Cretaceous carbonates of the Cogollo Group are important fractured reservoirs in the western Maracaibo Basin between Lake Maracaibo and the Perij Range. These strata were deformed and fractured during two phases of Tertiary compression which produced traps in the form of fault-propagation folds bounded by steep, basement-involved, reverse faults and, in at least one case, a detached fault-bend fold. Seven fields and drilled prospects in the area were studied using core, well-log data, detailed structure mapping, and geochemical methods to determine the controls on fracture distribution, orientation, and diagenesis. Normalized production data were used to assess the combined impact of fractures and matrix porosity on oil flow rates and sustainability. Empirical correlations indicate that key factors controlling reservoir quality in the area are matrix porosity, fault and fold intensity, uplift, and fracture diagenesis.

Fracture intensity measured in core is greater in mud-rich facies with less porosity and less siliciclastic material. Matrix porosity is best developed in grain-rich rocks of the upper highstand systems tract and basal trangressive systems tract of a single sequence. When calibrated to well logs, this information is extrapolated around the area using sequence stratigraphic concepts in order to produce fracture potential and matrix porosity maps. The main structural controls enhancing fracturing are fault proximity and greater throw for fault-propagation folds (Alturitas, San Jose, Alpuf), and higher curvature (tighter hinges) for the fault-bend fold (Rosario). Fracture intensity also decreases with depth of burial and increases with more uplift. Reservoir quality as measured by normalized flow rates depends on fracture intensity accounting for any diagenetic modifications and matrix porosity in communication with the fracture network.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah