Facies Control on Oil Producing, Naturally Fractured Reservoirs, A Case Study: Block I, Lake Maracaibo, Venezuela

A. Rodriguez, X. Marquez

In Block I, Lake Maracaibo, oil producing intervals in the Cretaceous have been found to be facies controlled. Detailed studies of the cored sections in various wells have shown that facies Ls2, characterized by an oolitic grainstone, is the main producing reservoir. This facies has been interpreted as deposited as an oolitic shoal in a shallowing-upward sequence. Other facies such as Ls1, represented by a bioclastic rudstone, and Ls, composed of a mudstone, are also present but do not show significant oil production.

The presence of subvertical, open, hairline fractures and centimeter-scaled wide, bridged fractures in facies Ls2 can explain its great producing capacity, while the presence of calcite mineralized fractures in Ls1 and absence of fractures in Ls account for the poor producing capability of these facies.

In contrast to what has been reported in other case studies, the grainstone facies shows the most fracture density and open fractures.

A facies model which predicts the lateral continuity of the facies present in Block I is proposed as a tool for oil exploration and exploitation for this area and other areas with similar sedimentological characteristics.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.