Western Gulf 47

East 

Texas 

Basin 

48 Louisiana-Mississippi 

Salt Basins 49

Florida 

Peninsula

50

Miami

Tampa

Austin

Dallas

Houston

Atlanta

Memphis

Wichita

Raleigh Nashville

Charlotte

Fort Worth

Birmingham

Louisville

San Antonio

New Orleans

Saint Louis

Oklahoma City

Corpus Christi

TEXAS

FLORIDA

GEORGIA

OKLAHOMA

KANSAS

ALABAMA

LOUISIANA

SOUTH CAROLINA

COLORADO

MISSISSIPPI

ARKANSAS

MISSOURI

ILLINOIS

TENNESSEE

KENTUCKY

NORTH CAROLINA

VIRGINIA

Gulf of Mexico

Atlantic

Ocean

 104° 102° 100° 98° 96° 94° 92° 90° 88° 86° 84° 82° 80° 78°

24°

26°

28°

30°

32°

34°

36°

38°

0 100 200 50 MILES

Wilcox Coalbed Gas

Cretaceous-Tertiary Coalbed Gas

Cretaceous Olmos Coalbed Gas

Rio Escondido Basin Olmos Coalbed Gas

MEXICO

UNITED

STATES

Using a geology-based assessment methodology, the 

U.S. Geological Survey estimated a mean of 4.06 trillion cubic feet of undiscovered, technically recoverable natural gas in Cretaceous-Tertiary coal beds of 

the onshore lands and State waters of the Gulf Coast.

Introduction

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

an assessment of the undiscovered continuous gas potential of 

Cretaceous-Tertiary coal beds of the onshore areas and State 

waters of the Gulf Coast (fig. 1). This continuous gas assessment was part of a broader assessment of the undiscovered oil 

and gas potential of Tertiary strata of the onshore areas and 

State waters of the Gulf Coast (Dubiel and others, 2007). The 

assessment of Cretaceous-Tertiary continuous gas resources 

is based on geologic elements such as hydrocarbon source 

rocks (the level of source-rock maturation and the timing of 

hydrocarbon generation and migration), the availability of suitable reservoir rocks (sequence stratigraphy and petrophysical 

properties), and hydrocarbon traps (trap formation and timing) 

in the three coalbed gas total petroleum systems (TPS) identified in the region by the USGS. Four continuous assessment 

units (AU) were defined within the TPSs, and estimates of the 

undiscovered gas resources in the AUs are presented in table 1. 

The Olmos Coalbed Gas TPS, located in south Texas and northeastern Mexico (fig. 1), contains two Upper Cretaceous AUs: 

the Cretaceous Olmos Coalbed Gas AU and the Rio Escondido 

Basin Olmos Coalbed Gas AU (figs. 1 and 2). The Wilcox 

Coalbed Gas TPS, which contains the Wilcox Coalbed Gas 

AU, is the primary coal gas petroleum system in the Gulf Coast 

area. The Wilcox Coalbed Gas TPS boundaries are defined by 

an approximate northern up-dip freshwater-saltwater transition 

Figure 1. Map of the Gulf Coast region showing the continuous coalbed gas assessment units (AUs). Province 

boundaries defined by the U.S. Geological Survey are indicated by red lines.

U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2007–3039

August 2007

National Assessment of Oil and Gas Fact Sheet

Assessment of Undiscovered Oil and Gas Resources in 

Cretaceous-Tertiary Coal Beds of the Gulf Coast Region, 2007

Printed on recycled paper
Figure 2. Generalized 

stratigraphic section 

showing the coalbed gas 

assessment intervals of 

the Gulf Coast region. 

Assessment interval colors 

match AU colors used on 

figure 1. Pal., Paleocene.

zone within the Wilcox Group (Paleocene and Eocene) (derived 

from Pettijohn, 1996) and a southern down-dip depth to the 

top of Wilcox of about 6,000 ft. The southeast boundary of the 

Wilcox Coalbed Gas TPS is defined by the State-Federal water 

boundary offshore, and the southwest limit is the border with 

Mexico. The Cretaceous-Tertiary Coalbed Gas TPS defines one 

AU (Cretaceous-Tertiary Coalbed Gas AU) that contains all 

other Cretaceous and Tertiary coal beds that are not part of the 

Cretaceous Olmos Coalbed Gas AU and the Wilcox Coalbed 

Gas TPS (fig. 2).

Resource Summary

The USGS assessment of undiscovered unconventional 

coalbed gas resources resulted in an estimated mean of 4.06 

trillion cubic feet of gas (TCFG) in the TPSs and AUs that were 

assessed (table 1). Nearly all of the undiscovered gas resource 

(95 percent, or 3.86 TCFG) is assessed within the Wilcox Coalbed Gas AU (fig. 2). The unconventional gas resources reside in 

coalbed reservoirs and in adjacent and interfingered sandstones Continuous Oil and Gas Resources

[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, CBG, coalbed 

gas. Gray shading indicates not applicable]

Not quantitatively assessed

Table 1. Gulf Coast Region unconventional coalbed gas assessment results.

Total Petroleum Systems 

(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

Olmos Coalbed Gas TPS (504702)

Cretaceous Olmos Coalbed Gas 

AU (50470281) CBG 37 70 133 75 0 0 1 0

Rio Escondido Basin Olmos 

Coalbed Gas AU (53000281) CBG 60 114 215 123 0 0 1 0

Wilcox Coalbed Gas TPS (504703)

Wilcox Coalbed Gas

AU (50470381) CBG 1,565 3,442 7,570 3,861 0 0 0 0

Cretaceous-Tertiary Coalbed Gas TPS (504704)

Cretaceous-Tertiary Coalbed 

Gas AU (50470481) CBG

Total Continuous

 Resources 1,662 3,626 7,918 4,059 0 0 2 0

deposited in fluvial and deltaic depositional environments. The 

assessment results reflect estimates based on vertical drilling 

technology. 

For Additional Information

Supporting geologic studies of total petroleum systems, 

assessment units, and the methodology used in the assessment 

of conventional resources in Tertiary strata and of unconventional coalbed gas resources in Cretaceous and Tertiary rocks of 

the Gulf Coast are in progress. Assessment results are available at the USGS Energy Team Website: http://energy.cr.usgs.

gov/oilgas/noga/

Gulf Coast Unconventional Gas Assessment 

Team

Peter D. Warwick (Task Leader; pwarwick@usgs.gov)

Review Committee: Ronald R. Charpentier, Troy A. Cook, Timothy R. Klett, Richard M. Pollastro, and Christopher J. Schenk

References Cited

Dubiel R.F., Pitman, J.K., Pearson, O.N., Warwick, P.D., 

Karlsen, A.W., Coleman, J.L., Hackley, P.C., Hayba, D.O., 

Swanson, S.M., Charpentier, R.R., Cook, T.A., Klett, T.R., 

Pollastro, R.M., and Schenk, C.J., 2007, Assessment of undiscovered oil and gas resources in Tertiary strata of the Gulf 

Coast, 2007: U.S. Geological Survey Fact Sheet FS-2007-

3066, 4 p.

Pettijohn, R.A., 1996, Geochemistry of ground water in the 

Gulf Coast aquifer systems, south-central United States: 

U.S. Geological Survey Water-Resources Investigations 

Report 96-4107, 158 p. [http://pubs.er.usgs.gov/usgspubs/wri/

wri964107].

TERTIARY CRETACEOUS

(part) (part)

Jackson Group

Claiborne Group

Wilcox Group

Midway Group

STRATIGRAPHIC

INTERVAL

EPOCH

PERIOD

LATE EOCENE PAL.

Olmos

Formation