Alteration and fracturing of siliceous mudstone during in-situ combustion, Orcutt field, California. Jason LORE1, Peter EICHHUBL*, and Atilla AYDIN Rock Fracture Project, Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305 1 Present address: BP-Amoco Corp., P.O. Box 3092, Houston TX 77253-3092. * Corresponding author Abstract. Changes in rock mineralogical composition and in fracture density and distribution resulting from natural in-situ combustion of hydrocarbons were characterized to infer comparable processes of alteration and fracturing during enhanced oil production from heavy oil reservoirs by in-situ combustion or fireflooding. Natural combustion alteration was studied in siliceous mudstone of the Miocene Sisquoc Formation at Orcutt oil field, California, where centers of most intense combustion alteration are composed of 1-2 m thick tabular zones of brecciated clinker. These centers are surrounded by 10-20 m wide alteration haloes of oxidized and sintered oxidized mudstone and an outer fringe of coked organic matter. Based on the stability of mineral phases around an individual combustion center, peak temperatures of combustion were estimated to have reached 1100°C at the center of combustion, tapering off to about 350°C at the outer edge of the coked zone. Changes in fracture density, distribution, and style were quantified based on fracture scan line measurements across alteration zones and in unaltered mudstone. With increasing alteration, newly formed fractures connect with and intersect pre-existing tectonic joints, providing an isotropic permeability structure for fluid flow. Addition of newly formed fractures to the existing joint systems is distinctly developed in oxidized mudstone, corresponding to alteration temperatures of about 750-800°C, and well developed in sintered oxidized mudstone that formed at inferred temperatures of about 900°C. Fractures with large aperture to length ratios in clinker are inferred to have formed at peak temperatures of about 1100°C. Based on alteration haloes around tectonic and combustion-induced fractures it is demonstrated that these fractures contributed significantly to flow of air or steam during combustion. Combustion zone centers are inferred to follow faults and joint zones that contained hydrocarbons that migrated into these migration conduits prior to and possibly during combustion. The natural combustion alteration is interpreted as the result of slowly outward moving alteration fronts around stationary combustion centers. The observed alteration distribution and associated pattern of induced fractures may thus be considered a natural outcrop analog of alteration associated with a well developed combustion front during fireflooding of heavy oil reservoirs. Although peak temperatures at Orcutt oil field likely exceeded temperatures characteristic of firefloods, fractures similar to those formed in the outer alteration zones may enhance the flow of oxidant to combustion fronts and of light hydrocarbons to production wells in firefloods. Keywords: In-situ combustion, high-temperature fractures, alteration, hydrocarbons, faulting, fireflooding.