U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2011–3105

September 2011

World Petroleum Resources Project

Assessment of Undiscovered Oil and Gas Resources of Libya 

and Tunisia, 2010

Printed on recycled paper

Introduction

The U.S. Geological Survey (USGS) assessed the potential 

for undiscovered conventional oil and gas fields within two geologic provinces of North Africa―Sirte Basin in Libya and Pelagian Basin in Tunisia and western Libya―as part of the USGS 

World Petroleum Resources Project (fig. 1). The Sirte Basin 

originated as a Cretaceous rift that evolved into a post-rift basin 

dominated by thermal subsidence; it is characterized by carbonate 

deposition on high blocks and fine-grained clastic deposition in

troughs (Ahlbrandt, 2001; Hallett, 2002). The Pelagian Basin was 

dominated by Mesozoic and Cenozoic subsidence related to tectonism along the northern margin of the African plate (Klett, 2001).

One total petroleum system (TPS) was defined in the Sirte 

Basin Province, and two TPSs were defined in the Pelagian 

Basin Province. The Sirte−Rachmat Composite TPS in the Sirte 

Basin Province contains the post-rift Coniacian−Campanian 

Sirte−Rachmat organic-rich shale/marl, which was deposited in 

troughs across the Sirte Basin during the early phase of thermal 

subsidence (Ahlbrandt, 2001; Hallett, 2002). Major reservoirs in 

the Sirte Basin Province include syn-rift continental sandstones 

and post-rift shallow-marine carbonates, with shales and evaporites acting as seals for hydrocarbon reservoirs (Ahlbrandt, 2001; 

Hallett, 2002). Two assessment units (AU) were defined within 

the Sirte−Rachmat Composite TPS: the Onshore Sirte Carbonate−Clastic AU and the Offshore Sirte Basin AU. 

Within the Pelagian Basin, two TPSs as defined by Klett 

(2001) were retained for this assessment. The Jurassic−Cretaceous 

Composite TPS consists of fluids from Jurassic and Cretaceous 

deep-marine shales that migrated into Jurassic−Cretaceous shallow marine limestones and Upper Cretaceous fractured deepwater chalks. Seals include Jurassic and Cretaceous shales and 

evaporites. One AU was defined for this TPS, the Jurassic−Cretaceous Structural/Stratigraphic AU, based on Klett (2001). The 

Bou Dabbous Cenozoic TPS contains the Eocene Bou Dabbous 

organic-rich shale, with hydrocarbons that migrated into lower 

and middle Eocene shallow-water limestones that are sealed by 

Using a geology-based assessment methodology, the U.S. 

Geological Survey estimated means of 3.97 billion barrels of 

undiscovered oil, 38.5 trillion cubic feet of undiscovered natural 

gas, and 1.47 billion barrels of undiscovered natural gas liquids 

in two provinces of North Africa.

Figure 1. Locations of the Sirte 

and Pelagian Basin Provinces, 

North Africa. AU, assessment unit.
overlying shales and marls. This TPS contains the Bou Dabbous−

Cenozoic Structural/Stratigraphic AU, as defined by Klett (2001).

The methodology for the assessment included a complete 

geologic framework description for each province, based mainly 

on published literature and the definition of petroleum systems 

and assessment units within these systems. Exploration and discovery history was a critical part of the methodology used to estimate sizes and numbers of undiscovered accumulations. In areas 

where there are few or no discoveries (for example, offshore Sirte 

Basin), geologic analogs were used as a basis for estimating volumes of undiscovered oil and gas resources. Each assessment unit 

was assessed for undiscovered oil and nonassociated gas accumulations, and coproduct ratios were used to calculate the volumes 

of associated gas (gas in oil fields) and natural gas liquids.

Resource Summary

The USGS assessed undiscovered conventional oil and gas 

resources within the three TPSs in the Sirte and Pelagian Basin 

Provinces (table 1). The mean total of undiscovered oil in these 

two provinces is 3,974 million barrels of oil (MMBO), with 

a range from 1,119 MMBO (95 percent probability) to 9,044 

MMBO (5 percent probability); for undiscovered gas the mean 

total is 38,509 billion cubic feet of gas (BCFG), with a range 

from 11,520 to 84,347 BCFG; and the mean total for natural gas 

is 1,466 million barrels of natural gas liquids (MMBNGL), with a 

range from 405 to 3,384 MMBNGL.

About 90 percent of the mean total of undiscovered oil 

(3,545 MMBO), 85 percent of the mean total of undiscovered gas 

(32,451 BCFC), and 89 percent of the mean total of undiscovered natural gas liquids (1,298 MMBNGL) are estimated to be 

in the Sirte Basin Province. Of these volumes, 64 percent of the 

undiscovered oil (2,267 MMBO), 80 percent of the undiscovered 

gas (25,609 BCFG), and 78 percent of the undiscovered natural 

gas liquids (1,010 MMBNGL) are in the Offshore Sirte Basin 

AU, with the remaining percentages in the Onshore Sirte Carbonate−Clastic AU. The higher percentage of undiscovered oil and 

gas resources assessed in the Offshore Sirte Basin AU reflects 

the relatively underexplored history of this part of the Sirte Basin 

Province.

Overall, the assessment indicates that (1) 80−90 percent 

of the undiscovered oil and gas resources are in the Sirte Basin 

Province, (2) there is significantly more total undiscovered gas

resource in both provinces (38,509 BCFG or 6,640 MMBOE) 

than total undiscovered oil resource (3,974 MMBO), and (3) there 

is almost twice as much undiscovered gas (25,609 BCFG or 4,415 

MMBOE) in the Offshore Sirte Basin AU as there is undiscovered 

oil (2,267 MMBO).

References Cited

Ahlbrandt, T.S., 2001, The Sirte Basin Province of Libya―Sirte−

Zelten total petroleum system: U.S. Geological Survey Bulletin 

2202-F, 29 p., http://geology.cr.usgs.gov/pub/bulletins/b2202-f/.

Hallett, Don, 2002, Petroleum geology of Libya: Amsterdam, Elsevier Inc., 503 p.

Klett, T.R., 2001, Total petroleum systems of the Pelagian Province, Tunisia, Libya, Italy, and Malta―The Bou Dabbous−Tertiary and Jurassic−Cretaceous Composite: U.S. Geological 

Survey Bulletin 2202-D, 27 p., http://geology.cr.usgs.gov/pub/

bulletins/b2202-d/.

For Further Information

Supporting studies of the geologic models and the methodology used in the assessment of North Africa basins are in progress. 

Assessment results are available at the USGS Central Energy 

Resources Science Center website, http://energy.cr.usgs.gov/

oilgas/.

North Africa Assessment Team: 

Katherine J. Whidden (Task Leader, kwhidden@usgs.gov), 

Michael Lewan, Christopher J. Schenk, Ronald R. Charpentier, 

Troy A. Cook, Timothy R. Klett, and Janet Pitman.

Total petroleum systems 

(TPS) 

and assessment units (AU)

Field

type

Largest 

expected 

mean

field size

Total undiscovered resources

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

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

Sirte-Rachmat Composite TPS

Onshore Sirte CarbonateClastic AU

Oil 432 364 1,087 2,823 1,278 418 1,338 4,035 1,673 22 74 240 96

Gas 2,042 1,267 4,179 12,569 5,169 44 151 478 192

Offshore Sirte Basin AU

Oil 857 563 1,838 5,457 2,267 633 2,250 7,677 2,972 34 124 454 170

Gas 6,843 6,591 19,540 49,077 22,637 233 709 1,903 840

Bou Dabbous-Cenozoic TPS

Bou Dabbous-Cenozoic 

Structural/Stratigraphic AU

Oil 60 130 283 552 305 45 113 274 131 1 3 8 4

Gas 616 1,443 2,933 5,405 3,119 37 75 140 80

Jurassic-Cretaceous Composite TPS

Jurassic-Cretaceous 

Structural/Stratigraphic AU

Oil 13 62 116 212 124 79 154 287 165 2 4 7 4

Gas 569 1,044 2,409 5,023 2,643 32 73 154 80

Total conventional

 resources 1,119 3,324 9,044 3,974 11,520 32,916 84,347 38,509 405 1,213 3,384 1,466

Table 1. Libya and Tunisia assessment results.

[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). Undiscovered gas resources are the sum of nonassociated and associated gas. F95 represents a 95-percent chance of 

at least the amount tabulated; other fractiles are defined similarly. Largest expected oil field in MMBO; largest expected gas field in BCFG. TPS, total petroleum system; AU, 

assessment unit. Gray shading indicates not applicable]