The deposition of organic carbon-rich sediments of the Late Cretaceous La Luna and Navay formations resulted in the formation of a world-class petroleum system in western Venezuela. Over 33 billion barrels of oil have been produced from reservoirs charged by oil from this system, with remaining recoverable reserves of at least 22 billion barrels. Assuming 14% average recovery of original oil in place, as much as 370 billion barrels of oil may have been trapped in the

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Maracaibo and Barinas/Apure basins following generation and expulsion from the La Luna and Navay formations.

The deposition of oil-prone source rocks in northwestern Maracaibo began following drowning of the Albian-Cenomanian Maraca Formation carbonate platform. La Luna/Navay deposition continued in southeastern Maracaibo after drowning of the more southerly Guayacan Member carbonate platform (Capacho and Escandalosa formations) during the late Cenomanian to early Turonian. The Maraca Formation and other shallow water Cogollo Group carbonates are important reservoirs in the western, central (Lake Maracaibo), and southern parts of the Maracaibo Basin. Arguably the best examples of this are the giant Mara-La Paz fields. Fracture-enhanced Coaollo Group reservoir there have produced over 850 MMBO since 1923. New discoveries in the central Barinas/Apure Basin may yield a similar (aggregate) volume from slightly younger Escandalosa Formation sandstones and carbonates.

The conditions under which organic carbon-rich sediments accumulated reflect complex paleoclimatic and paleoceanographic variables, including bathymetric restriction and bottom water anoxia. Specifically, paleobathymetric barriers (Santa Marta and Santander massifs, Paraguana Block, and ancestral Merida Andes) enhanced the development of anoxia by causing poor circulation and limited ventilation. Anoxia was also promoted by high evaporation and low precipitation rates (high salinity bottom water), and high levels of marine algal productivity (high organic matter flux).

Bottom water oxygen levels apparently increased from the late Santonian through the end of the Cretaceous, due primarily to changes in local and global climate. Increased tectonic uplift and higher rainfall rates in eastern Colombia during the Campanian and Maastrichtian contributed to lower TOC level, through delta progradation and siliciclastic dilution. Subsidence of the Paraguana paleobathymetric barrier facilitated exchange with better-oxygenated water from the Atlantic Ocean, thus ending source rock deposition in the Maracaibo Basin. Deposition of sealing Colon and Burguita Formation shales overlying the La Luna/Navay source rocks completed the key components of the Cretaceous petroleum system.

The best areas for the future discovery of large oil fields sourced from and reservoired within Cretaceous rocks may occur along the Sierra Perija Mountains of the western Maracaibo Basin, and along the central Merida Andes Mountain front in the western Barinas/Apure Basin.

Unnumbered Figure. Unconformable contact between the shallow water platform carbonate of the  Late Cenomanian Guayacan Member of the Escandalosa Formation (right of the arrows), and the organic carbon-rich middle shelf shales of the Turonian Navay Formation (left of the arrows). Location: Rio Santo Domingo, eastern Merida Andes, western Venezuela.

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