148 845 0 4 35 9

Torok South AU 1.0 Oil 11 27 161 50 10 32 196 60 0 0 2 1

Gas 550 1,539 5,220 2,020 4 12 41 16

Subtotals 1,754 6,960 21,537 8,727 3,561 16,865 72,503 24,545 31 152 671 226

2010 Beaufortian and Ellesmerian Assessment Units

Beaufortian Cretaceous

Topset North AU 0.9 Oil 0 0 48 8 0 0 36 6 0 0 1 0

Gas 0 592 1,685 670 0 6 18 7

Beaufortian Upper Jurassic 

Topset Northeast AU 1.0 Oil 0 26 80 33 0 28 82 34 0 0 1 0

Gas 829 2,415 5,254 2,638 8 23 50 25

Beaufortian Upper Jurassic 

Topset Northwest AU 1.0 Oil 0 0 0 0 0 0 0 0 0 0 0 0

Gas 251 967 2,211 1,047 3 12 27 13

Beaufortian Lower Jurassic 

Topset AU 1.0 Oil 0 0 0 0 0 0 0 0 0 0 0 0

Gas 866 4,074 9,839 4,552 8 39 95 44

Ellesmerian Ivishak AU 0.9 Oil 0 0 91 21 0 0 60 14 0 0 1 0

Gas 0 319 1,237 416 0 4 15 5

Ellesmerian Echooka

North AU 0.2 Oil 0 0 0 7 0 0 0 5 0 0 0 0

Gas 0 0 0 7 0 0 0 0

Ellesmerian Echooka

South AU 0.48 Oil 0 0 0 0 0 0 0 0 0 0 0 0

Gas 0 0 3,211 505 0 0 58 14

Ellesmerian Lisburne

North AU 0.54 Oil 0 0 60 8 0 0 44 6 0 0 1 0

Gas 0 0 938 146 0 0 12 2

Ellesmerian Lisburne

South AU 0.54 Oil 0 0 0 0 0 0 0 0 0 0 0 0

Gas 0 0 2,732 646 0 0 81 19

Ellesmerian Endicott

North AU 0.54 Oil 0 0 0 3 0 0 0 2 0 0 0 0

Gas 0 0 0 1 0 0 0 0

Ellesmerian Endicott 

South AU 0.6 Oil 0 0 0 0 0 0 0 0 0 0 0 0

Gas 0 0 3,156 522 0 0 41 7

Ellesmerian Structural AU 0.81 Oil 0 0 0 0 0 0 0 0 0 0 0 0

Gas 0 1,613 5,675 1,990 0 63 226 78

Thrust Belt AU 0.72 Oil 0 0 43 6 0 0 29 4 0 0 1 0

Gas 0 1,327 4,240 1,521 0 42 140 49

Subtotals 0 26 322 86 1,946 11,335 40,429 14,732 19 189 768 263

Total undiscovered 

conventional resources 1,754 6,986 21,859 8,813 5,507 28,200 112,932 39,277 50 341 1,439 489