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Fractures and Fracture Networks in Carbonate Reservoirs: A Geological Perspective

Abstract

Fracture (networks) play a key role in the development of naturally fractured reservoirs. Present knowledge is insufficient to predict the 3D architecture of fracture networks in buried reservoirs and permeability and flow patterns. This is particularly true for carbonate rocks which typically lithify close to the Earth surface and experience fracturing very early in their geological history. Detailed quantitative analysis of outcropping successions coupled with mechanical studies provide significant predictive tools and, thereby, minimize exploration and production risks. New acquisition, processing and interpretation tools are presented in this contribution with results from Cretaceous to Tertiary shallow water limestone of the Potiguar basin (Brazil) and of the Gafsa Basin (Tunisia). Carbonates of these two case studies experienced very early diagenesis and were lithified when they began their subsidence towards maximum burial depths. Subsidence took place in a stress regime characterized by a sub-horizontal tectonic stress. The first episode of fracturing occurred at depths of 500–1000m and led to the formation of sub-vertical stylolites and veins organized in conjugate sets with a sub-vertical intermediate principal stress σ2. Opening of fractures reduced pore pressure thereby favoring pressure solution and activated the long range circulation of fluids allowing for the arrival of meteoric waters in the buried carbonates and associated dissolution/cementation. With proceeding subsidence little deformation took place in the form of small normal faults. Carbonates of the Gafsa basin were folded during and following subsidence, thereby experiencing stresses resulting from the interaction between the fold-related and the regional stress fields. The type, position and architecture of syn-folding fractures critically depend on the mechanical stratigraphy of the folded sedimentary succession. In packages where layers are separated low-friction surfaces, new sets of conjugate fractures are formed in the hinge zone with an overall trend parallel to the fold axis. Layers which are thick and cannot easily slip with respect to each other are affected by fracturing controlled by flexural slip resulting in fractures ad stylolites oblique to bedding and concentrating away from the hinge zones. Outcrop models can then translated in permeability and, eventually, in flow models to test flow scenarios and sensitivity.