U.S. Gulf Coast Petroleum Systems Project

Assessment of Undiscovered Oil and Gas Resources in the Upper Cretaceous 

Austin Chalk and Tokio and Eutaw Formations, U.S. Gulf Coast, 2019

Using a geology-based assessment methodology, the U.S. Geological Survey estimated undiscovered, technically recoverable mean resources

of 6.9 billion barrels of oil and 41.5 trillion cubic feet of natural gas in conventional and continuous accumulations in the Upper Cretaceous 

Austin Chalk and Tokio and Eutaw Formations onshore and in State waters of the U.S. Gulf Coast region.

Introduction

The U.S. Geological Survey (USGS) assessed undiscovered, 

technically recoverable oil and gas in the Upper Cretaceous 

(Coniacian–Santonian) Austin Chalk and Tokio and Eutaw Formations in the subsurface of the Gulf Coast from southern Texas to 

the Florida Panhandle. The Austin Chalk and related stratigraphic 

units present onshore and in State waters of the U.S. Gulf Coast 

region are part of the Upper Jurassic–Cretaceous–Tertiary Composite 

Total Petroleum System (TPS) (Condon and Dyman, 2006).

Geologic Models for Assessment

The Austin Chalk, consisting of both chalk and marl, was 

deposited on a broad, low-relief marine shelf that deepened to the 

south and west during the Coniacian–Santonian marine transgression. 

Landward, to the northeast, the Austin Chalk transitions into sandstone 

and mudstone of the Tokio Formation in Arkansas and Louisiana and the 

Eutaw Formation in Mississippi and Alabama. The Tokio and Eutaw 

Formations were deposited in shallow to marginal marine depositional 

environments at the leading edge of the marine transgression. The 

source of hydrocarbons in Austin Chalk, Tokio, and Eutaw reservoirs 

varies spatially throughout the onshore Gulf Coast. Most of the 

hydrocarbons in the Austin Chalk are found in Texas and sourced 

from immediately underlying, thermally mature mudstone in the 

Figure 1. Map showing the Upper Jurassic–Cretaceous–Tertiary 

Composite Total Petroleum System and five conventional assessment 

units (AUs) in the Upper Cretaceous Austin Chalk and Tokio and Eutaw 

Formations of the U.S. Gulf Coast region.

ALABAMA

ARKANSAS

FLORIDA

LOUISIANA

MISSISSIPPI

OKLAHOMA

TEXAS

MEXICO

UNITED STATES

WESTERN GULF

BASIN PROVINCE 

GULF OF MEXICO

LOUISIANA-MISSISSIPPI

SALT BASIN PROVINCE 

EAST TEXAS

BASIN PROVINCE

GEORGIA

100° 98° 96° 94° 92° 90° 88° 86°

34°

32°

30°

28°

26°

EXPLANATION

Western Updip Fault Zone 

Conventional Oil AU

Eastern Updip Fault Zone 

Conventional Oil AU

Updip Salt Basins 

Conventional Oil and Gas AU

Middip Fault Zone and Salt 

Conventional Oil AU

Downdip Salt Basins 

Conventional Gas AU

Upper Jurassic–Cretaceous–Tertiary 

Composite Total Petroleum 

System (part)

0 100 300 KILOMETERS

0 100 200 300 MILES

200

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

TX

MEX

MEX

NM

OK

GA AL

AR

MS

TN

SC

FL LA

NC

Map GULF OF MEXICO

area

WESTERN

GULF BASIN

PROVINCE 

USA

EAST TEXAS

BASIN

PROVINCE

LOUISIANA-MISSISSIPPI

SALT BASIN PROVINCE KY WVVA

Figure 2. Map showing the Upper Jurassic–Cretaceous–Tertiary 

Composite Total Petroleum System and three continuous assessment 

units (AUs) in the Upper Cretaceous Austin Chalk of the U.S. Gulf 

Coast region.

ALABAMA

ARKANSAS

FLORIDA

LOUISIANA

MISSISSIPPI

OKLAHOMA

TEXAS

MEXICO

UNITED STATES

WESTERN

GULF BASIN

PROVINCE 

GULF OF MEXICO

LOUISIANA-MISSISSIPPI

SALT BASIN PROVINCE 

EAST TEXAS

BASIN

PROVINCE

GEORGIA

100° 98° 96° 94° 92° 90° 88° 86°

34°

32°

30°

28°

26°

EXPLANATION

Western Shelf Continuous Oil AU

Eastern Shelf Continuous Oil AU

Western Shelf Edge Continuous Gas AU

Upper Jurassic–Cretaceous–Tertiary 

Composite Total Petroleum 

System (part)

0 100 300 KILOMETERS

0 100 200 300 MILES

200

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

TX

MEX

MEX

NM

OK

GA AL

AR

MS

TN

SC

FL LA

NC

Map GULF OF MEXICO

area

WESTERN

GULF BASIN

PROVINCE 

USA

EAST TEXAS BASIN

PROVINCE

LOUISIANA-MISSISSIPPI

SALT BASIN PROVINCE KY WVVA

U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2020–3045

ver. 1.1, December 2020
Upper Cretaceous (Cenomanian–Turonian) Eagle Ford Group 

(a coupled Eagle Ford-Austin Chalk petroleum system). In northern 

areas of Texas, the Eagle Ford is thermally immature, and the deep 

Upper Jurassic (Oxfordian) Smackover Formation is a source of hydrocarbon. In eastern Texas and western Louisiana to the south where 

the Eagle Ford is thin to absent, hydrocarbons in the Austin Chalk are 

derived from sources in older Upper Cretaceous rocks (Hood and others, 

2002). Hydrocarbons migrated into conventional reservoirs in the Austin 

Chalk and into sandstones in the Tokio and Eutaw Formations along 

regional-to-local faults and (or) salt diapirs. The Austin Chalk shows 

continuous reservoir characteristics where it overlies the thermally 

mature Eagle Ford or age-equivalent (Cenomanian–Turonian) source 

rocks. Hydrocarbons expelled from Cenomanian–Turonian source 

rocks under high temperatures and pressures migrated into continuous 

reservoirs in the fractured, basal part of the Austin Chalk.

Assessment Units

Based on the petroleum-system framework, the USGS defined 

and quantitatively assessed eight assessment units (AUs), five conventional and three continuous (figs. 1 and 2). Strata in the defined 

AUs within these areas share similar stratigraphic, structural, and 

petroleum-charge histories.

The Western Updip Fault Zone Conventional Oil AU (fig. 1) 

is defined by Austin Chalk reservoirs and is bounded on the northwest 

by the updip Austin Chalk/Eagle Ford outcrop belt and regional faults 

(fig. 3). The AU extends west to the Texas border with Mexico 

and includes parts of the Sabine Uplift and East Texas Basin. The 

AU extends downdip to the south across the shelf to a depth and 

temperature corresponding to the thermal maturity threshold for 

oil generation based on vitrinite reflectance (Ro

=0.6 percent) in the 

Eagle Ford (Whidden and others, 2018).

The Eastern Updip Fault Zone Conventional Oil AU (fig. 1) 

is defined by Tokio and Eutaw sandstone lithofacies and is bounded 

on the northeast by outcrop and regional faults (fig. 3). The AU 

extends across the shelf toward the south and west and includes 

parts of the Sabine Uplift and North Louisiana and Mississippi Salt 

Basins (fig. 3). The southern boundary of the AU is the transition 

of the sandstone lithofacies with the northern updip limit of the 

Austin Chalk. This boundary generally corresponds to a depth and 

temperature range that defines the thermal maturity threshold for 

oil generation (Ro

=0.6 percent) in Cenomanian and Turonian source 

rocks beneath the Tokio and Eutaw Formations (Montgomery, 1996; 

Hackley and others, 2018). The interface between the Western and 

Eastern Updip Fault Zone Conventional Oil AUs occurs near the 

Texas and Louisiana State line where the Austin Chalk in Texas 

transitions to the Tokio Formation in Louisiana.

The Updip Salt Basins Conventional Oil and Gas AU (fig. 1) is 

defined by areas in which movement of the Jurassic Louann Salt has 

influenced the migration, entrapment, and sealing of hydrocarbons 

in reservoirs of the Austin Chalk and Tokio and Eutaw Formations. 

These areas include the diapir trend in the Brazos Basin and parts 

of the East Texas Basin and North Louisiana and Mississippi Salt 

Basins (fig. 3).

The Middip Fault Zone and Salt Conventional Oil AU (fig. 1) 

is bounded on the north by the Lower Cretaceous shelf edge and on 

the south by the Upper Cretaceous shelf edge (fig. 3). Hydrocarbon 

accumulations within this AU are associated with salt diapirs and 

large-scale faults that are part of the deep Tuscaloosa fault zone in 

Louisiana. The western boundary of this AU is closely coincident 

with the Louisiana-Texas State line where shelf edges transition 

from a less structurally complex shelf margin in Texas to an active 

and more structurally complex shelf margin in Louisiana.

The Downdip Salt Basins Conventional Gas AU (fig. 1), without 

well penetrations, contains previously unassessed reservoirs in the 

Austin Chalk. The northern limit of the AU is defined by the Lower 

and Upper Cretaceous shelf edges (fig. 3) and producing gas wells 

in the Eagle Ford of Texas. The southern limit is the boundary with 

Federal waters in each of the respective States.

100°

32°

102° 98° 96° 94° 92° 90° 88° 86° 84°

30°

28°

26°

34°

MEXICO

UNITED STATES

TEXAS

OKLAHOMA

ALABAMA

FLORIDA

GEORGIA

LOUISIANA

GULF OF MEXICO

East Texas

Basin

Brazos Basin

Maverick

Basin

North Louisiana

Salt Basin

Sabine

Uplift 

Monroe

Uplift 

Wiggins Uplift 

Lower Cretaceous shelf edge

ARKANSAS TENNESSEE

Jackson Dome

Uplift

LaSalle

Arch Uplift

Upper Cretaceous shelf edge

Mississippi Salt

Basin

Orogenic belt

MISSISSIPPI

EXPLANATION

Fault

Thrust fault—Dashed where 

location approximate

Upper Jurassic–Cretaceous– 

Tertiary Composite Total 

Petroleum System (part)

Austin/Eagle Ford outcrop

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

0 75 150 225 300 KILOMETERS

0 75 150 225 300 MILES

Figure 3. Map showing major geologic features of the greater onshore Gulf Coast area where the Upper Cretaceous Austin Chalk and Tokio 

and Eutaw Formations were assessed (modified from Ewing and Lopez, 1991; Schruben and others, 1998; and Galloway, 2008).
Assessment input data—

Conventional AUs

Western Updip Fault Zone Conventional Oil AU Eastern Updip Fault Zone Conventional Oil AU

Minimum Median Maximum Calculated mean Minimum Median Maximum Calculated mean

Number of oil fields 1 3 6 3.1 1 6 12 6.2

Size of oil fields (MMBO) 0.5 1 10 1.3 0.5 1 10 1.3

AU probability 1.0 1.0

Assessment input data—

Conventional AUs

Updip Salt Basins Conventional Oil and Gas AU Middip Fault Zone and Salt Conventional Oil AU

Minimum Median Maximum Calculated mean Minimum Median Maximum Calculated mean

Number of oil fields 1 4 12 4.3 1 4 12 4.3

Number of gas fields 1 2 6 2.1

Size of oil fields (MMBO) 0.5 2 10 2.3 0.5 1 10 1.3

Size of gas fields (BCFG) 3 6 70 7.9

AU probability 1.0 1.0

Assessment input data—

Conventional AUs

Downdip Salt Basins Conventional Gas AU

Minimum Median Maximum Calculated mean

Number of gas fields 1 50 150 53.2

Size of gas fields (MMBO) 5 10 1,000 24.9

AU probability 1.0

Assessment input data—

Continuous AUs

Western Shelf Continuous Oil AU Eastern Shelf Continuous Oil AU

Minimum Mode Maximum Calculated mean Minimum Mode Maximum Calculated mean

Potential production area of AU (acres) 100,000 7,250,000 14,445,000 7,265,000 1,000 1,000,000 11,000,000 4,000,333

Average drainage area of wells (acres) 80 120 160 120 80 120 160 120

Area untested in AU (%) 80 90 94 88 98 99 100 99

Success ratio (%) 86 90 94 90 20 40 60 40

Average EUR (MMBO) 0.08 0.134 0.18 0.135 0.02 0.04 0.1 0.043

AU probability 1.0 0.5

Assessment input data—

Continuous AUs

Western Shelf Edge Continuous Gas AU

Minimum Mode Maximum Calculated mean

Potential production area of AU (acres) 50,000 3,265,000 3,811,000 2,375,333

Average drainage area of wells (acres) 80 120 160 120

Area untested in AU (%) 87 97 98 94

Success ratio (%) 88 94 96 92.7

Average EUR (BCFG) 1.0 1.48 2 1.498

AU probability 1.0

Table 1. Key input data for five conventional and three continuous assessment units in the Austin Chalk and Tokio and Eutaw Formations of 

the U.S. Gulf Coast region.

[Well drainage area, success ratio, and estimated ultimate recovery are defined from analysis of the Austin, Tokio, and Eutaw wells. The average estimated ultimate 

recovery input is the minimum, median, maximum, and calculated mean. Gray shading indicates not applicable. AU, assessment unit; MMBO, million barrels of oil; 

BCFG, billion cubic feet of gas; %, percent; EUR, estimated ultimate recovery]

The Western Shelf Continuous Oil AU (fig. 2) is defined on 

the north by the updip limit of oil generation (Ro

=0.6 percent) in the 

Eagle Ford (Whidden and others, 2018). This threshold in thermal 

maturity is coincident with an observed transition from conventional 

reservoir field development of the Austin Chalk in the Western 

Updip Fault Zone Conventional Oil AU to continuous reservoir field 

development in the Western Shelf Continuous Oil AU. The transition 

is corroborated by comparing water-to-oil ratios of producing wells 

in the Austin Chalk in the two AUs over time. Water-to-oil ratios in 

the conventional AUs increase with time, whereas water-to-oil ratios 

in the continuous AUs overall remain constant with time. The 

southern boundary of this continuous oil AU corresponds to the 

thermal maturity threshold for gas generation (Ro

=1.3 percent) in 

the Eagle Ford (Whidden and others, 2018). The AU extends west 

across the Maverick Basin to the Texas border with Mexico.

The Eastern Shelf Continuous Oil AU (fig. 2) is defined on 

the north by the updip limit of the Austin Chalk and the thermal 

maturity threshold for oil generation (Ro

=0.6 percent) in the Upper 

Cretaceous (Turonian) Tuscaloosa marine shale (Hackley and 

others, 2018). The downdip limit is the thermal maturity threshold 

for gas generation (Ro

=1.3 percent) in the Tuscaloosa marine shale 

(Hackley and others, 2018).

The Western Shelf Edge Continuous Gas AU (fig. 2) is defined 

on the north by the thermal maturity threshold for gas generation 

(Ro

=1.3 percent) in the underlying Eagle Ford (Whidden and others, 

2018). The southern boundary closely approximates the Lower 

Cretaceous shelf edge (fig. 3) and the downdip limit of productive 

gas wells in the Eagle Ford. This gas AU has no producing wells in 

the Austin Chalk.

Table 1 lists input data used to calculate undiscovered 

resources in the eight AUs—five conventional and three continuous.

Undiscovered Resources Summary

The USGS assessed undiscovered, technically recoverable 

resources for five conventional and three continuous AUs in the 

Austin Chalk and Tokio and Eutaw Formations of the U.S. Gulf 

Coast region. The estimated mean totals for conventional and 

continuous oil and gas resources (table 2) are 6,867 million barrels 

of oil (MMBO), or 6.9 billion barrels of oil, with an F95–F5 range 

from 2,067 to 12,582 MMBO; 41,475 billion cubic feet of gas 

(BCFG), or 41.5 trillion cubic feet of gas, with an F95–F5 range 

from 13,178 to 71,883 BCFG; and 1,345 million barrels of natural 

gas liquids (MMBNGL), or 1.3 billion barrels of natural gas liquids, 

with an F95–F5 range from 423 to 2,353 MMBNGL. Values of 

0 at F95 reflect the chance that oil may not be present in the AU 

(geologic probability estimated to be less than 1).
References Cited

Condon, S.M., and Dyman, T.S., 2006, 2003 geologic assessment of 

undiscovered conventional oil and gas resources in the Upper Cretaceous Navarro and Taylor Groups, Western Gulf Province, Texas: 

U.S. Geological Survey Digital Data Series DDS–69–H, chap. 2, 

42 p.

Ewing, T.E., and Lopez, R.F., 1991, Plate 2—Principal structural features, Gulf of Mexico Basin in Salvador, A., ed., The Gulf of Mexico 

basin: The Geological Society of America, The geology of North 

America, v. J, 568 p.

Galloway, W.E., 2008, Depositional evolution of the Gulf of Mexico 

sedimentary basin, in Miall, A.D., ed., Sedimentary basins of the 

world, v. 5: Amsterdam, The Netherlands, Elsevier, p. 505–549.

Hackley, P.C., Enomoto, C.B., Valentine, B.J., Rouse, W.A., Lohr, 

C.D., Dulong, F.T., Hatcherian, J.J., Brennan, S.T., Craddock, W.H.,

Finn, T.M., Gaswirth, S.B., Le, P.A., Leathers-Miller, H.M., Marra,

K.R., Mercier, T.J., Paxton, S.T., Whidden, K.J., Woodall, C.A., and

Schenk, C.J., 2018, Assessment of undiscovered continuous oil and

gas resources in the Upper Cretaceous Tuscaloosa marine shale of the

U.S. Gulf Coast, 2018: U.S. Geological Survey Fact Sheet 2018–3043,

2 p., accessed August 31, 2018, at https://doi.org/10.3133/fs20183043.

Hood, K.C., Wenger, L.M., Gross, O.P., and Harrison, S.C., 2002, 

Hydrocarbon systems analysis of the northern Gulf of Mexico—

Delineation of hydrocarbon migration pathways using seeps and seismic imaging, chap. 2 of Schumacher, D., and LeSchack, L.A., eds., 

Surface exploration case histories—Applications of geochemistry, 

magnetics, and remote sensing: American Association of Petroleum 

Geologists, Studies in Geology, v. 48, and Society of Exploration 

Geophysicists, Geophysical References Series, v. 11, p. 25–40.

Montgomery, S.L., 1996, Mississippi Salt Dome Basin: Petroleum 

Frontiers, v. 13, no. 2, 96 p.

Schruben, P.G., Arndt, R.E., and Bawiec, W.J., 1998, Geology of the conterminous United States at 1:2,500,000 scale—A digital representation 

of the 1974 P.B. King and H.M. Beikman map: U.S. Geological Survey 

Data Series 11, 1 CD–ROM, release 2.

Whidden, K.J., Pitman, J.K., Pearson, O.N., Paxton, S.T., Kinney, S.A., 

Gianoutsos, N.J., Schenk, C.J., Leathers-Miller, H.M., Birdwell, J.E., 

Brownfield, M.E., Burke, L.A., Dubiel, R.F., French, K.L., Gaswirth, 

S.B., Haines, S.S., Le, P.A., Marra, K.R., Mercier, T.J., Tennyson, M.E., 

and Woodall, C.A., 2018, Assessment of undiscovered oil and gas 

resources in the Eagle Ford Group and associated Cenomanian–Turonian 

strata, U.S. Gulf Coast, Texas, 2018: U.S. Geological Survey Fact Sheet 

2018–3033, 4 p., accessed November 19, 2019, at https://pubs.usgs.gov/

fs/2018/3033/fs20183033.pdf.

Total petroleum system and

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

Upper Jurassic–Cretaceous–Tertiary Composite Total Petroleum System

Western Updip Fault Zone Conventional 

Oil AU 1.0 Oil 2 4 7 4 0 1 2 1 0 0 0 0

Eastern Updip Fault Zone Conventional 

Oil AU 1.0 Oil 4 7 13 8 0 1 1 1 0 0 0 0

Updip Salt Basins Conventional Oil and 

Gas AU 1.0

Oil 5 9 17 10 4 8 16 9 1 2 4 2

Gas 6 15 37 17 0 0 0 0

Middip Fault Zone and Salt Conventional

 Oil AU 1.0 Oil 2 5 10 5 7 15 33 17 1 1 3 1

Downdip Salt Basins Conventional 

Gas AU 1.0 Gas 534 1,216 2,489 1,324 3 8 18 9

Total undiscovered conventional 

resources 13 25 47 27 551 1,256 2,578 1,369 5 11 25 12

Western Shelf Continuous Oil AU 1.0 Oil 2,054 6,434 11,399 6,554 3,801 12,451 24,631 13,107 71 242 525 263

Eastern Shelf Continuous Oil AU 0.5 Oil 0 31 1,136 286 0 91 3,448 857 0 3 105 26

Western Shelf Edge Continuous Gas AU 1.0 Gas 8,826 26,910 41,226 26,142 347 1,061 1,698 1,044

Total undiscovered continuous 

resources 2,054 6,465 12,535 6,840 12,627 39,452 69,305 40,106 418 1,306 2,328 1,333

Total undiscovered resources 2,067 6,490 12,582 6,867 13,178 40,708 71,883 41,475 423 1,317 2,353 1,345

Table 2. Results for five conventional and three continuous assessment units in the Austin Chalk and Tokio and Eutaw Formations of the 

U.S. Gulf Coast region.

[Results shown are fully risked estimates. F95 represents a 95-percent chance of at least the amount tabulated; other fractiles are defined similarly. Gray shading 

indicates not applicable. MMBO, million barrels of oil; BCFG, billion cubic feet of gas; NGL, natural gas liquids; MMBNGL, million barrels of natural gas liquids]

For More Information

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

Austin Chalk and Tokio and Eutaw Formations Assessment Team

Janet K. Pitman, Stanley T. Paxton, Scott A. Kinney, Katherine J. Whidden, Seth S. Haines, Brian A. Varela, Tracey J. Mercier, 

Cheryl A. Woodall, Christopher J. Schenk, Heidi M. Leathers-Miller, Ofori N. Pearson, Lauri A. Burke, Phuong A. Le, Justin E. Birdwell, 

Nicholas J. Gianoutsos, Katherine L. French, Ronald M. Drake II, Thomas M. Finn, Geoffrey S. Ellis, Stephanie B. Gaswirth, 

Kristen R. Marra, Marilyn E. Tennyson, and Chilisa M. Shorten

ISSN 2327-6916 (print) ISSN 2327-6932 (online) https://doi.org/10.3133/fs20203045