The Venezuelan hydrocarbon habitat, Part 1: Tectonics, structure, palaeogeography and source rocks

Venezuela forms part of an important hydrocarbon province, defined by the p resence of prolific Cretaceous source rocks, which extends across northern South America. By early 1997, the country had produced 53 billion barrels o f oil. Reserves are estimated to total 73 billion barrels of oil and 146 TC F of gas with 250 billion barrels recoverable in the Heavy Oil Belt. Most r eserves are located within the intermontane Maracaibo and foreland Barinas- Apure and Eastern Venezuela Basins. They correspond to more than 1.5 trilli on BOE originally in place. The province's hydrocarbon history began with a broad passive margin over w hich the sea transgressed throughout much of the Cretaceous. Limestones and shales followed basal sands and included rich source rocks. Convergence be tween the distal part of the area and the Caribbean Plate created an active margin that migrated southwards, so that flysch and wildflysch followed th e transgressive facies. The process culminated in Late Cretaceous to Middle Eocene orogeny with the emplacement of southward-vergent nappes and the de velopment of northward-deepening foredeeps. Flysch and wildflysch formed in the north while important deltaic - paralic reservoir sands accumulated in the south. Major phases of hydrocarbon generation from Jurassic-Cretaceous source rocks occurred across the entire margin of northern South America d uring the orogeny. They are recorded by Jurassic - Middle Cretaceous graphi tic marbles, schists and quartzites (metamorphosed, organic limestones and shales and oil-bearing sandstones) in the Coastal and Northern Ranges of Ve nezuela and Trinidad. They probably charged giant fault and stratigraphic t raps analogous to today's Oficina-Temblador and Heavy Oil Belt accumulation s. From Late Eocene to Recent times, transpressive interaction between norther n South America and neighbouring parts of the Caribbean and the Pacific inv erted Mesozoic extensional systems below the remaining passive margin. The area became subdivided into a series of inter-montane, foreland and pull-ap art basins bounded by transpressional uplifts, the latter suffering conside rable shortening and strike-slip displacement. Sedimentation progressed fro m deep marine to deltaic and molassic facies, providing reservoir sands and local source rocks. Inverted faults and foreland flexuring and interplay b etween structuration and sedimentation produced abundant structural and str atigraphic traps. Hydrocarbons from earlier accumulations suffered further maturation in place, remigrated to younger traps or escaped to the surface. Further hydrocarbon generation, involving Upper Cretaceous source rocks, o ccurred in local foredeep kitchens. Minor contributions also came from Tert iary source rocks.