overlying shales and marls. This TPS contains the Bou Dabbous−Cenozoic Structural/Stratigraphic AU, as dened by Klett (2001).The methodology for the assessment included a completegeologic framework description for each province, based mainlyon published literature and the denition 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 reectsthe 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 signicantly 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
-
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/
.
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]