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ABSTRACT: Diagenesis and Reservoir Characterization of the Cretaceous-Tertiary Sequence, Eastern Venezuela

Bernabe Aquado, Santosh Ghosh, Andreina Isea

The giant El Furrial field, Maturin subbasin, is the most important oil field discovered in Venezuela in the last three decades. The average oil column has a thickness of 400 m and the reservoirs consist of essentially sandy siliciclastic sediments of nearshore-shallow marine origin. The oil's API gravity ranges from light to extra heavy and occurs in a stratified manner in the reservoirs.

A total of 1080 m of core from the producing sequence was studied through x-ray diffraction, scanning electron microscopy, and petrography. This data, along with petrophysical measurements, show a clear differentiation between the Upper Cretaceous and the Oligocene reservoirs. The Upper Cretaceous reservoirs are characterized by relatively fine and uniform grain size, subarkosic composition with common volcanic rock fragments, high degree of chemical and mechanical compaction, highly illitic mixed-layer I/S assemblage with less than 10% expandable layers, and ubiquitous baroque dolomite. Additionally, porosity and permeability values are persistently low. Clearly, the Cretaceous sediments are diagenetically mature and may indicate diagenetic transformation at greater depths or under a different thermal regime. In contrast, the coarser grained Oligocene reservoirs of quartz arenitic composition show a lesser diagenetic overprint, and

greater porosity and permeability. Porosity is dominantly secondary due to cement and grain (mostly quartz) dissolution, as well as tectonically induced grain fracturing. Common kaolinite and minor amounts of I/S with up to 20% of expandable layers attest to a lower diagenetic regime than in the Cretaceous reservoirs.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990