Focused ¯uid ¯ow along faults in the Monterey Formation, coastal California Peter Eichhubl* James R. Boles Department of Geological Sciences, University of California, Santa Barbara, California 93106, USA ABSTRACT ding. The inferred ¯ow geometry illustrates namics and associated transfer of mass and the combined effect of fault permeability heat, it is of interest to quantify the extent to Fluid ¯ow in fractured siliceous mud- structure, permeability anisotropy of the which conductive faults focus basinal ¯uid stone of the Miocene Monterey Formation surrounding formation, and hydraulic head ¯ow. In a sedimentary sequence undergoing of California is inferred to be highly fo- distribution in controlling basinal ¯uid ¯ow dewatering and ¯uid expulsion during pro- cused toward map-scale faults that locally in faulted sequences. grade burial and diagenesis, the extent of ¯ow contain extensive amounts of carbonate and focusing by the presence of faults may be minor silica cement. The distance of cross- Keywords: faults, ¯uids, strontium, iso- quanti®ed by the ratio of fault-perpendicular stratigraphic ¯ow, as inferred based on the topes, hydrocarbon, Monterey Formation. ¯ow distance over the distance of fault-par- strontium isotopic composition of carbon- allel ¯ow in the source region. In strati®ed ate fault cement, is close to the thickness of INTRODUCTION sequences and for faults inclined with respect the Monterey Formation of 700 m in one of to the strati®cation, it may be more practical two study locations, Jalama Beach, and less Faults may affect basinal ¯uid ¯ow as bar- to consider the ratio between the distance of than the formation thickness at another lo- riers or as preferred conduits, depending on formation-parallel ¯uid ¯ow DL over the dis- cation, Arroyo Burro Beach. Fluid is thus the difference in permeability between fault tance of cross-stratigraphic ¯ow DH within derived from within the Monterey Forma- zone and the surrounding formation. Faults the source unit, tion rather than from underlying older that are preferred conduits due to increased units. Based on mass-balance estimates of permeability parallel to the fault are likely to DL F 5 (1) the ¯uid volume required for fault cemen- control the localized precipitation of pore ce- DH tation at Jalama Beach, the minimum dis- ment and ore minerals and the migration and tance of formation-parallel ¯ow into the accumulation of hydrocarbons. Faults condu- (Fig. 1). Fault systems with a large focusing fault zone is 4 km and possibly .12 km. cive for ¯uid ¯ow can be subdivided into three ratio F will be more effective in channeling The inferred distance of ¯ow parallel to the hydraulic regimes (Fig. 1): (1) a source region ¯uid expulsion from the source rock to a po- formation into this fault thus exceeds the where pore ¯uid is drawn into the fault zone tential sink within the sequence or to the distance of cross-formational upward ¯ow from the surrounding formation of higher hy- Earth's surface. However, in petroleum sys- along the fault by at least a factor of six. draulic head; (2) a sink region of lower hy- tems faults with large F will also be more ef- The mass-balance estimate requires that draulic head where pore ¯uid is released back fective for leaking hydrocarbons out of ¯uid ¯ow along the fault is channeled into into the formation or onto the Earth's surface; breached reservoirs. a pipe-shaped conduit rather than distrib- and (3) an intermediate neutral conduit of no For any fault of given length, the ¯uid fo- uted along fault strike. Fluid ¯ow from the signi®cant ¯uid exchange with the surround- cusing factor F is a function of the perme- surrounding formation into fault pipes is ing formation that may separate the source ability anisotropy of the ¯uid source rock and inferred to follow a radial rather than uni- from the sink region. The effectiveness of ¯u- of the distance to adjacent faults (Fig. 1). or bilateral ¯ow symmetry, using bedding- id transport along a fault can be expressed as Large ratios of F would be expected in strat- con®ned sets of extension fractures and the total ¯uid volume that has migrated along i®ed source rocks with the highest permeabil- stratabound breccia bodies. Radial ¯uid the fault, the average ¯ow rate, the distance of ity parallel to bedding, transected by faults of ¯ow toward fault pipes requires fairly iso- fault-parallel ¯ow, and the distance perpendic- wide spacing. An isotropic permeability struc- tropic fracture permeability for ¯ow along ular to the fault over which ¯uid is focused ture of the formation and/or close fault spac- bedding and a low permeability across bed- toward the fault in the source region and dis- ing would lead to low ratios of F. The per- persed away from the fault in the sink region. meability anisotropy of the source rock may *Present address: Department of Geological and An additional parameter of interest is the be controlled by strati®cation within the for- Environmental Sciences, Stanford University, Stan- source volume, which is a function of ¯uid mation (intrinsic anisotropy) and by the for- ford, California 94305, USA, and Monterey Bay Aquarium Research Institute, Moss Landing, Cali- volume and source-rock porosity and mation boundaries to overlying and underly- fornia 95039, USA; e-mail: eichhubl@pangea. storativity. ing units (extrinsic anisotropy) (Fig. 1). stanford.edu. For the understanding of basinal hydrody- For a kilometer-scale fault system in the GSA Bulletin; November 2000; v. 112; no. 11; p. 1667±1679; 11 ®gures; 2 tables. 1667 EICHHUBL and BOLES Monterey Formation in southern California, we demonstrate that the distance of formation- parallel ¯uid ¯ow exceeds the distance of cross-stratigraphic ¯ow by a factor F $ 6. The distance of cross-stratigraphic ¯uid ¯ow DH will be estimated using the strontium isotopic composition of carbonate fault cement as a natural tracer. The formation-parallel ¯ow dis- tance DL is inferred based on mass-balance estimates of the ¯uid volume that has migrat- ed along the fault system. On the basis of the estimated distances of ¯uid ¯ow and the char- acteristic permeability structure of Monterey rocks as observed in outcrop, we infer that the expulsion of basinal ¯uid in the Monterey Formation is highly focused toward faults by the intrinsic, layer-controlled permeability an- isotropy of the formation (type A in Fig. 1). STRUCTURAL SETTING OF Figure 1. Geometry of upward-directed basinal ¯uid expulsion as controlled by faults and CONDUCTIVE FAULTS by the permeability anisotropy of sedimentary sequences. Conductive faults can be sub- divided into source and sink regions and an intermediate neutral ¯uid conduit of no or The Miocene Monterey Formation is an or- minimal ¯uid exchange with the surrounding formation. The ratio of formation-parallel ganic-rich, hemipelagic sequence of diatoma- over cross-formational ¯ow distance may be used to quantify the extent of ¯uid focusing ceous and phosphatic shale and organogenic toward faults in source units. Fluid focusing will be controlled by the permeability an- dolostone, deposited between 18 and 6 Ma in isotropy of the source unit, which can be intrinsic, i.e., controlled by bedding, or extrinsic, a series of continental-borderland basins (Pis- i.e., controlled by contacts to overlying and underlying formations of low permeability. ciotto and Garrison, 1981; Barron, 1986; Hor- Fluid ¯ow in the Monterey Formation is inferred to be strongly focused toward faults by na®us, 1994a). During burial, the organic-rich the intrinsic anisotropy of the formation (case A). diatomaceous sequence underwent a complex sequence of diagenetic alteration, dominated by the following reactions: (1) silica transfor- mation of diatom tests, from opal A via opal CT to quartz through two distinct steps of dis- solution-reprecipitation (Stein and Kirkpa- trick, 1976; Isaacs 1981); (2) degradation of organic matter and maturation of hydrocar- bons (Petersen and Hickey, 1987; Baskin and Peters, 1992); (3) precipitation of authigenic dolomite 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 to 34% (Isaacs and Petersen, 1987), the Monte- rey Formation is both source and reservoir rock for hydrocarbons (Ogle et al., 1987; MacKinnon, 1989). Due to the low matrix permeability, typically ,1 md, hydrocarbon migration and production critically depend on fractures as ¯ow 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 volumes of ¯uid ¯owing along these faults. At Jalama Beach, 100 km west of Santa Barbara (Fig. 2), two subparallel faults are cemented by dolo- Figure 2. Location map of Jalama and Arroyo Burro Beaches in south-central California. 1668 Geological Society of America Bulletin, November 2000 FOCUSED FLUID FLOW ALONG FAULTS mite and minor amounts of quartz. The south- ern fault forms a resistant wall of cemented fault breccia, ;6 m thick, 7 m tall, and ex- tending more than 35 m across the wavecut platform between mean low tide water line and the beach cliffs (Fig. 3). The second fault, ;300 m northwest of the ®rst one, is less well exposed in a cove. Both faults are characterized by a large amount of dolomite cement; 7±10-cm-thick layers of banded dolomite coat host rock frag- ments and aggregates of earlier dolomite ce- ment (Fig. 3). Dolomite fault cement can be traced into extension veins that branch off from both faults and extend over distances of a few hundred meters (Fig.