Journal of Petroleum Science and Engineering 36 (2002) 169–182 www.elsevier.com/locate/jpetscieng Alteration and fracturing of siliceous mudstone during in situ combustion, Orcutt field, California Jason S. Lore1, Peter Eichhubl*, Atilla Aydin Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305-2115, USA Received 19 October 2000; accepted 30 September 2002 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 jC at the center of combustion, tapering off to about 350 jC at the outer edge of the coked zone. Changes in fracture density, distribution, and style were quantified based on fracture scanline measurements across alteration zones and in unaltered mudstone. With increasing alteration, newly formed fractures connect with and intersect preexisting 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 jC, and well developed in sintered oxidized mudstone that formed at inferred temperatures of about 900 jC. Fractures with large aperture to length ratios in clinker are inferred to have formed at peak temperatures of about 1100 jC. 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. D 2002 Elsevier Science B.V. All rights reserved. Keywords: In situ combustion; High-temperature fractures; Alteration; Hydrocarbons; Faulting; Fireflooding * Corresponding author. Tel.: +1-650-723-4296; fax: +1-650- 1. Introduction 725-0979. E-mail address: [email protected] (P. Eichhubl). 1 Present address: BP-Amoco Corp., P.O. Box 3092, Houston, In situ combustion or fireflooding is one of several TX 77253-3092, USA. enhanced recovery techniques employed in heavy oil 0920-4105/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S0920-4105(02)00316-9 170 J.S. Lore et al. / Journal of Petroleum Science and Engineering 36 (2002) 169–182 reservoirs (Wu and Fulton, 1971; Moore et al., 1988; (Dusseault et al., 1988) and monitoring of micro- Islam et al., 1991). A sweeping combustion front seisms accompanying in situ combustion (Nyland cracks the crude oil, with the heavy fraction serving and Dusseault, 1983), prior studies did not address as fuel for combustion, and the light fraction driven to the distribution of fractures created during passage of the production well due to the injection of air or a combustion front and their effect on flow conditions oxygen at the injection borehole. The sweeping com- in the combustion system. bustion front results in a thermal pulse that can locally This study analyzed the extent of induced fractur- exceed temperatures of 800 jC (Gates and Sklar, ing as a function of host rock alteration and its impact 1971). The formation of transient stresses during the on in situ combustion based on an outcrop exposure passage of this thermal pulse may induce fracturing of natural combustion alteration in the Orcutt oil field, (Dusseault et al., 1988) and thus change the fluid flow California (Fig. 1). Alteration of organic-rich siliceous properties of the rock. Investigations into composi- shale of the Upper Miocene Sisquoc Formation at this tional changes associated with in situ combustion location has been inferred to result from the natural in have largely focused on laboratory-scale experiments situ combustion of hydrocarbons in the shallow sub- (Schulte and de Vries, 1985; Ranjbar and Pusch, surface (Bentor and Kastner, 1981; Cisowski and 1991) or examination of core samples (Hutcheon, Fuller, 1987; Eichhubl and Aydin, in press). Obser- 1984; Lefebvre and Hutcheon, 1986; Tilley and vations of steam discharge by Arnold and Anderson Gunter, 1988). With the exception of numerical sim- (1907) suggest active in situ combustion at this ulations of stresses associated with in situ combustion location as recent as 1906. Combustion alteration Fig. 1. Geologic map of Orcutt oil field showing areal extent of combustion alteration. Circled letters mark study sites as discussed in the text. Geology based on unpublished industry maps. J.S. Lore et al. / Journal of Petroleum Science and Engineering 36 (2002) 169–182 171 has been mapped for this study over an area 1 km long roughly planar zones that are parallel to the north- and 0.25 km wide (Fig. 1), following the E–W to west–southeast strike of regional faults that were NE–SW trending Orcutt anticline (Dibblee, 1989) mapped adjacent to the combustion alteration area that forms the oil producing structure in the under- (Fig. 1). At the location Red Rock Canyon (Fig. 1, lying Miocene Monterey and Point Sal Formations Site A), a stepped, 40-m-tall quarry face cuts perpen- (Dunham et al., 1991; Johnston and Wachi, 1994). dicularly across three 1–4 m wide brecciated com- The southeast boundary of the combusted area as bustion centers and associated, partially overlapping, exposed at the surface follows the unconformable alteration haloes (Fig. 2a,b). The stepped quarry face contact of the overlying Careaga sand of Pliocene allowed a nearly three-dimensional examination of age (Dibblee, 1989) (Fig. 1). Within the combustion thermal alteration and variations in fracturing within area, alteration is concentrated along steeply dipping, the alteration haloes. Fig. 2. (a) Siliceous mudstone of the Sisquoc Formation in Red Rock Canyon, Orcutt oil field, California, altered by natural combustion of hydrocarbons. Darkest zones correspond to the highest alteration at centers of combustion. (b) Outcrop map of alteration zones. See Table 1 for characterization of alteration zones. 172 J.S. Lore et al. / Journal of Petroleum Science and Engineering 36 (2002) 169–182 2. Compositional changes associated with and hydrocarbons into a volatile component and solid combustion alteration coke (Behar et al., 1988; Ranjbar and Pusch, 1991). Oxidized mudstone is characterized by a uniform Based on color, bulk density, and hardness, the yellowish-orange to orange coloration, due to the following alteration zones were mapped across the pervasive occurrence of hematite that coincides with quarry face: unaltered siliceous mudstone, coked the beginning instability of smectite-montmorillonite. mudstone, oxidized and bleached oxidized mudstone, (Table 1). Hematite formation may be the result of sintered oxidized mudstone, and clinker (Fig. 2; Table pyrite decomposition or release of Fe2+ from cation 1). Mineral composition data summarized in Table 1 exchange layers in smectite. Because pyrite is not are based on bulk rock X-ray diffraction (XRD) contained in unaltered mudstone to an amount detect- analyses by Eichhubl and Aydin (in press). able by XRD, smectite instability is the likely source Unaltered mudstone away from combustion alter- of Fe2+. Steam or air may have served as an oxidizing ation is friable, medium to dark gray on fresh outcrop agent. The rock texture is identical to that of unaltered surfaces, and white when weathered. Bedding is siliceous mudstone. Oxidized mudstone is locally indistinct, only recognizable by a faint preferred secondarily bleached, resulting in a patchy yellow- fissility. Opal-A and smectite are the main constitu- ish-orange, orange, red, and white coloration (Fig. ents, with minor opal-CT, kaolinite, illite, and detrital 3a). Bleaching is associated with the localized precip- quartz and feldspar (Table 1). itation of hematite along joints (Fig. 3a, arrow), likely Coked mudstone, forming the outermost zone of the result of secondary remobilization of hematite by alteration, is black to dark gray, with a friable texture infiltrating ground water or steam from the adjacent similar to that of unaltered Sisquoc Formation. The oxidized mudstone.