Effect of Faulting on Fluid Flow in Porous Sandstones: Geometry and Spatial Distribution

Abstract

We present a methodology to describe fault geometry at different scales and to characterize the distribution of these scales on the flanks of a salt intrusion in the Colorado Plateau (Arches National Park, United States). This methodology is based on the recognition of the physical processes of faulting and on the quantitative characterization of the structural and petrophysical properties of faults in porous sandstones. The methods used include a variety of mapping techniques (photography, aerial photography, string mapping, theodolite surveys, etc.), as well as techniques for determining fluid flow properties. The resulting study is a prototype for understanding seismic and subseismic scales of heterogeneity related to faulting and fracturing in subsurface reservoirs. > Faulting in porous sandstones on the flanks of the salt intrusion is developed at different stages, from simple deformation bands (1-20 mm shear offset) to slip planes (>1 m shear offset) and complex fault zones. We document that deformation-band outcrop geometry is characterized by a sinuous anastomosing pattern resulting from the linkage of quasitabular segments via ramp or eye structures. These connecting structures recur at different scales and provide lateral continuity of the deformation bands; therefore, deformation bands have good geometric sealing characteristics. Slip planes, which are not interconnected, may have poor geometric sealing characteristics. In the hanging wall of a major normal fault, the quantitative spatial distribution of the faults can be correlated with bending of the strata, probably associated with the salt intrusion. The number of deformation bands, the most ubiquitous element, is proportional to the amount of slip on a single major fault. Deformation bands also have a very high density (>100 m-1) in stepovers between slip planes. In these areas we find the largest anomalies in permeability. In zones of high strata curvature, the average layer-parallel permeability can drop one to two orders of magnitude with respect to the host rock; if complex fault zones are present, the average permeability can drop more than four orders of magnitude in the direction normal to the faults. Finally, by using outcrop and laboratory data that describe the effect of distinctive structural units on fluid flow, we quantify the three-dimensional distribution of permeability in a reservoir analog at any scale, and we show that such permeability distribution could be implemented in a geology-based reservoir simulator.