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Framework for the Exploration of Libya: An Illustrated Summary
Compiled by
Jingyao Gong1
Search and Discovery Article #10061 (2004)
1AAPG/Datapages, Inc., Tulsa, Oklahoma (jgong@aapg.org)
Recoverable reserves being produced in Libya, from more than 300 fields, exceed 50 billion barrels of oil and 40 trillion cubic feet of gas (Rusk, 2001, 2002). Even so, the Sirte (Sirt), Ghadamis, Murzuq, and Tripolitania basins (Figure 1) are yet to reach full maturity in exploration. Of the 24 giant fields, 20 were discovered prior to 1970. Deep plays are expected to be a large part of upcoming exploration efforts.
Rusk (2001. 2002), in describing the petroleum potential of the centers of Libyan basins, summarized very well the petroleum systems and plays in six basin-center sectors (Figures 1, 2, 3, and 4). This compilation uses the Rusk article as the foundation for presenting several other published maps, cross-sections, and a database, as well as some images in his article; together these should add to the working tool kit for those interested in exploration of Libya.
Maps presented here are in JPEG and PDF formats; for those interested in GIS applications, an expanded version, an atlas, has been prepared for GIS-UDRIL sponsors and other purchasers of AAPG digital products. In the expanded version, approximately 80 georeferenced maps show tectonic features, structural elements and their configuration, thicknesses and facies of key strata and reservoirs, and oil and gas fields, with links to databases and to other images.
The database of giant Libyan fields is from M.K. Horn (2003) in AAPG Memoir 78, Giant Oil and Gas Fields of the Decade 1990-1999. Other information is from various AAPG publications as well as Journal of Petroleum Geology (see Selected Bibliography).
The basin-center sectors in which significant petroleum systems have developed (Rusk, 2001, 2002) are:
Sirte Basin (Figure 2)
South Ajdabiya Trough
Maradah Graben
Southern Zallah Trough – Tumayam Trough
Ghadamis Basin (Figure 3)
Murzuq Basin (Figure 3)
Eastern Tripolitania Basin (Figure 4)
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Sirte BasinCommon features of the three underexplored elements of the Sirte Basin (Figure 2) are:
South Ajdabiya TroughThe underexplored parts of the Ajdabiya Trough cover 8500 km2. In addition to the Sirte-Rachmat Shale (Campanian-Coniacian in age), the Cenomanian-Turonian Etel Formation is an effective source rock, and the Lower Cretaceous lacustrine to lagoonal shale in the Nubian (Sarir) section probably should be considered a minor source (Figure 5). Reservoirs include the Nubian sandstones and Paleocene Lower and Upper Sabil carbonates.
Maradah GrabenThe underexplored part of Maradah Graben is 10,000 km2 in areal extent. The Sirte-Rachmat Shale and the Etel Formation are the known source rocks in this sector (Figure 5). Important reservoirs are the Nubian sandstones, carbonates of the Zelten Formation (equivalent to the Sabil), and Upper Cretaceous Bahi Sandstone, overlying the Nubian.
Southern Zallah Trough – Tumajam TroughThese troughs in the southern to southwestern part of the Sirte Basin contain 25,000 km2 that are underexplored. The Sirte-Rachmat Shale is the known source rock of this area (Figure 5). Nubian sandstones and the commonly indistinguishable Bahi Sandstone are important reservoirs, along with Paleocene Defa and Beda and lower Eocene Facha limestones. Also, as potential reservoirs are Upper Cretaceous sandstones in the Sirte-Rachmat interval.
Central Ghadamis BasinThe basin-center sector encompasses more than 20,000 km2. Two very good source rocks are distributed throughout the entire basin: Lower Silurian Tanezzuft and Middle to Upper Devonian Uennin formations (Figure 6). The main reservoirs are the Upper Silurian Acacus and the Lower Devonian Tadrart and Kasa formations. Three other objectives are the Middle Devonian Uennin Sandstone, Upper Devonian Tahara Formation, and the Triassic Ras Hamia Formation.
Central Murzuq BasinIn this area of more than 30,000 km2, up to the year 2000 only four wells had been drilled. In the area to the north, where oil reserves are some 1 billion barrels, the reservoirs are the sandstones of the Ordovician Memouniat Formation (Figure 6). The documented source rock is the Tanezzuft Shale, although some oil may have been sourced from the Uennin Shale. The potential reservoirs include the Acacus and Tadrart-Kasa sandstones, as well as the Memouniat.
Eastern Tripolitania BasinThis offshore basin, which is a highly faulted, deep trough, extends from the Gulf of Gabes to the northwestern margin of the Sirte Basin. The eastern sector is some 20,000 km2 in extent and is essentially unexplored, with only one dry hole (up to the year 2000); known accumulations are 100-150 km west of the area. Source rocks include the Turonian Bahloul limestone, Necomian-Cenomanian Sidi Kralif – Fahdene shales, Silurian Tanezzuft Shale (in the southwesternmost part), and lower Eocene Chouabine Limestone (in the western part) (Figure 7). Potential reservoirs are (1) limestone comprising the lower Eocene El Garia Formation (of the Metlaoui Group), the main pay in the basin, (2) the Jirani and Bilal carbonates, the equivalent of, or underlying, the El Garia, (3) carbonates of the Alagah and Makhabaz formations (or Lidam-Argub) (Cenomanian-Turonian), and (4) marine sandstones and rudist carbonates of the Lower Cretaceous Turghat-Kiklah sequence.
Table 1
Giant FieldsThe giant fields in Libya (Horn, 2003) are listed below according to basin; the database of selected features of these fields, prepared by M.K. Horn (2003), is presented in Table 1.
Ghadamis Basin Al Wafa
Murzuk BasinElephant
Tripolitania BasinBouri
Figure 8-19
Selected Sirte Basin FieldsProducing reservoirs in the giant fields of the Sirte Basin range from Precambrian basement (igneous rocks) to Oligocene sands. Fracture porosity is important not only in the basement rocks but also in Cambro-Ordovician sandstone at Amal field. Pre-Upper Cretaceous sandstones are important reservoirs in the Sarir, Messlah, Bu Attifel, and Masrab fields. Paleocene and Upper Cretaceous carbonates are the main reservoirs in the other Sirte Basin giant fields. The primary trap in the giant fields ranges from anticline (the most common type), nose, and fault block, to reef and wedge-out/truncation (Horn, 2003). Together, they reflect the tectonic history of the basin, with Mesozoic pre-graben arching, pre-Late Cretaceous faulting and nonmarine sedimentation, Late Cretaceous graben development, represented by several arms, and, to a less extent, Eocene-Neogene sag (Harding, 1984) (Figure 8). Below are sketch-like summaries of fields that together are fairly representative of the range of features the giant fields possess.
Amal Field (Figures 2, 9, and 10) Amal Field is on a north-northwest plunging nose, locally with more than 100 feet of closure at the Rakb (Upper Cretaceous) level. The nose is bounded by major platform/trough-bounding faults. The field is some 30 miles long and 10 miles wide. The main reservoirs are the Cambro-Ordovician Amal Formation, with its fractured quartzose sandstone, and the transgressive-marine sandstones of the Maragh Formation, probably Late Cretaceous in age.
Intisar D Field (Figures 2, 11, 12, and 13) Intisar D Field is one of five productive Paleocene pinnacle reefs that grew in an embayment bounded by three carbonate banks (Brady et al., 1980). It is approximately three miles in diameter. The reef consists largely of corals and algae, with grain- and mud-supported skeletal carbonates. Reef development was responsible for the spectacular reservoir (with 22% porosity) and trap (with 995-foot oil column).
Messlah (Messla) Field (Figures 2, 14, 15, 16, and 17) Messlah field, more than 25 miles long and 5 miles wide, is a stratigraphic trap located on the east flank of a broad Precambrian basement high (Clifford et al., 1980). The reservoir is Lower Cretaceous fluvial Sarir (Nubian) Sandstone, which wedges-out to the west onto the basement. It is truncated by the widespread unconformity at the base of the Upper Cretaceous section. It is similar in type and size to East Texas Field.
Nasser (Zelten) Field (Figures 2, 18, and 19) Nasser Field produces from highly porous Paleocene-Eocene limestone in a faulted anticline that borders the Maradah Graben. The field is more than 16 miles long (parallel to the fault) and 9 miles wide. The Paleocene Zelten “Member,” which is the main pay, experienced porosity enhancement due to groundwater leaching, and secondary porosity as high as 40% has been reported from three skeletal grain-supported shelf limestone facies (Bebout and Pendexter, 1975). These limestones characterized deposition on the platforms separating the arms of the Sirte Graben during some of the Late Cretaceous and much of the Paleocene, while deeper-water deposits characterized the troughs. During the sag phase, Eocene carbonates extended across trough and platform alike.
References and Source Documents for GIS AtlasAl-Shaieb, Z., and J.W. Shelton, 1978, Secondary ferroan dolomite rhombs in oil reservoirs, Chadra sands, Gialo field, Libya: AAPG Bulletin, v. 62, p. 463-468. Ambrose, G., 2000, The geology and hydrocarbon habitat of the Sarir Sandstone, SE Sirt basin, Libya: Journal of Petroleum Geology, v. 23, p. 165-191. |
Table 1.
Database for giant fields of Libya (from M.K. Horn, 2003).
