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.