World Petroleum Resources Project Assessment of Undiscovered Oil and Gas Resources of Libya and Tunisia, 2010

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Introduction

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

for undiscovered conventional oil and gas elds within two geo

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logic provinces of North Africa―Sirte Basin in Libya and Pela

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gian Basin in Tunisia and western Libya―as part of the USGSWorld Petroleum Resources Project (g. 1). The Sirte Basinoriginated as a Cretaceous rift that evolved into a post-rift basindominated by thermal subsidence; it is characterized by carbonatedeposition on high blocks and ne-grained clastic deposition introughs (Ahlbrandt, 2001; Hallett, 2002). The Pelagian Basin wasdominated by Mesozoic and Cenozoic subsidence related to tecto

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nism along the northern margin of the African plate (Klett, 2001).One total petroleum system (TPS) was dened in the SirteBasin Province, and two TPSs were dened in the PelagianBasin Province. The Sirte−Rachmat Composite TPS in the SirteBasin Province contains the post-rift Coniacian−CampanianSirte−Rachmat organic-rich shale/marl, which was deposited introughs across the Sirte Basin during the early phase of thermalsubsidence (Ahlbrandt, 2001; Hallett, 2002). Major reservoirs inthe Sirte Basin Province include syn-rift continental sandstonesand post-rift shallow-marine carbonates, with shales and evapo

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rites acting as seals for hydrocarbon reservoirs (Ahlbrandt, 2001;Hallett, 2002). Two assessment units (AU) were dened withinthe Sirte−Rachmat Composite TPS: the Onshore Sirte Carbon

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ate−Clastic AU and the Offshore Sirte Basin AU.Within the Pelagian Basin, two TPSs as dened by Klett(2001) were retained for this assessment. The Jurassic−CretaceousComposite TPS consists of uids from Jurassic and Cretaceousdeep-marine shales that migrated into Jurassic−Cretaceous shal

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low marine limestones and Upper Cretaceous fractured deep-water chalks. Seals include Jurassic and Cretaceous shales andevaporites. One AU was dened for this TPS, the Jurassic−Cre

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taceous Structural/Stratigraphic AU, based on Klett (2001). TheBou Dabbous Cenozoic TPS contains the Eocene Bou Dabbousorganic-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 Sirteand Pelagian Basin Provinces,North Africa. AU, assessment unit.

 

overlying shales and marls. This TPS contains the Bou Dabbous−Cenozoic Structural/Stratigraphic AU, as dened by Klett (2001).The methodology for the assessment included a completegeologic framework description for each province, based mainlyon published literature and the denition of petroleum systemsand assessment units within these systems. Exploration and dis

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covery history was a critical part of the methodology used to esti

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mate sizes and numbers of undiscovered accumulations. In areaswhere there are few or no discoveries (for example, offshore SirteBasin), geologic analogs were used as a basis for estimating vol

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umes of undiscovered oil and gas resources. Each assessment unitwas assessed for undiscovered oil and nonassociated gas accumu

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lations, and coproduct ratios were used to calculate the volumesof associated gas (gas in oil elds) and natural gas liquids.

Resource Summary

T

he USGS assessed undiscovered conventional oil and gas

resources within the three TPSs in the Sirte and Pelagian BasinProvinces (table 1). The mean total of undiscovered oil in thesetwo provinces is 3,974 million barrels of oil (MMBO), witha range from 1,119 MMBO (95 percent probability) to 9,044MMBO (5 percent probability); for undiscovered gas the meantotal is 38,509 billion cubic feet of gas (BCFG), with a rangefrom 11,520 to 84,347 BCFG; and the mean total for natural gasis 1,466 million barrels of natural gas liquids (MMBNGL), with arange 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 undiscov

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ered natural gas liquids (1,298 MMBNGL) are estimated to bein the Sirte Basin Province. Of these volumes, 64 percent of theundiscovered oil (2,267 MMBO), 80 percent of the undiscoveredgas (25,609 BCFG), and 78 percent of the undiscovered naturalgas liquids (1,010 MMBNGL) are in the Offshore Sirte BasinAU, with the remaining percentages in the Onshore Sirte Carbon

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ate−Clastic AU. The higher percentage of undiscovered oil andgas resources assessed in the Offshore Sirte Basin AU reectsthe relatively underexplored history of this part of the Sirte BasinProvince.Overall, the assessment indicates that (1) 80−90 percentof the undiscovered oil and gas resources are in the Sirte BasinProvince, (2) there is signicantly more total undiscovered gasresource in both provinces (38,509 BCFG or 6,640 MMBOE)than total undiscovered oil resource (3,974 MMBO), and (3) thereis almost twice as much undiscovered gas (25,609 BCFG or 4,415MMBOE) in the Offshore Sirte Basin AU as there is undiscoveredoil (2,267 MMBO).

References Cited

Ahlbrandt, T.S., 2001, The Sirte Basin Province of Libya―Sirte−Zelten total petroleum system: U.S. Geological Survey Bulletin2202-F, 29 p.,

 

http://geology.cr.usgs.gov/pub/bulletins/b2202-f/.Hallett, Don, 2002, Petroleum geology of Libya: Amsterdam, Else

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vier Inc., 503 p.Klett, T.R., 2001, Total petroleum systems of the Pelagian Prov

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ince, Tunisia, Libya, Italy, and Malta―The Bou Dabbous−Ter 

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tiary and Jurassic−Cretaceous Composite: U.S. GeologicalSurvey Bulletin 2202-D, 27 p., http://geology.cr.usgs.gov/pub/ bulletins/b2202-d/

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For Further Information

Supporting studies of the geologic models and the methodol

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ogy used in the assessment of North Africa basins are in progress.Assessment results are available at the USGS Central EnergyResources 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  typeLargestexpectedmeanfield 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 Carbonate-Clastic AU

Oil 

 

4323641,0872,8231,2784181,3384,0351,673227424096

Gas 

 

2,0421,2674,17912,5695,16944151478192Offshore Sirte Basin AU

Oil 

 

8575631,8385,4572,2676332,2507,6772,97234124454170

Gas 

 

6,8436,59119,54049,07722,6372337091,903840

Bou Dabbous-Cenozoic TPS

Bou Dabbous-CenozoicStructural/Stratigraphic AU

Oil 

 

60130283552305451132741311384

Gas 

 

6161,4432,9335,4053,119377514080

Jurassic-Cretaceous Composite TPS

Jurassic-CretaceousStructural/Stratigraphic AU

Oil 

 

1362116212124791542871652474

Gas 

 

5691,0442,4095,0232,643327315480

Total conventionalresources

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

T

 

able 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 ofat 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]