exhumed natural CO2 reservoir, Green River, Utah. 2. Max Wigley, Niko ..... Desert 30' x 60' quadrangles, Grand Emery Counties, Utah, and Mesa County,. 219.
Publisher: GSA Journal: GEOL: Geology Article ID: G32946 1
Fluid-mineral reactions and trace metal mobilization in an
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exhumed natural CO2 reservoir, Green River, Utah
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Max Wigley, Niko Kampman, Benoit Dubacq, and Mike Bickle
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Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge
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CB2 3EQ, UK
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ABSTRACT
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Red sandstones near Green River, Utah (United States), have been bleached by
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diagenetic fluids. Field relationships, modeling, fluid inclusion and isotopic data, suggest
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that the causal fluid was a CO2-charged brine, distinguishing this site from hydrocarbon-
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related bleaching elsewhere on the Colorado Plateau. Mineralogical and chemical profiles
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from unbleached to bleached sandstone show that bleaching is related to hematite
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dissolution and precipitation of a 1–2 cm band of secondary oxide and carbonate at the
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reaction front. Trace metals are mobilized by the fluid and concentrated near the reaction
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front. High flux fluid pathways are more heavily altered with large-scale secondary
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calcite and iron oxide precipitation. Changes may be modeled by a reaction with
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stoichiometry 20Fe2O3+5CH4+64CO2+19H2O+11H+ = 30Fe2++10FeHCO3+ +59HCO3-.
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The Fe-rich, reduced fluid precipitates iron-oxides and carbonate at the reaction front
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between bleached and unbleached sandstone. These findings enable the site to be used as
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an analogue for processes occurring over long timescales in geological carbon storage
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projects. Trace metals moblised by CO2-charged brines are likely to be rapidly re-
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precipitated at reaction fronts.
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INTRODUCTION
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Publisher: GSA Journal: GEOL: Geology Article ID: G32946 Scientific questions related to subsurface injection of CO2 for carbon storage
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include the nature of mineralogical changes in the host reservoir, and degree of
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contaminant mobilisation. This study examines an exhumed CO2 reservoir at Green
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River, Utah (United States). We show that CO2-charged fluids have bleached red aeolian
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sandstones, altering the mineralogy of the host formation, and mobilizing trace metals.
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CO2 injected into deep saline aquifers, as a buoyant supercritical fluid, may
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dissolve in formation fluids, forming acid brines. These may corrode reservoir or caprock
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minerals and, on long time scales, precipitate CO2 in carbonate minerals (Bickle, 2009).
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Previous CO2–rock interaction studies have documented dissolution of carbonate and
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oxide phases (e.g., Moore et al., 2005; Kharaka et al., 2006). Models predict dissolution
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of silicates and formation of secondary carbonate, dawsonite, and clay minerals (e.g.,
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Knauss et al., 2005; White et al., 2005), but are hindered by the complexity of the
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chemical systems and uncertain reaction kinetics (White and Brantley, 2003; Kampman
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et al., 2009). Natural analogues allow study of these fluid-mineral interactions over
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timescales inaccessible from laboratory experiments and free from modeling
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uncertainties. Models (Wilkin and DiGiulio, 2010; Zheng et al., 2009) and experiments
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(Lu et al., 2010; Little and Jackson, 2010), have also shown that CO2 can mobilise trace
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metals; however field experiments have not shown hazardous levels of contamination
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(Keating et al., 2010; Kharaka et al., 2010). Here we show that locally bleached
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sandstones at Green River, Utah, represent an analogue for reactions between CO2-
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charged brines and red sandstones. The CO2-bearing fluids cause bleaching by hematite
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dissolution, dissolution of original carbonate and silicate minerals, and growth of
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Publisher: GSA Journal: GEOL: Geology Article ID: G32946 secondary clays, with subsequent secondary carbonate formation. Trace metals are
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mobilized by the fluid, and concentrated at the reaction front.
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FIELD RELATIONSHIPS
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Across the Colorado Plateau, early-Middle Jurassic redbed units are bleached by
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diagenetic fluids, at a variety of scales. Typically, bleaching arises near faults, and the
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bleached regions crosscut stratigraphic boundaries. This has been attributed to multiple
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sources, including hydrocarbon fluids and gases, CO2, and H2S. Regionally, field
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evidence implicates a buoyant, reducing fluid, probably enriched in CH4, which dissolves
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the hematite and goethite coatings around sand grains (Chan et al., 2000; Garden et al.,
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2001; Beitler et al., 2003, 2005; Eichhubl et al., 2009).
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Near the town of Green River, Utah, CO2 has been escaping along the Little Wash
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and Salt Wash grabens for hundreds of thousands of years (Burnside 2010). The Entrada
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Sandstone, an aeolian dune deposit (Doelling, 2001), has been bleached by diagenetic
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fluids along the northern fault of Salt Wash Graben, particularly at the intersection of the
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fault and the crest of the Green River anticline. Bleaching occurs at the base of the unit in
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a broad domal structure, and is thickest (20 m) at the crest of the anticline which is cut by
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a fault that acted as a fluid pathway (Fig. 1B), the damage zone being highly altered.
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Laterally, the top of the bleaching cuts down across the stratigraphy and pinches out in
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diffuse fingers to the east. The western termination is not exposed. Bleaching also occurs
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upward around veins and fractures (Fig. 1A).
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PETROLOGY AND GEOCHEMISTRY
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The Jurassic Entrada Sandstone is a fine-to medium-grained quartz-arenite to subarkosic sandstone (76–89 wt% quartz, 8.5–16.5 wt% K-feldspar, 2.2–6.5 wt%
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Publisher: GSA Journal: GEOL: Geology Article ID: G32946 plagioclase, and trace muscovite, tourmaline, apatite and zircon). Locally, it comprises
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well sorted sediment of uniform grain size and mineralogy (Fig. 2A), deposited as large-
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scale crossbeds. Regional burial diagenesis includes development of hematite and
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goethite grain coatings (Cullers, 1995; Trimble, 1978), partial dissolution of feldspars,
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precipitation of grain coating illite-smectite (88%–91% illite) and minor kaolinite, and
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euhedral quartz overgrowths on earlier oxides and clays. There are three carbonate
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phases: a calcite (FeO 99 wt% TiO2), and 13% pure Fe-oxides (>98 wt% FeO) (Fig. 2D; Table DR3).
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In bleached samples, dissolution of iron oxide grain coatings is reflected by a
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stepped decrease in bulk Fe content (Fig. 3B) apparent in plane polarized light images
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(Figs. 2D and 2F). A decrease in primary calcite, and subsequent pore filling secondary
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calcite formation (up to 18 vol%) are observed in heavily altered zones (Figs. 1E and
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2C). Mixed Fe-Ti oxides are absent in bleached samples, and the proportion of pure Ti
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oxides increases (Fig. 2D), indicating a single step recrystallization from Fe-Ti to Ti
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oxides, (e.g., Janssen et al., 2010), contrary to other studies in natural systems which
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suggest a continuous transformation process (e.g., Morad and Adin Aldhan, 1986;
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Pownceby, 2010). Gypsum veins, 0.5–2.0 mm thick, occur within faults and fractures and
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cross cut, and are cut by, occasional 0.15–1.0 mm calcite veins. Both are paralleled by
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millimter-thick bands of coarse (1 mm) crystalline Fe-oxides that have occasional pyrite
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Publisher: GSA Journal: GEOL: Geology Article ID: G32946 cores. Late fibrous illite-smectite grain coatings formed due to bleaching, along with a
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pore filling illite/smectite and minor kaolinite. A second phase of quartz overgrows
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grains that have lost their hematite grain coatings. These locally include secondary calcite
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intergrown with gypsum. Samples from the base of the bleached zone contain minor
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pore-filling poikiloptic barite and sylvite. Sylvite-calcite intergrowths suggest
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contemporaneous formation during bleaching.
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A 1–2 cm band of a finely crystalline intergrown oxide and carbonate (up to 21
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vol%), coincident with spikes in bulk rock Ca and Fe content, occurs within 2 cm of the
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bleached-red contact (Figs. 1G, 3A, and 3B). It is absent in unaltered rock, but may
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persist in bleached samples (0–2 vol%). This is inferred to form due to gradients in fluid
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chemistry that set up spikes in hematite and calcite saturation indices, at the reaction
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front.
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Abdelouas et al. (1998) suggested that the Fe-oxide grain coatings in Jurassic red-
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bed sandstones elsewhere in the Paradox Basin (western United States) host the majority
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of mobile trace elements. Trace element profiles of Co, Cu, Zn, Ni, Pb, Sn, Mo, and Cr
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through the bleached to unbleached sediment exhibit large spikes in concentration above
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background Entrada concentrations over a 2–6 mm region, within 10 cm of the
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bleached/unbleached contact (Figs. 3C and 3D; Tables DR1 and D2). This is consistent
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with alteration by a low pH, reducing fluid that releases metals from Fe-oxides, and clays
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and re-deposits them at the front as pH rises, analogous to processes in acid rain affected
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sediments (Kjøller et al., 2004) and uranium roll front deposits (e.g., Hobday and
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Galloway, 1999).
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NATURE OF THE FLUID
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Publisher: GSA Journal: GEOL: Geology Article ID: G32946 Raman spectroscopy was carried out on fluid inclusions in quartz overgrowths
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and gypsum veins, petrographically linked to bleaching. Low homogenization
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temperatures, Brownian motion of vapor bubbles and the small size of the inclusions (2–
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10 µm), makes analysis challenging. More than 120 inclusions were analyzed, but the
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vast majority contained only water. The absence of an observed volatile phase in may be
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due to inaccurate targeting, volatile content below detection limits, or because no
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volatiles were present. Eight inclusions contained CO2, and minor (0%–28%) CH4 co-
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existed with CO2 in three (Fig. 4A; Table DR6) (Wopenka and Pasteris, 1987). Fluid
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salinity was estimated from last ice melting temperatures as 2.5–7.0 wt% (n = 15)
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(Bodnar, 2003). Homogenization temperatures suggest alteration at low temperatures
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(22.1–27.2 °C, n = 15; Table DR4).
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Bulk rock C and O isotopes (Fig. 4B; Table DR5) from the Green River site plot
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at heavier values than those from hydrocarbon related bleaching across the Colorado
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plateau (e.g., Eichhubl et al., 2009). The absence of isotopically light carbon in secondary
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carbonates precludes a major role of CH4 in their formation. The heavy oxygen isotopic
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composition of the cements suggests the parent fluid contained a significant fraction of
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basinal brine, derived from depth. This is consistent with the high salinities measured in
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fluid inclusions and the presence of gypsum, barite and sylvite cements but contrasts with
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many of the other sites of sandstone bleaching where the parent fluid was largely
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meteoric in origin.
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Localized bleaching toward the base of the formation and fluid flow features
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imply alteration by a dense fluid which migrated laterally as a gravity current. This
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contrasts with field evidence from other sites which implicate a buoyant CH4 enriched
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Publisher: GSA Journal: GEOL: Geology Article ID: G32946 fluid. While there is local gypsum veining in high fluid flux pathways, the general lack of
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sulfide minerals precludes the presence of significant H2S or highly reducing conditions.
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BATCH MODELING
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Reaction of a CO2 and CH4-bearing fluid (CO2 saturation index [SI] = −0.63–0.17
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[10–50 mmol/L C], CH4 1%–20% total C, CH4 SI 0.87 to −1.13) fluid at pressure = 1 bar
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and 25 °C with hematite was modeled using the thermodynamic program PHREEQC
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(Parkhurst and Appelo, 1999) with the WATEQ4F database (Ball and Nordstrom, 1991),
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amended to include (Fe-Ca)CO3 calcite and siderite solid solutions (XFe = 0–0.05 and XFe
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= 100–0.98). Changes to fluid Eh-pH were mapped as hematite dissolved (Fig. 5). Ca is
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assumed constant (10 mmol/L, based on compositions of fluids from the modern, CO2-
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charged groundwater of Kampman et al., 2009). Increasing the carbon content of the fluid
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has little effect on the morphology of mineral stability fields. PHREEQC solutions model
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dissolution of hematite by a combination of reactions with overall stochiometry 20Fe2O3
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+5CH4 +64CO2 +19H2O +11H+ = 30Fe2++ 10FeHCO3+ +59HCO3-, predominantly
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consuming CO2, with the path taken sub-parallel to methane activity contours. Trace CH4
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is needed to maintain reducing conditions, but increasing methane activity (aCH4) further
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has no effect on the amount of hematite dissolved. As hematite dissolves, pH and Fe
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activity (aFe) increase. As aFe increases the stability field of Fe minerals expands, and
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either a Fe-Ca carbonate or magnetite becomes stable (Fig. 5). If carbonate forms, the
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carbon content of the fluid drops, and the fluid moves to the magnetite-carbonate
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cotectic.
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CONCLUSIONS
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Publisher: GSA Journal: GEOL: Geology Article ID: G32946 Field relationships, fluid inclusion petrography, isotopic data and modeling show
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that bleaching of red sandstones near Green River is distinct from hydrocarbon related
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bleaching elsewhere on the Colorado Plateau, and is consistent with the causal fluid being
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a CO2-charged brine with minor CH4. The site is therefore an analogue for water-CO2-
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rock reactions in anthropogenic carbon storage sites. Trace metals, originally hosted in Fe
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oxy-hydroxide grain coatings, have been mobilized by the low pH fluid and re-deposited
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in large concentrations at the reaction front.
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ACKNOWLEDGMENTS
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We thank Jason Day for help with geochemical analyses, Doris Wager for
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help with bulk rock X-ray diffraction analysis (XRD), Nick Tosca for help with clay
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mineral XRD, and Chris Rochelle and Jonathan Pearce for useful discussion. Bruce
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Yardley helped with fluid inclusion microthermometry. Carbon research at
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Cambridge is supported by Natural Environment Research Council grant
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NE/F004699/1, part of the UK CRIUS (Carbon Research Into Underground Storage)
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consortium. Niko Kampman is a recipient of financial support from Shell
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International.
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APPENDIX
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Methods
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Profiles from bleached to unbleached sandstones were cored using an electric, diamond-
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edged 40 mm core plug drill. Samples were spaced at 2-15 cm intervals. Core plugs on or
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near to the contact were sub-sampled every 4 mm using a 2 mm tungsten-carbide drill bit.
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Between drilling, the bit and core were cleaned with compressed air. Bulk rock and clay
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fraction (
