Role of Tectonic and Sedimentary Heterogeneities in the Localization of Sub-Seismic Fracture Zones in Compressive Context

Bazalgette, L., J. P. Petit, Laboratoire Dynamique de la Lithosphère, Montpellier II University, Montpellier, France

Fractures zones, unlike background fractures (pervasive fractures with low vertical per­sistence) can constitute high permeability drains in reservoirs. The understanding of param­eters that control their presence is crucial for the characterization of reservoir dynamics. They consist in small faults, kinks or fracture corridors. They affect stiff piles, which define Mechanical Units. In folded areas, they are often fold articulations, i.e., narrow fracture con­centrations that accommodate fold curvature in a discontinuous way. Articulations are major actors of axial flow in reservoirs. Field studies in folded foothills and in tabular areas have shown the pre-eminent role of the reactivation of two kinds of heterogeneities: - Those of tectonic origin (small faults, stylolites concentrations, highly persistent fractures…). -Those of sedimentary origin (lenticular marly beds, local amalgamations). A study in Moroccan Cretaceous carbonates shows how kinks, accommodated by bed parallel slip and bed delamination, form in mechanical units from the tip of shear reactivated oblique joints. This kinematical model is confirmed by biaxial loading experiments in elastic/brittle paraffin multilayers. Experiments, as well as field observations, demonstrate that pre-existing oblique fractures can act as stress/strain concentrators during syn-folding shortening. They localize zones of strain, which can evolve towards fracture zones that can become fold artic­ulations or thrusts, all important in reservoir permeability. Other examples show that marly lenses in massive carbonate piles control kink localization. Here, stress/strain concentration is due to the presence of discontinuous soft inclusions within the stiff pile. That demon­strates a close link between sedimentary facies and fracture zone distribution.