Focused fluid flow along faults in the Monterey Formation ... › eichhubl › Reprints › GSABull00.pdfminor silica cement. The distance of cross-stratigraphic flow, as inferred

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GSA Bulletin; November 2000; v. 112; no. 11; p. 1667–1679; 11 figures; 2 tables.

Focused fluid flow along faults in the MontereyFormation, coastal California

Peter Eichhubl*James R. BolesDepartment of Geological Sciences, University of California, Santa Barbara, California 93106, USA

ABSTRACT

Fluid flow in fractured siliceous mud-stone of the Miocene Monterey Formationof California is inferred to be highly fo-cused toward map-scale faults that locallycontain extensive amounts of carbonate andminor silica cement. The distance of cross-stratigraphic flow, as inferred based on thestrontium isotopic composition of carbon-ate fault cement, is close to the thickness ofthe Monterey Formation of 700 m in one oftwo study locations, Jalama Beach, and lessthan the formation thickness at another lo-cation, Arroyo Burro Beach. Fluid is thusderived from within the Monterey Forma-tion rather than from underlying olderunits. Based on mass-balance estimates ofthe fluid volume required for fault cemen-tation at Jalama Beach, the minimum dis-tance of formation-parallel flow into thefault zone is 4 km and possibly .12 km.The inferred distance of flow parallel to theformation into this fault thus exceeds thedistance of cross-formational upward flowalong the fault by at least a factor of six.The mass-balance estimate requires thatfluid flow along the fault is channeled intoa pipe-shaped conduit rather than distrib-uted along fault strike. Fluid flow from thesurrounding formation into fault pipes isinferred to follow a radial rather than uni-or bilateral flow symmetry, using bedding-confined sets of extension fractures andstratabound breccia bodies. Radial fluidflow toward fault pipes requires fairly iso-tropic fracture permeability for flow alongbedding and a low permeability across bed-

*Present address: Department of Geological andEnvironmental Sciences, Stanford University, Stan-ford, California 94305, USA, and Monterey BayAquarium Research Institute, Moss Landing, Cali-fornia 95039, USA; e-mail: [email protected].

ding. The inferred flow geometry illustratesthe combined effect of fault permeabilitystructure, permeability anisotropy of thesurrounding formation, and hydraulic headdistribution in controlling basinal fluid flowin faulted sequences.

Keywords: faults, fluids, strontium, iso-topes, hydrocarbon, Monterey Formation.

INTRODUCTION

Faults may affect basinal fluid flow as bar-riers or as preferred conduits, depending onthe difference in permeability between faultzone and the surrounding formation. Faultsthat are preferred conduits due to increasedpermeability parallel to the fault are likely tocontrol the localized precipitation of pore ce-ment and ore minerals and the migration andaccumulation of hydrocarbons. Faults condu-cive for fluid flow can be subdivided into threehydraulic regimes (Fig. 1): (1) a source regionwhere pore fluid is drawn into the fault zonefrom the surrounding formation of higher hy-draulic head; (2) a sink region of lower hy-draulic head where pore fluid is released backinto the formation or onto the Earth’s surface;and (3) an intermediate neutral conduit of nosignificant fluid exchange with the surround-ing formation that may separate the sourcefrom the sink region. The effectiveness of flu-id transport along a fault can be expressed asthe total fluid volume that has migrated alongthe fault, the average flow rate, the distance offault-parallel flow, and the distance perpendic-ular to the fault over which fluid is focusedtoward the fault in the source region and dis-persed away from the fault in the sink region.An additional parameter of interest is thesource volume, which is a function of fluidvolume and source-rock porosity andstorativity.

For the understanding of basinal hydrody-

namics and associated transfer of mass andheat, it is of interest to quantify the extent towhich conductive faults focus basinal fluidflow. In a sedimentary sequence undergoingdewatering and fluid expulsion during pro-grade burial and diagenesis, the extent of flowfocusing by the presence of faults may bequantified by the ratio of fault-perpendicularflow distance over the distance of fault-par-allel flow in the source region. In stratifiedsequences and for faults inclined with respectto the stratification, it may be more practicalto consider the ratio between the distance offormation-parallel fluid flow DL over the dis-tance of cross-stratigraphic flow DH withinthe source unit,

DLF 5 (1)

DH

(Fig. 1). Fault systems with a large focusingratio F will be more effective in channelingfluid expulsion from the source rock to a po-tential sink within the sequence or to theEarth’s surface. However, in petroleum sys-tems faults with large F will also be more ef-fective for leaking hydrocarbons out ofbreached reservoirs.

For any fault of given length, the fluid fo-cusing factor F is a function of the perme-ability anisotropy of the fluid source rock andof the distance to adjacent faults (Fig. 1).Large ratios of F would be expected in strat-ified source rocks with the highest permeabil-ity parallel to bedding, transected by faults ofwide spacing. An isotropic permeability struc-ture of the formation and/or close fault spac-ing would lead to low ratios of F. The per-meability anisotropy of the source rock maybe controlled by stratification within the for-mation (intrinsic anisotropy) and by the for-mation boundaries to overlying and underly-ing units (extrinsic anisotropy) (Fig. 1).

For a kilometer-scale fault system in the

1668 Geological Society of America Bulletin, November 2000

EICHHUBL and BOLES

Figure 1. Geometry of upward-directed basinal fluid expulsion as controlled by faults andby the permeability anisotropy of sedimentary sequences. Conductive faults can be sub-divided into source and sink regions and an intermediate neutral fluid conduit of no orminimal fluid exchange with the surrounding formation. The ratio of formation-parallelover cross-formational flow distance may be used to quantify the extent of fluid focusingtoward faults in source units. Fluid focusing will be controlled by the permeability an-isotropy of the source unit, which can be intrinsic, i.e., controlled by bedding, or extrinsic,i.e., controlled by contacts to overlying and underlying formations of low permeability.Fluid flow in the Monterey Formation is inferred to be strongly focused toward faults bythe intrinsic anisotropy of the formation (case A).

Figure 2. Location map of Jalama and Arroyo Burro Beaches in south-central California.

Monterey Formation in southern California,we demonstrate that the distance of formation-parallel fluid flow exceeds the distance ofcross-stratigraphic flow by a factor F $ 6. Thedistance of cross-stratigraphic fluid flow DHwill be estimated using the strontium isotopiccomposition of carbonate fault cement as anatural tracer. The formation-parallel flow dis-tance DL is inferred based on mass-balanceestimates of the fluid volume that has migrat-ed along the fault system. On the basis of theestimated distances of fluid flow and the char-acteristic permeability structure of Montereyrocks as observed in outcrop, we infer that theexpulsion of basinal fluid in the MontereyFormation is highly focused toward faults bythe intrinsic, layer-controlled permeability an-isotropy of the formation (type A in Fig. 1).

STRUCTURAL SETTING OFCONDUCTIVE FAULTS

The Miocene Monterey Formation is an or-ganic-rich, hemipelagic sequence of diatoma-ceous and phosphatic shale and organogenicdolostone, deposited between 18 and 6 Ma ina series of continental-borderland basins (Pis-ciotto and Garrison, 1981; Barron, 1986; Hor-nafius, 1994a). During burial, the organic-richdiatomaceous sequence underwent a complexsequence of diagenetic alteration, dominatedby the following reactions: (1) silica transfor-mation of diatom tests, from opal A via opalCT to quartz through two distinct steps of dis-solution-reprecipitation (Stein and Kirkpa-trick, 1976; Isaacs 1981); (2) degradation oforganic matter and maturation of hydrocar-bons (Petersen and Hickey, 1987; Baskin andPeters, 1992); (3) precipitation of authigenicdolomite during early diagenesis and subse-quent dissolution and/or recrystallization(Burns and Baker, 1987; Compton, 1988; Ma-lone et al., 1994); and (4) transition of smec-tite to illite (Pollastro, 1990; Compton, 1991).With an initial organic matter content of up to34% (Isaacs and Petersen, 1987), the Monte-rey Formation is both source and reservoirrock for hydrocarbons (Ogle et al., 1987;MacKinnon, 1989). Due to the low matrixpermeability, typically ,1 md, hydrocarbonmigration and production critically depend onfractures as flow pathways (Crain et al., 1985;Roehl and Weinbrandt, 1985; Isaacs and Pe-tersen, 1987; MacKinnon, 1989).

Faults in the Monterey Formation are lo-cally cemented by massive volumes of car-bonate and quartz, indicative of large volumesof fluid flowing along these faults. At JalamaBeach, 100 km west of Santa Barbara (Fig. 2),two subparallel faults are cemented by dolo-

Geological Society of America Bulletin, November 2000 1669

FOCUSED FLUID FLOW ALONG FAULTS

Figure 3. Dolomite-cemented, left-lateral strike-slip fault at Jalama Beach. The south-eastern one of two exposed faults (Fig. 4) extends for ;35 m across the wavecut platform.A former slip surface forms the margin of the fault zone to the right (northwest), and asecond parallel slip surface is exposed on the left margin, to the left of the measuring stick(2 m long).

Figure 4. Map of cemented faults and associated vein systems at Jalama Beach, ;1.6 kmnorthwest of state beach.

mite and minor amounts of quartz. The south-ern fault forms a resistant wall of cementedfault breccia, ;6 m thick, 7 m tall, and ex-tending more than 35 m across the wavecutplatform between mean low tide water lineand the beach cliffs (Fig. 3). The second fault,;300 m northwest of the first one, is less wellexposed in a cove.

Both faults are characterized by a largeamount of dolomite cement; 7–10-cm-thicklayers of banded dolomite coat host rock frag-ments and aggregates of earlier dolomite ce-ment (Fig. 3). Dolomite fault cement can betraced into extension veins that branch offfrom both faults and extend over distances ofa few hundred meters (Fig. 4). The combinedthickness of dolomite cement of both faultsand connected veins, measured along a tra-verse across the strike of the fault-vein sys-tems, is ;8 m. Fragments of cement and hostrock fill interstitial cavities in fault cement.Sedimentary layering of infill and sorting offragments by grain size suggest transport ofdetrital fracture fill in suspension by rapidlymoving fluid along these faults (Eichhubl,1997). The fault system at Jalama Beach ishosted by siliceous dolostone and dolomiticopal CT porcelanite and minor amounts ofclay shale and chert.

A fault zone similar to that of Jalama Beachis exposed in coastal cliffs at Arroyo BurroBeach in Santa Barbara (Eichhubl and Behl,1998). Two subparallel faults and associatedveins are cemented by banded calcite of cu-mulative cement thickness similar to that ofJalama Beach. The host rock sequence at Ar-royo Burro Beach consists of dolomitic por-celanite and siliceous mudstone and only sub-ordinate dolostone.

Coastal sections of the Monterey Formationunderwent for the most part a single burial-exhumation cycle; exhumation started ca. 1Ma (Jackson and Yeats, 1982), while offshoresections of the Monterey Formation are stillparts of active basins undergoing burial. Theinferred maximum burial depth for the JalamaBeach section is ;1000 m (Eichhubl, 1997).The exposed fault system at Jalama Beach is;50 m below the top of the Monterey For-mation (Dibblee, 1988a, 1988b), which is;700 m thick at this location (Grivetti, 1982).The fault system at Arroyo Burro Beach cutsacross a bed of siliceous dolostone that marksthe Luisian-Mohnian stage boundary (Echols,1994; Hornafius, 1994c). On the basis of themeasured thickness of the Naples Beach sec-tion (Echols, 1994; Hornafius, 1994c), located22 km west of Arroyo Burro Beach, the faultsystem is ;300 m above the base of the Mon-terey Formation, which has a total thickness


EICHHUBL and BOLES

TABLE 1. 87Sr/86Sr COMPOSITION OF MONTEREY FAULT CEMENT

Sample number Sample location Description 87Sr/86Sr 62s

03/01/95-7 Jalama Beach, southeastern faultzone

Early, randomly oriented dolomitecement

0.708820 0.000014

03/01/95-6AB Jalama Beach, southeastern faultzone

Late banded, acicular dolomite 0.708815 0.000019

04/23/96-6 Jalama Beach, eastern margin ofsoutheastern fault zone

Gray clear dolomite in fault-parallelvein

0.708874 0.000025

02/26/95-2 Jalama Beach, vein 145 m westof southeastern fault

Early banded clear dolomite, veinconnected to fault (?)

0.708837 0.000014

02/26/95-2 Jalama Beach, vein 145 m westof southeastern fault

Late acicular dolomite, vein con-nected to fault (?)

0.708753 0.000014

02/11/95-5 Jalama Beach, veinlet 70 m eastof southeastern fault

Isolated, bedding-confined dolo-mite veinlet

0.708829 0.000014

03/14/95-5A Arroyo Burro Beach, eastern fault600 m east State Beach

Early cement band of large calcitebotryoid within fault

0.708765 0.000015

03/13/95-6 Arroyo Burro Beach, vein 20 meast eastern fault

Calcite vein, connected to fault 0.708749 0.000020

02/24/95-3B(B) Arroyo Burro Beach, strataboundvein 88 m west aluminum tube,1100 m east State Beach

Laminated calcite vein reactivatingearlier dolomicrite vein acrossdolostone marker bed

0.708774 0.000025

04/25/94-5 Arroyo Burro Beach, strataboundbreccia 4 m east aluminumtube, 1100 m east State Beach

Baroque dolomite cement inquartz-dolomite-calcite cement-ed dolostone breccia

0.708683 0.000014

Note: All 87Sr/86Sr values reported relative to NBS-987 standard 5 0.710235. For detailed location description,refer to Eichhubl and Behl (1998). Sample numbers are same as used for stable isotopic analyses reported inEichhubl and Boles (1998) and Eichhubl (1997).

TABLE 2. ELEMENTAL AND STRONTIUM ISOTOPIC COMPOSITION OF SILICEOUS DOLOSTONE HOST ROCK AT JALAMA BEACH

Sample Location SiO2 CaO MgO Al2O3 K2O Sr Rb HCl- Sr in residue 87Sr/86Sr Sr in carbonate† 87Sr/86Sr 87Sr/86Srnumber (wt%) (wt%) (wt%) (wt%) (wt%) (ppm) (ppm) insoluble

(ppm) (wt% ofof

(ppm)* (wt% of*of of

residuetotal Sr)*

residuetotal Sr)*

carbonate† bulk(wt%) rock*

04/23/96-3 88 m west ofsoutheasternfault

28.34 38.97 26.38 3.19 0.43 345 14 20 84.8 5.0 0.71104 6 3 411 95.0 0.70895 6 2 0.70905 6 5

01/22/98-1 180 m west ofsoutheasternfault§

51.86 26.35 13.68 1.97 0.39 300 13 39 43.6 5.7 0.71097 6 3 464 4.3 0.70891 6 1 0.70903 6 4

04/23/96-8 80 m east ofsoutheasternfault

41.89 31.83 18.70 2.93 0.55 244 19 35 44.3 6.3 0.71102 6 2 351 93.7 0.70899 6 2 0.70912 6 4

01/22/98-3 110 m east ofsoutheasternfault#

31.94 36.09 24.71 3.58 0.48 241 14 24 58.0 5.8 0.71160 6 2 299 94.2 0.70886 6 2 0.70902 6 4

Note: All 87Sr/86Sr values reported relative to NBS-987 standard 5 0.710235.*Calculated value.†Acetate leachate.§Approximately 20 m stratigraphically below sample 04/23/96-3.#15 m stratigraphically below sample 04/23/96-8.

of ;410 m at this location. The inferred max-imum burial depth of the Arroyo Burro Beachsection is 750 m (Eichhubl and Boles, 1998).

The faults and fault-related veins at Jalamaand Arroyo Burro Beach dip steeply and cutacross outcrop-scale folds, indicating fault andvein formation during shortening and exhuma-tion of the basin flanks. The late, postfoldingorigin of these fault-related veins is also docu-mented through untilted stratified detrital frac-ture fill. Abundant hydrocarbon fluid inclusionsand bitumen-cemented breccias indicate thatfault-related carbonate vein cementation is con-current with hydrocarbon migration. Striationsand drag of bedding indicate left-lateral strikeslip for both fault systems. Displacements areestimated to be ,100 m in both cases.

STRONTIUM ISOTOPICCOMPOSITION OF FAULTCARBONATE CEMENT AND HOSTDOLOSTONE

The strontium isotopic compositions of car-bonate vein and fault cement from JalamaBeach and Arroyo Burro Beach were analyzedin order to infer distances of mass transport.At Jalama Beach, microdrilled samples ofbanded, inclusion-poor dolomite cement fromthe eastern fault and from veins 145 m westand 70 m east of the eastern fault were ana-lyzed. At Arroyo Burro Beach, calcite cementwas sampled from the fault, from veins 20 and400 m to the east, and from dolomite ce-menting a dolostone breccia 500 m east of the

fault. In all cases, the composition and purityof carbonate cement were verified by thin-sec-tion petrography.

Carbonate samples were dissolved in 5 Macetic acid and the separated samples analyzedon a Finnigan MAT 261 adjustable five-col-lector mass spectrometer with a precision of,0.00003 (2s mean; Table 1). The accuracyof the measurements was monitored with theNBS 987 standard, which gave a mean valueof 0.710235 6 0.00002 (2s mean). Separa-tions and analyses were performed in the lab-oratories of James Mattinson and George Til-ton at the University of California, SantaBarbara.

In addition, four bulk rock samples of clay-poor, siliceous dolostone host rock were ana-lyzed to provide a strontium-stratigraphic ref-erence age of the section adjacent to the faultat Jalama Beach. Carbonate was dissolved us-ing 5 M acetic acid to minimize contaminationby clay dissolution and the 87Sr/86Sr ratio de-termined from the leachate (Table 2). Remain-ing carbonate in the dissolution residue wasremoved with 2.5 M hydrochloric acid untilno visible reaction occurred. The acid wassubsequently replaced two times to minimizestrontium contamination in exchangeable claylayers, and the resulting HCl-insoluble residuewas analyzed for 87Sr/86Sr (Table 2).

Strontium concentrations of the bulk rockand leachate obtained by X-ray fluorescencespectrometry (XRF) and thermal ionizationmass spectrometry (TIMS), respectively, in-dicate that 5%–6% of total strontium of thebulk dolostone host rock from Jalama Beachis radiogenic strontium contained in clays.Following results by Schultz et al. (1989, Ta-ble 2), we assume that the radiogenic stron-tium resides in the nonexchangeable clay lay-



Figure 5. Strontium isotopic composition of Monterey vein carbonate. Strontium valuesare plotted with respect to the biostratigraphic age of the host rock (Arroyo Burro Beach)and with respect to the strontium isotopic composition of the host dolostone on both sidesof the fault system (Jalama Beach). Horizontal bars on Jalama values represent the spreadin strontium isotopic composition of the dolostone host rock. Seawater curve is after Ho-warth and McArthur (1997; revised second edition of database, written commun., August1998). The 95% confidence interval of seawater curve is within line thickness. Time scaleis after Shackleton et al. (1994) for 0–7 Ma and Cande and Kent (1995) for 7–25 Ma.Vertical error bar approximates 2s standard deviation of vein cement analyses. All valuesare normalized to NBS 987 5 0.710235.

ers that are largely unaffected by the aceticacid leach, whereas strontium in the exchange-able layers mimics the isotopic composition ofthe carbonate. This assumption is based on thesimilarity of our radiogenic strontium ratios ofthe dissolution residue with that of smectite inSchultz et al. (1989) after clay treatment withammonium chloride, whereas the 87Sr/86Sr ra-tio of their untreated smectite samples com-pares to that of selectively leached carbonatepore cement. Negligible radiogenic strontiumcontamination during the selective acetic acidleach is also confirmed by the generally goodcorrespondence of the strontium stratigraphicposition of selectively leached carbonate sam-ples with their biostratigraphic or lithostrati-graphic position, as demonstrated by our sam-ples and by Miller (1995), and as discussed inthe following.

Strontium isotopic ratios for dolomite veinand fault cement at Jalama Beach, ranging be-tween 0.70875 and 0.70887, are distinctlylower than dolomite (i.e., acetate leachate)values, 0.70886 to 0.70899, of surroundinghost dolostone. Strontium isotopic ratios ofcalcite vein and fault cement at Arroyo BurroBeach are generally lower than vein and faultcement values from Jalama Beach, with valuesranging between 0.70875 and 0.70877. Thelowest 87Sr/86Sr ratio of 0.70868 is obtainedfrom a single analysis of dolomite vein ce-ment at Arroyo Burro Beach, 500 m east ofthe cemented fault. On the basis of crosscut-ting relations, this vein predates calcite faultand vein cement at Arroyo Burro Beach (Eich-hubl and Boles, 1998).

DISTANCE OF CROSS-STRATIGRAPHIC FLUID FLOW

For carbonate cement precipitating from anaqueous solution, the carbonate cement inher-its the strontium isotopic composition of theaqueous solution, making the strontium iso-topic composition of carbonate cement a use-ful natural tracer for basinal fluid flow(Schultz et al., 1989; Sullivan et al., 1990;Mountjoy et al., 1992; Chaudhuri and Clauer,1993; Feldman et al., 1993). Throughout Neo-gene and Quaternary time the seawater 87Sr/86Sr composition steadily increased to the pre-sent-day value of 0.709162 (Howarth andMcArthur, 1997) (Fig. 5). The strontium iso-topic composition of entrapped pore fluidwithin a Neogene sediment sequence is there-fore expected to decrease steadily with depth,provided the strontium isotopic compositionof the fluid remained unmodified by diagenet-ic processes. Upward migration of pore waterwould lead to lower 87Sr/86Sr ratios in carbon-

ate cement precipitating during or after fluidmigration as compared to cement precipitatedprior to fluid migration. The strontium isoto-pic composition of cement may thus be usedas a tracer for cross-stratigraphic flow of con-nate pore fluids, provided that diageneticchanges in strontium isotopic fluid composi-tion are negligible or predictable.

The strontium isotopic composition of theacetate-leached dolomite component in thefour host dolostone samples from JalamaBeach agrees well with the upper Miocenestratigraphic position of the sampled section,;50 m below the Monterey-Sisquoc contactas mapped by Dibblee (1988a, 1988b). Ap-parently, the selectively leached carbonate hasretained the primary strontium isotopic com-position of upper Miocene seawater. Similarly,Miller (1995) found a good correlation be-tween strontium isotopic composition of ace-tic-acid leached dolostone and biostratigraphicage of samples taken from a continuous Mon-terey core from the South Elwood offshore oilfield, 17 km west of Arroyo Burro Beach. Au-thigenic dolostone of the Monterey Formationhas been shown to precipitate during early

burial but is inferred to progressively recrys-tallize during burial, based on its oxygen iso-topic composition (Burns and Baker, 1987;Malone et al., 1994). Miller’s (1995) data in-dicate that, despite recrystallization, the do-lostone beds remain closed with respect tostrontium during recrystallization and oxygenisotopic resetting.

In contrast, the strontium isotopic values ofcalcite and dolomite vein and fault cementfrom Jalama Beach and Arroyo Burro Beachare distinctly lower than the expected seawaterstrontium isotopic values for the particularstratigraphic position of these locations (Fig.5). For Jalama Beach, where a biostratigraphicreference is not available, 87Sr/86Sr ratios offault and vein dolomite are compared to themeasured 87Sr/86Sr composition of the sur-rounding host dolostone. For Arroyo BurroBeach, the 87Sr/86Sr composition of vein andfault cement is compared to the biostratigraph-ic age of the dolostone marker bed that cor-responds to the Luisian-Mohnian stage bound-ary (Echols, 1994; Hornafius, 1994c) of ca.13.85 Ma (Barron, 1986; time scale revisedafter Cande and Kent, 1995).


EICHHUBL and BOLES

The average 87Sr/86Sr ratio of fault and veindolomite at Jalama Beach of 0.70882 is0.00011 lower than the mean 87Sr/86Sr valueof the surrounding host dolostone (0.70893).For a formation thickness at Jalama Beach of;700 m and assuming uniform sedimentationrate throughout the section, the difference sug-gests upward transport of strontium by ;245m (6100 m for 87Sr/86Sr ratios 60.00002 2s).The inferred distance of upward strontiumtransport at Arroyo Burro Beach is less andclose to the resolution limit of the method.Fault-related calcite cement at Arroyo BurroBeach has an average 87Sr/86Sr composition of0.708757 6 0.000020 that is 0.000031 60.000023 below the expected value of0.708788 6 0.000003 for this particular strati-graphic position. For a thickness of the un-derlying Monterey section of ;300 m, thisdifference in isotopic composition is equiva-lent to 70 m (650 m) of upward strontiumtransport. At both Jalama and Arroyo BurroBeach, the strontium isotopic composition ofvein cement points toward a fluid source be-low the surrounding host rock but within theformation rather than within deeper sectionsof the basin.

The interpretation of these numbers as dis-tance of upward fluid flow is contingent uponthe following three assumptions.

1. The Strontium Isotopic Composition ofthe Pore Fluid has not Significantly BeenChanged by Dissolution of DetritalMinerals

Mineral dissolution may change the stron-tium isotopic fluid composition. Dissolutionof clays and potassium feldspar tends to in-crease the 87Sr/86Sr ratio, whereas dissolutionof plagioclase tends to decrease the ratio of87Sr/86Sr. The extent to which these reactionsalter the pore fluid strontium isotopic com-position depends on the water-rock ratio, thestrontium concentration in the mineral phase,and the mineral abundance. Potassium feld-spar and plagioclase may contain 400–700ppm Sr21 (Schultz et al., 1989; Feldman et al.,1993), somewhat higher than Sr21 concentra-tions in dolomite of 300–460 ppm measuredat Jalama Beach (Table 2) and similar valuesreported previously (Faure, 1986; Burns et al.,1988). Potassium feldspar and calcic plagio-clase form only a minor detrital component inMonterey rocks, of ;2% and 0.5%weight, re-spectively (averages of 41 analyses; Compton,1991). Volcanic ash beds, which occasionallycontain Sr21 in excess of 1000 ppm (Horna-fius, 1994b), make up only ;1% of the totalMonterey section and are therefore unlikely to

affect the strontium isotopic composition ofpore fluid on a regional scale. Carbonate,however, is abundant as authigenic dolomiteand biogenic calcite (Isaacs, 1980; Burns andBaker, 1987; Compton, 1988).

Due to their higher abundance, clays havea potentially larger influence on the strontiumisotopic composition of Monterey Formationwater than feldspar. If we take (lacking a moresystematic data set) the four host dolostonesamples from Jalama Beach as a guide, com-plete dissolution of clay contained in these do-lostones would raise the pore-water 87Sr/86Srratio by ;0.00013 as compared to pore fluidwhose 87Sr/86Sr is controlled by carbonate dis-solution only. Addition of radiogenic stron-tium would lead to an apparent decrease ininferred fluid migration distance. Assumingcomplete clay dissolution, addition of radio-genic strontium would thus double the migra-tion distance of 245 m for Jalama Beach andquadruple the distance of 70 m for ArroyoBurro Beach. While the inferred fluid migra-tion distances would be significantly longer,both estimates would still be within the thick-ness of the Monterey Formation at both lo-cations, leaving the basic conclusion of an in-traformational fluid source as derived earlierunchanged. Dissolution of smectite, the pre-dominant clay phase in the Monterey Forma-tion, has been observed at burial temperaturesabove 80 8C only (Pollastro, 1990; Compton,1991). Thus, strontium release into the fluiddue to clay dissolution is unlikely to be sig-nificant except within the deepest parts of theformation. The lack of a significant radiogenicstrontium component in Monterey Formationwater is also suggested by the strontium iso-topic composition of present-day MontereyFormation water. Analyzed formation watersfrom three different Monterey reservoirs in theSanta Barbara and Santa Maria Basins, in-cluding the South Elwood offshore oil fieldstudied by Miller (1995) with respect to do-lomite strontium isotopes, range between0.70863 and 0.70826 (Eichhubl, 1997), andare even lower than the strontium isotopicrange predicted for connate Monterey For-mation water based on the seawater curve.

Lacking any indication of a significant ra-diogenic strontium component in MontereyFormation fluids, the following discussion onfluid migration distance is based on the firstestimates of 245 m for Jalama Beach and 70m for Arroyo Burro Beach.

2. The Strontium Composition of theFluid Corresponds to the StrontiumComposition of a Distinct Source Layer

Equating the difference in strontium isoto-pic composition between the vein cement and

some stratigraphic lower source layer with thedistance of strontium transport is tantamountto assuming that all the strontium is derivedfrom a single layer rather than from a depthrange, and no exchange of strontium has takenplace during upward flow. These assumptionsare justified if a distinct stratigraphic horizonserves as an aquifer supplying fluid to thefault zone, and provided that carbonate dis-solution or precipitation during upward flowdoes not cause significant exchange of stron-tium with the country rock. Because no singledistinct aquifer can be identified in the sec-tions at Jalama Beach and Arroyo BurroBeach, we assume that fluid is drawn into thefault uniformly from the underlying sectionover a finite depth interval and with uniformstrontium concentration. Assuming a uniformstrontium concentration of fluid entering thefault will be better justified by the uniformityof the dolostone-rich section at Jalama Beach(Grivetti, 1982) than for the more variablelithologic composition of the Arroyo BurroBeach section.

Assuming uniform fluid influx with uni-form strontium concentration over the depthinterval transected by the fault, the strontiumisotopic composition of the fluid and thus ofthe cement at the top of the fault is the inte-grated strontium isotopic composition over thedepth interval. If dissolved strontium entersthe fault and fracture system over the depthinterval htop–hbottom at equal amounts per unitdepth interval Dh, then the strontium isotopicratio of the fluid at the top of the depth inter-val is

87/86Srfluidtop1

87/865 Sr dh. (2)E formationh 2 htop bottom bottomIn equation 2, 87/86Sr stands for the strontiumisotopic ratio and htop and hbottom correspond tothe depth at top and bottom of the fault sys-tem, respectively. The isotopic composition offluid entering the fault zone can be assumedto correspond to the strontium isotopic ratioof the formation at this particular depth h.With 87/86Srfluid inferred through the measuredisotopic composition of the vein cement andd87/86Srformation/dh approximated through theseawater strontium isotopic curve in Figure 5,equation 2 can be solved numerically to obtainhbottom, the depth at the base of the depth in-terval drained by the fault system. The me-dium distance of mass transport thus obtainedis 535 m (6120 m, 62s) for Jalama Beachand 140 m (680 m, 62s) for Arroyo BurroBeach. For Jalama Beach in particular, and po-



Figure 6. Inferred distances of cross-stratigraphic mass transport, based on the 87Sr/86Srratio of vein cement, plotted against the stratigraphic section at Jalama Beach. The lowerestimate assumes a single source bed of fluid, and the higher estimate assumes that fluidis drawn into the fault uniformly over the depth interval. In both cases, a uniform sedi-mentation rate throughout the section is assumed, allowing substitution of the strontiumisotopic sequence of the section by the 87Sr/86Sr seawater curve.

tentially for Arroyo Burro Beach, these dis-tances are significant fractions of the thick-nesses of the underlying formation, which is;650 m at Jalama Beach (Fig. 6) and ;300m at Arroyo Burro Beach. However, the in-ferred distances still do not suggest that stron-tium mass transport originates significantlybelow the base of the Monterey Formation.

Inferring d87/86Srformation/dh from the seawaterstrontium isotopic curve requires knowledgeof variations in sedimentation rate within theparticular stratigraphic interval. The transportdistance obtained for Arroyo Burro Beach iscorrected for sedimentation rates given byEchols (1994) for Naples Beach. For JalamaBeach, where a detailed biostratigraphic anal-ysis is not available, we assumed a uniformsedimentation rate throughout the formationthickness. Because 535 m (6120 m) consti-tutes a large fraction of the total stratigraphicthickness of 700 m at Jalama Beach, varia-tions in sedimentation rate throughout the sec-tion are likely to cancel out when integratedover the depth range.

If part of the strontium is continuously re-moved from solution by precipitation duringupward flow in the fracture system, then thefinal strontium fluid composition in the frac-ture system can be estimated through the ex-pression

87/86Srfluid (3)top

87/86Sr (1 2 fh) dhE formationbottom

5 ,top

(1 2 fh) dhEbottom

where f is the fraction of strontium removedfrom solution through cement precipitationper unit flow distance.

The mass-balance calculation in the followingsection suggests that ;1.9 3 10–4 cm3 dolomiteper liter fluid per meter of flow distance precip-itates during upward fluid flow. With an averageconcentration of 300 ppm Sr21 in dolomite, ce-mentation removes ;1.6 3 10–4 mg/L strontiumper meter flow distance. Given an average stron-tium concentration of ;12 mg/L in MontereyFormation water (Eichhubl, 1997), the factor fbecomes ;1.3 3 1025 m21. With the factor fthat small, the fluid strontium isotopic ratio ob-tained through equation 3 differs from that ob-tained through equation 2 by ,1 3 10–6, whichis within the precision of the strontium isotopicanalyses—on average 62 3 10–5 (2s)—andtherefore within the uncertainties of the distanceestimates given herein.

As little as precipitation affects the esti-

mates of mass transport distance, dissolutionof host carbonate along the conduit wall willhave an equally small effect on the strontiumisotopic composition of the fluid. Fluid wouldhave to enter and react with the host rock fortens to hundreds of meters adjacent to the fluidconduits for any significant shift in fluid stron-tium composition to occur. Host rock alter-ation visible in thin section, such as dolomiterecrystallization, is restricted to within 1 madjacent to fault-related veins at JalamaBeach. If dolomite recrystallization and ac-companying strontium isotopic resetting of thehost dolostone would have been more perva-sive, the composition of the host rock sampleson which the transport distance estimates arebased would have been equally affected. Sig-nificant exchange in strontium isotopes be-tween fluid and host rock during upward flowcan thus be excluded.

3. The Distance of Strontium TransportCorresponds to the Distance of FluidTransport

The distances calculated herein refer to thetransport of strontium, corresponding to dis-tances of fluid flow only if strontium exchangebetween solution and country rock or earliervein cement during upward flow is negligible.The calculation suggests that this is the case,the distances thus representing true distancesof fluid flow. In sedimentary sequences, how-ever, stratigraphic differences in 87Sr/86Sr com-

position reflecting the seawater strontium iso-topic evolution during deposition will providea proxy only for fluid flow across bedding, butnot for bedding-parallel flow such as flow up-dip along the tilted flanks of the basin. Theflow distance parallel to bedding is estimatedin the following section based on a mass-bal-ance calculation that estimates the fluid vol-ume required for carbonate precipitation andby comparing the estimated fluid volume to alikely source volume.

VOLUME OF FLUID EXPELLED ANDTHE DISTANCE OF FORMATION-PARALLEL FLUID FLOW

The large volumes of carbonate cementcontained in the fault zones at Jalama Beachand Arroyo Burro Beach are indicative oflarge fluid volumes and potentially effectivefocusing of fluid flow along these faults. Inconjunction with the cross-stratigraphic flowdistance derived herein, an estimate of the flu-id volume necessary for precipitation of theobserved volume of carbonate cement mayprovide an estimate of the distance of bed-ding-parallel flow and thus the extent of fluidfocusing. The following discussion focuses ondolomite cementation at Jalama Beach but ap-plies similarly to calcite cementation at Ar-royo Burro Beach.

A mass-balance calculation of fluid involvedin precipitation of the observed volume of do-lomite requires information about the concentra-


EICHHUBL and BOLES

Figure 7. The variation of CO2aq concentration in an aqueous solution as a function ofdepth and mole fraction m of CO2gas. A CO2gas mole fraction of 0.07 is measured in gasproduced from the Hondo oil field (Exxon, Thousand Oaks, 1997, oral commun.), whereasa mole fraction of 0.01 is more characteristic for the temperature range of 80–100 8C(Smith and Ehrenberg, 1989). Curves are calculated for thermobaric gradients of 50 8C/100 atm (hydrostatic), 50 8C/240 atm (lithostatic), and 50 8C/130 atm, which is an upperlimit of characteristic gradients in the Monterey Formation. The mole fraction of CO2aqis calculated assuming a fugacity coefficient m 5 1 and using Henry’s Law coefficientsafter Ellis and Golding (1963) for 1 M NaCl solutions.

tions, or rather activities, of Ca21, Mg21, andCO322, and of pH, pressure, and temperature pri-or to and after upward fluid flow. Because theseparameters are not known to the desirable extent,order of magnitude estimates need to be ob-tained by limiting the number of variables. Thefirst estimate of fluid mass will be derived byassuming that cementation is controlled by theamount of Ca21 and Mg21 in solution. The sec-ond model assumes that Ca21 and Mg21 are notlimiting factors, but that precipitation is con-trolled by changes in CO2 partial pressure, otherfactors affecting pH being constant. In addition,reactions between a solid and an aqueous solu-tion may not reach equilibrium if the solution ismoving rapidly. A mass-balance calculationbased on the assumption of thermodynamicequilibrium may thus provide only a minimumestimate of the mass of fluid involved. A thirdestimate is therefore obtained through observedprecipitation rates in a moving solution undersimilar hydrodynamic conditions.

Cation-Limited Model

A conservative estimate can be obtainedbased on the assumption that dolomite precip-itation is limited by the concentration of Ca21

and Mg21 in solution only, and that dissolved

carbonate is available in excess. Concentra-tions of Ca21 and Mg21 in present-day Mon-terey Formation water in the Santa Barbaraand Santa Maria basins average ;60 mg/Land 20 mg/L, respectively (Eichhubl, 1997).If all the magnesium were consumed to formdolomite, these concentrations correspond to amaximum of 153 mg or 0.05 cm3 of dolomiteper liter solution. The entire fault system, hav-ing a cumulative cement thickness of 8 m andan estimated depth, based on the strontiumtransport, of 500 m, would thus require 8 3107 m3 fluid per meter fault length. Assumingthat the cemented zone extends 35 m alongstrike of the fault, a total volume of 2.8 3 109

m3 or 2.8 km3 fluid is required for dolomiteprecipitation.

Because only a fraction of Mg21 is likely tobe removed from solution during upward flow,this estimate is unrealistic. The pumping of for-mation water to the Earth’s surface during oilproduction and before sampling may resembleupward fluid flow in fault zones. During pump-ing, the formation water decompresses, thus de-gasses CO2, and possibly precipitates carbonateas scale in wells (Boles and Ramseyer, 1987).Ca21 and Mg21 concentrations are thereforelikely to be higher in the subsurface than theirvalues analyzed at the surface. The analyzed

formation water composition may therefore beconsidered the final fluid composition after rap-id upward flow rather than the initial compo-sition before flow.

Carbonate-Limited Model

A more viable approach to carbonate massbalancing is based on the assumption that theformation fluid is always saturated in Ca21

and/or Mg21 with respect to calcite or dolo-mite and that precipitation is controlled bychanges in CO2 partial pressure as fluid movesupward and decompresses. Initial saturationwith respect to dolomite or calcite appears rea-sonable for fluid that has been in contact withdolostone and calcite shell tests for severalmillion years. In a pure H2O-CO32–-Me21 sys-tem, carbonate cementation is controlled bychanges in temperature and CO2 partial pres-sure, with the tendency for carbonate disso-lution with decreasing temperature under con-stant , but for precipitation withPCO2decreasing under constant temperaturePCO2(Ellis, 1959). Under common geobaric gradi-ents, the pressure effect outweighs the tem-perature effect and carbonate precipitates dur-ing upward flow, to the saturationconcentration of available divalent metal cat-ions, primarily Ca21 and Mg21.

The CO2 concentration in solution can becalculated using Henry’s Law

fCO2K 5 , (4)h x

where Kh is Henry’s Law coefficient and x ismole fraction of CO2aq in solution. Under di-lute conditions, the fugacity, , approachesfCO2the partial pressure , which is equal to thePCO2total pressure times the mole fraction ofCO2gas. The change in CO2aq concentrationwith depth is shown in Figure 7 for a constantmole fraction CO2gas of 0.07, which is themole fraction of CO2gas in the gas phase pro-duced from the Monterey Formation in theoffshore Hondo oil field 40 km west of SantaBarbara (Exxon, Thousand Oaks, 1997, oralcommun.). Henry’s Law coefficients are takenfrom Ellis and Golding (1963) for an ionicstrength of 1 m and gas activity coefficientshave been approximated by unity.

Equilibrium concentrations of dissolvedCO2aq are calculated in Figure 7 for three ther-mobaric gradients, hydrostatic (50 8C/100atm), lithostatic (50 8C/240 atm), and in be-tween (50 8C/130 atm), the latter correspond-ing to a pressure gradient of 130 atm/km or0.6 psi/ft (12.7 MPa/km). Pore-fluid pressuregradients in Monterey hydrocarbon reservoirs



range between hydrostatic and 0.6 psi/ft (U.S.Geological Survey, 1983; Crain et al., 1985).Upward flow of 500 m to the depth of interest,;1000 m, and following a hydrostatic pres-sure gradient, leads to a drop in CO2aq con-centration of ;30 mg/L, corresponding to;0.02 cm3/L of dolomite precipitation. If thefluid were initially at a fluid pressure corre-sponding to a pressure gradient of 130 atm/km and were to drop to a hydrostatic pressure,then CO2aq concentration would drop by ;130mg/L, precipitating ;0.10 cm3/L dolomite.

With the latter number providing a mini-mum fluid volume, the entire fault systemwith a cumulative vein thickness of ;8 m andan estimated depth of 500 m requires a min-imum of 4 3 107 m3 of fluid per meter faultlength. Assuming this fluid volume corre-sponds to 5% of the source rock volume, thefault would drain a rock volume of at least 0.8km3 per meter fault length. Because the faultsat Jalama Beach are exposed only over a dis-tance of ;35 m along strike, these numberscan be interpreted in two ways. If the ce-mented zone is only 35 m long, the requiredfluid volume is 1.2 3 109 m3, correspondingto a source volume of ;23 km3 and a distanceof radial fluid flow into the fault of 4.3 km. Ifthe fault cementation were continuous alongstrike, focusing fluid bilaterally over a 500-m-thick depth interval, then the fault would haveto drain an 800-km-wide zone to each side ofthe fault, which is wider than the basin. Clear-ly, cementation along the fault must be dis-continuous along strike and the fault focusedfluid radially.

To see how sensitive the fluid volume es-timate is to the assumed pressure gradient, wemay assume that upward fluid flow followeda hydrostatic pressure gradient throughout. Inthis case, the total fluid volume required toprecipitate an 8 3 35 3 500 m3 dolomite-cemented zone is ;6.3 3 109 m3, four timesmore than obtained for the superhydrostaticcase herein. The drainage radius becomes 9km, twice the value, all other parameters beingheld constant.

A fluid pressure gradient close to hydrostaticis not very likely during fluid expulsion at Ja-lama Beach, however, because fluid migrationwith respect to the formation under hydrostaticconditions is possible only when the sectioncompacts through the stagnant water columnduring burial. One of us (Eichhubl, 1997) de-rived flow velocities of as much as 0.1 m/sbased on sedimentary structures of detrital frac-ture fill. These high flow velocities and the in-ferred timing of fluid flow during basin marginexhumation require above-hydrostatic fluidpressures to induce fluid flow, thus making flu-

id pressure conditions close to the hydrostaticgradient unlikely. Evidence for lithostatic fluidpressures, however, is not observed. Extensivebedding-parallel veins that would indicate lith-ostatic fluid pressures are not found in theMonterey Formation, with the exception ofsome water sills inferred to form during earlyburial based on crosscutting relations with foldsand faults. On the basis of these constraints andpresent-day fluid pressure gradients, an initialfluid pressure corresponding to an intermediatefluid pressure gradient of 130 atm/km may beconsidered a reasonable estimate of the fluidpressure before upward fluid flow.

In addition to fluid pressure, fluid volumeestimates depend critically on the CO2 partialpressure, which is a function of the mole frac-tion CO2gas. For a CO2gas mole fraction of 0.01,a more common value for hydrocarbon res-ervoirs in the 80–100 8C temperature rangethan 0.07 used herein (Smith and Ehrenberg,1989), the fluid volume becomes 10 km3, andthe radius of radial drainage becomes 11 km.The CO2gas mole fraction typically increases inhydrocarbon reservoirs with increasing burialtemperature and depth (Smith and Ehrenberg,1989). Curves of dissolved CO2gas plottedagainst burial depth would therefore have alower slope than the curves plotted for con-stant mole fraction CO2gas, as shown in Figure7. During rapid upward flow along fracturesin a formation of low matrix permeability, flu-id may not exchange CO2 easily with the for-mation water at shallower depth. The fluidmay therefore be expected to follow theclosed-system trajectories shown in Figure 7.Upward fluid flow through the matrix of per-meable formations, however, may follow tra-jectories for a decreasing mole fraction CO2gasand correspondingly precipitate more carbon-ate cement per unit fluid volume and flow dis-tance than would be expected for rapid flowalong faults.

These calculations were obtained assuming,for the sake of convenience, a pure H2O-CO32–-Me21 system. Pore water of the Monterey For-mation, however, is characterized by high con-centrations of organic acid (S.G. Franks, 1996,written commun.). The dissociation of organicacids, such as the reaction CH3COOH ↔CH3COO– 1 H1 for the acetate-acetic acid pair,may control pH in addition to . SolutionsPCO2with acetate concentrations below ;0.06 M(;3600 mg/L) at 100 8C would still precipitatecarbonate through CO2 exsolution, although to alesser extent than in a solution whose pH is sole-ly controlled by the carbonate system (Lunde-gard and Land, 1989). At higher acetate concen-trations, loss of CO2 leads to carbonatedissolution rather than precipitation. Acetate

concentrations of coastal Monterey Formationwaters are generally below this threshold, withan average concentration of 2500 mg/L (S.G.Franks, 1996, written commun.). Temporaryfluctuations in organic acid concentration abovethis threshold may lead to stages of carbonatedissolution intervening precipitation of vein ce-ment. Stages of dissolution in fault carbonate ce-ment were observed through cathodoluminesc-ence at Jalama and Arroyo Burro Beach(Eichhubl, 1997). In any case, due to the pres-ence of organic acid in Monterey Formation wa-ter, the fluid volume estimates and the inferredflow distances calculated here for systems freeof organic acid must be considered minimumestimates.

Two further assumptions are the pressureprofile over the vertical extent of the faultzone, and the ratio of expelled fluid to sourcerock volume. The preceding calculation as-sumes that the pressure and temperature dropis gradually distributed over 500 m. This as-sumption agrees with pore-pressure profileswith depth for the Monterey Formation (U.S.Geological Survey, 1983) that suggest a grad-ual pressure increase with depth. In addition,it is expected that, after initial rupture of aseal, the largest gradient in pore fluid pressureis presumably where the fluid leaves the for-mation and enters the fracture network. Fol-lowing the assumption made earlier, that fluidseeps into the fault zone over its entire depthrange, the fluid pressure drop would be equal-ly distributed over the estimated flow distancethickness of the cemented zone, which is as-sumed to be 500 m.

It is difficult to assess the ratio of expelledfluid to the volume of the source rock. Thisratio estimate is necessary to relate the cal-culated fluid volume to the flow distance. Theexpelled fluid volume may correspond to achange in pore volume during compaction.Because of the abundance of hydrocarbon in-clusions in Jalama Beach vein cement, fluidflow along the fault is likely to be the conse-quence of hydrocarbon maturation in the ad-jacent synclines of the basin. Ungerer et al.(1983) estimated a 57% volume gain for typeII kerogen during hydrocarbon maturation, al-lowing for selective escape of CO2 and H2S.For an initial average organic matter contentof 8% of the rock volume (Isaacs and Peter-sen, 1984), the possible volume gain amountsto ;5% of the initial source rock volume. Be-cause organic matter will be lost prior to theactivity of the fault system, 5% yield may beconsidered a maximum estimate, also leadingto a minimum estimate in flow distance.


EICHHUBL and BOLES

Figure 8. Fault-fracture mesh in alternating sequence of opal CT porcelanite and organic-rich, phosphatic opal A mudstone at Arroyo Burro Beach. Porcelanite fails brittlely byforming extension fractures. Extension is accommodated in overlying and underlying mud-stone by normal faults.

Figure 9. Schematic view of fault-fracturemesh, modified after Eichhubl and Behl(1998) and inspired by Sibson (1996) andGross (1995).

Kinetic Model

The preceding approach is based on the as-sumption that fluid is in thermodynamic equilib-rium with wall-rock carbonate prior to and dur-ing flow. While this assumption is justifiable asan initial condition with a long residence timeprior to fluid expulsion, sluggish reaction kinet-ics may prevent the solution from reaching equi-librium during upward flow. Long-term averageflow velocities for Jalama Beach are ;1 m/day,based on inferred rates of heat loss during up-ward flow (Eichhubl, 1987).

Experimental kinetic data for dolomite pre-cipitation at the temperatures of interest arenot available. A crude estimate of the fluidvolume required for precipitation of dolomitecement at Jalama Beach can be obtained, how-ever, from the observed rate of calcite scaleformation in well casings. Boles and Ramsey-er (1987) reported calcite scale with a cross-sectional area of 21 cm2 being precipitatedfrom 89.1 3 103 m3 of fluid over 3342 days(;9 yr) of hydrocarbon production at a depthof 2591 m in a Miocene sandstone reservoir.Precipitation is accompanied by a pressuredrop from 4900 psi (33.8 MPa) to 400 psi (2.8MPa) and degassing of the fluid. Mole percentCO2gas averaged 1.7% over this period, in-creasing from ;0.13% to 2.16% (data byArco, Bakersfield, provided to J.R. Boles, 1993).

Under the same conditions, 280 m2 of cal-cite, equivalent to the surface area of dolomiteat Jalama Beach, would require 12 3 109 m3

or 12 km3 of fluid, which is one order of mag-nitude higher than the best equilibrium esti-mate given herein. The corresponding radiusof fluid drainage is 12 km. Although the re-action kinetics may be slower for dolomite,this estimate is based, in contrast to the equi-librium thermodynamic calculation here, onactual precipitation rates in a moving fluid.The average calculated flow velocity in thewell casing is 0.3 L/s or ;20 cm/s.

DISCUSSION

The estimated distances of fluid flow, to650 m across bedding and a minimum of 4–12 km parallel to the formation, is equivalentto fluid focusing by a factor F $ 6 as definedin equation 1. This high degree of lateral fluidfocusing indicates that the permeability of theformation away from faults is highly aniso-tropic, the highest permeability being parallelto bedding.

Fluid flow may preferentially follow bed-ding due to the following types of permeabil-ity anisotropy.

1. Bedding-parallel fluid flow may be chan-

neled along beds of high matrix permeabilitythat alternate with beds of low permeability,as in an alternating sequence of sandstone andshale. Rock types of the Monterey Formationare characterized by equally low matrix per-meability (Crain et al., 1985; Roehl and Wein-brandt, 1985; Isaacs and Petersen, 1987;MacKinnon, 1989), with the exception ofsome dolostone beds that contain appreciablesecondary porosity we observed in core fromthe offshore Hondo oil field. Dolostone fromJalama and Arroyo Burro Beach, however, ap-pears to be tightly cemented throughout.

2. Fluid flow may follow partings parallel tobedding planes. Although siliceous rock types ofthe Monterey Formation typically part parallelto bedding in outcrop, it is not certain if bed-ding-parallel fractures are conductive for fluidflow in the subsurface. Mineralization alongbedding planes is infrequent and, where cross-cutting relations are observed, appears to predatefolding (Bartlett, 1994).

3. Fluid flow may follow bedding-confinedfractures. Extension fractures oriented at highangles to bedding and confined to the morecompetent units of porcelanite and dolostonein sequences alternating with clay shale of lowcompetence are characteristic of the MontereyFormation (Narr, 1991; Hickman and Dun-ham, 1992; Gross, 1995; Gross et al., 1998).Hydrocarbon fill and carbonate and silica ce-mentation of these bedding-confined veins in-dicate that they serve as fluid pathways in thesubsurface. Frequently, extension fractures inthe more competent beds lead into normalfaults in the overlying and underlying mud-stone beds (Figs. 8 and 9). The connected sys-

tem of normal faults and extension fractureswas termed fault-fracture mesh by Sibson(1996), following Hill (1977). Fluid flow islikely to occur preferentially along the exten-sion fractures, parallel to the bedding. Slipalong the normal faults would lead to contin-ued opening of the extension veins, possiblypreventing complete sealing of extensionveins by cementation. The permeability andconnectivity of bedding-confined fracturesmay also be enhanced due to shear reactiva-tion of fractures initially formed in extension(Dholakia et al., 1998).

4. Potential pathways for preferred fluid flowparallel to bedding are bedding-parallel brecciazones (Redwine, 1981; Roehl, 1981; Belfield et



Figure 10. Brittlely folded chert bed forming bedding-parallel breccia body. Hydrocarbonmatrix of chert breccia indicates that breccia serves as fluid migration pathway. Photo-graph courtesy of Richard Behl, California State University, Long Beach.

al., 1983). Dholakia et al. (1998) documentedthe formation of bedding-parallel brecciation inthe Monterey Formation due to differential slipbetween bedding planes, presumably caused byflexural slip in folds. Prominent breccia bodiesin excess of 1 m thickness are also observed asthe consequence of brittle folding of chert layers(Snyder et al., 1983; Hickman and Dunham,1992; Behl and Garrison, 1994) (Fig. 10). Typ-ically, diatomite and porcelanite layers overlyingand underlying folded chert beds are unfolded,giving chert folds the resemblance of synsedi-mentary folds. Behl and Garrison (1994) sug-gested that brittle chert folds are tectonic, withthe shortening accommodated in the overlyingand underlying diatomite and porcelanite layersby bulk shortening and porosity collapse, where-as low-porosity chert buckles, brecciates, and

folds. Brecciated chert layers are frequentlyfound saturated with hydrocarbons.

The observed control of bedding on thefracture permeability structure suggests thatthe inferred permeability anisotropy of the for-mation and resulting fluid focusing are intrin-sic to the formation (Fig. 1A) rather than con-trolled by formation boundaries (Fig. 1C).However, the degree to which the permeabilityanisotropy is intrinsic may vary on a regionalscale and with depth. Porcelanite and chert inthe quartz diagenetic stage may be sufficientlyfractured to allow flow across bedding. Thepermeability anisotropy of the formation maythen be controlled by diagenetic boundariesand by variations in silica and clay content ofthe formation with depth on the scale of strati-graphic members of the formation.

The mass-balance estimates require that fluidenters the faults in a radial rather than uni- orbilateral symmetry. It follows that faults are pre-ferred pathways over short segments, formingpipes rather than planar ducts within these faultzones (Fig. 11). Possible pathways for fluid floware releasing bends, extensional stepovers offault segments, and fault intersections. Radialflow into these fluid pathways requires that fluidflow along bedding is fairly isotropic within thebedding plane. A single dominant joint setwould likely result in a strong flow anisotropywithin bedding planes (Fig. 11B). Radial andthus isotropic flow requires at least two or morecrosscutting and mutually connected joint sets(Fig. 11C), as described by Gross (1993) fromthe Monterey Formation. Isotropic flow alongbedding may be best accommodated by bed-ding-parallel breccia zones. Belfield et al. (1983)and MacKinnon (1989) reported that oil produc-tion is most prolific from wells intersecting theregional, north-northeast–south-southwest–trend-ing joint set at high angles, a finding notsupported by others (Jon Schwalbach, Exxon,Thousand Oaks, 1996, oral commun.) and in-consistent with the radial flow geometry inferredfrom this study. This ambivalence may indicatea regional variability in the permeability anisot-ropy for flow parallel to bedding.

Within the tilted sequence on the basin flanks,fluid will be drawn preferentially from the centerof the basin or from adjacent synforms. Thissemicircle or arc of a circle geometry will extendthe minimum distances of formation-parallelflow of 4 to .12 km by at least a factor of 2.An updip migration path of hydrocarbons fromsynforms undergoing burial and catagenesis intouplifted structural highs was also inferred byOgle et al. (1987) based on the charge historyof Monterey reservoirs.

The effect of faults on basinal fluid migrationis thus governed by three geometric constraints:(1) the permeability structure of the fault zonesuch as the presence of zones of local extensionalong the fault; (2) the permeability anisotropyof the surrounding formation as controlled bystratification, jointing, and lateral stratigraphicvariations; and (3) the orientation of the regionalflow field as controlled by differences in hy-draulic head. An analysis of all three compo-nents is required for the prediction of basinalfluid flow in faulted sequences, as done routinelyby numerical modeling. Although the combina-tion of low matrix and high fracture permeabilityis unique to the siliceous shale of the MontereyFormation, bedding or layering, lateral strati-graphic variations, and the preferred orientationof regional joint sets will create a permeabilityanisotropy in most sedimentary sequences aswell as in basement rocks, thus affecting local-


EICHHUBL and BOLES

Figure 11. Control of fault structure and source bed permeability anisotropy on basinalfluid-flow geometry in fractured reservoirs. Fluid is depicted to flow updip within frac-tured source unit into conductive fault and leaves the fault into a fractured reservoir unitat a structurally higher level. A: Fault is uniformly conductive along strike; single fractureset directs fluid flow in single direction. Fault is highly effective in draining the sourcebed. B: Same as A, but fault is conductive only within stepover region. Fault is poorlyeffective in draining the source bed. C: Fault is conductive only within stepover region,but two sets of fractures in source and reservoir units allow radial flow pattern into andout of the fault fluid conduit. Fault is highly effective in draining source bed. Fluid flowin Monterey faults is inferred to follow case C. Radial flow in the Monterey Formationmay be accommodated by multiple fracture sets or by bedding-parallel brecciation.

ization of cementation and ore mineralization inany crustal environment.

CONCLUSION

Reservoir-scale faults in the Monterey For-mation contain locally large volumes of car-bonate and minor quartz cement, indicative oflarge volumes of fluid migrating along thesefaults. Using the strontium isotopic composi-tion of carbonate cement as a natural tracerfor fluid flow, the inferred distance of upwardcross-stratigraphic flow approaches the thick-ness of the Monterey Formation at JalamaBeach of 700 m and appears to constitute asignificant fraction of the underlying part ofthe Monterey Formation at Arroyo BurroBeach. At both locations, the inferred distanc-es suggest that fluid is derived from within theMonterey Formation rather than from under-lying older units. Mass-balance estimates ofthe volume of fluid required for fault cemen-tation at Jalama Beach indicate a minimumdistance of formation-parallel flow into thefault zone of 4 to .12 km. Fluid flow is thushighly focused toward this fault zone, the in-ferred distance of flow parallel to the forma-tion into the fault exceeding the distance ofcross-formational upward flow along this faultby at least a factor of six. The mass-balanceestimates also suggest that fluid flow along thefault is localized to pipe-shaped channels oflimited extent along fault strike, such as re-

leasing bends or stepover regions of overlap-ping fault segments. Outcrop observations in-dicate that likely conduits for fluid flow alongbedding are layer-confined sets of extensionfractures and bedding-parallel breccia bodies,leading to a permeability anisotropy that is in-trinsic to the formation rather than controlledby formation boundaries. Fluid flow parallelto bedding into the fault is inferred to followa radial rather than uni- or bilateral flow sym-metry. The inferred radial flow geometry to-ward fault channels or pipes requires that thepermeability structure for flow parallel to bed-ding is fairly isotropic, whereas flow acrossbedding is restricted. The isotropic permeabil-ity of bedding planes is likely to be caused bymultiple sets of extension fractures at high an-gles to bedding and by bedding-parallel brec-ciation. The inferred flow geometry illustratesthe combined effect of fault permeabilitystructure, permeability anisotropy of the sur-rounding formation, and hydraulic head dis-tribution in controlling basinal fluid flow infaulted sequences.

ACKNOWLEDGMENTS

This study benefited from discussions with JimMattinson, George Tilton, Dave Parkinson, JulieBryce, Rick Behl, Jon Schwalbach, Rick Sibson,Atilla Aydin, and Rob Knipe. An earlier version ofthis manuscript was reviewed by Art Sylvester.GSA Associate Editor Elizabeth Burton and review-ers Alden Carpenter and Isabel Montanez provided

detailed suggestions for clarification. This projectwas funded in part by National Science Foundationgrant EAR-93-04581 and a grant from the Geolog-ical Society of America.

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Barron, J.A., 1986, Updated diatom biostratigraphy for theMonterey Formation of California, in Casey, R.E., andBarron, J.A., eds., Siliceous microfossil and micro-plankton studies of the Monterey Formation and mod-ern analogs: Pacific Section, Society of Economic Pa-leontologists and Mineralogists Special Publication45, p. 105–119.

Bartlett, W.L., 1994, Field guide to the Monterey Formationat Ellwood Beach, Santa Barbara County, California,in Hornafius, S.J., ed., Field guide to the MontereyFormation between Santa Barbara and Gaviota, Cali-fornia: Pacific Section, American Association of Pe-troleum Geologists Guide Book, v. 72, p. 95–106.

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