U.S. Gulf Coast Petroleum Systems Project

Assessment of Undiscovered Conventional Oil and Gas Resources in the 

Downdip Paleogene Formations, U.S. Gulf Coast, 2017

Introduction

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

technically recoverable oil and gas resources in the conventional sandstone reservoirs of downdip Paleogene formations deposited along an 

arcuate extent from south Texas through Louisiana (Salvador, 1991). 

The USGS conducts geology-based assessments of undiscovered 

petroleum resources by evaluating components of a total petroleum 

system (TPS), including source and reservoir rocks, seals and traps, 

and petroleum products geohistory. The interval assessed here is part of 

the Upper Jurassic–Cretaceous–Tertiary Composite TPS from Warwick 

and others (2007) in onshore lands and State waters of the U.S. Gulf 

Coast region (fig. 1). Within a TPS, strata in an assessment unit (AU) 

share similar stratigraphic, 

structural, and petroleum-charge 

histories. In this update of previous assessment work (Schenk 

and Viger, 1996; Dubiel and 

others, 2007), the USGS outlined 

11 such AUs in the downdip 

Paleogene formations of the TPS 

(fig. 1). Eight AUs for conventional resources were quantitatively assessed, and three AUs 

for continuous (unconventional) 

resources were not quantitatively 

assessed.

Geologic Model for 

Assessment

Potential reservoirs in 

downdip Paleogene formations 

may exist as deep as 30,000 feet 

and may include paleoslope 

sandstones deposited as incised 

channel fills, slope fan channels, 

and ponded turbidites in intraslope minibasins. A paleoslope 

depositional environment was 

modeled for the AUs based on 

the stratigraphic interpretations of 

geophysical data, paleontologic 

picks, and combination of detrital 

zircon provenance studies with 

stratigraphic scaling relationships (Sømme and others, 2009). 

Potential seals consisting of 

fine-grained strata typical of continental slope deposits were interpreted as condensed sections on well logs and seismic lines. Trapping 

styles were interpreted to be both stratigraphic (for example, channel 

fill pinched out under distal overbank mudstones) and structural (for 

example, growth faulting in an expanded fault zone setting). AUs 

overlap with terrestrial source rocks interpreted in the west (Texas) of 

the TPS that transition to marine source rocks interpreted in the east 

(Lousiana), and thus, multiple Mesozoic–Paleogene source intervals 

are plausible (Hood and others, 2002), whereas thermal maturities 

are within the oil window or higher within the study area. Regardless, poor reservoir porosity, permeability, temperature, and pressure 

estimates may challenge further industry exploration.

Figure 1. Map showing boundaries of the eight assessment units (AUs) in the downdip Paleogene formations 

along the U.S. Gulf Coast region. AU offshore boundary lines are shown side-by-side for illustration purposes.

Upper Jurassic–Cretaceous–Tertiary

Composite Total Petroleum System 

boundary is shown in orange.

GULF OF

MEXICO

Map

area

MEXICO 

NM

TX

LA

CO KS MO

OK AR TN

KY IL IN WV VA

NC

SC

GA

AL MS

FL

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

EXPLANATION

Hackberry Slope Sandstones AU

Frio Formation Slope Sandstones AU

Anahuac Formation Slope Sandstones AU

Vicksburg Group Slope Sandstones AU

Jackson Group Slope Sandstones AU

Upper Claiborne Group Slope Sandstones AU

0 50 100 KILOMETERS

0 50 100 MILES

LOUISIANA

MISSISSIPPI

TEXAS

MEXICO 

GULF OF MEXICO

–98° –96° –94° –92° –90°

30°

28°

26°

Lower Claiborne Group Slope Sandstones AU

Wilcox Group Slope Sandstones AU

U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2018–3019

May 2018

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

conventional resources of 100 million barrels of oil and 16.5 trillion cubic feet of gas in the downdip Paleogene formations in onshore 

lands and State waters of the U.S. Gulf Coast region.
Assessment Units

Eight of 11 downdip Paleogene AUs were quantitatively 

assessed. These conventional AUs generally stack stratigraphically, 

and nearly all extend eastward from the United States-Mexico 

international border and northward from about the State-Federal 

waters limit. Table 1 lists input data used to calculate undiscovered 

resources in the eight conventional AUs.

The Hackberry Slope Sandstones AU incorporates upper 

Oligocene sandstone reservoirs of the middle part of the Frio 

Hackberry trend of eastern Texas and southwestern Louisiana (fig. 2). 

AU reservoirs comprise strata within rotated slide blocks and fill 

sequences in rotational fault accommodation spaces in updip areas 

and canyon and fan deposits farther downdip. Northern, western, 

and eastern AU boundaries are the limit of sand deposits in the 

Hackberry play, defined by Cossey and Jacobs (1992), and thus the 

limit of historical Hackberry production. The southern AU boundary 

is coincident with the northern boundary of the Frio Formation Slope 

Sandstones AU described below.

The Anahuac Formation Slope Sandstones AU comprises 

reservoirs in the downdip portion of the upper Oligocene Anahuac 

Formation. Paleodepositional systems of the reservoirs are sparse, 

shelf-fed channel, fan, and sheet sand deposits with the possibility of 

carbonate turbidites in eastern Louisiana. The updip AU boundary is 

the Frio paleoshelf margin, as interpreted by Galloway (2008), where 

transgressive facies of the Anahuac onlap the shelf.

Table 1. Key input data for eight conventional assessment units in the Tertiary slope sandstones of the downdip 

Paleogene formations, U.S. Gulf Coast region.

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

Assessment input data—

Conventional AUs

Hackberry Slope Sandstones AU Anahuac Formation Slope Sandstones AU

Minimum Median Maximum Calculated 

mean Minimum Median Maximum Calculated mean

Number of oil fields 1 3 6 3.1 1 6 30 6.8

Number of gas fields 5 12 30 12.6 1 25 125 28.5

Size of oil fields (MMBO) 0.5 0.7 20 1.1 0.5 1 100 2.5

Size of gas fields (BCFG) 3 7 300 13.1 3 10 1,000 26.7

AU probability 1.0 1.0

Assessment input data—

Conventional AUs

Frio Formation Slope Sandstones AU Vicksburg Group Slope Sandstones AU

Minimum Median Maximum Calculated 

mean Minimum Median Maximum Calculated mean

Number of oil fields 1 6 60 7.8 1 3 15 3.4

Number of gas fields 2 50 220 55.9 3 60 250 66.6

Size of oil fields (MMBO) 0.5 1 100 2.5 0.5 1 100 2.5

Size of gas fields (BCFG) 3 10 1,500 31.9 3 8 1,500 27.5

AU probability 1.0 1.0

Assessment input data—

Conventional AUs

Jackson Group Slope Sandstones AU Upper Claiborne Group Slope Sandstones AU

Minimum Median Maximum Calculated 

mean Minimum Median Maximum Calculated mean

Number of oil fields 1 3 15 3.4 1 7 50 8.5

Number of gas fields 1 30 200 36.0 8 80 500 94.9

Size of oil fields (MMBO) 0.5 1 100 2.5 0.5 1 100 2.5

Size of gas fields (BCFG) 3 10 1,000 26.7 3 10 1,000 26.7

AU probability 0.9 1.0

Assessment input data—

Conventional AUs

Lower Claiborne Group Slope Sandstones AU Wilcox Group Slope Sandstones AU

Minimum Median Maximum Calculated 

mean Minimum Median Maximum Calculated mean

Number of oil fields 1 3 20 3.6 1 5 30 5.9

Number of gas fields 1 50 250 57.0 2 120 1,000 150.7

Size of oil fields (MMBO) 0.5 1 100 2.5 0.5 1 100 2.5

Size of gas fields (BCFG) 3 10 1,000 26.7 3 12 2,500 45.4

AU probability 1.0 1.0

Hackberry

trend

 

Vicksburg Group

Eocene Paleocene

Jackson

Group

Moodys Branch

and Yazoo Formations

Midway Group

Claiborne

Group

Anahuac Formation

Oligocene

Paleogene

Tertiary

Wilcox Group

Queen City Sand

Frio

Formation

System Series Group or Formation

Cockfield, Cook

Mountain, and

Yegua Formations

Sparta Sand

Figure 2. Generalized stratigraphic section of the northern Gulf 

of Mexico coastal plain downdip of the Paleogene shelf margins, 

modified from Schenk and Viger (1996). 
Table 2. Results for eight conventional assessment units in the Tertiary slope sandstones of the downdip Paleogene formations, U.S. Gulf 

Coast region.

[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]

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

Hackberry Slope Sandstones AU 1.0

Oil 1 3 7 3 3 8 25 10 0 0 1 0

Gas 71 146 321 164 4 10 24 11

Anahuac Formation Slope Sandstones AU 1.0

Oil 3 12 50 17 3 16 85 30 0 1 3 1

Gas 187 635 1,759 759 6 23 77 30

Frio Formation Slope Sandstones AU 1.0

Oil 2 12 62 19 2 17 104 30 0 1 5 2

Gas 528 1,548 3,828 1,780 17 57 167 70

Vicksburg Group Slope Sandstones AU 1.0

Oil 1 5 27 8 1 7 45 13 0 0 2 1

Gas 571 1,609 3,853 1,833 19 59 169 72

Jackson Group Slope Sandstones AU 0.9

Oil 0 4 26 8 0 10 70 20 0 1 3 1

Gas 0 695 2,291 866 0 15 51 19

Upper Claiborne Group Slope

Sandstones AU 1.0

Oil 3 14 64 21 7 35 170 55 0 2 9 3

Gas 739 2,121 5,664 2,517 16 47 126 56

Lower Claiborne Group Slope

Sandstones AU 1.0

Oil 1 5 29 9 2 12 79 23 0 1 4 1

Gas 443 1,313 3,307 1,521 9 29 75 34

Wilcox Group Slope Sandstones AU 1.0

Oil 2 10 45 15 5 23 120 38 0 1 4 1

Gas 1,471 5,420 17,130 6,841 20 82 281 107

Total undiscovered conventional resources 13 65 310 100 4,033 13,615 38,851 16,500 91 329 1,001 409

The Frio Formation Slope Sandstones AU incorporates paleoslope sandstone reservoirs within lower to upper Oligocene Frio 

Formation strata. Paleodepositional systems of the reservoirs are both 

sand aprons in south Texas fed by the Norma and Norias Deltas and 

shelf-fed submarine channel and fan deposits throughout the AU. The 

updip AU boundary is the Frio shelf margin from Galloway (2008).

The Vicksburg Group Slope Sandstones AU consists of lower 

Oligocene Vicksburg Group sandstone reservoirs found downdip 

of the Vicksburg paleoshelf margin. Reservoirs are delta-fed aprons 

along the paleoshelf margin in Texas and sparsely distributed turbidite 

deposits throughout the AU, which therefore has a decreased probability. The updip AU boundary in south Texas is the Vicksburg shelf 

margin as defined by Coleman (1990), and the updip boundary in 

eastern Texas and southern Louisiana is the reinterpreted Vicksburg 

shelf margin and downdip limit of post-Vicksburg erosion.

The Jackson Group Slope Sandstones AU reservoirs comprise 

downdip equivalents of the upper Eocene and lower Oligocene 

Moodys Branch and Yazoo Formations. With a major paleodeposition center located downdip of the Rio Grande embayment, the AU 

only extends eastward to a facies change to primarily clay in the 

vicinity of the Texas-Louisiana State line. The Jackson paleoshelf 

margin from Galloway (2008) defines the northern AU limit.

The Upper Claiborne Group Slope Sandstones AU comprises 

downdip reservoirs of the middle Eocene Cockfield, Cook Mountain, 

and Yegua Formations and Sparta Sand (fig. 2). Paleodepositional 

systems included progradational shelf-fed and delta-fed aprons. The 

Yegua paleoshelf margin from Galloway (2008) defines the northern 

AU limit.

The Lower Claiborne Group Slope Sandstones AU comprises 

downdip reservoirs of the middle Eocene Queen City Sand. Sands 

of the Queen City were likely transported from the south Texas Rio 

Grande embayment to the paleoshelf edge and beyond, though they 

are not interpreted to extend east past the Texas-Louisiana State line. 

The Queen City paleoshelf margin from Galloway (2008) defined 

the northern AU limit.

The Wilcox Group Slope Sandstones AU comprises paleoslope 

sandstone reservoirs of the Paleocene–Eocene Wilcox Group strata. 

Paleodepositional systems of the reservoir strata are sandy deltafed aprons and shelf-fed aprons in onshore Texas and Louisiana 

(McDonnell and others, 2008). The upper Wilcox Group shelf 

margin from Galloway (2008) defined the northern AU limit. 

The continuous AUs reflect the extent of potentially selfsourcing shale oil and (or) gas resources. The Lower Claiborne 

Group Continuous AU, the Wilcox Group Continuous AU, and 

the Midway Group Continuous AU were defined where there is 

evidence of organic-rich, marine kerogen-rich mudstones in the 

oil-window or higher thermal maturity zone. There is no known 

development of continuous resources in this TPS from these three 

groups, and the source rock potential of these formations is not 

well known. Therefore, these three AUs were not quantitatively 

assessed.

Undiscovered Resources Summary

The USGS assessed undiscovered, technically recoverable 

resources for eight conventional oil and gas AUs in the downdip 

Paleogene formations (table 2). The estimated mean totals are 

100 million barrels of oil (MMBO) with an F95–F5 range from 13 to 

310 MMBO; 16,500 billion cubic feet of gas (BCFG), or 16.5 trillion 

cubic feet of gas, with an F95–F5 range from 4,033 to 38,851 BCFG; 

and 409 million barrels of natural gas liquids (MMBNGL) with an 

F95–F5 range from 91 to 1,001 MMBNGL.
Photograph of an outcropping of Eocene Jackson Group rocks at Lake Somerville, Texas. These sandstones may be considered general shallow-water equivalents to the 

slope sandstones investigated in this Paleogene assessment. Photograph by James L. Coleman, Jr., is also used as banner image.

Downdip Paleogene Formations Assessment Team

Marc L. Buursink, Colin A. Doolan, Catherine B. Enomoto, William H. Craddock, James L. Coleman, Jr., Michael E. Brownfield, 

Stephanie B. Gaswirth, Timothy R. Klett, Phuong A. Le, Heidi M. Leathers-Miller, Kristen R. Marra, Tracey J. Mercier, Ofori N. Pearson, 

Janet K. Pitman, Christopher J. Schenk, Marilyn E. Tennyson, Katherine J. Whidden, and Cheryl A. Woodall

For More Information

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

References Cited

Coleman, J.M.C., 1990, Depositional systems and tectonic/

eustatic history of the Oligocene Vicksburg episode of the 

northern Gulf Coast: University of Texas at Austin, Ph.D. 

dissertation, 722 p.

Cossey, S.P.J., and Jacobs, R.E., 1992, Oligocene Hackberry formation of southwest Louisiana—Sequence stratigraphy, sedimentology, and hydrocarbon potential: American Association 

of Petroleum Geologists Bulletin, v. 76, no. 5, p. 589–606.

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 2007–3066, 

4 p. [Also available at https://pubs.usgs.gov/fs/2007/3066/.]

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

Mexico sedimentary basin, chap. 15 of Miall, A.D., ed., The 

sedimentary basins of the United States and Canada, volume 

5: The Netherlands, Elsevier, Sedimentary Basins of the 

World, p. 505–549.

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, in Schumacher, D., and 

LeShack, L.A., eds., Surface exploration case histories—

Applications of geochemistry, magnetics, and remote sensing: American Association of Petroleum Geologists, Studies 

in Geology No. 48, and Society of Economic Geologists, 

Geophysical References Series No. 11, p. 25–40.

McDonnell, A., Loucks, R.G., and Galloway, W.E., 2008, 

Paleocene to Eocene deep-water slope canyons, western Gulf 

of Mexico—Further insights for the provenance of deepwater offshore Wilcox Group plays: American Association of 

Petroleum Geologists Bulletin, v. 92, no. 9, p. 1169–1189.

Salvador, A., ed., 1991, The Gulf of Mexico Basin: Boulder, 

Colo., Geological Society of America, Decade of North 

American Geology, v. J, 568 p. 

Schenk, C.J., and Viger, R.J., 1996, Western Gulf Province (047), 

in Gautier, D.L., Dolton, G.L., Takahashi, K.I., and Varnes, 

K.I., eds., 1995 national assessment of United States oil and gas 

resources—Results, methodology, and supporting data [release 

2]: U.S. Geological Survey Digital Data Series DDS–30, 44 p. 

[Also available at https://certmapper.cr.usgs.gov/data/noga95/

prov47/text/prov47.pdf.]

Sømme, T.O., Helland-Hansen, W., Martinsen, O.J., and 

Thurmond, J.B., 2009, Relationships between morphological 

and sedimentological parameters in source-to-sink systems—

A basis for predicting semi-quantitative characteristics in 

subsurface systems: Basin Research, v. 21, no. 4, p. 361–387.

Warwick, P.D., Coleman, J.L., Hackley, P.C., Hayba, D.O., 

Karlsen, A.W., Rowan, E.L., and Swanson, S.M., 2007, USGS 

assessment of undiscovered oil and gas resources in Paleogene strata of the U.S. Gulf of Mexico Coastal Plain and State 

waters, in Kennan, L., Pindell, J., and Rosen, N.C., The Paleogene of the Gulf of Mexico and Caribbean Basins—Processes, 

events, and petroleum systems: 27th Annual Gulf Coast 

Section Society for Sedimentary Geology (GCSSEPM) Bob F. 

Perkins Research Conference, December 2–5, 2007, Houston, 

Texas [Proceedings], CD–ROM, p. 2–44.

ISSN 2327-6916 (print) ISSN 2327-6932 (online) https://doi.org/10.3133/fs20183019