ANALYSIS OF EXTENSIONAL FAULT CORES – FAULT FACIES CHARACTERISTICS OF SLIP-SURFACES AND ROCK LENSES

Faults represent three-dimensional zones characterized by significant petrophysical and structural heterogeneity, which represent a major challenge in fluid flow assessments. In our study we apply Fault facies definitions based in field data on fault elements (dimensions, geometry, internal structure, petrophysical properties, and spatial distribution), which facilitate pattern recognition and statistical analysis. Fault facies can be organized hierarchically and scale-independent. The majority of the dataset on fault cores are from moderately consolidated sandstones in Sinai (Egypt) and Utah (USA).

Analyses of cores of extensional faults in sandstone reservoirs establish common fault facies associations of discrete structures (slip surfaces, fractures, and deformation bands), fault rock membranes, and lenses. In this study we focus on slip surfaces and lenses, since these represent major uncertainties in fault seal prediction as they may form permeable conduits within otherwise flow-retarding fault core lithologies. Slip surfaces are non-cohesive semi-planar discontinuities that have seen significant displacement. Small throw faults offer a network of irregular slip surfaces. With larger throw, a principal slip surface (PS) develops into a straight and continuous element throughout the exposure; however, this PS is commonly surrounded by a halo of smaller slip surfaces, potentially offering a network for fluid flow. We investigate lenses by addressing; 1) the position of the lenses, 2) their geometry and shape, and 3) the influence of lithology. The shape of the lenses can be analysed by normalizing length (measured in the dip-direction) and width (measured in the strike-direction) to maximum thickness (c:a and b:a-ratios). The average c:a-ratio for lenses included in the datasets is in the range of 9:1 to 12,5:1, but varies. Lithology is seen to affect the minimum/maximum sizes of the lenses; and the break-down of lenses commonly causes a change in the lens c:a-ratios, before the rock disintegrates into fault-rock.