U.S. Gulf Coast Petroleum Systems Project

Assessment of Undiscovered Oil and Gas Resources in the Eagle Ford Group 

and Associated Cenomanian–Turonian Strata, U.S. Gulf Coast, Texas, 2018

Introduction

The U.S. Geological Survey (USGS) 

assessed undiscovered, technically 

recoverable hydrocarbon resources in 

self-sourced continuous reservoirs of 

the Upper Cretaceous Eagle Ford Group 

and associated Cenomanian–Turonian 

strata, which are present in the subsurface 

across the U.S. Gulf Coast region, Texas 

(fig. 1). The USGS completes geologybased assessments using the elements 

of the total petroleum system (TPS), 

which include source rock thickness, 

organic richness, and thermal maturity for 

self-sourced continuous accumulations. 

Assessment units (AUs) within a TPS 

are defined by strata that share similar 

structural and petroleum-charge histories 

along with lithology and stratigraphy. 

Total Petroleum System and 

Geologic Models for Assessment

The Eagle Ford Group contains one 

of the most prolific continuous accumulations of oil and gas in the United States 

(fig. 2); its composition is predominantly 

mudstone and calcareous mudstone (marl) 

with organic-rich intervals. These marine 

strata were deposited in outer shelf and 

upper slope environments during the 

Cenomanian–Turonian ages (Denne and 

Breyer, 2016). The assessed rock interval 

Figure 2. Bubble map showing the relative 

sizes of the top five U.S. Geological Survey 

assessment results through 2018 for continuous 

oil and gas resources in the United States. 

Quantitative assessment results are from 

Coleman and others (2011), Kirschbaum and 

others (2012), Gaswirth and others (2013, 2016), 

Marra and others (2015, 2017), Hawkins and 

others (2016), and Paxton and others (2017a, b).

EXPLANATION

Eagle Ford Marl 

 Continuous Oil AU

Eagle Ford Marl Continuous 

 Gas AU

Submarine Plateau-Karnes 

 Trough Continuous Oil AU

Submarine Plateau–Karnes 

 Trough Continuous Gas AU 

Cenomanian–Turonian 

 Mudstone Continuous 

 Oil AU

Cenomanian–Turonian 

 Mudstone Continuous 

 Gas AU

Cenomanian–Turonian

 Downdip Continuous 

 Gas AU

−102° −100° −98° −96° −94° −92° −90°

32°

30°

28°

26°

0 100 200 KILOMETERS

0 100 200 MILES

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

TEXAS

MEXICO

LOUISIANA

MISSISSIPPI

GULF OF MEXICO

GULF OF MEXICO

TX

OK

AL

AR

LA

MS

TN

GA NM

MEXICO

FL

Map

area

Upper Jurassic–Cretaceous–Tertiary 

Composite Total Petroleum System 

boundary (part) shown in orange.

Figure 1. Map showing the extent of the seven assessment units 

(AUs) in the Eagle Ford Group and associated Cenomanian–

Turonian strata in the U.S. Gulf Coast region, Texas.

 Top five USGS assessment results for mean continuous oil and gas resources

(Undiscovered, technically recoverable resources)

Total mean gas: 573 TCFG

 (trillion cubic feet of gas)

Total mean oil: 41 BBO

 (billion barrels of oil)

EXPLANATION

Total mean gas

per assessment 

(TCFG)

50 to 75

75 to 100

>100

Total mean oil

per assessment

(BBO)

0 to 5

5 to 10

10 to 20

MARCELLUS 

APPALACHIAN BASIN

UTICA

APPALACHIAN BASIN

EAGLE FORD 

U.S. GULF COAST, TEXAS

HAYNESVILLE-BOSSIER

(COMBINED)

 U.S. GULF COAST

WOLFCAMP

MIDLAND BASIN SPRABERRY

MIDLAND BASIN

MANCOS-NIOBRARA 

PICEANCE BASIN

BARNETT 

BEND ARCH-FORT 

WORTH BASIN

BAKKEN AND THREE FORKS

WILLISTON BASIN

66

66

7.4

84

20 4.2

53

8.5

304

.94

U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2018–3033

June 2018

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

mean resources of 8.5 billion barrels of oil and 66 trillion cubic feet of gas in continuous accumulations in the Upper Cretaceous 

Eagle Ford Group and associated Cenomanian–Turonian strata in onshore lands of the U.S. Gulf Coast region, Texas.
includes mudstone strata that may be 

slightly older than Eagle Ford Group strata 

but are not well understood in terms of age 

or distribution. Therefore, these units are 

referred to as “associated Cenomanian–

Turonian strata.” To better capture the 

resource heterogeneity of this interval, 

Eagle Ford Group marl strata are defined 

as having less than 25 percent clay, and 

Cenomanian–Turonian mudstone strata 

are defined as having more than 25 percent 

clay, based on work by Donovan and 

others (2017). The Eagle Ford Group and 

associated Cenomanian–Turonian strata 

are part of the Upper Jurassic–Cretaceous–

Tertiary Composite TPS in onshore lands 

of the U.S. Gulf Coast region, Texas 

(Dubiel and others, 2012).

Assessment Units

Seven continuous AUs (fig. 1) were 

defined for the Eagle Ford Group and 

associated Cenomanian–Turonian strata 

across the study area, based on lithology, 

stratal thickness, thermal maturity, 

regional geologic features, and spatial 

distribution of productive fairways.

The Eagle Ford Marl Continuous 

Oil AU is defined by the United StatesMexico border, the 25-percent-clay line, 

and the thermal maturity window for 

oil (0.6–1.3 percent modeled vitrinite 

reflectance) (fig. 1). Within this AU is 

the Submarine Plateau-Karnes Trough 

Continuous Oil AU, which is an area 

of thicker Eagle Ford Group strata 

(greater than 120 feet), as mapped by 

Hammes and others (2016) (fig. 1). The 

thicker interval is interpreted to have 

provided additional source rock and 

reservoir potential. The Cenomanian–

Turonian Mudstone Continuous Oil 

AU is bounded by the 25-percent-clay 

line, the eastern extent of production 

of Cenomanian–Turonian oil (at about 

the Texas-Louisiana State line), and 

the thermal maturity window for oil 

(0.6–1.3 percent modeled vitrinite 

reflectance) (fig. 1).

The Eagle Ford Marl Continuous 

Gas AU is defined by the United 

States-Mexico border, the 25-percent-clay 

line, the updip limit of gas generation 

(1.3 percent modeled vitrinite reflectance), 

and the Lower Cretaceous shelf margin as 

illustrated by Donovan and others (2015) 

(fig. 1). The Submarine Plateau-Karnes 

Trough Continuous Gas AU is defined as 

the area within the Eagle Ford Marl Continuous Gas AU that has Eagle Ford Group 

strata greater than 120 feet thick, which is 

interpreted to have additional source rock 

and reservoir potential. The Cenomanian–

Turonian Mudstone Continuous Gas AU 

is defined by the 25-percent-clay line, the 

updip limit of gas generation (1.3 percent 

modeled vitrinite reflectance), and the outboard expression of the Upper Cretaceous 

shelf margin as illustrated by Galloway 

(2008) (fig. 1). The Cenomanian–Turonian 

Downdip Continuous Gas AU (fig. 1), 

which extends to the State-Federal waters 

boundary, was not quantitatively assessed 

because of a lack of data. 

Table 1 lists input data used to calculate undiscovered resources in the six 

quantitatively assessed AUs.

Table 1. Key input data for six assessment units in the Eagle Ford Group and associated Cenomanian–Turonian strata in the U.S. Gulf Coast 

region, Texas.

[AU, assessment unit; %, percent; EUR, estimated ultimate recovery per well; MMBO, million barrels of oil; BCFG, billion barrels of gas. The average EUR input is the minimum, 

median, maximum, and calculated mean. Shading indicates not applicable]

Assessment input data—

Continuous AUs

Eagle Ford Marl Continuous Oil AU Submarine Plateau-Karnes Trough Continuous Oil AU

Minimum Mode Maximum Calculated 

mean Minimum Mode Maximum Calculated mean

Potential production area of AU (acres) 3,783,000 5,021,000 5,661,000 4,821,667 320,000 406,000 497,000 407,667

Average drainage area of wells (acres) 60 100 120 93.3 60 100 120 93.3

Percentage of area untested in AU 63 73 76 70.7 4 22 36 20.7

Success ratio (%) 85 90 95 90 95 97 99 97

Average EUR (MMBO) 0.05 0.15 0.3 0.156 0.11 0.2 0.35 0.207

AU probability 1.0 1.0

Assessment input data—

Continuous AUs

Cenomanian–Turonian Mudstone Continuous Oil AU Eagle Ford Marl Continuous Gas AU

Minimum Mode Maximum Calculated 

mean Minimum Mode Maximum Calculated mean

Potential production area of AU (acres) 2,600,000 4,210,000 7,270,000 4,693,333 1,100,000 2,000,000 2,850,000 1,983,333

Average drainage area of wells (acres) 80 120 140 113.3 80 120 140 113.3

Percentage of area untested in AU 97 98 99 98 88 93 95 92

Success ratio (%) 50 70 90 70 80 85 90 85

Average EUR (MMBO, oil; BCFG, gas) 0.01 0.11 0.2 0.113 1 2.5 4.5 2.579

AU probability 1.0 1.0

Assessment input data—

Continuous AUs

Submarine Plateau-Karnes Trough Continuous Gas AU Cenomanian–Turonian Mudstone Continuous Gas AU

Minimum Mode Maximum Calculated 

mean Minimum Mode Maximum Calculated mean

Potential production area of AU (acres) 270,000 372,000 436,000 359,333 1,000 1,500,000 3,000,000 1,500,333

Average drainage area of wells (acres) 80 120 140 113.3 120 140 160 140

Percentage of area untested in AU 84 88 90 87.3 100 100 100 100

Success ratio (%) 90 95 99 94.7 10 50 90 50

Average EUR (BCFG) 1.5 3 5 3.079 0.3 0.9 3 1.012

AU probability 1.0 1.0
Undiscovered Resources Summary

The USGS assessed undiscovered, technically 

recoverable oil and gas resources for six continuous AUs 

in the Eagle Ford Group and associated Cenomanian–

Turonian strata, U.S. Gulf Coast region, Texas (table 2). 

The estimated mean totals for oil and gas resources in the 

Eagle Ford Marl, Submarine Plateau-Karnes Trough, and 

Cenomanian–Turonian Mudstone Continuous Oil and 

Gas AUs are 8,515 million barrels of oil (MMBO), or 

8.5 billion barrels of oil, with an F95–F5 range from 5,266 

to 12,846 MMBO; 65,981 billion cubic feet of gas (BCFG), 

or 66 trillion cubic feet of gas, with an F95–F5 range from 

37,984 to 104,048 BCFG; and 1,891 million barrels of 

natural gas liquids (MMBNGL) with an F95–F5 range from 

1,032 to 3,093 MMBNGL (table 2). These assessment results 

for the Eagle Ford Group and associated Cenomanian–

Turonian strata are among the top five largest continuous 

resources for both oil and gas assessed by the USGS in the 

United States (fig. 2).

Upper Eagle Ford Group at Lozier Canyon, near Del Rio, Texas. Photograph by Stanley T. Paxton, U.S. Geological Survey.

Table 2. Results for six assessment units in the Eagle Ford Group and associated Cenomanian–Turonian strata in the U.S. Gulf Coast region, Texas.

[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. 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]

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

Eagle Ford Marl Continuous Oil AU 1.0 Oil 3,397 4,962 7,443 5,129 5,689 9,810 16,328 10,240 100 192 354 205

Submarine Plateau-Karnes Trough Continuous 

Oil AU 1.0 Oil 77 178 302 182 223 526 941 546 6 15 30 16

Cenomanian–Turonian Mudstone Continuous 

Oil AU 1.0 Oil 1,792 3,064 5,101 3,204 3,134 6,008 11,037 6,412 56 118 236 128

Total undiscovered continuous oil resources 5,266 8,204 12,846 8,515 9,046 16,344 28,306 17,198 162 325 620 349

Eagle Ford Marl Continuous Gas AU 1.0 Gas 21,922 34,303 52,061 35,304 614 1,019 1,634 1,057

Submarine Plateau-Karnes Trough Continuous 

Gas AU 1.0 Gas 5,908 7,943 10,843 8,100 224 316 449 324

Cenomanian–Turonian Mudstone Continuous 

Gas AU 1.0 Gas 1,108 4,464 12,838 5,379 32 132 390 161

Cenomanian–Turonian Downdip Continuous 

Gas AU Gas Not quantitatively assessed

Total undiscovered continuous gas resources 28,938 46,710 75,742 48,783 870 1,467 2,473 1,542

Total undiscovered continuous resources 5,266 8,204 12,846 8,515 37,984 63,054 104,048 65,981 1,032 1,792 3,093 1,891

Inoceramid shells on a 

bedding plane in the 

U.S. Geological Survey 

Gulf Coast #1 West Woodway 

research core through the 

Eagle Ford Group; core was 

taken near Waco, Texas. 

Photograph by 

Stanley T. Paxton, 

U.S. Geological Survey.
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Eagle Ford Group Assessment Team

Katherine J. Whidden, Janet K. Pitman, Ofori N. Pearson, Stanley T. Paxton, Scott A. Kinney, Nicholas J. Gianoutsos, Christopher J. Schenk, 

Heidi M. Leathers-Miller, Justin E. Birdwell, Michael E. Brownfield, Lauri A. Burke, Russell F. Dubiel, Katherine L. French, Stephanie B. Gaswirth, 

Seth S. Haines, Phuong A. Le, Kristen R. Marra, Tracey J. Mercier, Marilyn E. Tennyson, and Cheryl A. Woodall

For More Information

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

Banner image shows organic-rich (darker) interval in the lower Eagle Ford Group at Lozier Canyon, near Del Rio, Texas. Photograph by Stanley T. Paxton, U.S. Geological Survey.

ISSN 2327-6916 (print) ISSN 2327-6932 (online) https://doi.org/10.3133/fs20183033