alteration and fracturing of siliceous mudstone … and fracturing of siliceous mudstone during in...

$: Alteration and fracturing of siliceous mudstone … and fracturing of siliceous mudstone during in situ combustion, Orcutt field, California ... Combustion zone centers are inferred$
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

1. Introduction

In situ combustion or fireflooding is one of several

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

* Corresponding author. Tel.: +1-650-723-4296; fax: +1-650-

725-0979.

E-mail address: [email protected] (P. Eichhubl).1 Present address: BP-Amoco Corp., P.O. Box 3092, Houston,

TX 77253-3092, USA.

www.elsevier.com/locate/jpetscieng

Journal of Petroleum Science and Engineering 36 (2002) 169–182

reservoirs (Wu and Fulton, 1971; Moore et al., 1988;

Islam et al., 1991). A sweeping combustion front

cracks the crude oil, with the heavy fraction serving

as fuel for combustion, and the light fraction driven to

the production well due to the injection of air or

oxygen at the injection borehole. The sweeping com-

bustion front results in a thermal pulse that can locally

exceed temperatures of 800 jC (Gates and Sklar,

1971). The formation of transient stresses during the

passage of this thermal pulse may induce fracturing

(Dusseault et al., 1988) and thus change the fluid flow

properties of the rock. Investigations into composi-

tional changes associated with in situ combustion

have largely focused on laboratory-scale experiments

(Schulte and de Vries, 1985; Ranjbar and Pusch,

1991) or examination of core samples (Hutcheon,

1984; Lefebvre and Hutcheon, 1986; Tilley and

Gunter, 1988). With the exception of numerical sim-

ulations of stresses associated with in situ combustion

(Dusseault et al., 1988) and monitoring of micro-

seisms accompanying in situ combustion (Nyland

and Dusseault, 1983), prior studies did not address

the distribution of fractures created during passage of

a combustion front and their effect on flow conditions

in the combustion system.

This study analyzed the extent of induced fractur-

ing as a function of host rock alteration and its impact

on in situ combustion based on an outcrop exposure

of natural combustion alteration in the Orcutt oil field,

California (Fig. 1). Alteration of organic-rich siliceous

shale of the Upper Miocene Sisquoc Formation at this

location has been inferred to result from the natural in

situ combustion of hydrocarbons in the shallow sub-

surface (Bentor and Kastner, 1981; Cisowski and

Fuller, 1987; Eichhubl and Aydin, in press). Obser-

vations of steam discharge by Arnold and Anderson

(1907) suggest active in situ combustion at this

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) 169–182170

has been mapped for this study over an area 1 km long

and 0.25 km wide (Fig. 1), following the E–W to

NE–SW trending Orcutt anticline (Dibblee, 1989)

that forms the oil producing structure in the under-

lying Miocene Monterey and Point Sal Formations

(Dunham et al., 1991; Johnston and Wachi, 1994).

The southeast boundary of the combusted area as

exposed at the surface follows the unconformable

contact of the overlying Careaga sand of Pliocene

age (Dibblee, 1989) (Fig. 1). Within the combustion

area, alteration is concentrated along steeply dipping,

roughly planar zones that are parallel to the north-

west–southeast strike of regional faults that were

mapped adjacent to the combustion alteration area

(Fig. 1). At the location Red Rock Canyon (Fig. 1,

Site A), a stepped, 40-m-tall quarry face cuts perpen-

dicularly across three 1–4 m wide brecciated com-

bustion centers and associated, partially overlapping,

alteration haloes (Fig. 2a,b). The stepped quarry face

allowed a nearly three-dimensional examination of

thermal alteration and variations in fracturing within

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.

J.S. Lore et al. / Journal of Petroleum Science and Engineering 36 (2002) 169–182 171

2. Compositional changes associated with

combustion alteration

Based on color, bulk density, and hardness, the

following alteration zones were mapped across the

quarry face: unaltered siliceous mudstone, coked

mudstone, oxidized and bleached oxidized mudstone,

sintered oxidized mudstone, and clinker (Fig. 2; Table

1). Mineral composition data summarized in Table 1

are based on bulk rock X-ray diffraction (XRD)

analyses by Eichhubl and Aydin (in press).

Unaltered mudstone away from combustion alter-

ation is friable, medium to dark gray on fresh outcrop

surfaces, and white when weathered. Bedding is

indistinct, only recognizable by a faint preferred

fissility. Opal-A and smectite are the main constitu-

ents, with minor opal-CT, kaolinite, illite, and detrital

quartz and feldspar (Table 1).

Coked mudstone, forming the outermost zone of

alteration, is black to dark gray, with a friable texture

similar to that of unaltered Sisquoc Formation. The

black color is likely due to coked organic material,

forming in response to thermal breakdown of kerogen

and hydrocarbons into a volatile component and solid

coke (Behar et al., 1988; Ranjbar and Pusch, 1991).

Oxidized mudstone is characterized by a uniform

yellowish-orange to orange coloration, due to the

pervasive occurrence of hematite that coincides with

the beginning instability of smectite-montmorillonite.

(Table 1). Hematite formation may be the result of

pyrite decomposition or release of Fe2 + from cation

exchange layers in smectite. Because pyrite is not

contained in unaltered mudstone to an amount detect-

able by XRD, smectite instability is the likely source

of Fe2 +. Steam or air may have served as an oxidizing

agent. The rock texture is identical to that of unaltered

siliceous mudstone. Oxidized mudstone is locally

secondarily bleached, resulting in a patchy yellow-

ish-orange, orange, red, and white coloration (Fig.

3a). Bleaching is associated with the localized precip-

itation of hematite along joints (Fig. 3a, arrow), likely

the result of secondary remobilization of hematite by

infiltrating ground water or steam from the adjacent

oxidized mudstone.

Sintered oxidized mudstone is of uniform reddish-

orange color. The rock texture is distinctly harder and

Table 1

Characteristics of alteration zones

Alteration zone Thickness (m) Color Mineral

composition

Estimated maximum

alteration temperature (jC)

Unaltered siliceous

mudstone

country rock

(>20 m from

center of alteration)

light gray opal-A, smectite,

illite, kaolinite,

minor opal-CT,

(detrital) quartz,

feldspar

40–50

Coked mudstone 1–4 medium to dark

gray, black

same 350–500

Bleached oxidized

mudstone

0–10 spotty orange,

preferred oxidation

and reduction

along joints

opal-A, illite,

hematite, (detrital)

quartz, feldspar;

hematite precipitation

along fractures

650

Oxidized mudstone 2–10 yellow-orange;

oxidation fronts

adjacent to joints

opal-A, illite, hematite,

(detrital) quartz, feldspar

750–800

Sintered oxidized

mudstone

0.2–2 bright orange;

uniform oxidation

cristobalite, hematite,

illite, quartz, feldspar

900

Clinker 1–3 dark red to purple

and black

anorthite, tridymite,

cordierite, hematite,

ilmenite, cristobalite

1100

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.


less friable than unaltered mudstone though still easily

scratched with a knife. The fissile fabric of the mud-

stone is largely obliterated. The contact between

oxidized and sintered oxidized mudstone is sharp

and characterized by the instability of opal-A and

beginning precipitation of cristobalite (Table 1).

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.


Clinker is dark red to purple and reddish-brown in

color, and in hardness and density similar to fired clay

brick. Clinker is characterized by the instability of

illite and beginning instability of quartz, and by the

formation of tridymite, cordierite, and calcic plagio-

clase (Table 1). Under the microscope, plagioclase

Table 2

Mineral stability criteria, modified after Perry and Gillott (1982)

Mineral Transformation Temparature

range (jC)Comments Reference

Smectite-montmorillonite loss of interlayer H2O 100–300 reversible D92

(1 2= Ca, Na)0.7(Al, Mg, Fe)4 dehydroxylation (300), 500 XRD unchanged

[(Si, Al)8O20] (OH)4�nH2O irreversible collapse 650

180 103–104 years

geothermal

W79

decomposition 750 D92

200 103–104 years

geothermal

W79

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 SiO2�nH2O decomposition to 1000 8 days JS71

cristobalite

to opal-CT

900 100 days

40–50 106–107 years KI85

Cristobalite SiO2 formation 1470 inversion temperature,

but formation also below

D92

Tridymite SiO2 formation 870 inversion temperature,

but formation also below

D92

Quartz SiO2 a–h quartz 573 reversible D92

decomposition

to tridymite

870 very sluggish D92

Cordierite Al3(Mg, Fe2 +)

2[Si5AlO18]

formation

(A-cordierite)800–900 from glass under

atmospheric pressure

D78

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

from 2 kbar based

on data for albite

E01

Hematite Fe2O3 formation 750 Fe2 + from smectite

decomposition

D92

dissociation to Fe3O4 1390

Coke C formation by thermal

cracking of hydrocarbons

350–500 minimum temperature B88,

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).


and tridymite form a eutectic intergrowth texture,

indicative of co-precipitation from a partial melt

(Eichhubl et al., 2001).

3. Inferred temperature distribution across

combustion centers

In order to infer the interaction of fluid and heat

transfer with the fracture system, peak 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-

ation zone based on the occurrence or disappearance

of minerals as listed in Table 1, in comparison to

published experimental data of mineral stability at

high temperature and low pressure (Table 2). The

lowest temperature estimate related to combustion

alteration is provided by the formation of coke, under

experimental conditions starting at 350 jC (Behar et

al., 1988; Verkoczy and Jha, 1988; Ranjbar and

Pusch, 1991). The outer edge of the coked zone is

equated with the 350 jC isotherm assuming that this

contact represents the onset of coke formation. A

small opal-CT component in the unaltered siliceous

mudstone host rock is likely the result of burial

diagenesis at 40–50 jC (Keller and Isaacs, 1985),

predating and independent of combustion alteration.

Beginning irreversible collapse of smectite-montmor-

illonite suggests temperatures of about 650 jC in the

bleached oxidized mudstone (Weaver, 1979; Deer et

al., 1992). In the XRD analyses, the bulk occurrence

of hematite appears to correlate inversely with smec-

tite decomposition, indicating temperatures of around

750–800 jC (Weaver, 1979; Deer et al., 1992).

The next distinct change in mineralogical composi-

tion is the instability of opal-A and the formation of

cristobalite. Experiments of opal stability over 100

days indicate opal-A instability at about 900 jC (Jones

and Segnit, 1971), distinctly higher than the opal-A to

opal-CT transformation at 40–50 jC during burial

diagenesis (Keller and Isaacs, 1985). The highest, and

most distinct, mineral transformation involves the

instability of illite and formation of cordierite and

plagioclase. Laboratory experiments suggest begin-

ning cordierite formation at temperatures of around

900 up to 1100 jC. Similarly, the apparent onset of

quartz instability requires temperatures in excess of 870

jC. The eutectic texture of plagioclase and tridymite

indicates that the solidus of plagioclase was reached, in

the presence of quartz at atmospheric pressure at

temperatures of about 1100jC (Eichhubl et al., 2001).

The alteration temperatures inferred from the min-

eralogical composition are interpreted as peak temper-

atures attained in each zone. Earlier alteration

products formed at lower temperature would have

been overprinted by subsequent higher temperature

alteration.

4. Changes in fracture pattern associated with

combustion alteration

4.1. Jointing and faulting in unaltered mudstone

To quantify the extent of fracturing attributable to

combustion alteration, fracture density, length, and

orientation were measured along two scanlines inside

and outside the combustion alteration area. Fracture

data of altered Sisquoc Formation were measured

along a scanline across a combustion center and its

associated alteration halo at the base of the quarry

(scanline in Fig. 2). Fracturing in unaltered Sisquoc

Formation was measured at a location 200 m away

from the combustion area (Site B in Fig. 1). Site B is

situated immediately outside the combustion-altered

area in a similar structural position as the quarry and is

assumed to provide a measure of fracture style and

density of the rock units exposed in the quarry prior to

combustion alteration.

Joints in unaltered mudstone at site B occur in

two sets (Fig. 4a): A dominant joint set, referred to

as Set 1, dips steeply and strikes approximately

N50jE. These joints are typically clustered, with

longer joints exhibiting narrower spacing than

shorter ones (Fig. 4b). A second set of steeply

dipping joints, referred to as Set 2, is wider spaced

and shorter compared to Set 1 and strikes N20jE(Fig. 4a). Based on abutting relations, Set 2 post-

dates Set 1. A third, poorly developed set, strikes

N60jW (Fig. 4a). Set 1 joints typically have aper-

tures of 1–2 mm, with some reaching 15 mm, and

are commonly hydrocarbon-stained whereas the

other joint sets are typically barren of hydrocarbons.

Hydrocarbons are most abundant in clusters of Set 1

(Figs. 3b and 4b) and along faults (Fig. 3c). The


joint sets measured at the surface correlate with a

dominant steeply dipping regional joint sets striking

approximately N45–60jE and a secondary set strik-

ing N30jW as observed by Johnston and Wachi

(1994) in nearby wells penetrating underlying Mon-

terey and Sisquoc formations.

4.2. Fracturing in combustion-altered mudstone

A second fracture scanline was measured across

the alteration zones at the base of the Red Rock quarry

to assess the extent of fracturing associated with

combustion alteration (Fig. 5a–f). With increasing

alteration, apparent fracture length and spacing

decreases (Fig. 5a), with a marked decrease at the

outer contact of sintered oxidized mudstone (Fig.

5a,b). In addition, the pattern of two steeply dipping

joint sets in unaltered mudstone (Figs. 3a and 5c) is

replaced by a nearly uniform distribution of fracture

orientations in sintered oxidized mudstone and clinker

(Fig. 5e,f). Fractures in clinker have characteristically

blunt tips (Fig. 3d), apertures that exceed 5 mm, and

ratios of aperture over length frequently exceeding

1:10 (Eichhubl and Aydin, in press). These fractures

frequently intersect at angles approaching 90j (Fig.

3d) forming a well-connected fracture network. At the

center of clinker zones, these fractures are sufficiently

dense to form isolated rock fragments and to brecciate

the formation (Fig. 3e). Interstitial voids of the breccia

exceed 10 cm in diameter and are locally filled with

vesiculated material. Using image analysis void space

in brecciated clinker was determined to be 26F 5%

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.


and up to 53F 5% in brecciated and vesiculated

clinker.

Joints in bleached oxidized zone were mapped

(Fig. 6a,b) to document the evolution of intense

jointing associated with combustion assuming that

the bleached oxidized zone represents an intermediate

stage of combustion alteration. Jointing in the

bleached oxidized zone is dominated by steeply dip-

ping set of 1–2 m long joints that correlates with the

regional Set 1 outside the combustion-altered zone

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.


(Figs. 4a and 5c). Occasionally, these joints have

small amounts of normal offset. Set 2 fractures (Fig.

6b) dip shallowly, and truncate against Set 1 fractures

to form angular blocks of rock approximately 30 cm

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 sym-

metric 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 fre-

quently cut across alteration compartments without

affecting alteration.

5. Discussion

5.1. Fracture formation and combustion alteration

Based on textural and microanalytical studies,

Eichhubl and Aydin (in press) showed that the large-

aperture fractures observed in clinker formed during

the formation of high-temperature mineral phases.

They demonstrated that these fractures resulted from

the growth and coalescence of pores, with pores

originating as molds after the dissolution of opal-A

diatoms. Subsequent growth and coalescence of pores

was attributed by Eichhubl et al. (2001) to the ten-

dency of the partially molten rock to eliminate sub-

Fig. 6. (a) Alteration in the bleached oxidized mudstone. Arrows

indicate asymmetric alteration gradients bound by Set 1 and Set 2

fractures. (b) Map of joins in (a). Classification of joints in three sets

is based on cross-cutting relationships.


micron-sized pores at the expense of larger ones. In

analogy to similar processes during firing of ceramics,

they inferred that the fracture-like elongation of pores

resulted from a tensile sintering stress stemming from

the tendency of the partially molten system to reduce

the surface free energy of pore and grain surfaces. This

reduction in surface free energy results in the tendency

of the porous material to shrink or, if constrained by

the surrounding formation, to form opening-mode

contraction fractures. The association of large-aperture

fractures with high-temperature minerals indicated that

these fractures formed when combustion approached

peak temperatures of about 1100 jC. The large inter-

stitial space observed in brecciated clinker likely

resulted from collapse that may be attributed to the

reduction in porosity during partial melting and min-

eral neoformation and to the remobilization of melt.

Evidence for collapse was observed by downward drag

of partially detached fragments along the outer boun-

daries of the brecciated zones.

Assuming that the joints observed at site B repre-

sent the extent of jointing prior to combustion alter-

ation, it can be inferred that joint Set 3 observed in

oxidized mudstone (Fig. 6) formed during or after

combustion alteration. Set 3 joints could have formed

as a result of shear activation of Set 1 fractures. Shear

along Set 1 joints is frequently observed in the

combustion-altered area and can be attributed to the

volume reduction within combustion centers during

combustion. This explanation is consistent with the

occurrence of microseismic events during in situ

combustion as observed by Nyland and Dusseault

(1983). Set 3 joint formation by slip along Set 1 joints

would result in a consistent tail or wing crack geom-

etry, however, similar to that observed along tectonic

joints (Willemse and Pollard, 1998). This geometry is

not characteristic of Set 3 joints. Instead, they are

uniformly distributed (Fig. 5d–f) rather than forming

a set of distinct orientation as expected for tail cracks.

The preferred explanation of Set 3 joints is thus that

they formed as a result of mineralogical and textural

changes during combustion similar to the large-aper-

ture fractures in clinker. Unlike large-aperture frac-

tures in clinker, fractures in oxidized and sintered

oxidized mudstone retained their joint-like appearance

consistent with the lesser extent of mineral alteration

in oxidized mudstone. The first distinct occurrence of

Set 3 fractures in bleached oxidized mudstone sug-

gests that joint formation correlates with the disap-

pearance of smectite at about 650 jC. The well-

developed occurrence of Set 3 fractures in sintered

oxidized mudstone likely relates to the opal-A dis-

solution and the formation of cristobalite at 900 jC.Both reactions involve dehydration. Structural water

of smectitic clay can amount to 10–15 vol.% (Burst,

1969), opal-A contains up to 17 wt.% water (Hurd and

Theyer, 1977). In both cases, water would be released

as steam during combustion and leave the system. It is

thus conceivable that Set 3 fractures formed as natural

hydraulic fractures due to steam expansion. Alterna-

tively, or in addition, Set 3 fractures could have

formed due to the contraction of the rock associated

with mineral neoformation similar to that inferred by

Eichhubl et al. (2001) for the formation of large-

aperture fractures in clinker.

The interaction of some Set 3 joints with alteration

gradients suggests joint formation during combustion

and is inconsistent with formation due to thermal

contraction after combustion had ceased. Formation

of large-aperture fractures in clinker is accompanied

by textural reorganization associated with the forma-

tion of high-temperature mineral phases (Eichhubl et

al., 2001) and thus occurred during combustion.

5.2. Combustion geometry and fracture–fluid flow

interaction

In situ combustion involves the migration of fuel

and of oxygen in the form of air to the combustion site.

Based on the observation of hydrocarbon-stained

joints (Fig. 3b) and faults (Fig. 3c) along strike of the

combustion zones, it is inferred that combustion fol-

lowed these hydrocarbon conduits and that the hydro-

carbons acting as fuel for combustion were already

largely in place when combustion started. Although

hydrocarbons may have migrated into the combustion

centers during combustion, the lack of hydrocarbons in

brecciated clinker indicates that migration did not

continue after combustion had ceased.

Assuming that combustion started at the Earth’s

surface, it is inferred that combustion propagated

downdip along the hydrocarbon-stained joint and fault

zones. Arnold and Anderson (1907) suggested that in

situ combustion started at the surface by lightening or

bush fires. Spontaneous ignition in the subsurface by

the exothermic oxidation of pyrite, suggested by Math-


ews and Bustin (1984) for some pyrite- and organic-

rich mudstones, is unlikely due to the low pyrite con-

tent of the formation. Pyrite was not detected by XRD.

Once combustion propagates into the subsurface,

the rate of combustion is likely to be controlled by the

flow of air or steam to the combustion site. Potential

conduits for air or steam are inactive brecciated clinker

zones along strike of active combustion sites. Air or

steam may also have been drawn through the fractured

formation. Evidence for flow of steam through the

fractured formation is observed as asymmetric oxida-

tion haloes around joints and by the localized precip-

itation of hematite along joints in bleached oxidized

mudstone. Hematite precipitation presumably reflects

the flow of steam released by dehydration reactions of

opal-A and clays or by the flow of meteoric water into

combustion zones. Asymmetric alteration patterns are

interpreted to result from pervasive infiltration of the

mudstone matrix by oxidizing air or steam, brought

into the system along joints, and the loss in oxidation

potential as the oxidant reacts with organic matter in

the matrix. The systematic asymmetry of oxidation on

only one side of joints indicates that infiltration of

oxidant into the matrix occurred by advective flow,

rather than by diffusion that is expected to have

resulted in a symmetric alteration pattern.

Unaltered siliceous mudstone is characterized by

low matrix permeability, thus joints and faults acted as

preferred conduits for fluid flow, providing a highly

anisotropic permeability structure. In combustion-

altered rocks, joint orientations change to an increas-

ingly isotropic permeability structure. This isotropic

permeability structure is observed out to approxi-

mately 8 m from the heating source, with the strongest

overprinting in the innermost 3 m, corresponding to

the sintered oxidized mudstone and clinker with

maximum alteration temperatures exceeding 900 jC.

5.3. Significance for heavy oil production by in situ

combustion

During fireflooding of heavy oil reservoirs, in situ

combustion results in cracking of long-chain organic

molecules. Whereas the heavier hydrocarbon fraction

serves as fuel for the migrating combustion front, the

light fraction is produced from a production well. In

forward combustion, the combustion front migrates

toward the production well, in reverse combustion

toward the injection well (Chu, 1987; Moore et al.,

1988; Greaves and Ibrahim, 1991). In both cases,

combustion consumes hydrocarbons that are con-

tained throughout the porous formation between the

wells allowing the combustion front to sweep through

the formation.

Similar to fireflooding, the natural combustion

processes resulting in alteration at Orcutt are inferred

to have consumed the heavy hydrocarbon fraction,

whereas a light fraction may have left the system as

a volatile phase. The coked zone forming the outer-

most alteration halo is interpreted as a remnant of the

heavy hydrocarbon fraction that formed immediately

ahead of the combustion front but remained pre-

served when in situ combustion seized. Whereas

fireflooding typically combusts hydrocarbons that

are distributed in the produced formation, it is

inferred that natural in situ combustion at Orcutt

Oil field consumed hydrocarbons that migrated up

faults and joint zones and that impregnated the

immediate vicinity of these migration conduits. Com-

bustion would thus have been localized around the

hydrocarbon-impregnated conduits rather than

sweeping across the formation as in fireflooding.

Although the center of combustion would have been

localized within the fault zone, the alteration zones

would slowly have migrated outward into unaltered

formation as combustion progressed and heat was

conducted or advected into the surrounding forma-

tion. Assuming that heat was dissipated purely by

conduction, Lore (1999) estimated that combustion

lasted about 9 years for the alteration halo along the

fracture scanline (Fig. 1) based on a one-dimensional

finite element numerical simulation.

Based on the interpretation of the alteration haloes

at Red Rock quarry as the result of localized in situ

combustion, it is suggested that the observed alter-

ation and fracture patterns are a natural outcrop

analog of the zonation of alteration associated with

a sweeping combustion front at any fully developed

stage of a fireflood. The formation of fractures in

association with alteration at Orcutt oil field suggests

that firefloods may be aided by newly formed

fracture permeability behind and immediately in

front of the combustion front. In forward combus-

tion, the newly formed fractures behind the combus-

tion front would aid in the flow of oxidant to the

combustion front. In reverse combustion, newly


formed fractures would aid in the flow of hydro-

carbons to the production well.

The temperatures usually reached in firefloods, up

to 800 jC (Gates and Sklar, 1971), are clearly below

those estimated for clinker of about 1100 jC. Con-ditions during fireflooding may thus approach those

during formation of oxidized and bleached oxidized

mudstone where newly formed fractures (Set 3) are

distinctly developed (Figs. 5d and 6).

6. Conclusions

Natural in situ combustion of hydrocarbons at

Orcutt oil field resulted in alteration of siliceous mud-

stone to oxidized and sintered oxidized mudstone and,

at the centers of combustion, to clinker. Alteration

haloes extend up to 20 m away from tabular, steeply

dipping combustion centers. Based on the stability of

minerals, inferred peak alteration temperatures are 800

jC for oxidizedmudstone, 900 jC for sintered oxidized

mudstone, and 1100 jC for clinker. By comparing joint

patterns in unaltered siliceous mudstone with those in

combustion-altered mudstone, it is demonstrated that

alteration was associated with the formation of addi-

tional fractures. These newly formed fractures are

distinctly developed in bleached oxidized and oxidized

mudstone, corresponding to 800 jC, and well devel-

oped in sintered oxidized mudstone and clinker, corre-

sponding to peak temperatures of 900–1100 jC.Newly formed fractures in clinker are characterized

by high apertures and blunt tips and are sufficiently

connected to brecciate the formation. Newly formed

fractures in altered mudstone and clinker are inter-

preted as syn-combustion and associated with mineral

alteration and dehydration reactions.

Natural combustion alteration at Orcutt oil field is

considered a natural analog for rock alteration and

induced fracturing during fireflooding of heavy oil

reservoirs. Although peak temperatures in firefloods

are typically lower than those inferred for combustion

centers at Orcutt oil field, jointing similar to that

observed in oxidized mudstone may be expected to

occur in association with firefloods. These alteration-

induced joints may result in an isotropic fracture

permeability that aids the flow of oxidant to the

fireflood, and the flow of higher hydrocarbons to the

production well.

Acknowledgements

The authors gratefully acknowledge the assistance

of Roger Canady and Greg Yvarra at Nuevo-Torch

Operating Company for access to Orcutt oil field and

for permission to use the geologic map. Reviews by

Ian Hutcheon of an early version of this manuscript,

by the managing editor, and an anonymous journal

reviewer are thankfully acknowledged.

References

Arnold, R., Anderson, R., 1907. Geology and oil resources of the

Santa Maria Oil District, Santa Barbara County, California. U.S.

Geol. Surv. Bull. 322, 1–161.

Behar, F., Ungerer, P., Audibert, A., Villalba, M., 1988. Experimen-

tal study and kinetic modeling of crude oil pyrolysis in relation

to thermal recovery processes. In: Meyer, R.F., Wiggins, E.J.

(Eds.), Fourth UNITAR/UNDP International Conference on

Heavy Crude and Tar Sands, Edmonton, Canada, pp. 748–759.

Bentor, Y.K., Kastner, M., 1981. Combustion metamorphism of

bituminous sediments and the formation of melts of granitic

and sedimentary composition. Geochim. Cosmochim. Acta 45,

2229–2255.

Burst, J.R., 1969. Diagenesis of Gulf Coast clayey sediments and its

possible relation to petroleummigration. AAPGBull. 53, 73–93.

Chu, C., 1987. Thermal recovery. In: Bradley, H.B. (Ed.), Petro-

leum Engineering Handbook. Society of Petroleum Engineers,

Richardson, TX, pp. 46-1–46-46.

Cisowski, S.M., Fuller, M., 1987. The generation of magnetic

anomalies by combustion metamorphism of sedimentary rock,

and its significance to hydrocarbon exploration. Geol. Soc.

Amer. Bull. 99, 21–29.

Deer, W.A., Howie, R.A., Zussman, J., 1978. Rock-Forming Min-

erals, 2nd ed. Disilicates and Ring Silicates, vol. 1B. Wiley,

New York. 629 pp.

Deer, W.A., Howie, R.A., Zussman, J., 1992. An Introduction to the

Rock-Forming Minerals. Longman, Hong Kong. 696 pp.

Dibblee, T.W.J., 1989. Geologic Map of the Casmalia and Orcutt

Quadrangles. Dibblee Foundation, Santa Barbara, CA.

Dunham, J.B., Bromely, B.W., Rosato, V.J., 1991. Geologic con-

trols on hydrocarbon occurrence within the Santa Maria Basin

of western California. In: Gluskoter, H.J., Rice, D.D., Taylor,

R.B. (Eds.), Economic Geology-US Geological Society of

America, Boulder, CO, pp. 431–445.

Dusseault, M.B., Wang, Y., Simmons, J.V., 1988. Induced stresses

near a fireflood front. AOSTRA J. Res. 4, 153–170.

Eichhubl, P., Aydin, A., 2003. Ductile opening-mode fracture by

pore growth and coalescence during combustion alteration of

siliceous mudstone. J. Struct. Geol. 25, 121–139.

Eichhubl, P., Aydin, A., Lore, J., 2001. Opening-mode fracture in

siliceous mudstone at high homologous temperature—effect of

surface forces. Geophys. Res. Lett. 28, 1299–1302.

Gates, C.F., Sklar, I., 1971. Combustion as a primary recovery

process—Midway Sunset field. J. Pet. Technol. 23, 981–986.


Greaves, M., Ibrahim, G.M.S., 1991. The development of in situ

combustion projects: techno-commercial aspects and strategy. In:

Meyer, R.F. (Ed.), Heavy Crude and Tar Sands—Hydrocarbons

for the 21st Century. 5th UNITAR International Conference on

Heavy Crude and Tar Sands, Caracas, Venezuela, pp. 281–296.

Hurd, D.C., Theyer, F., 1977. Changes in the physical and chemical

properties of biogenic silica from the central equatorial Pacific:

Part II. Refractive index, density, and water content of acid-

cleaned samples. Am. J. Sci. 277, 1168–1202.

Hutcheon, I., 1984. A review of artificial diagenesis during ther-

mally enhanced recovery. Clastic diagenesis. In: McDonald,

D.A., Surdam, R.C. (Eds.), Clastic Diagenesis. AAPG Memoir,

vol. 37, pp. 413–429.

Islam, M.R., Verma, A., Farouq Ali, S.M., 1991. In situ combus-

tion—the essential reaction kinetics. In: Meyer, R.F. (Ed.),

Heavy Crude and Tar Sands—Hydrocarbons for the 21st Cen-

tury. 5th UNITAR International Conference on Heavy Crude

and Tar Sands. UNITAR Centre for Heavy Crude and Tar Sands,

Caracas, Venezuela, pp. 343–353.

Johnston, P., Wachi, N., 1994. Estimation of natural fracture orien-

tation using borehole imaging logs and vertical seismic profiles

at Orcutt oil field, California, USA. 14th World Petroleum Con-

gress, Stavanger, Norway.

Jones, J.B., Segnit, E.R., 1971. The nature of opal: I. Nomenclature

and constituent phases. J. Geol. Soc. Australia 18, 57–67.

Keller, M.A., Isaacs, C.M., 1985. An evaluation of temperature

scales for silica diagenesis in diatomaceous sequences including

a new approach based on the Miocene Monterey Formation,

California. Geo Mar. Lett. 5, 31–35.

Lefebvre, R., Hutcheon, I., 1986. Mineral reactions in quartzose

rocks during thermal recovery of heavy oil, Lloydminter, Sas-

katchewan, Canada. Appl. Geochem. 1, 395–405.

Lore, J.S., 1999. Thermal fractures in basalt and burned shale. PhD

thesis, Stanford University, California.

Mathews, W.H., Bustin, R.M., 1984. Why do the Smoking Hills

smoke? Can. J. Earth Sci. 21, 737–742.

Moore, R.G., Bennion, D.W., Ursenbach, M.G., 1988. A review of

in situ combustion mechanisms. In: Meyer, R.F., Wiggins, E.J.

(Eds.), The Fourth UNITAR/UNDP International Conference on

Heavy Crude and Tar Sands, Proceedings. In Situ Recovery,

Edmonton, Canada, pp. 775–784.

Nyland, E., Dusseault, M.B., 1983. Microseismic monitoring: re-

sults and potential for process control. J. Can. Pet. Technol.,

62–68.

Perry, C., Gillott, J.E., 1982. Mineralogical transformations as in-

dicators of combustion zone temperatures during in situ com-

bustion. Bull. Can. Pet. Geol. 30, 34–42.

Ranjbar, M., Pusch, G., 1991. Experimental studies of crude oil

pyrolysis, fuel formation and combustion in relation to in-situ

combustion. In: Meyer, R.F. (Ed.), Heavy Crude and Tar

Sands—Hydrocarbons for the 21st Century. 5th UNITAR Inter-

national Conference on Heavy Crude and Tar Sands, Caracas,

Venezuela, pp. 297–305.

Schulte, W.M., de Vries, A.S., 1985. In-situ combustion of naturally

fractured heavy oil reservoirs. Soc. Pet. Eng. J. 25, 67–77.

Tilley, B.J., Gunter, W.D., 1988. Mineralogy and water chemistry of

the burnt zone from a wet combustion pilot in Alberta. Bull.

Can. Pet. Geol. 36, 25–38.

Verkoczy, B., Jha, K.N., 1988. Behavior of heavy oil core material

under simulated fireflood conditions. In: Meyer, R.F., Wiggins,

E.J. (Eds.), The Fourth UNITAR/UNDP International Confer-

ence on Heavy Crude and Tar Sands Proceedings, Edmonton,

Canada, pp. 713–726.

Weaver, C.E., 1979. Geothermal alteration of clay minerals and

shales: diagenesis. Technical Report 70. Office of Nuclear Waste

Isolation Battelle, Columbus, OH, 176 pp.

Willemse, E.J.M., Pollard, D.D., 1998. On the orientation and pat-

terns of wing cracks and solution surfaces at the tips of a sliding

flaw or fault. J. Geophys. Res. 103, 2427–2438.

Wu, C.H., Fulton, P.F., 1971. Experimental simulation of the

zones preceding the combustion front of an in-situ combustion

process. Soc. Pet. Eng. 11, 38.


alteration and fracturing of siliceous mudstone … and fracturing of siliceous mudstone during in...

Documents