venerdì 16 dicembre 2011

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

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The U.S. Geological Survey (USGS) assessed the potential
for undiscovered conventional oil and gas elds within two geo
logic provinces of North Africa―Sirte Basin in Libya and Pela
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
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
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
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
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
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
covery history was a critical part of the methodology used to esti
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
umes of undiscovered oil and gas resources. Each assessment unitwas assessed for undiscovered oil and nonassociated gas accumu
lations, and coproduct ratios were used to calculate the volumesof associated gas (gas in oil elds) and natural gas liquids.
Resource Summary
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
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
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.,, Don, 2002, Petroleum geology of Libya: Amsterdam, Else
vier Inc., 503 p.Klett, T.R., 2001, Total petroleum systems of the Pelagian Prov
ince, Tunisia, Libya, Italy, and Malta―The Bou Dabbous−Ter 
tiary and Jurassic−Cretaceous Composite: U.S. GeologicalSurvey Bulletin 2202-D, 27 p., bulletins/b2202-d/
For Further Information
Supporting studies of the geologic models and the methodol
ogy used in the assessment of North Africa basins are in progress.Assessment results are available at the USGS Central EnergyResources Science Center website,
North Africa Assessment Team:
Katherine J. Whidden (Task Leader,,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 Oi(MMBO)Ga(BCFG)NG(MMBNGL
Sirte-Rachmat Composite TPS
Onshore Sirte Carbonate-Clastic AU
2,0421,2674,17912,5695,16944151478192Offshore Sirte Basin AU
Bou Dabbous-Cenozoic TPS
Bou Dabbous-CenozoicStructural/Stratigraphic AU
Jurassic-Cretaceous Composite TPS
Jurassic-CretaceousStructural/Stratigraphic AU
Total conventionalresources
1,113,329,043,974 11,5232,9184,3438,50401,213,381,466
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]

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