The Venezuelan land area embraces some 920,000 sq km lying between the Equator and latitude 10 degrees north. The western part of the country comprises two northerly trending mountain chains which are the result of the bifurcation of the Andes. Between them lie two major bodies of water, Lake Maracaibo and the Gulf of Venezuela.

The northern Caribbean coast is marked by an east-west trending chain of mountains which was formed as a result of movements at the junction of the South American and the Caribbean tectonic plates. To the south of this range, the Orinoco and Apure rivers drain from the west and the south-west before debouching via a large delta into the Atlantic Ocean.

The area south of the Orinoco is composed of mountains in which basement rocks of the Guyanan shield are cut by a series of northerly draining tributaries of the Orinoco.

The Orinoco-Apure river system effectively divides the country in two. Most of the southern half is non-prospective while the northern half, along with Lake Maracaibo, a total area of about 350,000 sq km, contains the world's greatest hydrocarbon reserves. The bitumen-rich area, along the northern bank of Orinoco, is often referred to as the "Faja" or the "Orinoco Tar Belt". Step-out drilling has taken place around fields in the north-west and south-east of Lake Maracaibo and northern Monagas. These concepts rendered in 2002 a major new discovery called Ceuta-Tomoporo.

The three major petroliferous basins of Venezuela are, from west to east, the Maracaibo Basin, the Barinas-Apure Basin and the Oriental Basin. A fourth basin, the Falcon, lies to the east of Lake Maracaibo but is insignificant compared to the others.

The petroleum geological history of Venezuela is linked to the opening of the Atlantic in the Mesozoic age. The subsequent development of basins and the neighbouring orogenic uplifts have been associated with the movements of the South American and Caribbean tectonic plates in the period between the Tertiary to Recent ages.

Of major importance to geologists was the deposition throughout the sedimentary area in the Late Cretaceous of organic-rich, bituminous shales and limestones which have provided prolific source rocks in all four basins. Subordinate source rocks are found in the Tertiary which are often gas-prone.

PDVSA geologists are also excited by a new play that some believe has the potential to rival that of North African success stories such as Algeria and Sudan. Based on the premise of Venezuela's ancient geological link with North Africa, PDVSA has focused on an under-explored south-west region of Venezuela known as the Western Llanos. Outcrop studies over the last four years have identified some pre-Cretaceous source rocks and oil seepage in the Andean region. Some of the Palaeozoic formations tend to run too deep for commercial purposes but an encouraging trend at about 12,000 feet or less has been identified.

Reservoir rocks in Venezuela range from fractured basement to Late Tertiary and comprise, for the most part, sandstones with subordinate limestones. The sandstone depositional environments range from being open marine through deltaic to fluviatile.

There is a wide variety of traps from those associated with normal faulting on the passive side of basins to complex fold-fault traps where overthrusting has resulted from mountain building at active basin margins. Many traps also have a stratigraphic element in them.

The Maracaibo Basin, with an area of 50,000 sq km, is the most important in Venezuela and almost 40 bn barrels of oil have been extracted from it. In recent years the basin has produced more than half a billion barrels per annum. The centre of the basin is occupied by the shallow waters of Lake Maracaibo.

The main producing areas extend from the shore out into the lake, with the greatest concentration of fields being in the northern portion. The onshore fields contain both heavy and medium oils, while in the offshore medium and light crude oils predominate.

The petroleum geological history of Venezuela is linked to the opening of the Atlantic in the Mesozoic age. The subsequent development of basins and the neighbouring orogenic uplifts have been associated with the movements of the South American and Caribbean tectonic plates in the period between the Tertiary to Recent ages.