The Maracaibo basin lies mainly in northwestern Venezuela and occupies the V-shaped depression between the diverging Andes de Merida and Sierra de Perija, two offshoots of the main Cordilleran system of South America. On the southwest the basin extends slightly into eastern Colombia.

The area of the basin is approximately 61,450 square kilometers (23,572 square miles), of which about 12,900 square kilometers (5,000 square miles) are covered by Lake Maracaibo, a large body of brackish water whose outlet is into the Caribbean Sea by way of the Gulf of Venezuela. Large areas surrounding the lake are covered by swamps and heavily wooded flats and nowhere within the basin proper is the elevation more than 100 meters (328 feet).

The presence of oil in the Maracaibo basin has been known for centuries, but it was not until the years immediately preceding the first World War that the major oil companies took an active interest in the area. The Shell Company led the way and in 1914 discovered the Mene Grande field, east of Lake Maracaibo. During the next 2 years the same company discovered the Rio de Oro and Las Cruces (Tarra) fields, southwest of the lake, and in 1917 drilled the discovery well of the Bolivar Coastal field. This company's success aroused the interest of others and it was not long until most of the major American oil companies and a number of independents were active in western Venezuela. At the present time only the Creole Petroleum Corporation (Standard Oil Company (New Jersey)), the Shell Grou , the Mene Grande Oil Company (Gulf Oil Corporation), the Orinoco Oil Company (Pure Oil Company), and the Richmond Exploration Company (Standard Oil of California) are active in the Venezuelan part of the basin, while the Colombian Petroleum Company (Socony Vacuum-Texas) is active in the Colombian part.

About 85 per cent of the basin floor is covered by the waters of Lake Maracaibo and recent deposits, but the bordering highlands expose a geologic section extending from the pre-Cambrian to Recent. The Ordovician, Devonian, Permo-Carboniferous, and Triassic were periods of rather widespread deposition, but each was followed by a long interval of uplift and erosion so that only remnants of the original deposits are now present in the uplifted areas.

The pre-Cambrian is represented by the igneous and metamorphic rocks of the Perija and Iglesias series, which form the cores of the bordering mountain ranges. These are followed by the largely metamorphosed Mucuchachi series of Upper Cambrian to Upper Ordovician age. The Devonian is well developed in the Sierra de Perija, where more than 2,438 meters (8,000 feet) of the fossiliferous sediments of the Cachiri group are exposed along the Rio Cachiri. The Palmarito series of the Permo-Pennsylvanian is distributed extensively throughout the Merida Andes and along the eastern slopes of the Sierra de Perija. The greatest thickness is in the type area of the Merida Andes, where 1,800 meters (5,910 feet) have been measured. The redbeds of the Upper Triassic La Quinta formation are generally l mited to the mountain regions and are best developed in Tachira and Merida, where thicknesses up to 3,500 meters (11,482 feet) have been noted.

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Rocks of Cretaceous age are distributed widely around the margin of the basin. The Lower Cretaceous is represented by the thick sandstones and conglomerates of the Rio Negro formation and has a limited distribution in the west central part of the District of Perija, Zulia. The Middle Cretaceous is marine in origin and has an over-all average thickness of about 700 meters (2,297 feet). Its component formations are the Apon, Aguardiente, and Capacho, all of which are distributed widely throughout the basin. The Apon formation, formerly included in the Cogollo, is defined here for the first time. The Upper Cretaceous has an average over-all thickness of about 650 meters (2,133 feet). It is largely of marine origin and comprises the La Luna, Colon, Mito Juan, and Catatumbo formations.

The Paleocene is represented by the shallow marine sediments of the Guasare formation, which has an average thickness of 400 meters (1,312 feet) and occupies a limited belt extending southeastward across the northern part of the basin.

The Eocene sequence, which is largely of marine origin, has an over-all thickness in excess of 3,700 meters (12,140 feet). Its component members are distributed widely throughout the basin and comprise the Marcelina, Misoa, Pauji, Ambrosio, and equivalent formations.

The post-Eocene Tertiary section has a thickness exceeding 1,600 meters (5,249 feet). The fresh-water and terrestrial deposits of the Oligocene El Fausto, and the equivalent Icotea formation, have a limited areal distribution. The La Rosa and Lagunillas formations of the Lower and Middle Miocene are largely of marine and brackish-water origin and, with their equivalents, are distributed widely throughout the area. The Betijoque formation of the Upper Miocene and Cerro Vigia formation of the Pliocene are fresh-water and terrestrial deposits. The former has a rather extensive distribution but the latter is limited to the northwestern part of the basin.

At one place or another within the basin, commercial accumulation of oil has been discovered in all but two of the seventeen formations comprising the geologic section which extends from the base of the Middle Cretaceous to the top of the Middle Miocene. This sequence has an over-all thickness in excess of 7,000 meters (23,000 feet).

Numerous oil seepages and asphalt deposits are present around the edges of the basin and along the crests of truncated anticlines where the Cretaceous and Eocene sediments crop out.

The Maracaibo basin occupies the structural depression which resulted from the uplift of the bordering Merida Andes and Sierra de Perija. It has been subjected to the recurrent Andean orogenies which began at the close of the Eocene and culminated in the late Pleistocene when the ranges were elevated to their present heights and the basin received its present outline. Throughout a long part of Oligocene time the area remained above sea-level and was severely eroded. The major unconformity of the post-Cretaceous is along this break.

A number of structural trends, parallel and subordinate to the bordering highlands, are present along the eastern and western flanks, and it is along these that the outlying oil fields of the basin are located. The Bolivar Coastal field, largest in the area and outstanding among the major oil fields of the world, occupies a position on the lakeward-dipping northeastern limb of the basin. The structure is monoclinal and the post-Eocene sediments are deposited on the eroded and partially peneplaned Eocene surface, which was tilted gently to the southwest in late Pleistocene time. The oil in the post-Eocene sediments represents updip shore-line accumulation, controlled largely by the type of sedimentation. Accumulation in the underlying Eocene may be either stratigraphic or structural. >

At the present time there are thirteen active fields in the basin. Eight of these are located in Venezuela and five in Colombia. In 1945 the total production for all fields was 221,730,000 barrels, approximately 9 per cent of the reported world total. On December 31, 1945, the cumulative total production was 2,634,430,000 barrels.

The Bolivar Coastal field produced 186,861,000 barrels during 1945 and, on December 31, 1945, had a total cumulative production of 2,188,339,000 barrels.

The total remaining proved reserves of the basin are estimated to be in excess of 5,500,000,000 barrels.