Numerous major hydrocarbon accumulations are related to burial diagenesis in carbonate reservoirs; however, three-dimensional (3-D) modeling of such reservoirs is a major challenge. The geometries of diagenetic fronts are poorly understood, and few quantitative data on field and outcrop examples for these diagenetic zones exist. The Early Cretaceous Cogollo carbonates (Urdaneta West field, northwest Venezuela) are an example of deeply buried (>5000 m; >1600 ft), diagenetically altered limestones producing from a few high-porosity streaks and fracture corridors. Reservoir “sweet spots” formed because of the interplay of structural evolution and diagenesis. Structural deformation not only generated a fracture network but also influenced primary facies distribution, circulation of diagenetic fluids, and distribution of early charged paleohighs in the reservoir.
The prediction of reservoir sweet spots is of key importance for economic success in the Cogollo carbonates but constitutes a major challenge. This article outlines the genetic relationships between structural evolution and sweet-spot distribution, as well as the workflow developed to transfer these concepts into 3-D geocellular models. These 3-D models include a novel approach to fracture permeability prediction based on a geomechanical approach.
The reservoir model predicts sweet spots along leached faults, which are best preserved in early charged paleohighs. The Cogollo and other reservoirs suggest that favorable structural development can generate and retain significant matrix porosity even at great depth. The workflow established and applied might help to maximize field potential in similar settings.