THE VENEZUELAN HYDROCARBON HABITAT, PART 1: TECTONICS, STRUCTURE, PALAEOGEOGRAPHY AND SOURCE ROCKS

K. H. James*

*Consultant Geologist, 1111 Briar Bayou Drive, Houston, Texas 77077, USA.

Venezuela forms part of an important hydrocarbon province, defined by the presence of prolific Cretaceous source rocks, which extends across northern South America. By early 1997, the country had produced 53 billion barrels of oil. Reserves are estimated to total 73 billion barrels of oil and 146 TCF of gas with 250 billion barrels recoverable in the Heavy Oil Belt. Most reserves are located within the intermontane Maracaibo and foreland Barinas-Apure and Eastern Venezuela Basins.They correspond to more than 1.5 trillion BOE originally in place.

The province's hydrocarbon history began with a broad passive margin over which the sea transgressed throughout much of the Cretaceous. Limestones and shales followed basal sands and included rich source rocks. Convergence between the distal part of the area and the Caribbean Plate created an active margin that migrated southwards, so that flysch and wildflysch followed the transgressive facies. The process culminated in Late Cretaceous to Middle Eocene orogeny with the emplacement of southward-vergent nappes and the development 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 during the orogeny. They are recorded by Jurassic - Middle Cretaceous graphitic marbles, schists and quartzites (metamorphosed, organic limestones and shales and oil-bearing sandstones) in the Coastal and Northern Ranges of Venezuela and Trinidad. They probably charged giant fault and stratigraphic traps analogous to today's Oficina-Temblador and Heavy Oil Belt accumulations.

From Late Eocene to Recent times, transpressive interaction between northern South America and neighbouring parts of the Caribbean and the Pacific inverted Mesozoic extensional systems below the remaining passive margin. The area became subdivided into a series of intermontane, foreland and pull-apart basins bounded by transpressional uplifts, the latter suffering considerable shortening and strike-slip displacement. Sedimentation progressed from deep marine to deltaic and molassic facies, providing reservoir sands and local source rocks. Inverted faults and foreland flexuring and interplay between structuration and sedimentation produced abundant structural and stratigraphic 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, occurred in local foredeep kitchens. Minor contributions also came from Tertiary source rocks.

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