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SWANSON, DONALD C., Swanson Consulting Services, Houston, TX, and CRISALIDA TARACHE, Lagoven, S.A.

ABSTRACT: The Cerro Negro Accumulation of Venezuela's Orinoco Belt -- The Favorable Convergence of Several Geological Processes

The Cerro Negro Area is a major part of eastern Venezuela's Orinoco Belt. Here upper Eocene fluvial-deltaic deposits of the Oficina Fm. reservoir billions of barrels of heavy oil, much of which is in valley-fill deposits. Maturation, migration and accumulation of these hydrocarbons in thick, porous and permeable sandstones were the logical conclusion to several major geological events in eastern Venezuela during the Tertiary.

In the Cerro Negro Area, Cretaceous clastics were deposited on an igneous and metamorphic basement after which the sea withdrew northward toward the axial part of the Eastern Venezuelan Basin. The basement and Cretaceous deposits were weathered and eroded during the Eocene, Oligocene, and early Miocene, forming the unconformity on which the Oficina Formation is deposited. Historic reconstruction begins with this unconformity, a paleotopographic surface strongly influencing the character and distribution of the overlying Oficina Formation. As relative sea level fell and gradients increased, streams incised into the shelf while transporting great amounts of coarse clastic load northward. At Cerro Negro, a mature topography of low ridges and hills were developed with differential elevati ns of several hundred feet.

During the Miocene, a sea transgressed across the stream-etched unconformity. Streams carrying large amounts of clastic load encountered an elevating sea level. They consequently dropped their coarse load, forming long, linear, transgressive, valley-fill deposits. The resulting conduits of excellent porosity and permeability extended from the basin southward up on the shelf.

Climate and tectonic conditions varied as sea level rose, and periodic regressive lobes of deltaic sediments prograded northward from their stream focal points. Cyclic regressive deltaic deposition continued eventually filling the basin, all in a stratigraphic framework of onlap.

By Late Miocene, hydrocarbons generated in the deeper basin began to migrate southward through the long linear fluvial-deltaic clastic conduits that were separated laterally and vertically into complex "plumbing systems." As the hydrocarbons moved shelfward, normal faults cut the conduits into numerous reservoir segments. The timing between migration and faulting is critical to present-day hydrocarbon distribution in these segments.

AAPG Search and Discovery Article #90988©1993 AAPG/SVG International Congress and Exhibition, Caracas, Venezuela, March 14-17, 1993.