--> Characteristics of Faults in Sandstone and Their Effect on Fluid Flow, by M. A. Antonellini; #90986 (1994).

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Abstract: Characteristics of Faults in Sandstone and Their Effect on Fluid Flow

Marco A. Antonellini

A comprehensive study of faults in sandstone by means of video image analysis, CAT scanner tomography, minipermeametry, field mapping, standard microscopic techniques, and distinct elements numerical analysis indicates that fault zones strongly affect fluid flow. In this study I have examined sandstones from world over, but I have focused on the Mesozoic Entrada and Navajo Sandstones outcropping in SE Utah. The results suggest that stress state and rock properties such as porosity, mineralogy and clay content influence the style of deformation. Low porosity (<10%) and low confining pressures (<5 MPa) promote dilatancy with no cataclasis. High porosity (>10%) and high confining pressures (>5 MPa) promote compaction and cataclasis. If faulting causes compaction and cataclasi , the capillary pressure of the fault zone may increase 1 to 2 orders of magnitude with respect to the host rock and the permeability can decrease 1 to 7 orders of magnitude with respect to the host rock.

These results are applied to interpret outcrop exposures of the Mio-Pliocene Pismo Formation which is a good reservoir rock in Central California. The outcrops show spectacular evidence of fault-controlled tar concentrations in thick sandstone deposits. Most faults have offsets ranging from a few mm to a few dm and separate laterally and vertically tar-saturated from tar-free sandstone compartments. Small faults density increases in proximity to km-scale photo-lineaments interpreted to be the traces of thrust faults with tens of meters offset. The observed compartmentalization of the sandstone can be explained in terms of the petrophysical properties of the faults.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994