LIBYA - 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 plateau land where major oil reservoirs have been found. The greater part of the country lies to the south in the Sahara which is also rich in oil.

Libya remains largely unexplored. It has four main oil fairways onshore: the Sirte Basin in the centre, by far 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 continental 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 oilfields, is the richest oil province in North Africa. It stretches southward from the Gulf of Sirte and contains structures with strata of marine clastic and carbonate sediments. These structures 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 and in sandstones and limestones.

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's regime imposed tough terms on the companies before some of them were nationalised. As a result, the Sirte Basin has remained under-explored, with its deeper horizons untouched.

In addition to the many folded and faulted traps, there is a variety of stratigraphic traps that are 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 gas to be more common in the north-central part of the 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 few 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, such as the membrane separation process, their size is too small to attract foreign E&P investors.

The Ghadames Basin: Basement in Libya rises to crop out in the south and south-west. In the western part of the country the Ghadames elements seen in Algeria and Tunisia cross into Libya. Among interesting discoveries in Libya close to the Algerian border is the Al-Wafaa gas/condensate field. The field was found in 1991 by the state-owned National Oil Co. (NOC) and is an extension of Alrar gas field in Algeria. Al-Wafaa is estimated to have recoverable reserves of 100m barrels of condensates and 2,000 BCF of natural gas. The field, developed jointly by NOC and Agip, is one of the sources of gas being exported to Italy by pipeline.

Early exploration by foreign companies in the Ghadames Basin resulted in small oil finds which were not developed, for lack of infrastructure. In the 1970s, NOC discovered several oilfields in the Hammadah Al-Hamra plateau. Although they were small, the fields were close to one another and this warranted their development by NOC. The oil is reservoired in Palaeozoic sandstones.

Another petroliferous province has been established in the south-west of that region extending from the Palaeozoic Illizi Basin of Algeria. Oil is reservoired in Palaeozoic sandstones sourced from the same prolific Silurian shales which have contributed so much oil in North Africa. There are traps in that region which occur in folded and faulted structures, as in the case of the Hammada Al-Hamra Basin to the north.

The Murzuk Basin: Separated from Ghadames by the Qarqaf uplift, the Murzuk Basin lies in a remote province formerly called Fezzan. It is about 800 km south-west of the Libyan coast. Early geological surveys there point to a strong possibility of reserves of up to 12,000m barrels of oil equivalent, of which 83% are likely to be gas. Al-Sharara field there is a giant, found in 1985 by Rompetrol of Romania which then said its recoverable reserves were about 2 bn barrels. In late 1993, NOC said that its recoverable reserve exceeded 5 bn barrels. The field, operated by Repsol/YPF, came on stream in late 1996 and will be producing up to 250,000 b/d eventually. Another Murzuk giant, Elephant field, was found in October 1997 by Lasmo of the UK (now part of Agip) and should be producing 150,000 b/d (see Part 2).

Most Murzuk fields have a Palaeozoic reservoir similar to that of Illizi in Algeria. A northern extension was found in the 1980s by a Bulgarian firm but this failed to develop the structure because it lacked the funds and in 1996 its rights were sold to Canadian Occidental. The oil reserves there were originally estimated at about 600m barrels. Repsol/YPF has made several important oil discoveries in Blocks NC-186 and NC-190 in this basin (See Repsol profile in Part 2).

The Kufrah Basin: In the remote and unexplored south-east, the Kufrah Basin contains partly marine Palaeozoic strata. Until recently, no firm showed interest in this basin, with NOC saying it would be similar to Murzuk's.

In May and June 2003 Repsol/YPF of Spain in partnership with OMV of Austria, and RWE-Dea of Germany took three blocks in Kurfra. Some firms are considering exploration in Chad, close to Kufra. Kufra's underwater reservoir forms part of the so-called Great Man-made River (GMR).