The Venezuelan hydrocarbon habitat, part 2: Hydrocarbon occurrences and generated-accumulated volumes

Venezuela's most important hydrocarbon reserves occur in the intermontane M aracaibo Basin and in the Eastern Venezuela foreland basin. Seeps are abund ant in these areas. Lesser volumes occur in rite Barinas-Apure foreland bas in. Most of the oil in these basins was derived from the Upper Cretaceous L a Luna Formation in the west and its equivalent, the Querecual Formation, i n the east. Minor volumes of oil derived from Tertiary source rocks occur i n the Maracaibo and Eastern Venezuela Basins and in the Falcon area. Offshore, several TCF of methane with some associated condensate are presen t in the Carupano Basin, and gas is also present in the Columbus Basin. Oil reserves are present in La Vela Bay and in the Gulf of Paria, and oil has been encountered in the Cariaco Basin. The Gulf of Venezuela remains undril led. The basins between the Netherlands and Venezuelan Antillian Islands se em to lack reservoirs. Tertiary sandstones provide the most important reservoirs, but production c omes also from fractured basement (igneous and metamorphic rocks), from bas al Cretaceous sandstones and from fractured Cretaceous limestones. Seals ar e provided by encasing shales, unconformities, faults and tar plugs. There is a wide variety of structural and stratigraphic traps. The Orinoco Heavy Oil Belt of the Eastern Venezuela Basin, one of the World's largest accumul ations (1.2 x 10(12) brl) involves stratigraphic trapping provided by onlap and by tar plugging. Stratigraphic trapping involving unconformities and t ar plugging also plays a major role also in the Bolivar Coastal complex of fields along the NE margin of Lake Maracaibo. Many of the traps elsewhere i n the Maracaibo Basin were influenced by faulting. The faults played an ext ensional role during Jurassic rifting and subsequently suffered inversion a nd strike-slip reactivation. This created anticlines as well as fracture po rosity and permeability: and influenced the distribution of sandstone reser voirs, unconformities and related truncation traps. The faults probably als o provided migration paths as well as lateral seals. This is very likely th e case also in the large, thrust-related traps of the Furrial Trend in East ern Venezuela. Normal faults, many antithetic to basement dip, provide impo rtant traps in the Las Mercedes, Oficina and Ternblador complexes on the so uthern flanks of the Eastern Venezuela Basin. Similar faults seem to contro l the Sinco-Silvestre complex of the Barinas-Apure Basin. Much of Venezuela's crude (around 1.5 trillion brls original STOIIP) has be en degraded and is heavy. Perhaps two to three trillion brls of precursor l ighter ail existed. While the known Upper Cretaceous La Luna and Querecual Formations are known to include prolific source racks, a reasonable generat ion/accumulation efficiency of 10% implies volumes too large to have come f rom the reported kitchens. The country's vast reserves are perhaps better e xplained by recognizing that the present clay basins are remnants of much b roader sedimentary ureas. The source rocks originally had a much more regio nal distribution. They suffered widespread, earlier phases of generation th at probably charged early-formed traps on a regional scale. These, together with more recent kitchens, provided oil to the present-day accumulations. This history involved long-distance migration and remigration.