Alteration and Fracturing of Siliceous Mudstone During in Situ Combustion, Orcutt Field, California

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. black color is likely due to coked organic material, Sintered oxidized mudstone is of uniform reddish- forming in response to thermal breakdown of kerogen orange color. The rock texture is distinctly harder and

Table 1 Characteristics of alteration zones Alteration zone Thickness (m) Color Mineral Estimated maximum composition alteration temperature (jC) Unaltered siliceous country rock light gray opal-A, smectite, 40–50 mudstone (>20 m from illite, kaolinite, center of alteration) minor opal-CT, (detrital) quartz, feldspar Coked mudstone 1–4 medium to dark same 350–500 gray, black Bleached oxidized 0–10 spotty orange, opal-A, illite, 650 mudstone preferred oxidation hematite, (detrital) and reduction quartz, feldspar; along joints hematite precipitation along fractures Oxidized mudstone 2–10 yellow-orange; opal-A, illite, hematite, 750–800 oxidation fronts (detrital) quartz, feldspar adjacent to joints Sintered oxidized 0.2–2 bright orange; cristobalite, hematite, 900 mudstone uniform oxidation illite, quartz, feldspar Clinker 1–3 dark red to purple anorthite, tridymite, 1100 and black cordierite, hematite, ilmenite, cristobalite Mineral composition based on X-ray diffraction analyses by Eichhubl and Aydin (in press). Temperature estimates are based on mineral stability criteria listed in Table 2. J.S. Lore et al. / Journal of Petroleum Science and Engineering 36 (2002) 169–182 173

Fig. 3. (a) Bleached oxidized mudstone at Site D (Fig. 1) is characterized by spotty yellow-white coloration of otherwise red hematite-stained mudstone and the local precipitation of hematite cement along joints (arrow). (b) Hydrocarbon-stained joints in unaltered Sisquoc Formation at Site B in Fig. 1. Hydrocarbon staining is observed in the longer, more clustered, steeply dipping joints. (c) Normal fault contact of hydrocarbon- impregnated Careaga Fm. against jointed Sisquoc Fm., Site C in Fig. 1. (d) Opening-mode fractures in clinker are characterized by blunt tips (arrow) and large apertures. (e) Brecciated clinker (right) and sintered oxidized mudstone (left). Notice high density of short, connected fractures in both alteration zones. less friable than unaltered mudstone though still easily oxidized and sintered oxidized mudstone is sharp scratched with a knife. The fissile fabric of the mud- and characterized by the instability of opal-A and stone is largely obliterated. The contact between beginning precipitation of cristobalite (Table 1). 174 J.S. Lore et al. / Journal of Petroleum Science and Engineering 36 (2002) 169–182

Clinker is dark red to purple and reddish-brown in illite and beginning instability of quartz, and by the color, and in hardness and density similar to fired clay formation of tridymite, cordierite, and calcic plagio- brick. Clinker is characterized by the instability of clase (Table 1). Under the microscope, plagioclase

Table 2 Mineral stability criteria, modified after Perry and Gillott (1982) Mineral Transformation Temparature Comments Reference range (jC)

Smectite-montmorillonite loss of interlayer H2O 100–300 reversible D92 (1= 2Ca, Na)0.7(Al, Mg, Fe)4 dehydroxylation (300), 500 XRD unchanged [(Si, Al)8O20] (OH)4ÁnH2O irreversible collapse 650 180 103 –104 years W79 geothermal decomposition 750 D92 200 103 –104 years W79 geothermal Kaolinite dehydroxylation 400–525 XRD unchanged D92

Al4[Si4O10](OH)8 irreversible collapse 800 decomposition 900–1000 80–220 106 –107 years, dependent on fluid composition

Illite K1.5 – 1.0Al4 dehydroxylation 350–600 XRD unchanged D92 [Si6.5 – 7.0Al1.5 – 1.0 O20](OH) decomposition 900–1000 Opal-A SiO2ÁnH2O decomposition to 1000 8 days JS71 cristobalite 900 100 days to opal-CT 40–50 106 –107 years KI85

Cristobalite SiO2 formation 1470 inversion temperature, D92 but formation also below

Tridymite SiO2 formation 870 inversion temperature, D92 but formation also below

Quartz SiO2 a–h quartz 573 reversible D92 decomposition 870 very sluggish D92 to tridymite 2+ Cordierite Al3(Mg, Fe ) formation 800–900 from glass under D78 2[Si5AlO18] (A-cordierite) atmospheric pressure formation (indiolite) 900–1250 from glass under atmospheric pressure melting 1465 Plagioclase (An 0.6) formation decomposition of clays D92

(Ca0.6Na0.4)Al solidus 820 PH2O = 2 kbar D92 (Al0.6Si0.4)Si2O8 f 1100 P = 1 atm, extrapolated E01 from 2 kbar based on data for albite 2+ Hematite Fe2O3 formation 750 Fe from smectite D92 decomposition

dissociation to Fe3O4 1390 Coke C formation by thermal 350–500 minimum temperature B88, cracking of hydrocarbons V88, R91 Bold numbers were used as temperature constraints for the temperature estimates in Table 1. XRD: X-ray diffraction. References: JS71: Jones and Segnit (1971); D78: Deer et al. (1978); W79: Weaver (1979); KI85: Keller and Isaacs (1985); B88: Behar et al. (1988); V88: Verkoczy and Jha (1988); R91: Ranjbar and Pusch (1991); D92: Deer et al. (1992); E01: Eichhubl et al. (2001). J.S. Lore et al. / Journal of Petroleum Science and Engineering 36 (2002) 169–182 175 and tridymite form a eutectic intergrowth texture, jC. The eutectic texture of plagioclase and tridymite indicative of co-precipitation from a partial melt indicates that the solidus of plagioclase was reached, in (Eichhubl et al., 2001). the presence of quartz at atmospheric pressure at temperatures of about 1100jC (Eichhubl et al., 2001). The alteration temperatures inferred from the min- 3. Inferred temperature distribution across eralogical composition are interpreted as peak temper- combustion centers atures attained in each zone. Earlier alteration products formed at lower temperature would have In order to infer the interaction of fluid and heat been overprinted by subsequent higher temperature transfer with the fracture system, peak alteration alteration. temperatures were estimated for the alteration profile along the base of the Red Rock quarry (Fig. 2b). Alteration temperatures were estimated for each alter- 4. Changes in fracture pattern associated with ation zone based on the occurrence or disappearance combustion alteration of minerals as listed in Table 1, in comparison to published experimental data of mineral stability at 4.1. Jointing and faulting in unaltered mudstone high temperature and low pressure (Table 2). The lowest temperature estimate related to combustion To quantify the extent of fracturing attributable to alteration is provided by the formation of coke, under combustion alteration, fracture density, length, and experimental conditions starting at 350 jC (Behar et orientation were measured along two scanlines inside al., 1988; Verkoczy and Jha, 1988; Ranjbar and and outside the combustion alteration area. Fracture Pusch, 1991). The outer edge of the coked zone is data of altered Sisquoc Formation were measured equated with the 350 jC isotherm assuming that this along a scanline across a combustion center and its contact represents the onset of coke formation. A associated alteration halo at the base of the quarry small opal-CT component in the unaltered siliceous (scanline in Fig. 2). Fracturing in unaltered Sisquoc mudstone host rock is likely the result of burial Formation was measured at a location 200 m away diagenesis at 40–50 jC (Keller and Isaacs, 1985), from the combustion area (Site B in Fig. 1). Site B is predating and independent of combustion alteration. situated immediately outside the combustion-altered Beginning irreversible collapse of smectite-montmor- area in a similar structural position as the quarry and is illonite suggests temperatures of about 650 jC in the assumed to provide a measure of fracture style and bleached oxidized mudstone (Weaver, 1979; Deer et density of the rock units exposed in the quarry prior to al., 1992). In the XRD analyses, the bulk occurrence combustion alteration. of hematite appears to correlate inversely with smec- Joints in unaltered mudstone at site B occur in tite decomposition, indicating temperatures of around two sets (Fig. 4a): A dominant joint set, referred to 750–800 jC (Weaver, 1979; Deer et al., 1992). as Set 1, dips steeply and strikes approximately The next distinct change in mineralogical composi- N50jE. These joints are typically clustered, with tion is the instability of opal-A and the formation of longer joints exhibiting narrower spacing than cristobalite. Experiments of opal stability over 100 shorter ones (Fig. 4b). A second set of steeply days indicate opal-A instability at about 900 jC (Jones dipping joints, referred to as Set 2, is wider spaced and Segnit, 1971), distinctly higher than the opal-A to and shorter compared to Set 1 and strikes N20jE opal-CT transformation at 40–50 jC during burial (Fig. 4a). Based on abutting relations, Set 2 post- diagenesis (Keller and Isaacs, 1985). The highest, and dates Set 1. A third, poorly developed set, strikes most distinct, mineral transformation involves the N60jW (Fig. 4a). Set 1 joints typically have aper- instability of illite and formation of cordierite and tures of 1–2 mm, with some reaching 15 mm, and plagioclase. Laboratory experiments suggest begin- are commonly hydrocarbon-stained whereas the ning cordierite formation at temperatures of around other joint sets are typically barren of hydrocarbons. 900 up to 1100 jC. Similarly, the apparent onset of Hydrocarbons are most abundant in clusters of Set 1 quartz instability requires temperatures in excess of 870 (Figs. 3b and 4b) and along faults (Fig. 3c). The 176 J.S. Lore et al. / Journal of Petroleum Science and Engineering 36 (2002) 169–182

Fig. 4. (a) Rose diagram of joints and poles to joint orientation from exposure of unaltered Sisquoc Formation approximately 200 m from the combustion area (Site B in Fig. 1). (b) Scanline showing joint trace length (in meters) (solid lines) and width of hydrocarbon staining (in millimeters) (dotted lines) in unaltered Sisquoc Fm. at site B. The spatial distribution shows a clustering of the longer joints, and an association between joint clusters and hydrocarbon staining. joint sets measured at the surface correlate with a 5a,b). In addition, the pattern of two steeply dipping dominant steeply dipping regional joint sets striking joint sets in unaltered mudstone (Figs. 3a and 5c) is approximately N45–60jE and a secondary set strik- replaced by a nearly uniform distribution of fracture ing N30jW as observed by Johnston and Wachi orientations in sintered oxidized mudstone and clinker (1994) in nearby wells penetrating underlying Mon- (Fig. 5e,f). Fractures in clinker have characteristically terey and Sisquoc formations. blunt tips (Fig. 3d), apertures that exceed 5 mm, and ratios of aperture over length frequently exceeding 4.2. Fracturing in combustion-altered mudstone 1:10 (Eichhubl and Aydin, in press). These fractures frequently intersect at angles approaching 90j (Fig. A second fracture scanline was measured across 3d) forming a well-connected fracture network. At the the alteration zones at the base of the Red Rock quarry center of clinker zones, these fractures are sufficiently to assess the extent of fracturing associated with dense to form isolated rock fragments and to brecciate combustion alteration (Fig. 5a–f). With increasing the formation (Fig. 3e). Interstitial voids of the breccia alteration, apparent fracture length and spacing exceed 10 cm in diameter and are locally filled with decreases (Fig. 5a), with a marked decrease at the vesiculated material. Using image analysis void space outer contact of sintered oxidized mudstone (Fig. in brecciated clinker was determined to be 26 F 5% J.S. Lore et al. / Journal of Petroleum Science and Engineering 36 (2002) 169–182 177

Fig. 5. (a) Variation in fracture trace length measured for all fractures observed crossing the scanline in combustion altered rock. A trend to shorter trace lengths with decreasing distance to the combustion center is observed, reflecting a higher density of intersecting joints. (b) Outcrop photograph of fracture scanline location. Stereonet plots of poles to fracture surfaces of (c) unaltered/coked mudstone, (d) bleached oxidized/ oxidized mudstone, (e) sintered oxidized mudstone, and (f) clinker. Notice increasingly uniform fracture distribution with increasing alteration. andupto53F 5% in brecciated and vesiculated the bleached oxidized zone represents an intermediate clinker. stage of combustion alteration. Jointing in the Joints in bleached oxidized zone were mapped bleached oxidized zone is dominated by steeply dip- (Fig. 6a,b) to document the evolution of intense ping set of 1–2 m long joints that correlates with the jointing associated with combustion assuming that regional Set 1 outside the combustion-altered zone 178 J.S. Lore et al. / Journal of Petroleum Science and Engineering 36 (2002) 169–182

across. Numerous smaller fractures (Set 3) of variable orientation in-fill these angular blocks. Joints of Set 3 in some cases truncate against Sets 1 and 2, but in other cases cross fractures of Sets 1 and 2. Unlike Set 1 joints at Site B 200 m away from the combustion area boundary, no hydrocarbon staining was observed in the combustion-altered rock within Red Rock Canyon. This suggests that oil migration into the burnt zone had ceased before or at the same time as combustion ceased.

4.3. Interaction between chemical alteration and fractures

In oxidized and bleached oxidized mudstone, two types of interaction between chemical alteration and fractures are observed: (1) Fractures form sharp boundaries of red oxidized and yellowish to gray reduced mudstone (Fig. 6a). Between fractures a transition from oxidized to reduced mudstone is observed that typically extends over 10–50 cm. These fractures thus compartmentalize asymmetric alteration gradients. (2) A second type of interaction results in reducing or oxidizing alteration haloes that are symmetric around fractures. Symmetric oxidation haloes are frequently associated with precipitate of hematite within fractures (Fig. 3a). Both types of interaction with chemical alteration are associated with joints of Sets 1 and 2. Joints of Set 3 form occasionally boundaries of asymmetric alteration haloes but frequently cut across alteration compartments without affecting alteration.

5. Discussion

5.1. Fracture formation and combustion alteration

Based on textural and microanalytical studies, Fig. 6. (a) Alteration in the bleached oxidized mudstone. Arrows Eichhubl and Aydin (in press) showed that the large- indicate asymmetric alteration gradients bound by Set 1 and Set 2 fractures. (b) Map of joins in (a). Classification of joints in three sets aperture fractures observed in clinker formed during is based on cross-cutting relationships. the formation of high-temperature mineral phases. They demonstrated that these fractures resulted from the growth and coalescence of pores, with pores (Figs. 4a and 5c). Occasionally, these joints have originating as molds after the dissolution of opal-A small amounts of normal offset. Set 2 fractures (Fig. diatoms. Subsequent growth and coalescence of pores 6b) dip shallowly, and truncate against Set 1 fractures was attributed by Eichhubl et al. (2001) to the ten- to form angular blocks of rock approximately 30 cm dency of the partially molten rock to eliminate sub- J.S. Lore et al. / Journal of Petroleum Science and Engineering 36 (2002) 169–182 179 micron-sized pores at the expense of larger ones. In gests that joint formation correlates with the disap- analogy to similar processes during firing of ceramics, pearance of smectite at about 650 jC. The well- they inferred that the fracture-like elongation of pores developed occurrence of Set 3 fractures in sintered resulted from a tensile sintering stress stemming from oxidized mudstone likely relates to the opal-A dis- the tendency of the partially molten system to reduce solution and the formation of cristobalite at 900 jC. the surface free energy of pore and grain surfaces. This Both reactions involve dehydration. Structural water reduction in surface free energy results in the tendency of smectitic clay can amount to 10–15 vol.% (Burst, of the porous material to shrink or, if constrained by 1969), opal-A contains up to 17 wt.% water (Hurd and the surrounding formation, to form opening-mode Theyer, 1977). In both cases, water would be released contraction fractures. The association of large-aperture as steam during combustion and leave the system. It is fractures with high-temperature minerals indicated that thus conceivable that Set 3 fractures formed as natural these fractures formed when combustion approached hydraulic fractures due to steam expansion. Alterna- peak temperatures of about 1100 jC. The large inter- tively, or in addition, Set 3 fractures could have stitial space observed in brecciated clinker likely formed due to the contraction of the rock associated resulted from collapse that may be attributed to the with mineral neoformation similar to that inferred by reduction in porosity during partial melting and min- Eichhubl et al. (2001) for the formation of large- eral neoformation and to the remobilization of melt. aperture fractures in clinker. Evidence for collapse was observed by downward drag The interaction of some Set 3 joints with alteration of partially detached fragments along the outer boun- gradients suggests joint formation during combustion daries of the brecciated zones. and is inconsistent with formation due to thermal Assuming that the joints observed at site B repre- contraction after combustion had ceased. Formation sent the extent of jointing prior to combustion alter- of large-aperture fractures in clinker is accompanied ation, it can be inferred that joint Set 3 observed in by textural reorganization associated with the forma- oxidized mudstone (Fig. 6) formed during or after tion of high-temperature mineral phases (Eichhubl et combustion alteration. Set 3 joints could have formed al., 2001) and thus occurred during combustion. as a result of shear activation of Set 1 fractures. Shear along Set 1 joints is frequently observed in the 5.2. Combustion geometry and fracture–fluid flow combustion-altered area and can be attributed to the interaction volume reduction within combustion centers during combustion. This explanation is consistent with the In situ combustion involves the migration of fuel occurrence of microseismic events during in situ and of oxygen in the form of air to the combustion site. combustion as observed by Nyland and Dusseault Basedontheobservationofhydrocarbon-stained (1983). Set 3 joint formation by slip along Set 1 joints joints (Fig. 3b) and faults (Fig. 3c) along strike of the would result in a consistent tail or wing crack geom- combustion zones, it is inferred that combustion fol- etry, however, similar to that observed along tectonic lowed these hydrocarbon conduits and that the hydro- joints (Willemse and Pollard, 1998). This geometry is carbons acting as fuel for combustion were already not characteristic of Set 3 joints. Instead, they are largely in place when combustion started. Although uniformly distributed (Fig. 5d–f) rather than forming hydrocarbons may have migrated into the combustion a set of distinct orientation as expected for tail cracks. centers during combustion, the lack of hydrocarbons in The preferred explanation of Set 3 joints is thus that brecciated clinker indicates that migration did not they formed as a result of mineralogical and textural continue after combustion had ceased. changes during combustion similar to the large-aper- Assuming that combustion started at the Earth’s ture fractures in clinker. Unlike large-aperture frac- surface, it is inferred that combustion propagated tures in clinker, fractures in oxidized and sintered downdip along the hydrocarbon-stained joint and fault oxidized mudstone retained their joint-like appearance zones. Arnold and Anderson (1907) suggested that in consistent with the lesser extent of mineral alteration situ combustion started at the surface by lightening or in oxidized mudstone. The first distinct occurrence of bush fires. Spontaneous ignition in the subsurface by Set 3 fractures in bleached oxidized mudstone sug- the exothermic oxidation of pyrite, suggested by Math- 180 J.S. Lore et al. / Journal of Petroleum Science and Engineering 36 (2002) 169–182 ews and Bustin (1984) for some pyrite- and organic- toward the injection well (Chu, 1987; Moore et al., rich mudstones, is unlikely due to the low pyrite con- 1988; Greaves and Ibrahim, 1991). In both cases, tent of the formation. Pyrite was not detected by XRD. combustion consumes hydrocarbons that are con- Once combustion propagates into the subsurface, tained throughout the porous formation between the the rate of combustion is likely to be controlled by the wells allowing the combustion front to sweep through flow of air or steam to the combustion site. Potential the formation. conduits for air or steam are inactive brecciated clinker Similar to fireflooding, the natural combustion zones along strike of active combustion sites. Air or processes resulting in alteration at Orcutt are inferred steam may also have been drawn through the fractured to have consumed the heavy hydrocarbon fraction, formation. Evidence for flow of steam through the whereas a light fraction may have left the system as fractured formation is observed as asymmetric oxida- a volatile phase. The coked zone forming the outer- tion haloes around joints and by the localized precip- most alteration halo is interpreted as a remnant of the itation of hematite along joints in bleached oxidized heavy hydrocarbon fraction that formed immediately mudstone. Hematite precipitation presumably reflects ahead of the combustion front but remained pre- the flow of steam released by dehydration reactions of served when in situ combustion seized. Whereas opal-A and clays or by the flow of meteoric water into fireflooding typically combusts hydrocarbons that combustion zones. Asymmetric alteration patterns are are distributed in the produced formation, it is interpreted to result from pervasive infiltration of the inferred that natural in situ combustion at Orcutt mudstone matrix by oxidizing air or steam, brought Oil field consumed hydrocarbons that migrated up into the system along joints, and the loss in oxidation faults and joint zones and that impregnated the potential as the oxidant reacts with organic matter in immediate vicinity of these migration conduits. Com- the matrix. The systematic asymmetry of oxidation on bustion would thus have been localized around the only one side of joints indicates that infiltration of hydrocarbon-impregnated conduits rather than oxidant into the matrix occurred by advective flow, sweeping across the formation as in fireflooding. rather than by diffusion that is expected to have Although the center of combustion would have been resulted in a symmetric alteration pattern. localized within the fault zone, the alteration zones Unaltered siliceous mudstone is characterized by would slowly have migrated outward into unaltered low matrix permeability, thus joints and faults acted as formation as combustion progressed and heat was preferred conduits for fluid flow, providing a highly conducted or advected into the surrounding forma- anisotropic permeability structure. In combustion- tion. Assuming that heat was dissipated purely by altered rocks, joint orientations change to an increas- conduction, Lore (1999) estimated that combustion ingly isotropic permeability structure. This isotropic lasted about 9 years for the alteration halo along the permeability structure is observed out to approxi- fracture scanline (Fig. 1) based on a one-dimensional mately 8 m from the heating source, with the strongest finite element numerical simulation. overprinting in the innermost 3 m, corresponding to Based on the interpretation of the alteration haloes the sintered oxidized mudstone and clinker with at Red Rock quarry as the result of localized in situ maximum alteration temperatures exceeding 900 jC. combustion, it is suggested that the observed alteration and fracture patterns are a natural outcrop 5.3. Significance for heavy oil production by in situ analog of the zonation of alteration associated with combustion a sweeping combustion front at any fully developed stage of a fireflood. The formation of fractures in During fireflooding of heavy oil reservoirs, in situ association with alteration at Orcutt oil field suggests combustion results in cracking of long-chain organic that firefloods may be aided by newly formed molecules. Whereas the heavier hydrocarbon fraction fracture permeability behind and immediately in serves as fuel for the migrating combustion front, the front of the combustion front. In forward combus- light fraction is produced from a production well. In tion, the newly formed fractures behind the combus- forward combustion, the combustion front migrates tion front would aid in the flow of oxidant to the toward the production well, in reverse combustion combustion front. In reverse combustion, newly J.S. Lore et al. / Journal of Petroleum Science and Engineering 36 (2002) 169–182 181 formed fractures would aid in the flow of hydro- Acknowledgements carbons to the production well. The temperatures usually reached in firefloods, up The authors gratefully acknowledge the assistance to 800 jC (Gates and Sklar, 1971), are clearly below of Roger Canady and Greg Yvarra at Nuevo-Torch those estimated for clinker of about 1100 jC. Con- Operating Company for access to Orcutt oil field and ditions during fireflooding may thus approach those for permission to use the geologic map. Reviews by during formation of oxidized and bleached oxidized Ian Hutcheon of an early version of this manuscript, mudstone where newly formed fractures (Set 3) are by the managing editor, and an anonymous journal distinctly developed (Figs. 5d and 6). reviewer are thankfully acknowledged.

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