Petroleum exploration in Libya started in late 1953 with a regional reconnaissance of surface geology that was carried out under special exploration permits. In June 1955 the Libyan Petroleum Law (Law no 25/1955), which governs Petroleum Exploration and Exploitation, came into effect, and by the end of that year 47 concessions covering 519,816 km2 (the equivalent of almost 90 North Sea quadrants) distributed throughout the country's sedimentary basins, had been granted to a number of different companies.
Major tectonic elements of the Sirt Basin
Bahi - good
The first Libyan petroleum exploration well (A1-18) was spudded on April 19, 1956, on a large surface structure in the Jabal Akhadar Cyrenaica Platform in the north-east part of the country, but this was a dry hole. The first Libyan hydrocarbon discovery was Atshan 1 B2-1 in the western Murzak Basin (compare map), which tested non-commercial oil from Devonian Sandstones on December 27, 1957.
The first commercial discovery was made in the Sirt Basin in 1958 with well A1-32 (Bahi Field), which tested 704 bpd of 40° API oil from basal Upper Cretaceous Sandstones overlying a major pre-Cretaceous unconformity (Hercynian Unconformity). Excitement at the first hydrocarbon discovery was expressed by the drilling crew shouting the word "Bahi", meaning good in Arabic, giving us both the formation name Bahi Sandstones and the Bahi Field.
The first giant discovery, the Zelten Field, was made with well C1-6, testing 17,500 bopd from Upper Paleocene limestone.
Oil and gas fields in the Sirt Basin
A significant petroleum province
In the same year (1958), another well, B1-32, was drilled about 40 km south-east of A1-32. This resulted in a giant hydrocarbon discovery, the Dahra Field, in Paleocene carbonates, and the huge potential of Waha Oil Company's Concession 32 was confirmed. Today the total estimated hydrocarbon reserves in-place for Concession 32 adds up to approximately 7 billion barrels of oil (Bbo) and about 2 trillion cubic feet of gas (Tcfg).
Following these discoveries, systematic exploration drilling along the major structural features was carried out. To date 22 giant fields, more than 25 large fields, and many small accumulations, with total known recoverable reserves of 42 Bbo and 33 Tcfg, have been discovered in the Sirt Basin, ranking the basin 15th among the world's premier petroleum provinces.
The Upper Cretaceous Sirte Shale, which is thickly developed in the deep trough areas (compare structural cross-section), is the major source of hydrocarbons. At least seven stratigraphic pay zones, varying in depth from 600 - 4700 m (15,500 - 2,000 ft) and ranging in age from Precambrian fractured basement to Oligocene, are known to be present in the Sirt Basin.
Generalized correlation chart of the western part of Sirt Basin where A1-32 was discovered
Exploration activities in the Sirt basin have until very recently concentrated mainly on the relatively shallow and easy targets found on the structural highs, within structural and stratigraphic plays around the basement highs, and on a few structures in the relatively shallow troughs, while the deep troughs remain virtually unexplored.
When we consider the petroleum source rocks available in the Sirt Basin, the quantity of hydrocarbons generated and the volume found so far, supported by the lack of surface oil seepages in the basin, it is reasonable to believe that the basin has enormous potential reserves still waiting to be discovered.
The deep trough areas, the basement rims and the barely explored stratigraphic traps are all thought to have a significant unexplored potential. This should be released by recent advances in seismic technology, modern improvements in data interpretation, and developments in deep drilling technology.
Structural cross section over the Dahra Platform.