Abstract: Depositional History of the Cerro Negro Region in the Orinoco Tar Belt, Venezuela

By

Donald C. Swanson

The Orinoco Tar Belt in eastern Venezuela holds one of the world's largest accumulations of unexplored energy reserves. Although the oil is low gravity and will present difficult and expensive production problems, the size of the reserves alone should make the tar belt or "Faja" of considerable interest to petroleum geologists. Estimates vary, but everyone agrees that there are hundreds of billions of barrels of oil held in the fluvial-deltaic-clastics that make up the reservoir facies.

The oil is contained in sandstones of the Miocene Oficina Formation, the basal unit of a thick wedge of Tertiary clastic sediments which thins southward over the stable southern shelf of the Eastern Venezuelan Basin. It terminates near the surface along the Orinoco River just north of the Guyana Shield. Structure in the area is relatively simple and consists principally of normal faults. The trapped hydrocarbons move updip from the basin southward through "conduits" consisting of fluvial-deltaic and fluvial valley fill deposits (similar to Fisk's substratum in the Mississippi River trench).

The stratigraphic framework of the Oficina Formation includes onlap onto an unconformity consisting of stream dissected Cretaceous and igneous rocks. Although the stratigraphic pattern is one of onlap, the actual shoreline either remained stationary or often prograded basinward as the result of laterally shifting regressive deltaic lobes or "tongues". The principal stratigraphic facies sequence was one of transgressing valley fill followed by numerous episodes of regressive deltaic sedimentation which filled a stratigraphic "skeleton" of onlap.

Facies and stratigraphic relationships are markedly similar between the Orinoco Tar Belt accumulations and those in Alberta, Canada. Both occur in essentially thick sandstones which lie upon and are molded by a stream-dissected paleotopographic surface.

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