Gas–water–rock interactions in sedimentary basins: CO2 sequestration in the Frio Formation, Texas, USA

To investigate the potential for the geologic storage of CO₂ in saline sedimentary aquifers, 1600 ton of CO₂ were injected at 1500 m depth into a 24-m sandstone section of the Frio Formation — a regional reservoir in the US Gulf Coast. Fluid samples obtained from the injection and observation wells before, during and after CO₂ injection show a Na–Ca–Cl type brine with 93,000 mg/L TDS and near saturation of CH₄ at reservoir conditions. As injected CO₂ gas reached the observation well, results showed sharp drops in pH (6.5 to 5.7), pronounced increases in alkalinity (100 to 3000 mg/L as HCO₃) and Fe (30 to 1100 mg/L), and significant shifts in the isotopic compositions of H₂O and DIC. Geochemical modeling indicates that brine pH would have dropped lower, but for buffering by dissolution of calcite and Fe oxyhydroxides. Post-injection results show the brine gradually returning to its pre-injection composition.     

CO<Subscript>2</Subscript> source-sink matching in the lower 48 United States,with examples from the Texas Gulf Coast and Permian Basin

Documenting geographic distribution and spatial linkages between CO₂ sources and potential sinks in areas with significant levels of CO₂ emissions is important when considering carbon-management strategies such as geologic sequestration or enhanced oil recovery (EOR). For example, the US Gulf Coast overlies a thick succession (>6,000 m [>20,000 ft]) of highly porous and permeable sandstone formations separated by thick, regionally extensive shale aquitards. The Gulf Coast and Permian Basin also have a large potential for EOR, in which CO₂ injected into suitable oil reservoirs could be followed by long-term storage of CO₂ in nonproductive formations below reservoir intervals. For example, >6 billion barrels (Bbbl) of oil from 182 large reservoirs is technically recoverable in the Permian Basin as a result of miscible-CO₂ flooding_. The Gulf Coast also contains an additional 4.5 Bbbl of oil that could be produced by using miscible CO₂. Although the CO₂ pipeline infrastructure is well-developed in the Permian Basin, east Texas and the Texas Gulf Coast may have a greater long-term potential for deep, permanent storage of CO₂ because of thick brine-bearing formations near both major subsurface and point sources of CO₂.     

Saline brines and metallogenesis in a modern sediment-filled rift: the Salton Sea geothermal system, California, U.S.A.

The Salton Sea geothermal system (SSGS) is the site of active hydrothermal metamorphism and metallogenesis in the delta of the Colorado River, which partially fills the Salton Trough rift zone at the head of the Gulf of California. Growth of the delta across the rift has isolated the northern part of the Salton Trough since the Pleistocene, forming the evaporative Salton Sea basin whose sediments host the SSGS. More than 70 commercial geothermal wells, including a 3.2 km deep research borehole drilled as part of the Salton Sea Scientific Drilling Project (SSSDP), are yielding a wealth of new data from this system.Within the SSGS, active greenschist facies metamorphism is occuring at temperatures 365°C at only 2–3 km depth, by reaction of NaCaKFeMnCl brines with the deltaic and lacustrine Pilo-Pleistocene sediments. Two kinds of base metal ore mineralization occur at depth: stratabound diagenetic Fe sulfides, and epigenetic vertical veins containing FeZnCuPb sulfides and Fex oxides. The vein mineralization occurs as two types: an older, reduced carbonate-sulfide assemblage, and a modern, oxidized, silicate-hematite-sulfide assemblage. The earlier assemblage formed at temperatures up to 100°C higher than the ambient temperatures measured in the wells today, implying that cooler, oxidized fluids have displaced hot, reduced fluids.A sharp interface between shallow fluids containing <12 wt % TDS and deep hypersaline brines containing 15–27 wt % TDS exists in the SSGS. The deeper hypersaline brines are rich in base metals (Fe 1500 ppm, Mn 1000 ppm, Zn 500 ppm) whereas the overlying lower-salinity fluids contain less than 100 ppm each of Fe, Mn and Zn. The modern silicate-hematite-sulfide vein assemblage is precipitating where the two fluids appear to be mixing. The two fluid types also have distincty different δ¹⁸O and δD relations. The shallow lower-salinity fluids have only partially-exchanged oxygen with deltaic and lacustrine host rocls. The deep hypersaline brines have exchanged oxygen extensively at 250°C with the deltaic sediments. δD values of the hypersaline brines are typically lighter than either the lower-salinity fluids or the modern evaporating groundwaters occupying the Salton Trough, implying a different source for the original fluids.The high salinities of the geothermal brines are derived from a combination of evaporation of fossil lakewaters, groundwater dissolution of shallow lacustrine evaporites, and subsurface hydrothermal metamorphism of buried lacustrine evaporites. Episodic filling and desiccation of the closed Salton Sea basin has allowed cold saline brines to form and percolate down into the sedimentary section. In some wells anyhydrite meta-evaporites and interbedded solution collapse breccias occur at 1 km depth. The anyhydrite contains fluid inclusions that are saturated in halite at their homogenization temperatures of 300°C, recording the hydrothermal dissolution of bedded salt. Based on Sr and Pb isotopic data and whole-rock chemical data, the source of the metals in the hypersaline brines is from leaching of the host sediments. However, the origin of reduced S for ore mineralization remains an enigma. δ³⁴S values for vein sulfides cluster uniformly around zero per mil, implying that an isotopically-homogeneous source of reduced sulfide exists in the brines. It is possible that the vein sulfides receive a constribution from a magmatic S source.Movement and mixing of brines of different chemistry and oxidation states play a major role in ore genesis. Our model envisages an early stage in which a deep brine pool accumulated at depth in the sedimentary section by partial section by partial evaporation of basin and dissolution of bedded salts. Intrusion of rift-related basaltic magma into the base of the sedimentary caused heatingand fracturing of the sediments, resulting in precipitation of the carbonate-sulfide veins during pore fluid expulsion. Heating also caused a diapir of the hypersaline brine to rise and displace colder, less saline, shallower pore fluids. This brine intrusion was accompanied by pervasive and extensive mineralization. As this diapir cooled it began to move downward, drawing in shallow, more oxidized fluids and causing the formation of the modern silicate-hematite-sulfide vein ore zone.     

An Upper Cretaceous (middle Campanian) marine chondrichthyan and osteichthyan fauna from the Rattlesnake Mountain sandstone member of the Aguja Formation in West Texas

A thin phosphate-granule conglomerate within the Upper Cretaceous (middle Campanian) Rattlesnake Mountain sandstone member of the Aguja Formation preserves a diverse chondrichthyan and osteichthyan fauna. This highly fossiliferous deposit (the ‘Ten Bits Microsite’) yielded about 5000 teeth, spines, and denticles in a small amount of matrix (c. 150 kg). About 30 identifiable species of sharks, rays, and bony fishes are recognized. Two of the three most abundant chondrichthyan species at Ten Bits (Scapanorhynchus texanus and Ischyrhiza mira) are also the most common species in other middle to late Campanian marine vertebrate faunas along the Gulf and Atlantic Coastal Plain. The myliobatiform rays Brachyrhizodus and Rhombodus that occur at Ten Bits also appear to be characteristic of the Gulf and Atlantic Coast, as are lamniform sharks such as Cretalamna and Serratolamna. These taxa are entirely absent or extremely rare in Western Interior Campanian faunas. In contrast, some small orectolobiform sharks (Cantioscyllium, Chiloscyllium, Columbusia) and small rays (Protoplatyrhina) found at Ten Bits are common in shallow water faunas of the Western Interior and Texas Coastal Plain, but rarely reported from the eastern Gulf or Atlantic Coast. The common Western Interior ray Myledaphus bipartitus does not occur at Ten Bits or in any Gulf or Atlantic Coast fauna. Ptychotrygon agujaensis is abundantly represented in the Ten Bits fauna, but unknown in correlative marine faunas. Although Ptychotrygon occurs in all Western Interior, Gulf and Atlantic Coastal Plain faunas, it is represented elsewhere by apparently endemic species at each collection site. The differences between Western Interior, Gulf, and Atlantic Coastal Plain faunas probably reflect latitudinal variation in water temperature or salinity, or different oceanic water circulation patterns between the Western Interior Seaway and the Gulf or Atlantic Coast that restricted the distributions of some marine fish species. The Ten Bits fauna shares typical species with both Western Interior and Gulf and Atlantic Coast faunas, reflecting its position at the border between these provinces; however, the dominant taxa found at Ten Bits are the same as those found on the Gulf and Atlantic Coast, and indicate that western Texas was more closely allied biogeographically with that province than with the Western Interior of North America. One species tentatively identified in the Ten Bits fauna on the basis of a single tooth, Igdabatis cf. I. indicus, is otherwise known only from southern Europe and Asia, although a similar large myliobatid ray also occurs rarely in Texas Coastal Plain faunas. These occurrences indicate that western Texas may have been near the northern limit of the range for some tropical Tethyan marine vertebrate species.     

Evidence of regional subsidence and associated interior wetland loss induced by hydrocarbon production, Gulf Coast region, USA

Analysis of remote images, elevation surveys, stratigraphic cross-sections, and hydrocarbon production data demonstrates that extensive areas of wetland loss in the northern Gulf Coast region of the United States were associated with large-volume fluid production from mature petroleum fields. Interior wetland losses at many sites in coastal Louisiana and Texas are attributed largely to accelerated land subsidence and fault reactivation induced by decreased reservoir pressures as a result of rapid or prolonged extraction of gas, oil, and associated brines. Evidence that moderately-deep hydrocarbon production has induced land-surface subsidence and reactivated faults that intersect the surface include: (1) close temporal and spatial correlation of fluid production with surficial changes including rapid subsidence of wetland sediments near producing fields, (2) measurable offsets of shallow strata across the zones of wetland loss, (3) large reductions in subsurface pressures where subsidence rates are high, (4) coincidence of orientation and direction of displacement between surface fault traces and faults that bound the reservoirs, and (5) accelerated subsidence rates near producing fields compared to subsidence rates in surrounding areas or compared to geological rates of subsidence. Based on historical trends, subsidence rates in the Gulf Coast region near producing fields most likely will decrease in the future because most petroleum fields are nearly depleted. Alternatively, continued extraction of conventional energy resources as well as potential production of alternative energy resources (geopressured-geothermal fluids) in the Gulf Coast region could increase subsidence and land losses and also contribute to inundation of areas of higher elevation.     

Multiple fluid components of salt diapirs and salt dome cap rocks, Gulf Coast, U.S.A.

Salt diapirs contain a few percent of anhydrite that accumulated as residue to form anhydrite cap rocks during salt dissolutions. Reported ⁸⁷Sr/⁸⁶Sr ratios of these salt-hosted and cap rock anhydrites in the Gulf Coast, U.S.A., indicate their derivation from Middle Jurassic seawater. However, a much wider range of ⁸⁷Sr/⁸⁶Sr ratios, incorporating a highly radiogenic component in addition to the Middle Jurassic component, has been found in several Gulf Coast salt domes. This wide range of ⁸⁷Sr/⁸⁶Sr ratios of anhydrite within the salt stocks records Sr contributions from both marine water and formation water that has equilibrated with siliciclastics. During cap rock formation this anhydrite either recrystallized in the presence of, or was cemented by, a low-Sr fluid with a Late Cretaceous seawter-type Sr isotope ratio or simply lost Sr during recrystallization. Later, the cap rock was invaded by warm saline brines with high Sr isotope ratios from which barite and metal sulfides were precipitated. Subsequently, low-salinity water hydrated part of the anhydrite bringing to six the total number of fluids that interacted througout the history of salt dome and cap rock growth. The progenitor of these salt diapirs, the Louann Formation, is generally thought to have formed from marine water evaporated to halite and, rarely, higher evaporite facies. Salt domes in the East Texas, North Louisiana, and Mississippi Salt Basins have ⁸⁷Sr/⁸⁶Sr and δ³⁴S values that corroborate a Mid-Jurassic age for the mother salt. However, salt domes in the Houston and Rio Grande Embayments of the Gulf Coast Basin have ⁸⁷Sr/⁸⁶Sr ration ranging to values higher than both Middle Jurassic seawater and all Rb-free marine Phanerozoic rocks. These anomalous ⁸⁷Sr/⁸⁶Sr ratios are probably derived from radiogenic Sr-bearing fluids that equilibrated with siliciclastic rocks and invaded the salt either prior to, or during, diapirism. Potential sources of the radiogenic ⁸⁷Sr component include clay and/or feldspar (located either in older units beneath the Louann Formation or younger units flanking the salt diapirs) and K-salts within the Louann evaporites. Because partial Sr exchange in anhydrite had to take place in a fluid medium, admittance of radiogenic ⁸⁷Sr-bearing fluids into the salt may have led to diapirism by lowering the shear strength of the crystalline salt. The slight number of anomalous ⁸⁷Sr/⁸⁶Sr values in the interior basins indicates that anomalous values are related to areally discrete structural or stratigraphic controls that affected only the Gulf Coast Basin.     

A survey of oil and gas wells in the Texas Gulf Coast,USA, and implications for geological sequestration of CO<Subscript>2</Subscript>

Subsurface sequestration of CO₂ in oil and gas provinces where permanence of hydrocarbon accumulations has proven the reliability of potential traps is rightly seen as a solid option for containment of CO₂ atmospheric concentrations. However, one of the most promising provinces for carbon storage in North America, the Texas Gulf Coast, has also been heavily drilled for more than a century, puncturing many otherwise perfectly sound seals (>125,000 wells over ~50,000 km²). As a result, boreholes and, in particular, older abandoned wells could be major leakage pathways for sequestered CO₂. This article presents statistics on well spatial and depth distribution that have been drawn from public domain sources and relates these data to historical plugging and abandonment regulations in the Texas Gulf Coast. Surface-well density averages of 2.4 wells/km² can be locally much higher—but also much lower in larger areas. Average well penetration density drops to 0.27 and 0.05 well/km² below a depth of 2,440 and 3,660 m, respectively. Natural mitigating factors such as thief zones and heaving “shales” could also play a role in limiting the impact of these direct conduits to the shallow subsurface and surface.     

The geochemistry of Ca,Sr, Ba and Ra sulfates in some deep brines from the Palo Duro Basin,Texas

The geochemistry of Ca, Sr, Ba and Ra sulfates in some deep brines from the Palo Duro Basin of north Texas, was studied to define geochemical controls on radionuclides such as ⁹⁰Sr and ²²⁶Ra. Published solubility data for gypsum, anhydrite, celestite, barite and RaSO₄ were first reevaluated, in most cases using the ion interaction approach of Pitzer, to determine solubility products of the sulfates as a function of temperature and pressure. Ionic strengths of the brines were from 2.9 to 4.8 m, their temperatures and pressures up to 40°C and 130 bars. Saturation indices of the sulfates were computed with the ion-interaction approach in one brine from the arkosic granite wash fades and four from the carbonate Wolfcamp Formation. All five brines are saturated with respect to gypsum, anhydrite and celestite, and three of the five with respect to barite. All are undersaturated by from 5 to 6 orders of magnitude with respect to pure RaSO₄. ²²⁶Ra concentrations in the brines, which ranged from 10^?11.3 to 10^?12.7 m, are not controlled by RaSO₄ solubility or adsorption, but possibly by the solubility of trace Ra solid solutions in sulfates including celestite and barite.     

Land subsidence along the northeastern Texas Gulf coast: Effects of deep hydrocarbon production

The Texas Gulf of Mexico coast is experiencing high (5–11 mm/yr) rates of relative sea level (RSL) rise that are the sum of subsidence and eustatic sea level (ESL) rise. Even higher rates are associated with areas of groundwater pumping from confined aquifers. We investigate the possibility of deep petroleum production as a cause for the high regional rates of subsidence. The northeast Texas coast was chosen for the study because it has a high rate of RSL rise, very limited groundwater production, and a long history of petroleum production. We examine in detail the Big Hill and Fannett fields, for which adequate bottom hole pressure (BHP) and well log data are available. The hypothesis of deep petroleum production is tested in three ways. First, industry BHP tests show many of the fields are depressurized to far below hydrostatic pressures. Second, analysis of BHP data over time in the Big Hill and Fannett fields indicates that some Zones in these fields were below hydrostatic when production commenced. This indicates that depressurization from production in neighboring fields or zones within the same field is not limited to the production zone. Third, three models for subsidence (a general 1-D regional model, an intrareservoir model, and a reservoir bounding layer model), using reasonable hydrogeological parameters, predict subsidence within the inferred range of data. The latter two models use data from the Big Hill and Fannett fields. Additional verification of the hypothesis that deep petroleum production is causing or accelerating regional subsidence will require the collection and analysis of data on the subsurface hydrogeological parameters and detailed measurements of the spatial and temporal distribution of subsidence along the Texas Coast.     

四川盆地某地富矿卤水水文地球化学特征及其成因资源意义

四川盆地某地富矿卤水以深层卤水形式赋存于地下４０００余米深的中三叠统雷口权组四段（Ｔ＿２ｌ￣４）盐系的碳酸盐岩储层中。富矿卤水与海水各浓缩阶段相比，其中Ｋ￣＋含量异常高，构成世界罕见的液态钾盐资源；Ｂｒ￣－、Ｉ￣－、Ｂ￣（３＋）、Ｌｉ￣＋等有用组分也远远超过综合利用工业品位，为优质化工原料水。富矿卤水为沉积变质和钾盐溶滤的复合成因，具有资源及固液态钾盐找矿的指示意义。本文为四川某地固液态找钾提供了有价值的线索。     

Evolution of isotopic composition and deuterium excess of brines in the Sichuan Basin, China

The isotopic composition and parameters for deuterium excess of brines, which were sampled in the Si-chuan Basin, show obvious regularities of distribution. The brine isotopic composition shows distinct two systems of marine and terrestrial deposits, with the Middle Triassic strata as the boundary. Brine hydrogen isotopic composition of marine deposits is lower while oxygen isotopic composition is higher than that of the SMOW, respectively, indicating that the brines were derived from seawater with different evaporating degrees at different times. From the Sinian strata, up to the Cambrian, Permian Maokou Formation and the Triassic Jialingjiang Formation, the δD values of brines tend to become relatively positive with the strata becoming younger. Brines of terrestrial deposits are considered to have been derived from precipitation and their isotopic composition is close to the globe meteoric water line (GMWL). Brines of transitional deposits between marine and terrestrial ones (the Upper Triassic Xujiahe Formation) have δD and δ18O values falling between the two end members of marine deposit brines and precipitation, indicating that the brines are a mixture of precipitation and vaporing seawater. Water samples from the brine-bearing strata of different ages show various deuterium excesses (d) with an evident decreasing trend as the age of strata gets older and older. Brine-bearing strata of the Triassic Leikoupo-Jialingjiang Formation, the Permian Maokou Formation, the Cambrian and Sinian strata are all carbonate rocks which have experienced intensive water/rock reaction and the deuterium excess essentially changes with time. All brine-bearing-strata surrounding the basin or faults, as well as those brine wells exploited for resources, have been obviously influenced by the precipitation supply. Therefore, the deuterium excesses of their brines have increased to different extents, depending on the amount of involvement of meteoric water. The variation and distribution of d values of the brines from different Triassic strata are related to the embedded depth of the strata. The deuterium excesses of brines become lower with increasing burial depth of the strata.     

Diagenesis in tertiary sandstones from Kettleman North Dome. California—II. Interstitial solutions: Distribution of aqueous species at 100°C and chemical relation to the diagenetic mineralogy

Interstitial brines from the Temblor and the McAdams sandstones at Kettleman are essentially NaCaCl solutions with subsidiary SO₄ and the total salinities are roughly 30,000 and 10,000 ppm, respectively. Activities of H⁺ and all other aqueous species have been calculated for 100°C (the in situ temperatures of the brines) from chemical analyses of the brines and 100-degree dissociation constants alone. The brine alkalinities measured at surface temperature appear to be too low when comparing them against alkalinities calculated from the measured pHs of the brines. Consequently, alkalinities calculated for 25°C were substituted for the measured ones in the calculation of the distribution of aqueous species at 100°C.Although the brines are nearly neutral (pH 6·3–d7·9) at surface temperature, their pHs calculated for 100°C range from 8·1 to 8·7 (± 0·35). These pHs and the 100-degree activities of the other aqueous species permit graphic representation of the brines on activity diagrams. Most brines fall at or near the boundaries between the stability fields of quartz, albite, microcline, mica, montmorillonite and anhydrite. Because these minerals are present as authigenic phases in the sandstones, the calculations suggest that the minerals are in stable equilibrium with the brines. By contrast, the calculations suggest that the brines are supersaturated by about three orders of magnitude with respect to calcite, also present in the sandstones. One possible explanation for this is kinetic inhibition of calcite crystallization by Mg²⁺ and SO₄^2? ions in the brines. Phosphatic pellets, glauconite and probably dolomite, pyrite and some kaolinite are early authigenic minerals preserved in the sandstones and they are not now in equilibrium with the brines, which are supersaturated with respect to dolomite and pyrite. The chemical relationship between the brines and the diagenetic minerals laumontite and sphene, also present in the Temblor Formation, cannot be assessed reliably until the thermodynamic properties of laumontite and of aqueous titanium complexes are well known.     

A strontium, oxygen and hydrogen isotopic composition of brines, Michigan and appalachian basins, Ontario and Michigan

⁸⁷Sr/⁸⁶Sr ratios of brine from samples from the Michigan and Appalachian Basins, in Ontario and Michigan, covering the stratigraphic interval from the Cambrian to Mississippian, vary from 0.708 to 0.711. With the exception of the salt units of the Salina Formation (Silurian), most values are greater than seawater for the time in question, indicating water-rock interaction. The sources of the radiogenic Sr has not been identified. All samples plot below the GMWL in δ¹⁸O−δ²H space, with the Cambrian and Ordovician samples closest to the line. Mixing of brines meteoric and glacial (Pleistocene) water is indicated in some cases. The more concentrated brines from each stratigraphic unit show a very narrow spread in values. All the Ordovician brines show a narrow range over a 200 km area for Sr, O and H isotopes, indicating extensive lateral migration of the fluids.Strontium in the brine has not equilibrated isotopically with its host rock. In some cases the late-stage minerals saddle dolomite, calcite and anhydrite have the same ⁸⁷Sr/⁸⁶Sr ratios as the brine, indicating that they precipitated from the brine in isotopic equilibrium.     

The use of bromide and chloride mass ratios to differentiate salt-dissolution and formation brines in shallow groundwaters of the Western Canadian Sedimentary Basin

In the Western Canadian Sedimentary Basin, the petroleum industry handles two geochemically distinctive brines that are traceable in the environment: formation brines extracted along with hydrocarbons from the basin, and salt-dissolution brines, produced by dissolving deep halite formations to create caverns for petroleum product storage. The concentrations of the conservative ions chloride (Cl) and bromide (Br) in many formation brines plot closely to the seawater evaporation trajectory of previous studies. These brines contain Cl/Br mass ratios of around 300, while salt-dissolution brines are relatively Br depleted, having Cl/Br mass ratios in excess of 20,000. An oilfield site in central Alberta had experienced nearby releases of both salt-dissolution and formation brines. Geochemical mixing trends were defined by theoretically mixing samples of local salt-dissolution and formation brine sources with background shallow groundwater. Most site monitoring wells and local surface water samples plotted directly on a salt-dissolution brine dilution trend, while results from four monitoring wells, all located directly downgradient of formation brine spills, suggested the mixing of formation brines into shallow groundwater. This work indicates that there is a large-scale salt-dissolution brine plume beneath the site and reinforces the use of Cl and Br concentrations and mass ratios as environmental tracers.     

Major,minor element chemistry and oxygen and hydrogen isotopic compositions of Marun oil‐field brines,SW Iran: Source history and economic potential

Cation and anion concentrations and oxygen and hydrogen isotopic ratios of brines in the Asmari Formation (Oligocene–early Miocene) from the Marun oil field of southwest Iran were measured to identify the origin of these brines (e.g. salt dissolution vs. seawater evaporation) as well as the involvement of water–rock reaction processes in their evolution. Marun brines are characterized by having higher concentrations of calcium (11 000–20 000 mg/L), chlorine (120 000–160 000 mg/L) and bromide (600–1000 mg/L) compared to modern seawater. Samples are also enriched in ¹⁸O relative to seawater, fall to the right of the Global Meteoric Water Line and local rain water, and plot close to the halite brine trajectory on the δD versus δ¹⁸O diagram. Geochemical characteristics of Marun brines are inconsistent with a meteoric origin, but instead correspond to residual evaporated seawater modified by water–rock interaction, most significantly dolomitization. In addition, anhydrite precipitation or sulphate reduction appears to be important in chemical modification of the Marun brines, as indicated by lower sulphate contents relative to evaporated seawater. Extensive dolomitization, the presence of anhydrite nodules and high salinity fluid inclusions in the upper parts of the Asmari Formation fit a model whereby the Marun brines likely originated from the seepage reflux of concentrated seawater during the deposition of the overlying Gachsaran Formation evaporites in the Miocene. Copyright © 2010 John Wiley & Sons, Ltd.     

Facies change in upper devonian and lower carboniferous rocks of Southern Ireland

In south-west Ireland 8,000 ft (2,440 m) of marine sandstones and mudstones, the Cork Beds (?Upper Famennian to E Zone, Carboniferous, in age), overlie the Old Red Sandstone. Farther north the Old Red Sandstone is succeeded by thin Lower Limestone Shales overlain by thick Waulsortian bank limestones. A critical section (North Ringabella) west of Cork Harbour, in which 6,500 ft (1,981 m) of Old Red Sandstone and Cork Beds is exposed, is described and divided into ten formations. By comparison with sections to the south the upper beds of the Old Red Sandstone are shown to pass southward into marine sandstones (Cork Beds) of ?Upper Famennian age. The successions in the Cloyne and Cork Synclines are described and reveal the progressive northward change in the Lower Carboniferous from the argillaceous Cork Facies through a zone of isolated bank limestones (Cloyne) to a thick, 4,000 ft (1,219 m), Waulsortian bank complex (Cork). Finally an attempt is made with use of isopachyte maps to reconstruct the palaeogeography of southern Ireland in Upper Devonian and Lower Carboniferous times.     

Fingerprinting groundwater salinity sources in the Gulf Coast Aquifer System,USA

Understanding groundwater salinity sources in the Gulf Coast Aquifer System (GCAS) is a critical issue due to depletion of fresh groundwater and concerns for potential seawater intrusion. The study objective was to assess sources of groundwater salinity in the GCAS using ～1,400 chemical analyses and ～90 isotopic analyses along nine well transects in the Texas Gulf Coast, USA. Salinity increases from northeast (median total dissolved solids (TDS) 340 mg/L) to southwest (median TDS 1,160 mg/L), which inversely correlates with the precipitation distribution pattern (1,370– 600 mm/yr, respectively). Molar Cl/Br ratios (median 540–600), depleted δ²H and δ¹⁸O (?24.7‰, ?4.5‰) relative to seawater (Cl/Br ～655 and δ²H, δ¹⁸O 0‰, 0‰, respectively), and elevated ³⁶Cl/Cl ratios (～100), suggest precipitation enriched with marine aerosols as the dominant salinity source. Mass balance estimates suggest that marine aerosols could adequately explain salt loading over the large expanse of the GCAS. Evapotranspiration enrichment to the southwest is supported by elevated chloride concentrations in soil profiles and higher δ¹⁸O. Secondary salinity sources include dissolution of salt domes or upwelling brines from geopressured zones along growth faults, mainly near the coast in the northeast. The regional extent and large quantities of brackish water have the potential to support moderate-sized desalination plants in this location. These results have important implications for groundwater management, suggesting a current lack of regional seawater intrusion and a suitable source of relatively low TDS water for desalination.     

Potential environmental issues of CO2 storage in deep saline aquifers: Geochemical results from the Frio-I Brine Pilot test,Texas, USA

Sedimentary basins in general, and deep saline aquifers in particular, are being investigated as possible repositories for large volumes of anthropogenic CO₂ that must be sequestered to mitigate global warming and related climate changes. To investigate the potential for the long-term storage of CO₂ in such aquifers, 1600 t of CO₂ were injected at 1500 m depth into a 24-m-thick “C” sandstone unit of the Frio Formation, a regional aquifer in the US Gulf Coast. Fluid samples obtained before CO₂ injection from the injection well and an observation well 30 m updip showed a Na–Ca–Cl type brine with ∼93,000 mg/L TDS at saturation with CH₄ at reservoir conditions; gas analyses showed that CH₄ comprised ∼95% of dissolved gas, but CO₂ was low at 0.3%. Following CO₂ breakthrough, 51 h after injection, samples showed sharp drops in pH (6.5–5.7), pronounced increases in alkalinity (100–3000 mg/L as HCO₃) and in Fe (30–1100 mg/L), a slug of very high DOC values, and significant shifts in the isotopic compositions of H₂O, DIC, and CH₄. These data, coupled with geochemical modeling, indicate corrosion of pipe and well casing as well as rapid dissolution of minerals, especially calcite and iron oxyhydroxides, both caused by lowered pH (initially ∼3.0 at subsurface conditions) of the brine in contact with supercritical CO₂.     

Dissolution kinetics of Devonian carbonates at circum-neutral pH, 50 bar pCO2, 105 °C,and 0.4 M: The importance of complex brine chemistry on reaction rates

The dissolution kinetics of carbonate rocks sampled from the Keg River Formation in Northeast British Columbia were measured at 50 bar pCO₂ and 105 °C, in both natural and synthetic brines of 0.4 M ionic strength. Natural brines yielded reaction rates of −12.16 ± 0.11 mol cm⁻² s⁻¹ for Log R_Ca, and −12.64 ± 0.05 for Log R_Mg. Synthetic brine yielded faster rates of reaction than natural brines. Experiments performed on synthetic brines, spiked with 10 mmol of either Sr or Zn, suggest that enhanced reaction rates observed in synthetic brines are due to a lack of trace ion interaction with mineral surfaces. Results were interpreted within the surface complexation model framework, allowing for the discrimination of reactive surface sites, most importantly the hydration of the >MgOH surface site. Dissolution rates extrapolated from experiments predict that CO₂ injected into the Keg River Formation will dissolve a very minor portion of rock in contact with affected formation waters.     

Arsenic enrichment in unconfined sections of the southern Gulf Coast aquifer system, Texas

Groundwater arsenic concentrations exceeding the federal drinking water standard are common in the southern Gulf Coast aquifer system in Texas, including in aerobic, unconfined groundwater which provides much of the municipal and domestic water supplies for the region. The objective of this study was to determine geochemical factors affecting the occurrence and distribution of groundwater As in unconfined portions of the southern Gulf Coast aquifer system through a comparative transect study of groundwater across three major hydrostratigraphic units (the Catahoula Formation, Jasper aquifer and Evangeline aquifer) and analysis of regional water quality data. Results show that As concentrations decrease with increasing distance from the Catahoula Formation, which is consistent with Miocene volcanic ash as the main source of As to groundwater in the region. Arsenic concentrations correlate with V, SiO₂ and K, all of which were released during weathering of volcanic sediments and their degradation products. In all three units, carbonate weathering and active recharge in the unconfined zones result in circum-neutral pH and oxidizing groundwater, which are typically amenable to As immobilization by adsorption of arsenate onto mineral oxides and clays. However, As concentrations exceed 10 μg/L in approximately 30% of wells. Silica that was co-released with As may compete for sorption sites and reduce the capacity for arsenate adsorption.