U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2011–3046

June 2011

National Assessment of Oil and Gas Fact Sheet

Assessment of Undiscovered Oil and Gas Resources of 

the Upper Cretaceous Austin Chalk and Tokio and Eutaw 

Formations, Gulf Coast, 2010

Printed on recycled paper

Introduction

The U.S. Geological Survey (USGS) recently completed 

a geology-based assessment of the undiscovered, technically 

recoverable oil and gas resources in the Austin Chalk and Tokio 

and Eutaw Formations of the U.S. Gulf Coast region, which 

includes parts of Texas, Louisiana, Arkansas, Mississippi, 

Alabama, and Florida (fig. 1). The assessment was based on 

the geologic elements and petroleum processes used to define a 

total petroleum system (TPS), which includes petroleum source 

rocks (source-rock maturation and petroleum generation and 

migration), reservoir and seal rocks (sequence stratigraphy and 

petrophysical properties), and petroleum traps (trap formation, 

timing, and seals). Using this petroleum-system framework, 

the USGS defined four assessment units (AUs): (1) the Austin–

Tokio–Eutaw Updip Oil and Gas AU, (2) the Austin–Eutaw 

Middip Oil and Gas AU, (3) the Austin Downdip Gas AU, and 

(4) the Austin Pearsall–Giddings Area Oil AU.

Geologic Summary

The Upper Cretaceous Austin Chalk is a low to moderate 

primary porosity and low primary permeability reservoir that relies on 

interconnected fracture networks for production of continuous-type 

accumulations; it also contains conventional-type accumulations. 

When assessing undiscovered accumulations, it is critical to infer the 

locations of: (1) conventional traps, such as faults and salt structures, 

and, (2) structures that are associated with fractures, such as anticlines 

and the Lower Cretaceous shelf edge. The Upper Cretaceous Tokio 

and Eutaw Formations have higher permeabilities and porosities than 

the Austin Chalk, do not require fracture networks for production, and 

contain conventional-type accumulations. Trap type influences accumulation size in conventional reservoirs, where small traps, such as 

fault segments, are associated with small accumulations. In addition, 

conventional-type accumulations tend to have well-defined boundaries and hydrocarbon-water contacts. In low-permeability continuous 

reservoirs, large-scale structures, such as broad anticlines and fracture 

networks, influence the distribution of accumulations that have diffuse 

boundaries and lack obvious traps and seals. The underlying Upper 

Cretaceous Eagle Ford Shale is the principal source rock for Austin 

Chalk hydrocarbons and may partially source oil and gas in Tokio and 

Eutaw reservoirs; it is included in the Upper Jurassic–Cretaceous–Tertiary Composite Total Petroleum System. Source rock quality of the 

Eagle Ford is inconsistent throughout the region, and some reservoirs 

in the Tokio and Eutaw may contain Jurassic-sourced hydrocarbons.

Austin-Eutaw Middip Oil and Gas AU

Gulf of Mexico

98° 94° 92° 88° 86°

Austin Pearsall-Giddings 

Area Oil AU

50490168 Austin Downdip Gas AU

50490132 50490132

Austin-Tokio-Eutaw Updip Oil and Gas AU

50490130

50490131

Austin-Eutaw Middip Oil and Gas AU

50490131

102° 96° 90°

34°

32°

30°

28°

26°

TEXAS

FLORIDA

OKLAHOMA

ALABAMA

LOUISIANA

MISSISSIPPI

ARKANSAS

MEXICO

100°

GULF OF MEXICO

UNITED

STATES 0 100 MILES

0 100 KILOMETERS

Figure 1. Map of the Gulf Coast region 

showing part of the Upper Jurassic–

Cretaceous–Tertiary Composite Total 

Petroleum System (TPS) (blue line), 

assessment unit (AU) boundaries (black 

lines), and the Lower Cretaceous shelf 

edge (red line). Lower Cretaceous shelf 

edge after Martin (1980) and Ewing and 

Lopez (1991).

Using a geology-based assessment methodology, the 

U.S. Geological Survey estimated means of 957 million 

barrels of undiscovered oil, 3.6 trillion cubic feet of undiscovered natural gas, and 363 million barrels of undiscovered 

natural gas liquids in the Austin Chalk and Tokio and 

Eutaw Formations in onshore lands and State waters of the 

Gulf Coast.
Resource Summary

The USGS assessed undiscovered, technically recoverable oil and gas resources in four assessment units (table 1). 

For conventional resources, the Austin–Tokio–Eutaw Updip Oil 

and Gas AU was assessed to contain a mean of 20 million barrels of oil (MMBO), 53 billion cubic feet of gas (BCFG), and 

about 1 million barrels of natural gas liquids (MMBNGL). The 

Austin–Eutaw Middip Oil and Gas AU was assessed to contain 

a total mean of 45 MMBO, 677 BCFG, and 66 MMBNGL. The 

Austin Downdip Gas AU was assessed to contain a total mean 

of 13 MMBO, 1.6 trillion cubic feet of gas (TCFG), and 190 

MMBNGL. The Austin Chalk and Tokio and Eutaw Formations 

conventional resource mean totals are: 78 MMBO, 2.3 TCFG, 

and 257 MMBNGL. For continuous resources, the USGS 

estimated a total mean of 879 MMBO, 1.3 TCFG, and 106 

MMBNGL for the Austin Pearsall–Giddings Area Oil AU. The 

assessment was based on 2008 IHS well and production data 

(IHS Energy Group, 2009a, 2009b) and 2006 Nehring field data 

(Nehring Associates, 2007).

For Further Information

Supporting geologic studies and the methodology used 

in the 2010 Jurassic and Cretaceous Gulf Coast Assessment 

are in progress. Assessment results are available at the 

USGS Central Energy Resources Science Center website: 

http://energy.cr.usgs.gov/oilgas/noga

References

Ewing, T.E., and Lopez, R.F., 1991, Principal structural 

features, in Salvador, A., ed., The Gulf of Mexico Basin: 

The Geological Society of America, The Geology of North 

America, v. J, plate 2.

IHS Energy Group, 2009a, PI/Dwights PLUS U.S. production data: Englewood, Colo., database available from IHS 

Energy Group.

IHS Energy Group, 2009b, PI/Dwights PLUS U.S. well data: 

Englewood Colo., database available from IHS Energy Group.

Martin, R.G., 1980, Distribution of salt structures, Gulf of 

Mexico: U.S. Geological Survey Miscellaneous Field Studies, 

Map MF-1213, scale 1:2,500,000.

Nehring Associates, 2007, The significant oil and gas fields of 

the United States: Colorado Springs, Colo., database available 

from Nehring Associates, Inc.

Assessment Team

Gulf Coast Assessment Team: Krystal Pearson 

(kpearson@usgs.gov), Russell F. Dubiel, Ofori N. Pearson, 

and Janet K. Pitman.

Review Committee: Ronald R. Charpentier, Troy A. Cook, 

Timothy R. Klett, Richard M. Pollastro, and Christopher J. 

Schenk.

Upper Jurassic-Cretaceous-Tertiary Composite TPS (50490100)

Austin-Tokio-Eutaw Updip Oil

and Gas AU (50490130)

Oil 6 19 34 20 1 3 6 3 0 0 0 0

Gas 17 47 92 50 0 1 2 1

Austin-Eutaw Middip Oil and

Gas AU (50490131)

Oil 10 41 95 45 28 118 300 135 2 10 27 12

Gas 109 477 1,192 542 10 46 124 54

Austin Downdip Gas AU

(50490132)

Oil 3 11 32 13 10 43 130 53 1 4 14 5

Gas 406 1,429 3,013 1,542 46 167 385 185

Total Conventional

 Resources 19 71 161 78 571 2,117 4,733 2,325 59 228 552 257

Austin Pearsall-Giddings Area

Oil AU (50490168) Oil 507 839 1,389 879 674 1,233 2,255 1,319 49 97 193 106

Total Continuous

Resources 507 839 1,389 879 674 1,233 2,255 1,319 49 97 193 106

Total Undiscovered

Oil and Gas Resources 526 910 1,550 957 1,245 3,350 6,988 3,644 108 325 745 363

[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). 

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. TPS, total petroleum system; AU, assessment unit. Gray 

shading indicates not applicable] 

 

Table 1. Austin Chalk assessment results. Conventional Oil and Gas Resources Continuous Oil and Gas Resources 

(TPS) 

and Assessment Units (AU)

Field 

Type

Total Undiscovered Resources

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

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