Extensional vs. Shear Fractures: Implications for Reservoir Characterisation

Abstract

ABSTRACT Fractured reservoirs are typically strongly anisotropic and heterogeneous, irrespective of whether the fractures are extensional or shearing-induced. Fracture network data are notoriously hard to obtain. Here, we present new results on fracture-network characteristics from outcrop and experimental studies. In contrast to current views that fault and fracture patterns are of a fractal nature, these results demonstrate that extensional fractures (joints) are non-fractal. Shear fractures and faults, on the other hand, were found to agree with fractal concepts on a wide range of scales. They form scale-invariant patterns. Both fracture networks consisting of shear fractures and those consisting of joints have a high degree of fracture connectivity. Extensional fractures tend to link with fractures of the same family and to abut against pre-existing joints. Blunting of joint tips at intersections also improves the connectivity of the fracture network. Monte Carlo simulation techniques have been used to determine the effective permeability of networks of extensional fractures. An outcrop-derived model yielded a markedly anisotropic bed-parallel permeability. Localised shear failure in porous sandstones occurs by particulate flow or, more frequently, by cataclasis. Particulate flow leads to significant permeability increase whereas cataclasis may result in permeability increase or decrease, the latter being more common. Shear fractures are characterised by zones of anastomosing cataclastic deformation bands with enclosed lenses of undeformed matrix material. Thus, there will be no tortuous flow paths through these lenses around the tips of isolated deformation bands across the shear zone. Reservoir compartmentalisation may result from deformation bands which will have, at best, very subtle seismic expression.