Pore Water Evolution Research Articles

Managing Pore-Water Quality in Mine Tailings by Inducing Microbial Sulfate Reduction

A field-scale experiment was conducted to evaluate the potential for inducing microbial sulfate reduction as a passive in situ technique for managing water quality in mine tailings deposits. Sulfide- and carbonate-rich minetailings, characterized by near-neutral pH pore water, were amended with < 1 dry wt. % organic carbon. The geochemical evolution of pore water was monitored for four years. The results demonstrate that organic carbon supported dissimilatory sulfate reduction (DSR) in the vadose zone. Decreases in dissolved SO4 and S2O3 were accompanied by H2S production, increased populations of sulfate-reducing bacteria (SRB), 34S-SO4 enrichment, and undersaturation of pore water with respect to gypsum [CaSO4 x 2H2O]. The mass of dissolved S decreased by > 45% during the monitoring period, which coincided with the removal of Zn, Sb, and Tl. Mobilization of Fe and As occurred initially; however, subsequent decreases in aqueous concentrations were observed. Mineralogical investigation confirmed the presence of secondary Fe-S and Zn-Fe-S phases. Amendment of tailings with a small and dispersed mass of organic carbon resulted in a general decrease in mass transport of sulfide oxidation products.

Kinetic fractionation of Fe isotopes during transport through a porous quartz-sand column

Sorption and desorption processes are an important part of biological and geochemical metallic isotope cycles. Here, we address the dynamic aspects of metallic isotopic fractionation in a theoretical and experimental study of Fe sorption and desorption during the transport of aqueous Fe(III) through a quartz-sand matrix. Transport equations describing the behavior of sorbing isotopic species in a water saturated homogeneous porous medium are presented; isotopic fractionation of the system (Δ sorbed metal-soln) being defined in terms of two parameters: (i) an equilibrium fractionation factor, α e; and (ii) a kinetic sorption factor, α 1. These equations are applied in a numerical model that simulates the sorption-desorption of Fe isotopes during injection of a Fe(III) solution pulse into a quartz matrix at pH 0–2 and explores the effects of the kinetic and equilibrium parameters on the Fe-isotope evolution of porewater. The kinetic transport theory is applied to a series of experiments in which pulses of Na and Fe(III) chloride solutions were injected into a porous sand grain column. Fractionation factors of α e = 1.0003 ± 0.0001 and α 1 = 0.9997 ± 0.0004 yielded the best fit between the transport model and the Fe concentration and δ 56Fe data. The equilibrium fractionation (Δ 56Fe sorbed Fe-soln) of 0.3‰ is comparable with values deduced for adsorption of metallic cations on iron and manganese oxide surfaces and suggests that sandstone aquifers will fractionate metallic isotopes during sorption-desorption reactions. The ability of the equilibrium fractionation factor to describe a natural system, however, depends on the proximity to equilibrium, which is determined by the relative time scales of mass transfer and chemical reaction; low fluid transport rates should produce a system that is less dependent on kinetic effects. The results of this study are applicable to Fe-isotope fractionation in clastic sediments formed in highly acidic conditions; such conditions may have existed on Mars where acidic oxidizing ground and surface waters may have been responsible for clastic sedimentation and metallic element transport.

Coupled thermal, hydraulic and geochemical evolution of pyritic tailings in unsaturated column experiments

The evolution of pore-water and the composition of solid phases in the vadose zone of pyritic tailings was studied by means of unsaturated column experiments. Several columns of water-saturated mine tailings were dried during 125 days under controlled laboratory conditions. The columns were dismantled at four successive drying stages and the evolution of pore-water, mineralogy, water content and temperature was characterized. Sulfide and aluminosilicate minerals present in the waste dissolved, releasing sulfate and other solutes (mainly Fe, Zn, Cu, Al, Mg and Ca) to the pore-water. Evaporation caused a crust of efflorescent, water-soluble sulfates to develop over the complete top surface of the columns and into the pores of the underlying waste material. This crust, which has also been identified in the field, changed the hydraulic properties of the tailings and produced a decrease in the evaporation rate of the columns. Moreover, these water-soluble precipitates (mainly rozenite, szomolnokite, halotrichite, hexahydrite, mirabilite and gypsum) acted as temporary sinks for Cd, Pb, Co and Ni, which could be released to the surface run-off or the groundwaters during rainfall events under field conditions. Pore-water evolution was determined not only by geochemical processes (dissolution of sulfides and aluminosilicates, precipitation of secondary phases) but also by thermal and hydraulic processes. Progressive dilution was observed in the lower part of the columns. Dilution was caused by the thermally driven vapor flux from the top of the column to its colder bottom and subsequent condensation therein. This process, which may also occur in tailings under sub-arid climate, played a key role on the evolution of pore-water with increasing drying.

Modelling of long-term dynamic leaching tests applied to solidified/stabilised waste

The paper aims at simulating the closed-system dynamic leaching of a cement-based monolith containing lead with the numerical reactive transport code HYTEC in a 3D-cylindrical geometry. The model considers, simultaneously, the chemical evolution of pore water, the progression of mineralogical alteration fronts, and the concomitant release of elements from the S/S waste. In good agreement with the experiment, element releases were found to be mainly controlled by either diffusion (Na, K, and, to a lesser extent, Cl), by surface dissolution (Ca, Si) or by a mixed evolution (Pb, SO 4). All of the calculated mineralogical transformations take place in a thin layer beyond the monolith surface. Consequently, modelling of Ca, Si and SO 4 releases was quite sensitive to the node size of the simulation grid and was improved by taking into account the increase of porosity and effective diffusion coefficient due to mineral dissolution in the leached layer. In agreement with experimental results, the deepest front corresponds under closed-system conditions to portlandite dissolution and calcium silicate hydrates CSH 1.8 transformation into CSH of lower Ca/Si ratio. A second, distinct and intermediate, front is made by ettringite dissolution. The network of CSH is globally preserved in the leached layer, complete dissolution occurring over a very small thickness only. Finally, hydrotalcite precipitation in the leached layer is expected by modelling due to pH drop.

Impact of basin evolution, depositional environment, pore water evolution and diagenesis on reservoir-quality of Lower Paleozoic Haima Supergroup sandstones, Sultanate of Oman

ABSTRACTSandstones of the Early Paleozoic Miqrat Formation and Barik Sandstone Member (Haima Supergroup) are the most prolific gas/condensate containing units in the northern part of the Interior Oman Sedimentary Basin (IOSB). The reservoir-quality of these sandstones, buried to depths exceeding 5 km, is critically related to the depositional environment, burial-related diagenetic reactions, the timing of liquid hydrocarbon charge and the replacement of liquid hydrocarbon by gas/condensate.The depositional environment of the sandstones controls the net-sand distribution which results in poorer reservoir properties northwards parallel to the axis of the Ghaba Salt Basin. The sandy delta deposits of the Barik Sandstone Member have a complex diagenetic history, with early dolomite cementation, followed by compaction, chlorite formation, hydrocarbon charge, quartz and anhydrite precipitation and the formation of pore-filling and pore-lining bitumen. In the Miqrat Formation sandstone, which is comprised of inland sabkha deposits, similar authigenic minerals occur, but with higher abundances of dolomite and anhydrite, and less quartz cement. The deduced pore water evolution from deposition to recent, in both the Miqrat Formation and the Barik Sandstone Member, reflects an early addition of saline continental waters and hydrocarbon-burial related mineral reactions with the likely influx of lower-saline waters during the obduction of the Oman Mountains.Four structural provinces are recognized in the IOSB based on regional differences in the subsidence/uplift history: the Eastern Flank, the Ghaba and Fahud Salt Basins and the Mabrouk-Makarem High. In the Fahud Salt Basin, biodegradation of an early oil charge during Late Paleozoic uplift resulted in reservoir-quality degradation by bitumen clogging of the pore space. On the Eastern Flank and the Mabrouk-Makarem High, however, the early oil bypassed the area. In contrast, post-Carboniferous liquid hydrocarbons were trapped in the Mabrouk-Makarem High, whereas on the Eastern Flank surface water infiltration and loss of hydrocarbons or biodegradation to pore occluding bitumen occurred. In the Ghaba Salt Basin, post-Carboniferous hydrocarbon charge induced a redox reaction to form porosity/permeability preserving chlorite in the reservoirs. The liquid hydrocarbons were replaced since the obduction of the Oman Mountains by gas/condensate which prevented the deep parts (&amp;gt;5,000 m) of the Ghaba Salt Basin from pore occluding pyrobitumen and thus deterioration of the reservoir quality.

Regional paleohydrologic and paleoclimatic settings of wetland/lacustrine depositional systems in the Morrison Formation (Upper Jurassic), Western Interior, USA

During deposition of the Upper Jurassic Morrison Formation, water that originated as precipitation in uplands to the west of the Western Interior depositional basin infiltrated regional aquifers that underlay the basin. This regional groundwater system delivered water into the otherwise dry continental interior basin where it discharged to form two major wetland/lacustrine successions. A freshwater carbonate wetland/lacustrine succession formed in the distal reaches of the basin, where regional groundwater discharged into the Denver–Julesburg Basin, which was a smaller structural basin within the more extensive Western Interior depositional basin. An alkaline–saline wetland/lacustrine complex (Lake T'oo'dichi') formed farther upstream, where shallower aquifers discharged into the San Juan/Paradox Basin, which was another small structural basin in the Western Interior depositional basin. These were both wetlands in the sense that groundwater was the major source of water. Input from surface and meteoric water was limited. In both basins, lacustrine conditions developed during episodes of increased input of surface water. Inclusion of wetlands in our interpretation of what had previously been considered largely lacustrine systems has important implications for paleohydrology and paleoclimatology. The distal carbonate wetland/lacustrine deposits are well developed in the Morrison Formation of east-central Colorado, occupying a stratigraphic interval that is equivalent to the “lower” Morrison but extends into the “upper” Morrison Formation. Sedimentologic, paleontologic, and isotopic evidence indicate that regional groundwater discharge maintained shallow, hydrologically open, well oxygenated, perennial carbonate wetlands and lakes despite the semi-arid climate. Wetland deposits include charophyte-rich wackestone and green mudstone. Lacustrine episodes, in which surface water input was significant, were times of carbonate and siliciclastic deposition in scarce deltaic and shoreline deposits. Marginal lacustrine deposits include ooid and skeletal packstone–grainstone, siltstone, and sandstone. Distal lacustrine units are skeletal mudstone–wackestone, microbialites, and laminated (siliciclastic) mudstone. Differentiation between wetlands and distal lacustrine units is not always possible. Palustrine features, Magadi-type chert (MTC), and evaporites record episodes of increased aridity and exposure. Farther upstream, during deposition of the upper part of the Brushy Basin Member, the ancestral Uncompahgre Uplift imposed a barrier to shallow, eastward-flowing groundwater that discharged into the San Juan/Paradox Basin on the upstream side of the uplift. This created the closed hydrologic setting necessary for development of an alkaline–saline wetland/lacustrine complex (“Lake” T'oo'dichi'). Silicic volcanic ash, delivered by prevailing winds from calderas west and southwest of the basin, contributed to the pore-water evolution in the sediments. A distinctive lateral hydrogeochemical gradient, reflecting increasing salinity and alkalinity in the pore waters, altered the ash to a variety of authigenic minerals that define concentric zones within the basin. The basinward progression of diagenetic mineral zones is smectite→clinoptilolite→analcime±potassium feldspar→albite. The groundwater-fed wetlands were shallow and frequently evaporated to dryness. Scarce laminated gray mudstone beds record distinct episodes of freshwater lacustrine deposition that resulted from intermittent streams that carried detritus well out into the basin.

Linking basement carbonate vein compositions to porewater geochemistry across the eastern flank of the Juan de Fuca Ridge, ODP Leg 168

Leg 168 of the Ocean Drilling Program (ODP) investigated the heat flow, fluid chemistry and crustal alteration associated with ridge flank hydrothermal systems. Ten sites were drilled on the eastern flank of the Juan de Fuca Ridge, along an 80 km transect, between 20 and 100 km east of the spreading centre. Recovered cores consisted of 100–500 m of sediment with shallow penetration (1.7–48.1 m) into the underlying igneous basement (0.8–3.6 Ma). Here we use the composition of calcium carbonate minerals, from veins within the upper basement, to reconstruct the evolving chemistry of hydrothermal fluids with increasing crustal age and sediment cover thickness. We show for the first time a clear link between the alteration of the basement rocks as recorded by secondary minerals, and the near-basement sedimentary pore fluids, which are often assumed to be representative of the basement fluids responsible for low temperature alteration of the upper crust. Carbonates precipitated from basement fluids that ranged in strontium isotopic composition from near-modern seawater ( 87Sr/ 86Sr≈0.70918) to the near-basement pore fluid values at any one site. 87Sr/ 86Sr ratios are independent of mineralogy with both aragonite and calcite precipitating from variably evolved fluids with the range in carbonate 87Sr/ 86Sr increasing with crustal age. A parallel geochemical evolution of basement fluids and sediment porewaters is shown since 87Sr/ 86Sr ratios of near-basement pore fluids decrease from 0.709013 to 0.707108 away from the ridge axis. A correlation exists between 87Sr/ 86Sr ratios and δ 18O-calculated fluid temperatures, with more geochemically evolved carbonates having precipitated from warmer fluids. Basement fluid compositions, calculated from carbonate Sr, Mg, Fe and Mn concentrations combined with suitable partition coefficients, are also temperature-dependent. Given an observed increase in basement temperature with age, from 16°C to 64°C along the transect, a progressive chemical development of basement fluid is demonstrated. Carbonate veins in volcanic basement from ODP Holes 504B and 896A, on the Costa Rica Rift, record the same temperature compositional evolution of basement fluid as those from the Juan de Fuca Ridge flank. Although these locations have different thermal histories and therefore must have experienced different temporal geochemical evolution of basement fluid, basement temperature appears to be the dominant control on basement fluid composition.

Stable isotope evidence for multiple fluid regimes during carbonate cementation of the Upper Tertiary Hazeva Formation, Dead Sea Graben, southern Israel

Stable carbon- and oxygen-isotope compositions of calcite and dolomite cements have been used to understand porewater evolution in the Upper Tertiary Hazeva Formation within the Dead Sea Graben, southern Israel. Sandstone samples were obtained from four boreholes in three tectonic blocks of the graben over depths of 253–6448 m, a variation that largely reflects differential subsidence of individual fault-bounded blocks. Early carbonate cements dominate diagenesis. Calcite occurs at <1600 m, but was replaced by dolomite at greater depths. Dolomite at ∼1600–2700 m is Fe-poor (<0.8 mol% FeCO 3), and at ∼4700–6200 m, Fe-rich (0.5–7.2 mol% FeCO 3). Magnesite, anhydrite and halite are the final diagenetic phases. Calcite has positively correlated δ 18O (+21‰ to +25‰) and δ 13C (−6‰ to −2‰) values that generally decrease with depth. Dolomite has a wider variation in δ 18O (+18‰ to +30‰) and δ 13C (−8‰ to −1‰) values, which also generally are lower with increasing depth. However, the δ 13C and δ 18O values of dolomite from the uppermost ∼400 m of the Hazeva Formation in the Sedom Deep-1 borehole are anomalous in spanning the entire range of stable carbon and oxygen isotopic compositions over this relatively small interval. The decreasing dolomite δ 13C values likely indicate an increased contribution of carbon from organic sources with increasing depth. Except for the uppermost 400 m, Hazeva Formation dolomite in the Sedom Deep-1 borehole has stable carbon-isotope compositions that imply initial dolomitization at much shallower levels, prior to the preferential subsidence of this tectonic block. The oxygen isotopic compositions of the calcite cement are best explained by equilibration at present burial temperatures (≤55 °C) with porewater of meteoric origin. Its δ 18O values increased from ∼−5‰ at the shallowest depths to ∼0‰ at ∼1600 m. The dolomite oxygen isotopic compositions also reflect equilibration at present burial temperatures with porewaters ranging from ∼0‰ at ∼1600 m to ∼+7‰ at 3600 m (∼100 °C). In the deepest fault block (Sedom Deep-1 borehole), however, increasingly Fe-rich dolomite has (re)equilibrated with porewater whose δ 18O values decreased from ∼+9‰ at ∼4750 m (∼120 °C) to ∼+1‰ to +2‰ by ∼6200 m (∼150 °C). Much of the dolomite likely formed at relatively shallow depths from saline brines derived from precursors to the Dead Sea. These infiltrated the Hazeva Formation, mixing with and largely displacing meteoric water, and dolomitizing calcite. Rock–water ratios tended to be high during these processes. However, the upper ∼400 m of the Hazeva Formation in the deepest fault block were likely deposited during its rapid tectonic subsidence, and largely escaped the initial style of dolomitization pervasive elsewhere in the study area. These sediments were also capped by evaporites. This relatively thin interval likely became a preferential conduit for brines that escaped underlying and overlying strata, including the Fe-rich, lower 18O fluids (evolved seawater?) present in the deepest part of the graben. These rocks present the most promising target for the passage and accumulation of hydrocarbons in the study area.

Pore water evolution in oilfield sandstones: constraints from oxygen isotope microanalyses of quartz cement

Oxygen isotope microanalyses of authigenic quartz, in combination with temperatures of quartz precipitation constrained by fluid inclusion microthermometry and burial history modelling, are employed to trace the origin and evolution of pore waters in three distinct reservoirs of the Brae Formation in the Miller and Kingfisher Fields (North Sea). Oxygen isotope ratios of quartz cements were measured in situ in nine sandstone thin sections with a Cameca ims-4f ion microprobe. In conjunction with quartz cement paragenesis in the reservoirs, constrained from textural and cathodoluminescence (CL) microscopy studies, pore water evolution was reconstructed from the time of deposition of the sandstones in the Upper Jurassic until the present. CL photomicrographs of quartz overgrowths in the Brae Formation sandstones show three cement zones (A, B and C) which can be related to different oxygen isotope compositions: (1) the earliest, and thinnest, zone A (homogeneous CL pattern with probable δ 18O values between +23‰ and +26‰—direct measurements were not possible) precipitated in the sandstones at temperatures <60 °C; (2) the second zone B (complex CL pattern and directly measured δ 18O values between +15‰ and +18‰) precipitated in the sandstones most likely between 70 and 90 °C; (3) the third zone C (homogeneous CL pattern and directly measured δ 18O values between +16‰ and +22‰) precipitated in the sandstones most likely at temperatures >90 °C. Calculated oxygen isotope compositions of pore waters show that zone A quartz cements, and enclosing concretionary calcite, precipitated from a meteoric-type fluid (∼−7‰) during shallow burial (<1.5 km). Zone B quartz cements precipitated from fluids which evolved in composition from a meteoric-type fluid ( δ 18O −7‰) to a more 18O-enriched fluid ( δ 18O −4‰) as burial continued to ∼3.0 km. Data from zone C quartz cements are consistent with further fluid evolution from δ 18O −4‰ to basinal-type fluids with δ 18O similar to the present-day formation water oxygen isotope composition (+0.6‰ at 4.0 km burial). A similar pore water evolution can be derived for all three reservoirs studied, indicating that hydrogeologic evolution was similar across sandstones of the whole Brae Formation. The quartz cement zones observed in the Brae Formation sandstones, and the pore water history derived for the area studied, is analogous to published petrographic and pore water evolution data from the nearby Brent Group reservoirs and from reservoirs located in the Haltenbanken area on the Atlantic margin offshore Norway. Considering quartz cement is a major porosity-occluding phase in many reservoir sandstones, and because pore waters both dissolve quartz and carry the dissolved silica to cementation sites, the data presented are valuable for improving the understanding and prediction of reservoir quality development in sandstones globally.

Diagenesis and paleofluid flow in the Devonian Southesk-Cairn carbonate complex in Alberta, Canada

One of the largest geological features in the deep part of the Alberta Basin in western Canada is the Middle to Upper Devonian Southesk-Cairn carbonate complex (SCCC). The SCCC lies at depths of about 5000m in the vicinity of the disturbed belt of the Rocky Mountains and is rising to depths of 2000m in a northeasterly direction over a distance of 150km.The complex contains several natural gas pools, which contain up to about 30% H2S. The major focus of this study is to construct a detailed diagenetic history and concurrent pore fluid flow over time. The Southesk-Cairn complex reveals a complicated diagenetic history that consists of five phases of pore water evolution: (1) synsedimentary to shallow burial, (2) intermediate burial, (3) deep burial, (4) maximum burial, and (5) present depth. Each of these phases influenced the character and the quality of the reservoir rocks.Phase 4 is of special interest because the strontium isotope data of late diagenetic calcite cements suggest the influence of basin-external fluids. The 87Sr/86Sr-isotope ratios of these calcite cements are high (up to 0.7320) directly adjacent to the disturbed belt and they decrease with increasing distance towards the foreland basin (0.7100). The radiogenic strontium signal can be traced for about 100km into the foreland basin. Circumstantial evidence suggests that the radiogenic strontium was carried by fluids that were expelled during the Laramide orogeny from the Rocky Mountain Main Ranges and/or the underlying basement.

Origin of Authigenic Mn-Fe Carbonates and Pore-Water Evolution in Marine Sediments: Evidence from Cenozoic Strata of the Arctic Ocean and Norwegian-Greenland Sea (ODP Leg 151)

Early diagenetic Mn-Fe carbonates, occurring in thin layers, nodules, and burrows, were recovered from Cenozoic silts and clays from the Arctic Ocean and the Norwegian-Greenland Sea during Ocean Drilling Program Leg 151. At Sites 909 (Fram Strait; Miocene-Quaternary) and 911 (Yermak Plateau; Pliocene-Quaternary), fine-crystalline siderite, enriched in Ca and/or Mg, is the predominant carbonate. Variable δ13C values (-21.8 to +7.9‰ PDB) suggest that dissolved carbon was derived from the Fe-reduction suboxic zone, the oxidation of marine organic matter in the bacterial sulfate-reduction zone, and the early stages of methanogenesis. The range of δ18O values for siderite (-6.3 to +5.1‰ PDB) indicates precipitation over a temperature range of 4-56°C during successive burial. At Site 913 (East Greenland Margin; Eocene-Quaternary), concentrically zoned microspherules and rhombohedra of ferroan rhodochrosite and manganoan siderite, 20-600 μm in diameter, are the main authigenic carbonates. Although they have low δ13C values (-22.3 to -13.8‰ PDB), elevated pore-water sulfate concentrations indicate low sulfate-reducing activity and suggest that dissolved carbon may have been partially derived from thermogenic methane. The variable δ18O values for rhodochrosite (-11.9 to +1.5‰ PDB) and siderite (-11.4 to -10.3‰ PDB) suggest precipitation at elevated temperatures (60-100°C) during progressive burial. The close association with barite cement suggests that carbonate precipitation at Site 913 may have been influenced by hydrothermal fluids. However, active circulation of hydrothermal fluids is precluded by the occurrence of geochemical gradients in Site 913 pore waters. Pore waters at the three sites reflect various evolutionary pathways that are partly due to variations in the extent of interaction with volcanic ash layers and the underlying basaltic crust, which led to an overall decrease in Mg2+, K+, and δ18O and an increase in Ca2+ with depth. However, pore waters at Site 913 have evolved more extensively to Na-Ca-Cl brines. The sources of freshwater that cause the downhole decrease in Cl- and Na+ concentrations at Sites 909 and 913 are poorly constrained. The possible influence of meteoric-water incursion is suggested by the δD-δ18O relationship of pore waters and the low δ18O values of Site 913 authigenic carbonates.

Origin of Authigenic Mn-Fe Carbonates and Pore-Water Evolution in Marine Sediments: Evidence from Cenozoic Strata of the Arctic Ocean and Norwegian-Greenland Sea (Odp Leg 151)

Origin of Authigenic Mn-Fe Carbonates and Pore-Water Evolution in Marine Sediments: Evidence from Cenozoic Strata of the Arctic Ocean and Norwegian-Greenland Sea (Odp Leg 151)

Claystone Constraints on Models of the Long-Term Chemical Evolution of Buffer Porewaters

Geochemical models of clay-water interaction are evaluated using data characterizing the hydrochemical, isotopic and mineralogical properties of a natural claystone that is similar to bentonite buffers that have evolved over long periods of time in a deep geologic repository for nuclear wastes. Model predictions in general compare favorably with the mineralogy and hydrochemistry of this formation if increases in pH due to partial CO2(g) exsolution from porewater samples is accounted for. This suggests the models can be used with enhanced confidence to accurately predict the chemistry of pore fluids in bentonite buffers, enabling more defensible estimates to be made of radioelement solubilities that partially define the source term in performance assessments. Modifications are needed, however, to improve the accuracy of thermodynamic data supporting the models and to account for potential effects on porewater compositions of reactions that are kinetically inhibited from attaining equilibrium over experimental time scales.

Pore water evolution in sandstones of the Groundhog Coalfield, northern Bowser Basin, British Columbia

Abstract The succession of sandstone cements in chert and volcanic lithic arenites and wackes from the northern Bowser Basin of British Columbia comprises a record of diagenesis in shallow marine, deltaic, and coastal plain siliciclastic sediments that pass through the oil window and reach temperatures near the onset of metamorphism. The succession of cements is consistent with seawater in the sandstones mixing with acid waters derived from dewatering of interbedded organic rich muds. Sandstone cement paragenesis includes seven discrete cement stages. From earliest to latest the cement stages are: (1) pore-lining chlorite; (2) pore-lining to pore-filling illite; (3) pore-filling kaolinite; (4) oil migration through some of the remaining connected pores; (5) chlorite dissolution; (6) quartz cement; and (7) calcite cement. These seven cement stages are interpreted as a record of the evolution of pore waters circulating through the sandstones after burial. The earliest cement stages, as well as the depositional environments, are compatible with seawater as the initial pore fluid. Seawater composition changed during transport through the sandstones, first by loss of Mg2+ and Fe2+ during chlorite precipitation (stage 1). Dewatering of interbedded organic-rich mudstones probably added Mg2+ and Fe2+ to partially buffer the loss of these cations to chlorite. Acids produced during breakdown of organic matter are presumed to have mixed into sandstone pore fluids due to further compaction of the muds, leading to reduction of initial alkalinity. Reduction in alkalinity, in turn, favours change from chlorite to illite precipitation (stage 2), and finally to kaolinite (stage 3). Pore waters likely reached their peak acidity at the time of oil migration (stage 4). Chlorite dissolution (stage 5) and quartz precipitation (stage 6) occurred when pores were filled by these hydrocarbon-bearing and presumably acidic fluids. Fluid inclusions in fracture-filling quartz cements contain petroleum, high-pressure methane, and methane-rich aqueous solutions. Homogenization temperatures from primary two-phase inclusions are consistent with quartz cementation during progressive heating between approximately 100 and 200°C. Following quartz precipitation, alkaline pore waters were re-established, as evidenced by late-stage calcite cement (stage 7).

Diagenetic reactions in reservoir strata and geochemical properties of pore fluid and its origin in Songliao Basin

The reservoirs of the Songliao Basin are composed of typical unstable sandstones, with high percentages of volcanic fragments and feldspar. In the course of sedimentation and burying, a series of physical and chemical reactions took place between minerals and pore water and water-rock reactions and ion exchange caused changes in ion assemblage of pore water. Hydration-hydrolysis, dissolution and the albitization of feldspar made many ions free from their framework and inter into the pore water, and induced the precipitation of a large amount of authigenic minerals such as smectite and chlorite. During the diagenesis of sandstone, diagenetic reactions involved several stages with increasing depth, and so did the precipitation of authigenic minerals and the transformation of minerals. The migration of ions is related with the precipitation, transformation and dissolution of authigenic minerals. Thus, to deepen our study on sandstone diagenesis is an important link for the analysis of ion migration in the evolution of pore water. The origin and evolution of pore water could be tracked in terms of the geochemistry of fluid inclusions in authigenic minerals. And the isotopic composition of the authigenic mineral calcite can provide its genetic information.

Strontium isotopic evidence on the chemical evolution of pore waters in the Milk River Aquifer, Alberta, Canada

Strontium isotope ratios were measured on 13 rock, 18 leachate and 28 pore-water samples from the Milk River aquifer, the confining argillaceous formations, and the glacial till mantling the recharge area. Strontium isotope ratios ( 87Sr/ 86Sr) of pore waters from the aquifer, confining units, and the glacial till ranged from 0.7069 to 0.7082. The 87Sr/ 86Sr ratios in aquifer pore waters decrease with increasing distance from the aquifer recharge area, and this is interpreted to be the result of mixing and water–rock interaction within the aquifer. The solute composition of the recharging groundwater is modified by the local lithology, causing distinct geochemical patterns along different flow paths within the aquifer. Whole-rock 87Sr/ 86Sr ratios indicate that the shales and till are generally more radiogenic than the aquifer sandstone. The authigenic carbonate cements and rock-forming minerals comprising the major lithologic units had little apparent influence on the pore-water Sr chemistry. Carbonate cement leachates from the till and the aquifer sandstone are more radiogenic than those from the confining shale formations. Feldspar separates from the aquifer sandstone have relatively radiogenic Sr isotope ratios, whereas bentonites from the Milk River and Colorado Shale Formations have whole-rock and leachate Sr isotope ratios that are relatively unradiogenic. Ratios of most Milk River aquifer pore waters are lower than those of any leachates or whole rocks analyzed, except the bentonites. The 87Sr/ 86Sr ratios of exchangeable Sr in the bentonites are similar to ratios found in the more evolved pore waters. Simple rock–water interaction models calculated for the whole-rock, leachate, and exchangeable-ion/pore-water pairs indicate that ion exchange with bentonite clays within the Milk River and Colorado Shale Formations appears to influence the isotopic evolution of the pore-water Sr in each of these units.

Local Sulphide Oxidation Drives Carbonate Dissolution in Modern Shallow Marine Carbonates: Evidence from Oxygen and Sulphur Isotope Composition of Pore Water Sulphate

We have investigated sulphur cycling in modem carbonate sediments from the Florida Platform, U.S.A. Relations among pore water chemistry (SO24 , ZCO2, Ca2+/C1 ) and oxygen and sulphur stable isotope composition of SO 2require a direct coupling between sulphur redox cycling and syndepositional carbonate dissolution. Importantly, despite rapid rates of sulphate reduction, organic-rich carbonate sediment pore waters typically maintain SO2-/C1 ratios within 5% of seawater values, suggesting local reoxidation of H2S. Shallow shelf carbonates comprise 93% of the entire Phanerozoic carbonate rock record and about 38% of the modem oceanic carbonate accumulation, so quantifying sulphur and carbon coupling during the early marine diagenesis is vital in deciphering biogeochemical cycling (e.g. Ronov, 1980; Milliman, 1993). In low-Fe carbonates, H2S-O2 reactions can lower pH values and pore water saturation states promoting carbonate dissolution (e.g. Boudreau, 1991). Given that platform carbonate dissolution fluxes are roughly half of gross biogenic carbonate production, sulphur cycling may greatly affect carbonate preservation and accumulation (Walter and Burton, 1990; Walter et al., 1993). Pore water sulphate ~180 and 834S values were markedly shifted from the established seawater values of +9.5%o (SMOW) and +21.0%o (CDT), respectively. Chemical evolution was least in environments where the sediment-pore water system was relatively 'open' to the overlying seawater. The greatest chemical evolution was observed in bioturbated sediments colonized by Thalassia grass. Here, there was a ~lSOso 4 shift of 5%0, var iab le ~348SO 4 values (+17.7 to +23.3%o), and exceptionally high Ca2+/C1 ratios. These shifts in pore water sulphate isotopic compositions exist despite the apparent lack of net sulphate reduction (Fig. 1). The natural system isotope trend is in marked contrast to the closed system isotope evolution observed in sediment incubation experiments (Fig. 1). Importantly, sulphide oxidation is required as an acid source because pore water Ca+2/C1 and ZCO2 values exceed those attributable to the utilization of dissolved oxygen and the small amount of net sulphate reduction. The observed degree of carbonate dissolution (up to 2.3 mM excess Ca 2+) and sulphate isotopic values are best explained by a redox cycle where bacterial sulphate reduction is followed by efficient H2S oxidation resulting in relatively invariant SO24-/C1 ratios. The 02 required to locally oxidize pore water H2S may come from oxygenated seawater advection or from marine grass rhizome systems which enhance oxygen supply rates. A mass balance model demonstrates that the observed 81SOso4 values are consistent with measured sulphate reduction rates and the short pore water residence times. Pore water geochemistry and sediment incubation experiments confirm that sulphur cycling has a profound effect on the early marine diagenesis of shallow platform carbonates. The important diagenetic processes are oxic respiration, bacterial sulphate reduction, sulphide oxidation, and carbonate dissolution. In most sediments, pore water SO2-/C1 ratios are near overlying ocean values, yet in many cases SO24 has been recycled from HzS. The regularity of pore water evolution and sediment environment becomes more apparent when the oxygen and sulphur isotope compositions of SO 2are examined. Diagenetic effects are most

Natural weathering of pulverized fuel ash and porewater evolution

Borehole samples of pulverized fuel ash (PFA) were taken from the unsaturated zone in a disposal mound at a decommissioned power station in the UK. The aim was to investigate the long-term natural weathering reactions of PFA and the chemical evolution of the contained porewaters. Concentrations of most species, including Al, Na, K, Ca, SO 4, B, Co, Cr, Li, Mo, Ni, Pb and Sr in the porewaters, increase with borehole depth, consistent with these elements being released from the PFA through continued weathering reactions with infiltrating porewaters. The concentration of Ba shows a near-constant value throughout the depth range investigated and this element is thought to have achieved equilibrium with respect to a sulphate phase. The Ca and S in the PFA show depletion near-surface, consistent with the higher porewater concentrations with depth. Using mass balance calculations for these two elements, approximate infiltration rates are obtained. Other elements which are depleted in near-surface samples are Cu, Mn, Ni, Pb and Zn. Higher concentrations, particularly of Na 2O and K 2O, in near-surface borehole samples demonstrate, however, that the ash was probably not homogeneous at the time of emplacement. Other elements in solution, such as Cl and NO 3, show peak concentrations in the depth profiles, which are thought to represent a time-dependent migration of an anthropogenic input, probably fertiliser. No significant changes were detected in the mineralogy using XRD and SEM. Porewater analyses were processed using a geochemical modelling program, WATEQ4F, to investigate equilibrium relationships and to identify potential solubility controlling solid phases. Several solid phases were identified, including Al(OH) 3 for Al, Fe(OH) 3(am) for Fe and CaSO 4.2H 2O (gypsum) for Ca and SO 4.

Diagenesis of a mixed siliciclastic/evaporitic sequence of the Middle Muschelkalk (Middle Triassic), the Catalan Coastal Range, NE Spain

ABSTRACTThe Middle Muschelkalk (Middle Triassic) of the Catalan Coastal Range (north‐east Spain) comprises sandstone, mudstone, anhydrite and minor carbonate layers. Interbedded sandstones and mudstones which are dominant in the north‐eastern parts of the basin are terminal alluvial fan deposits. South‐westward in the basin, the rocks become dominated by interbedded evaporites and mudstones deposited in sabkha/mudflat environments. The diagenetic and pore water evolution patterns of the Middle Muschelkalk suggest a strong facies control. During eodiagenesis, formation of microdolomite, anhydrite, baryte, magnesite, K‐feldspar and mixed‐layer chlorite/smectite was favoured within and adjacent to the sabkha/mudflat facies, whereas calcite, haematite, mixed‐layer illite/smectite and quartz formed mainly in the alluvial facies. Low δ18OSMOW values for microdolomite (+23.7 to +28.4%) and K‐feldspar overgrowths (+17.3 to +17.7%) suggest either low‐temperature, isotopic disequilibrium or precipitation from low‐18O porewaters. Low‐18O waters might have developed, at least in part, during low‐temperature alteration of volcanic rock fragments. During mesodiagenesis, precipitation of quartz overgrowths and coarse dolomite occurred in the alluvial sandstones, whereas recrystallization of microdolomite was dominant in the sabkha/mudflat facies. The isotopic compositions of these mesogenetic phases reflect increasing temperature during burial. Upon uplift and erosion, telogenetic calcite and trace haematite precipitated in fractures and replaced dolomite. The isotopic composition of the calcite (δ18OSMOW=+21.5 to +25.6%o; δ13C= 7.7 to ‐ 5.6%o) and presence of haematite indicate infiltration of meteoric waters.

Genesis of siderite in the Upper Miocene offshore Sarawak: constraints on pore fluid chemistry and diagenetic history

The cored sequence of the upper Miocene reservoir of Baram field, offshore Sarawak, consists of cyclic quartz-rich sandstones and mudstones interpreted to have been deposited during storm events in shallow to midshelfwater depths. The sequence is intercalated with minor tidal intervals. Authigenic siderite is common in sandstones throughout the sequence. Siderite cemented zones are up to 2 m thick. The cement is found in five different sandstone types: laminated sandstone, massive sandstone, bioturbated sandstone, heterogeneous sandstone, and in association with mud intraclasts and shell fragments horizon. Whole-rock XRD gives estimate of 20 to 40% of siderite in bioturbated and heterogeneous sandstones and 10 to 25% for the others. Petrographic analysis reveals that diagenetic siderite occurs in four different crystal morphologies: rhombic, bundle, acicular and cylindrical. The rhombic siderite, which commonly occurs in bioturbated and heterogeneous sandstone, has the most adverse effect on the poroperm characteristics of the sandstones, reducing porosity to 10% and permeability to 2 md. Bl3C and BIBO plots show groupings based on morphology. Bundled and acicular siderite show ranges of Bl3CpDB of -15 to -25 and BIBOpDB of 0 to -1. The late Miocene seawater BIBO estimate for the region is -O.S PDB. This would give the bundle and acicular siderite a temperature of formation range of 25° to 30°C. The Bl80 values are compatible with precipitation at shallow burial depth from unaltered sea­ water; Bl3C values are inherited from sulfate reduction horizons. Rhombic siderite has ranges of Bl3CpDB of -5 to -15 and Bl80pDB of -3 to -4. The range of Bl3C indicates that siderite diagenesis occurred within both the shallow sulfate reduction zone and at deeper levels within the zone of decarboxylation. The maximum temperature of formation here is 3So to 4SoC at depths of about -450 to -SOO m, assuming precipitation from unaltered seawater. These results are consistent with those ofICP geochemical values which indicate that the rhombic siderite are Mg-rich relative to the acicular and bundle types. The cylindrical siderite shows isotope values between these two extremes. These results reveal the pathways of early pore water evolution and diagenetic history of the Upper Miocene succession of Baram Field.