PETROPHYSICAL STUDY OF FAULTS IN SANDSTONE USING PETROGRAPHIC IMAGE-ANALYSIS AND X-RAY COMPUTERIZED-TOMOGRAPHY

Petrographic image analysis (PIA) and X-ray computerized tomography (C T) provide local determinations of porosity in sandstone. We have inve stigated small faults called deformation bands in porous sandstones us ing these techniques. Because the petrophysical properties of the faul t rock vary at a small scale (mm scale), the ability of PIA and CT to determine porosity in small volumes of rock and to map porosity distri bution in two and three dimensions is crucial. This information is use d to recognize the processes involved in fault development and the dif ferent kinds of microstructures associated with dilatancy and compacti on. The petrophysical study of fault rock in sandstone permits one to make predictions of the hydraulic properties of a fault acid thereby e valuate the sealing or fluid transmitting characteristics of faulted r eservoirs and aquifers. The results of this study indicate that faulti ng in sandstone alters the original porosity and permeability of the h ost rock: the porosity is reduced by an order of magnitude and the per meability is reduced by one to more than seven orders of magnitude for faults associated with compaction.