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SWANSON, DONALD C., Swanson Consulting Services, Houston, TX, and JULIAN SALAZAR, Lagoven, S.A.

ABSTRACT: Eocene Upper-B Reservoirs of the Bolivar Coastal Field, Venezuela -- A Classic Stratigraphic Trap With A Strongly Preferential Reservoir Orientation

Eocene Upper-B reservoirs of Lake Maracaibo, Venezuela are textbook examples of truncated, overlapped, and sealed stratigraphic traps. They have produced an appreciable part of the more than 10 billion barrels of oil produced from the Lagunillas Area of the Bolivar Coastal Field. In these fields, correlation of Upper-B reservoirs has always been very difficult - so difficult that detailed geological description was very arduous.

In 1981 it was found that the Upper-B Eocene section correlated perfectly in an east-west direction. This preferential correlation direction was evident in fine detail as well as in gross patterns. Correlation in any direction more than a few degrees different from east-west deteriorates rapidly. The excellent east-west correlation grain allowed a more accurate development of the stratigraphic, facies, and structural framework.

The correlation phenomenon is a result of geological factors. Environmental facies of cores indicate the deposits are nearshore tidal deposits located in an overall prograding deltaic setting. Many of the reservoirs consist of parallel, narrow, and long east-west trending tidal bars, oriented due east-west. They are separated by inter-bar laminated claystone, silty claystone, siltstone, and thin sandstones.

Discovery of the Upper B's preferential geologic and correlation direction was a breakthrough that allowed a more accurate geological description of the reservoirs. Routine testing for correlation grain or direction should be an early procedure in geological reservoir studies.

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