LIBYA - Libya's Geology.

With a population of less than 5m, Libya occupies 1.76m sq km of territory between Tunisia and Algeria to the west and Egypt to the east. On both sides, the Libyan geology is interesting, with rocks being the sources of high quality oil.

The northern part of the country consists of arable

Libya remains largely unexplored. It has four main oil fairways onshore: the Sirte Basin in the centre, the richest; the Cyrenaica Basin in the east; the Ghadames Basin in the west; and the Murzuk Basin in the south-west. A fairly rich area lies in the Atchan Saddle of al-Qarqaf, between the south of Ghadames and Murzuk.

Most parts of the Kufrah Basin in the south-east are said to lack suitable source rocks. In the south-east, Libya has lost its claim to a 114,000 sq km strip rich in uranium called Aouzou, which it occupied during successive battles with Chad in the past three decades but which in 1994 was awarded to Chad by the International Court of Justice.

Offshore, there is a relatively narrow shelf and slope in the Mediterranean and the Gulf of Sirte, which widens to the west in the Gulf of Gabes, where the Pelagian Basin is the richest zone for oil and gas. In the northern part of the Gabes Gulf, an area called "November Seven" zone, rich in oil and gas and long disputed by Libya and Tunisia, is being exploited by a joint Libyan-Tunisian oil company, as part of a territorial settlement reached in late 1990.

The Sirte Basin, the location of most of Libya's producing fields, is the richest oil province in Africa. This basin holds 22% of Africa's 300 bn barrels of oil reserves. It stretches south from the Gulf of Sirte and contains structures with strata of marine clastic and carbonate sediments. These were formed during transgressions and regressions of the seas across the relatively flat North African Shelf in prehistoric times. Oil there is found in a variety of reservoirs ranging in age from Cambrian to Tertiary.

Because of the difficulties of dating the many barren sandstones, debate on the age of many of the producing reservoirs lasted through most of the early years of exploration in the 1950s.

The basin's oil reservoirs are generally carbonates, with shale providing the caprock. But those in the south-east of the basin, such as the Sarir field in the Calenscio Sand Sea, are sandstones of Cretaceous or Cambrian age, overlain by thick marine Cretaceous shales.

There are Palaeozoic basins which have been partly explored. In the 1960s, foreign companies which found major reserves of high quality oil in the Sirte Basin did not explore deeper strata or look for oil in the more remote parts of Libya. They became more reluctant to explore in the early 1970s, when the government under Qadhafi imposed tough terms on IOCs before some of their assets were nationalised. As a result, the Sirte Basin has remained under-explored.

In addition to the many folded and faulted traps, there is a variety of stratigraphic traps yet to be properly studied. These include carbonate reefs and sand pinchouts both against older highs and as lenses.

With Tertiary and Upper Cretaceous shales forming the major source rocks, much of the oil must have leaked across unconformities into geologically older strata. There is a tendency for natural gas to be more common in the north-central part of the Sirte Basin and this may be a depth-dependent feature.

Offshore, a relatively small amount of drilling has been done in the Gulf of Sirte. The successes have been mostly gas discoveries which may be related to the nearby onshore parts of the Sirte Basin. The main success is the Pelagian Basin (or Tripolitanian Shelf) in the Gulf of Gabes, where Libya's only producing offshore field is el-Bouri. Oil was found under a thick gas cap, both reservoired in Tertiary limestones, possibly sourced from late Cretaceous to Tertiary shales.

Offshore prospects in areas other than the Gulf of Gabes do not seem so promising. This is mainly in view of the relatively small shelf area and the results of exploration to date. All discoveries there, each made by a different company and on a different block, have been small in relative terms. Some of the wells have tested gas with very high CO2 and H2S content. Although they can be exploited commercially with the use of advanced technology, their size is too small to attract foreign E&P investors.