The petrophysical properties of deformation bands in relation to their microstructure

Abstract

Deformation bands are significant discontinuities in reservoir sandstones providing baffles to the migration of fluids and acting as seals for hydrocarbon accumulations. Their contribution to these processes is dependent upon a range of inter-related factors including lateral continuity, relative displacement, reservoir geohistories and rock microstructure. Deformation bands have millimetre-scale displacements, which are smaller than the seismic resolution, and hence the study of their microstructure from reservoir cores is a necessary tool in the prediction of their fault seal potential. The latter is only achieved if detailed petrophysical measurements of fault microstructure are carried out using techniques, which are suitable for measurement of very low permeability fault rock at the relevant scale. Conventional techniques of fault rock porosity and permeability determination invite host rock bias in measurement and inevitably underestimate the potential of these structures as fluid barriers. Pressure decay probe permeametry used in this study has the advantage of measuring small volumes of rock at finer spatial resolutions and detailed permeability distributions around common types of deformation bands are obtained. Together with supporting data provided by conventional core analysis techniques, these measurements show that the greatest differences in petrophysical properties relative to host rock occur in those bands, which have experienced cataclasis. These results provide valuable input into reservoir simulation models and help reduce much of the uncertainty regarding the role of different types of deformation bands in fault seal.