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_Journal of Structural Geology 32 (2010) 1768–1782_ Contents lists available at ScienceDirect Journal of Structural Geology journal homepage: www.elsevier.com locate jsg Structural controls on leakage from a natural CO2 geologic storage site: Central Utah, U.S.A. Ben Dockrill *, Zoe K. Shipton Department of Geology, Trinity College Dublin, Dublin 2, Ireland. Department of Geographical and Earth Sciences, University of Glasgow, Glasgow G12 8QQ, Scotland Article info Article history: Received 26 May 2009 Received in revised form 26 November 2009 Accepted 19 January 2010 Available online 28 January 2010 Keywords: Fault zone Damage zone Fluid flow CO2 Hydrocarbons Abstract Faults and associated fracture networks can significantly influence regional flow of groundwater, hydrocarbons and other fluids. The distribution of CO2 springs and seeps along the Little Grand Wash fault and Salt Wash faults in central Utah is controlled by along-fault flow of CO2-charged groundwater from shallow aquifers (<1 km deep). The same faults are likely conduits that charge the shallow aquifers with CO2 from depth. We document fault zone trace geometry and architecture, and evidence for paleo-fluid flow within the footwalls of both faults. Evidence for paleo-fluid flow consists of extensive bleaching of sandstones and some siltstones, mineralization of carbonates and celestine veins, and minor hydrocarbon staining. The field evidence shows that the pathways for multiple phases of fluid flow were structurally controlled utilizing the fracture network developed in the damage zone of the faults. To investigate the likely effect of these faults on the regional fluid-migration pathways at depth, a 3D model of the faulted system was generated and a fault seal analysis applied to predict the cross-fault sealing capabilities of the studied faults. Due to the scarcity of subsurface data, the results are not conclusive but suggest probable multiple cross-fault leak points for fluids to migrate across the fault, in contrast to field observations that indicate fault-parallel flow. This comparison of field observations to the modeling approach demonstrates the inability of conventional seal analysis techniques to predict fault-parallel fluid leakage and highlights the effects fracture networks in the damage zone, especially at structural complexities along the fault, have in producing pathways for vertical flow. Multiple fluids have utilized similar fault-parallel pathways over geological time demonstrating that such pathways have the potential to cause long-term leakage from hydrocarbon reservoirs and CO2 storage sites. © 2010 Elsevier Ltd. All rights reserved. 1. Introduction Faults and associated fracture networks can play a significant role in the subsurface migration of various fluids. Focusing of flow related to fault geometry has been demonstrated in geothermal fields (Curewitz and Karson, 1997; Rowland and Sibson, 2004), hydrothermal epithermal systems (Breit and Meunier, 1990; Micklethwaite, 2009) and petroleum systems (Chan et al., 2000; Garden et al., 2001; Gartrell et al., 2004). In most studies, flow is concentrated in the fracture network (commonly referred to as the damage zone) that is developed around a main zone of slip. Complexities along a fault related to terminations and/or linkages between fault segments are commonly domains of high fracture density and connectivity and are therefore likely to focus flow. * Corresponding author. Present address: Chevron Australia, 250 St Georges Terrace, Perth, WA, Australia. Tel.: +61 8 9216 4141; fax: +61 8 9216 4103. E-mail address: ben.dockrill@chevron.com (B. Dockrill). 0191-8141 $ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsg.2010.01.007 (Curewitz and Karson, 1997; Anderson and Fairley, 2008; Eichhubl et al., 2009). Models that predict fault properties based on the throw and host rocks cut by the fault (Yielding et al., 1997; Yielding, 2002; Bretan et al., 2003) generally rely on oversimplified fault geometries and complexities, leading to the possibility of underestimating likely leakage points due to fault throw partitioning and simplified fault linkages (Childs et al., 1996, 1997), and do not account for along-fault flow. This study investigates a natural leaking CO2-rich system at the northern end of the Paradox Basin in central Utah, United States. In this locality, the Little Grand Wash fault and northern fault of the Salt Wash graben provide lateral barriers to present-day cross-fault flow, but provide pathways through the cap rock via damage zone fractures to allow CO2 and additional fluid regimes to leak to the surface (Shipton et al., 2004, 2005). Multiple mineralization and diagenetic products associated with past and present migration of fluids along both faults demonstrate structural controls influencing multiple fluid regimes to vertically migrate through a thick interbedded sandstone–shale stratigraphy. By combining outcrop B. Dockrill, Z.K. Shipton Journal of Structural Geology 32 (2010) 1768–1782 1769 analysis of the fault zone and associated fluid-migration products with analysis of fault rock integrity through modeling, we have examined constraints on leakage from shallow reservoirs. Faults can form barriers to flow either by juxtaposing reservoir rock against low-permeability clay-rich non-reservoir rock to create a juxtaposition seal, or when processes of fault rock generation form a low-porosity and low-permeability fault rock. By analyzing the fault rocks and the processes and factors that contribute to the failure of this system, we can identify and highlight the roles faults can play in trapping and transmitting fluids. These results demonstrate the impact fault zones can have on fault-parallel leakage from a robust structural trap and highlight the potential risks when assessing seal integrity for structural traps in the hydrocarbon and emerging CO2 geologic storage industries. 2. Geological setting The field area is located at the northern end of the Paradox Basin in the Colorado Plateau region of the United States (Fig. 1), a late Paleozoic to Mesozoic intracratonic basin filled with a thick sequence of evaporite, carbonate and clastic sediments (Hintze, 1993). The basin is defined by the areal extent of the Pennsylvanian Paradox Formation, which contains nearly 2 km of evaporates (Doelling et al., 1988). The basin has been investigated for hydrocarbons (Peterson, 1973, 1989; Hansley, 1995; Huntoon et al., 1999) and mineral resources (Breit and Meunier, 1990; Morrison and Parry, 1986). Multiple reservoirs have accumulated CO2 for extended periods of time (Allis et al., 2001, 2005; Moore et al., 2005; Shipton et al., 2004, 2005; White et al., 2005). Some of these are now currently being exploited, predominantly for enhanced oil recovery (i.e. Bravo and McElmo domes – Allis et al., 2001). Other reservoirs leak CO2 due to the influence of faults (Springerville-St Johns – Moore et al., 2005; Hurricane fault – Nelson et al., 2009) or boreholes (Woodside – Doelling, 1994). The stratigraphy in the Paradox basin ranges from the Pennsylvanian Paradox Formation to the Mid Cretaceous Mancos Shale, though only the Upper Jurassic to Mid Cretaceous succession crops out in the field area (Fig. 1). The Pennsylvanian and Permian formations consist of marine carbonates and shales that are potential sources of the CO2 (Heath et al., in press; Wilkinson et al., 2008) and hydrocarbons (Peterson, 1973, 1989; Huntoon et al., 1999; Chan et al., 2000; Garden et al., 2001). The aeolian reservoir sandstones of the Permian White Rim Sandstone are capped by fluvial and lacustrine shales of the Triassic Moenkopi and Chinle formations. The aeolian Lower Jurassic Wingate and Navajo sandstones are important regional aquifers separated by the fluvial Kayenta Formation aquitard. Forming a seal above the Navajo Sandstone is the Mid-Jurassic Carmel Formation, a complex sabkha sequence of sandstone, siltstone, mudstone, limestone, anhydrite and gypsum. The youngest reservoir units in the basin are the MidJurassic Entrada and Curtis aeolian to marginal marine sandstones, overlain by marine siltstones and shales of the Middle Jurassic Summerville Formation. The remaining stratigraphic sequence is dominated by shales with small, disconnected reservoir units. The Upper Jurassic Morrison Formation consists of stacked fluvial sandstone channels, interspersed and overlain by lacustrine shales. The lower Cretaceous Cedar Mountain Formation lacustrine shales are overlain by conglomeritic fluvial channels from the lower Cretaceous Dakota Formation. The youngest rocks exposed in the field area are marine marls of the Middle Cretaceous Mancos Shale. The shallowly north-to-northwest plunging, open Green River anticline is one of a series of northwest-trending folds that have growth histories related to salt movement in the Paradox Formation since the Permian (Doelling et al., 1988). The Green River anticline is cut by the Little Grand Wash fault and the Salt Wash graben. Timing of movement along both faults is poorly constrained, with the youngest faulted stratigraphy being the Mid Cretaceous Mancos Shale, though evidence presented by Pevear 39°00’ 25 uJ throw (m) 110°15’ 6 3 Green River 110°00’ 300 200 100 2 4 5 0 4 uJ 3 0 Little Grand b c Wash faultK Fig. 2a K 7 Salt Wash graben 3 a Fig. 2b 3 mJ uJ Ten Mile 5 graben2 Salt 260m 10 20 30 distance along strike (km) Q Valley Q uJ graben Courthouse Q Quaternary P Permian Cutler UTAH Tr Triassic Chinle and Moenkopi lJ Lower Jurassic Navajo, Kayenta and Wingate mJ Mid Jurrasic Entrada and Carmel N uJ Upper Jurassic Morrison, Summerville, Curtis K Cretaceous Mancos and Cedar Mountain Fault (tick on downthrown side) Fold (arrow in plunge direction) throw (m) Green River Anticline 400 300 200 100 0 0 366m R 210m 165m 154m R B 10 20 30 distance along strike (km) syncline K uJ mJ lJ 0 Kilometres 10 Moab fault mJ lJ 38°45’ uJ mJ P Tr Fig. 1. Geological map Ключевые слова: paradox formation, cap rock, northern fault, early tertiary, dockrill, fault bend, data, wash fault, allis, heath, childs, bed, morrison formation, fracture orientation, smearing algorithm, vrolijk, subsurface, graben, fault zone, survey, ?uids, pathway, enhanced permeability, uid-ow product, curtis, geological survey, fault rock, completely bleached, bleaching, maximum change, navajo sandstone, eventual leakage, cap, cross-fault, mcknight, uids migrating, entrada, wash graben, relay, fracture analysis, geological, surface, paradox basin, travertine deposit, slip, calcite vein, studied fault, uid, throw, sandstone, faulted, grand wash fault, wingate sandstone, grand, aquifer, fracture, steeply-dipping fracture, hydrocarbon, study, argillaceous smear, sequence, bounding fault, damage, host rock, fault strand, branch point, natural reservoir, reduction halo, vertically migrate, allan diagram, eld observation, smear, modelling, across-fault, combined throw, rock, structure, chan, representative cross, structural, cm, extended period, structural setting, structural control, complexity, subsurface data, eld area, moenkopi formation, geology, hydrocarbon staining, watterson, colour change, hydrocarbon migration, clay, dockrill shipton, study area, rich, ancient travertines, shipton journal, ramp, structural geology, southern fault, anticline, basin, juxtaposition, uid migration, main strand, bleaching uid, paradox, carbon, area, fault seal, green, fault core, damage zone, potential, structural closure, migrating parallel, burial history, condon, uid regime, footwall, multiple uids, wa, sensitivity analysis, northern, cross-fault leakage, entrada sandstone, hurricane fault, cretaceous siliciclastics, williams, permeability, seal, bleached halo, outcrop study, eichhubl, studies, ha, bulletin, formation, long, migration pathway, salt, travertine, curtis sandstone, travertine mineralisation, utah, throw partitioning, relay zone, caine, gouge, clay-rich, zone, doelling, free phase, sibson, ross, ssf algorithm, reduction, leakage, evans, association, ?uid, shipton, sgr, navajo, journal structural, lacustrine shale, river, migration, natural leaking, geo?uids, jurassic, grand wash, fracture network, source, along-fault, iron-oxide reduction, reservoir, signicant proportion, publication, salt wash, analysis, jurassic wingate, east-central utah, fault trace, green river, carmel formation, dip, multiple aquifer, model, natural, shale, fault salt, american, structural complexity, knipe, strand, petroleum, special, regional aquifer, walsh, hansley, celestine vein, ?ow, elsevier, peterson, white, gure legend, relay ramp, multiple, yielding, parry, journal structural geology, wash, fault, journal, faulted stratigraphy, fault-parallel, curtis formation, survey bulletin, deposit, fault segment, fold axis, ?eld