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Combining Numerical Models and Outcrop Analog Studies to Enhance Predictions in Fractured Atoll-Like Carbonate Reservoirs

Bertotti, Giovanni *1; Boro, Herman 2
(1) Delft University of Technology, Delft, Netherlands. (2) VU University, Amsterdam, Netherlands.

Kilometre-scale atoll-like carbonate platforms form major hydrocarbon reservoirs worldwide and their permeability is frequently associated with fracturing. They are characterized by a sub-rounded shape, and by the juxtaposition of major units, the platform interior, margin and slope, with very different spatial organization of beds and lithologies resulting in contrasting mechanical (elastic and plastic) properties. Once subjected to loading, changes in directions and magnitudes of the principal stresses and in proximity-to-failure are expected producing potentially a very heterogeneous fracturing pattern. In addition, both mechanical properties and stress conditions are expected to change through time. Physical properties of rocks will be modified by diagenesis. Stress conditions will change because of increasing overburden and variable tectonic stresses. Combining finite element numerical models and analysis of outcropping analogs, we predict the distribution of fractured domains, the nature and orientation of fractures as well as the network they form.

Numerical experiments envisaging a platform margin stronger than the interior and slope predict that, in the absence of tectonic stresses, fracturing is limited until the platform subsides to depths of few hundreds meters. Fracturing affects initially the platform margin and gradually expands to the interior and slope and at depths >1lm the entire platform is fractured. Fractures are generally mode I (=opening) and subvertical (subhorizontal σ3) with strike parallel or perpendicular to the platform margins. In the common situation that tectonic stresses are present, the platform experiences fracturing already at very early stages and below a few hundred of meters, the entire platform is predicted to be fractured. In contrast with the gravity-only scenario, fractures are expected to be aligned with the direction of the principal stresses overruling the sub-rounded shape of the platform. For high tectonic stresses, fractures tend to be sub-horizontal.

Significant differences are predicted in the fracture geometries and in the resulting networks. In the platform interior, fractures are typically higher than bed thicknesses and only 30-60% of terminations are at bed boundaries. Spacing distances are weakly dependent on lithology. Fractures in the slope are poorly organized, higher and more widely spaced than coeval features in the platform interior.

 

AAPG Search and Discovery Article #90141©2012, GEO-2012, 10th Middle East Geosciences Conference and Exhibition, 4-7 March 2012, Manama, Bahrain