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MURAT, BRUNO, and ISASKUN AZPIRITXAGA, Maraven S.A., Caracas, Venezuela

ABSTRACT: Application of Sequence Stratigraphy to Reservoir and Hydrocarbon Source Rock Prediction in the Cretaceous Carbonate Platforms of Maracaibo Basin, Venezuela

Prediction of reservoir and source rocks is enhanced by an understanding of the sequential organization of the sedimentary units.

In the Maracaibo Basin, the carbonate Cogollo Group and the basal part of the shaly La Luna Formation (Upper Barremian to Lower Cenomanian) have been subdivided into a hierarchy of cycles ranging from parasequences (4th and 5th order) up to Regressive-Transgressive cycles (2nd order).

Sedimentation during this period on a passive platform under the influence of eustatic sea level fluctuations, led to a succession of about twenty 3rd order sequences (depending on their location on the platform) composed of Transgressive Systems Tracts (TST) and Highstand Systems Tracts (HST). Their boundaries and maximum flooding surfaces can be traced on wireline logs and on cored material. These sequences belong to three Regressive-Transgressive 2nd order cycles showing a 3-stage evolution of infill, aggradation and backstepping.

Sedimentary facies vary laterally within systems tracts and vertically from one cycle to another. Most basal TST units display high energy sediments prone to porosity development, whereas the basal HST units are generally characterized by muddier sediments. The best reservoirs are at the top of HST units, with development of both early dolomite and grainy packstones with moderate reservoir quality.

Maximum oil productivity occurs where matrix porosity is associated with fractures, which are always best developed within the aggrading stage. Finally, source-rock intervals coincide with the maximum flooding surfaces which limit second order cycles.

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