Abstract
Faulting in a sandstone introduces properties which are different from the country rock. Previous work has shown that these new properties can significantly impede the flow of hydrocarbons through the country rock. This thesis seeks to analyze the effect on sandstone where the fault displacements are measured in centimeters. The study area consists of three roadcuts in Arroyo Grande, California. These roadcuts show a number of anastomosing faults in sandstone that have heavily hydrocarbon saturated rocks on only one side. The other side of the fault is unsaturated, with the fault appearing to be a barrier. The couture rock is a medium grained, quartzose sandstone that is moderately sorted. The gouge zones have a fine gained matrix consisting of comminuted grains filling the pores. Fault gouge properties, obtained using optical microscopy, SEM and capillary pressure tests, are compared with properties of the surrounding county rock to determine the reasons why faults are acting as barriers to hydrocarbon migration. Porosity in the undeformed rock is as high as 40 %, while porosity in the gouge is as low as 4 % due to the fine gained matrix. This fine gained matrix affects the permeability and capillary pressures of the rocks as well. Calculated permeabihties are seen to drop three orders of magnitude and measured displacement pressures from 17 psi to 4 psi when comparing the gouge to the country rock. These faults will seal columns of oil of 0.77 meters.
Switek, Daniel Paul (1994). Faults as potential hydrocarbon barriers, Arroyo Grande, California. Master's thesis, Texas A&M University. Available electronically from
https : / /hdl .handle .net /1969 .1 /ETD -TAMU -1994 -THESIS -S979.