Carbon-rich Shales Research Articles

Experimental investigation of acid imbibition on matrix and fractured carbonate rich shales

Matrix and fracture permeability of carbonate rich tight cores from Niobrara, Bakken, and Three Forks formations were measured before and after exposing the core samples to spontaneous imbibition using 10 wt % KCl and 1 wt % HCl acid. A hydraulic fracture was modeled by saw-cutting a given core sample. Sand proppant of 0.02 lbs/ft2 concentration and 30/80 mesh size with monolayer packing was used for the propped fracture cores. Permeability was measured at varying net stress of 1000 psia up to 6000 psia. Before and after dilute HCl acid imbibition matrix and fracture permeability vs. stress and rock hardness measurements were investigated. Brine and dilute acid imbibition effect on proppant embedment, rock softening/weakening, and fracture roughness were assessed. Brine imbibition before and after dilute acid imbibition was also studied to investigate possible wettability alteration.After acid soaking/imbibition, up to twenty fold increase in matrix permeability was observed for carbonate rich Niobrara cores. A matrix permeability decrease was observed in clay rich Three Forks cores. On the other hand, permeability decrease after dilute HCl acid imbibition was observed on both propped and un-propped fractured Niobrara cores. Matrix permeability increase after acid imbibition is attributed to the enhancement of pore throat connectivity and nano- and micro-fracture etching. Permeability decrease for the un-propped fractured Niobrara cores is attributed to fracture surface softening leading to decrease in fracture roughness and fracture aperture at in-situ stress conditions. And, permeability decrease for the propped fracture cores is attributed to decrease in fracture width or aperture caused by proppant embedment as brine and acid weakens the fracture face fabric. It is concluded that dilute acid injection of pre and post hydraulic fracturing stimulation in ultra-tight carbonate rich mudrocks could improve the overall deliverability of stimulated reservoir volume. This is because, despite decrease in fracture permeability after acid imbibition, matrix permeability increase is more significant for the overall deliverability of ultra-tight carbonate rich mudrocks. Moreover, favorable wettability alteration can be achieved by dilute acid imbibition.

Hydrothermal vent complexes offshore Northeast Greenland: A potential role in driving the PETM

Continental rifting is often associated with voluminous magmatism and perturbations in the Earth's climate. In this study, we use 2D seismic data from the northeast Greenland margin to document two Paleogene-aged sill complexes ≥18000 and ≥10000 km2 in size. Intrusion of the sills resulted in the contact metamorphism of carbon-rich shales, producing thermogenic methane which was released via 52 newly discovered hydrothermal vent complexes, some of which reach up to 11 km in diameter. Mass balance calculations indicate that the volume of methane produced by these intrusive complexes is comparable to that required to have caused the negative δ13C isotope excursion associated with the PETM. Combined with data from the conjugate Norwegian margin, our study provides evidence for margin-scale, volcanically-induced greenhouse gas release during the late Paleocene/early Eocene. Given the abundance of similar-aged sill complexes in Upper Paleozoic–Mesozoic and Cretaceous–Tertiary basins elsewhere along the northeast Atlantic continental margin, our findings support a major role for volcanism in driving global climate change.

Isotope and elemental geochemistry of black shale‐hosted fossiliferous concretions from the Cretaceous Santana Formation fossil Lagerstätte (Brazil)

AbstractCarbonate concretions hosted within organic carbon‐rich shale sequences represent unique archives of often exceptionally preserved fossil biota. Besides providing high‐fidelity preservation, their geochemical signatures can provide insight into the physical and chemical processes during early and later‐stage concretion growth. Here, two fossiliferous carbonate concretions of the late Early Cretaceous Santana Formation (Araripe Basin, north‐east Brazil) are analysed with an integrative geochemical approach including μ‐XRF scanning, δ13C, δ18O, 87Sr/86Sr and Δ47 (clumped isotope thermometry). Individual concretions show a concentric internal zonation with the outermost layer being composed of millimetre thick cone‐in‐cone calcite. A strong covariance of δ13C and δ18O values of the fine‐crystalline concretion body indicates mixing of two different carbonate phases and supports a scenario of temporally separated pervasive growth stages. Microbially‐mediated formation of an early porous calcite framework was controlled by the combined processes of fermentation and methanogenesis around the decaying carcass, forming localized environments within a zone of sulphate reduction. Microbial sulphate reduction is indicated by the concentric enrichment of pyrite in the outer part of the concretion body and by high pyrite abundance in the surrounding shale. Information on the later‐stage diagenetic processes affecting the Santana concretions can be derived from the outermost fringing cone‐in‐cone calcite. The carbonate precipitating fluid was characterized by a more or less marine δ18O composition (calculated δ18Oporewater = −1·0 to −1·8‰) and by radiogenic Sr‐isotope ratios (up to 0·713331 ± 7·0*10−6), the latter probably reflecting modification due to interaction with the surrounding shale or, alternatively, with underlying evaporitic sulphate deposits influenced by strong continental inflow or with crystalline basement rocks. The Δ47‐derived temperature estimates range between 37°C and 42°C ± 5, indicating precipitation of the cone‐in‐cone calcite at a depth of 650 to 850 m, which is only half as much as the maximum burial depth derived from existing fission‐track data. Overall, the study of fossiliferous carbonate concretions in organic carbon‐rich sedimentary sequences can reveal a complex growth history spanning incipient microbially‐influenced precipitates as well as later‐stage burial diagenetic phases.

A simple method for quantitative analysis of elements by WD-XRF using variable dilution factors in fusion bead technique for geologic specimens

A common approach in the quantitative analysis of geological samples by X-ray fluorescence is to establish calibration lines for elements of interest by using several reference materials (RMs) and/or the combination of RMs and pure chemicals. Herein, we introduce an alternative to use only two RMs, to establish a calibration application. Variation of the dilution factor is employed to generate a dynamic range of concentrations for each RM and to evenly furnish the calibration lines to analyze certain matrices. A wide range of dilution factors were employed from 2–54 times dilution (with respect to the flux to sample ratios). Calibration lines for the major elements including: Si, Al, Ca, Fe, Mg, Na, Mn, and Ti show an extremely high level of linearity with all elements. R2 values greater than 0.9990 were obtained for each analyzed element. The calibration application was validated by checking against a variety of geological RMs including petroleum and carbonate rich shale (SGR-1), Muscovite rich marine shale (SBC-1), metamorphic rock (SDC-1), carbonatite (COQ-1), and types of igneous rocks (GSP-2, BCR-2, AGV-2, QLO-1, and W-2). Mixtures of Alumina and Silica (ARG-1 and ARG-2) and pure SiO2 beads were also analyzed to further check the application. Rigorous statistical analysis on the RMs confirms the reliability of the calibration application for the employed matrices. Copyright © 2016 John Wiley & Sons, Ltd.

Cu isotopes in marine black shales record the Great Oxidation Event

The oxygenation of the atmosphere ∼2.45-2.32 billion years ago (Ga) is one of the most significant geological events to have affected Earth's redox history. Our understanding of the timing and processes surrounding this key transition is largely dependent on the development of redox-sensitive proxies, many of which remain unexplored. Here we report a shift from negative to positive copper isotopic compositions (δ(65)CuERM-AE633) in organic carbon-rich shales spanning the period 2.66-2.08 Ga. We suggest that, before 2.3 Ga, a muted oxidative supply of weathering-derived copper enriched in (65)Cu, along with the preferential removal of (65)Cu by iron oxides, left seawater and marine biomass depleted in (65)Cu but enriched in (63)Cu. As banded iron formation deposition waned and continentally sourced Cu became more important, biomass sampled a dissolved Cu reservoir that was progressively less fractionated relative to the continental pool. This evolution toward heavy δ(65)Cu values coincides with a shift to negative sedimentary δ(56)Fe values and increased marine sulfate after the Great Oxidation Event (GOE), and is traceable through Phanerozoic shales to modern marine settings, where marine dissolved and sedimentary δ(65)Cu values are universally positive. Our finding of an important shift in sedimentary Cu isotope compositions across the GOE provides new insights into the Precambrian marine cycling of this critical micronutrient, and demonstrates the proxy potential for sedimentary Cu isotope compositions in the study of biogeochemical cycles and oceanic redox balance in the past.

Multiproxy constraints on alteration and primary compositions of Ediacaran deep-water carbonate rocks, Yangtze Platform, South China

The occurrence of shallow and deep-water sedimentary facies has established the Yangtze Platform in South China as a key site for the study of Neoproterozoic ocean oxidation and Ediacaran animal evolution following the Marinoan glaciation. The Yanwutan section in Hunan Province is one of the few coherent sections on the Yangtze Platform where Ediacaran deep-water carbonate sediments (predominantly dolostones) are preserved together with organic carbon-rich shales. Here we present new major and trace element abundance data as well as Sr-, O- and C-isotope compositions of leachates from carbonates of the Doushantuo Formation. We evaluate the role of diagenetic modification of the carbonate rocks and constrain the redox evolution of Ediacaran seawater in space and time. 87Sr/86Sr systematically varies with δ18Ocarb, Sr- and Ba abundances, indicating variable but mostly strong modification of fluid-mobile elements by continental basin fluids. In contrast, REE+Y patterns have preserved seawater-like compositions. Cap dolostones (unit I) on top of the Nantuo diamictites differ from cap dolostones at shallow-water sections on the Yangtze Platform in that they show no Ce-anomalies, and little alteration near the top (87Sr/86Sr=0.7078, δ18O=−4.0, δ13Ccarb=1.1), suggesting that δ13Ccarb and δ18O of cap dolostones at many other sections were compromised by hydrothermal alteration. The overlying organic carbon poor micritic dolostone (unit II) shows negative Ce-anomalies that disappear towards the top of the unit. No Ce-anomalies occur in subsequent organic carbon-rich muddy dolostone units (units III to IV). These observations, enrichments in TOC that correlate with variations in redox-sensitive metals in the carbonates, negative δ13Ccarb in units II to IV and the decoupling of δ13Ccarb from δ13Corg argue for the existence of mostly anoxic deep-water at the Yangtze passive continental margin during the Ediacaran. The negative Ce-anomalies at the base of unit II (with negative δ13Ccarb) may reflect fluctuations towards suboxic or oxic conditions or an allochthonous origin of this unit. However, trace metal enrichments in carbonates of the same unit argue for reducing conditions in pore-water, whereas the carbonates may have preserved the REE+Y signatures inherited from suboxic- to oxic seawater. The trace element and negative δ13Ccarb values in units II to IV are consistent with a stratified basin model with a large partially remineralised organic matter reservoir in anoxic bottom and pore-waters.

Heterogeneity of the Lower Silurian Longmaxi marine shale in the southeast Sichuan Basin of China

This study characterizes the vertical heterogeneity of the Lower Silurian Longmaxi marine shale in the southeast Sichuan Basin based on core description, thin sections observation, geological chemistry data interrogation, and QEMSCAN image analysis. Lithofacies associations, mineralogy, graptolite species occurrence, rock fabrics, as well as TOC (Total Organic Carbon) and gas content, change noticeably and regularly in the Lower Silurian Longmaxi marine shale within sequence stratigraphic framework. Lithofacies associations mainly include in-situ graptolite rich and siliceous shale in the transgressive systems tract (TST), micro-offsite graptolite rich shale and shale interbedded with siltstone laminaes in the early highstand systems tract (EHST), and micro-offsite graptolite rich and carbonate rich shale in the late highstand systems tract (LHST). The TST has a greater abundance of Diplogratus than EHST and LHST, and has a lesser level of Monograptus, which is conversely higher in the EHST and LHST. The TST shale is silica rich, while the LHST shale is generally clay rich. The EHST shale is more balanced in its silica and clay content than the TST and LHST are. The sedimentary structure and fabric of shale shows that shale is homogenous in the TST, with laminaes well developed in the EHST but decreasing in the LHST. TOC and gas content are highest in the TST and decrease upward. The TST, with high TOC, high gas content, high brittle minerals (silica rich) and anoxic-suboxic to dysoxic sedimentary environment, is the most potential shale gas target interval in the study area. Our research also shows that the TOC mainly controls shale gas accumulation and that the redox conditions mainly control the preservation of organic matter. The silica also plays a significant role in shale gas content due to its contribution to hydraulic fracturing. Finally, the study proposes a depositional and heterogeneity model for the Lower Silurian Longmaxi marine shale in the study area.

Authigenic dolomites in the Eocene–Oligocene organic carbon-rich shales from the Polish Outer Carpathians: Evidence of past gas production and possible gas hydrate formation in the Silesian basin

Eocene–Oligocene dolomite concretions and beds from the Grybów and Dukla units of the Polish Outer Carpathians were studied. These rocks occur in the organic carbon-rich, marine and fine-grained deposits of hemipelagic or turbiditic origin. Mineralogic, elemental and stable C and O isotopic composition of the dolomites was determined. Results indicate that the rocks were formed by precipitation of predominantly Fe-rich dolomite cement close to the sediment-water interface prior to significant compaction. The main source of bicarbonate for dolomite formation was bacterial methanogenesis as evidenced by the high δ13C values up to 16.6‰. The main source of alkalinity was probably weathering of silicate minerals which might have also liberated Ca and Mg ions for the dolomites to form. The distribution of these dolomites indicates that microbial methane production was widespread in the Silesian basin. Moreover, formation of some dolomites in the Eastern part of the Dukla unit was probably associated with gas hydrates as suggested by the elemental and oxygen isotopic composition of dolomitic matrix. Therefore, the dolomites may serve as a proxy of areas where biogenic methane was produced, where the rocks had high hydrocarbon potential, and where hydrates could have existed.Detailed mineralogic and petrographic analyses allowed for the reconstruction of the diagenetic sequence and the evolution of pore fluids. Textural relationships between successive cement generations indicate that the central parts of the composite dolomite crystals experienced corrosion and that the latest ankerite cement filled the secondary intragranular cavities within those crystals. This observation shows that reconstructions of pore fluid evolution based on core-to-rim analyses of such composite crystals may lead to wrong interpretations. Septarian cracks developed in the dolomites are often filled with multistage cements. The earliest generations are ferroan dolomite and ankerite cements which precipitated within the cracks simultaneously to the ferroan dolomite and ankerite cements from the matrix of the dolomitic rocks which shows that septarian cracking occurred very early, during the final stages of concretionary formation. These cements were followed by the late-diagenetic precipitates, mainly quartz, kaolinite and blocky calcite. This calcite is commonly associated with bitumen which shows that it precipitated during or after oil migration in the decarboxylation zone.

Redox Conditions and Metal-Organic Carbon Relations of Eocene Bituminous Shales (Veliler/Mengen-Bolu/Turkey)

The proposed bituminous shale sequences in the Eocene sediments are located at the vicinity of Veliler (Mengen/Bolu) in northwest Turkey. The total section is 11.87 m thick, consisting mainly of organic carbon-rich marl and shale, which is 8.17 m thick. Organic-carbon rich sediments are enriched in several redox-sensitive and stable sulfide-forming trace elements. Bituminous shales were geochemically analyzed in order to find total organic carbon (1.07–15.17 wt%), major elements (Si, Al, Fe, Mg, Mn, Ca, Ti, Na, K, P), and trace elements (V, Ni, Co, Cr, Cu, and Zn) of the whole rock. The results are discussed based on the organic and inorganic association of trace elements and their use as redox conditions of sedimentation. Whole rock concentrations are indicative of changes in the dysoxic sedimentation conditions according to V/V+Ni ratio (0.46) and authigenic U (10.19) to suboxic-anoxic conditions according to U/Th ratio (6.46). The most important sediment sources of the trace elements are authigenic (Mo, Ni, Co, As, U, Sr, Cr, and Mg) and detrital (Ba, V, Pb, Fe, Cu, Si, and Ti) content. Enrichment factors of Corg, Mo, Cu, Pb, Zn, Ni, Co, Mn, Fe, As, U, Th, Sr, V, P, Cr, Mg, Ba, Na, and K are calculated with recpect to their abundance of average shale, black shale, recent and ancient upwelling sediments, and sapropels, such as Mediterranean sapropels, Monterey Formation, Namibian mud lens, Gulf of California and Peru margin.

REE and their relevance to the development of the Kupferschiefer copper deposit in Poland

Over one hundred samples, representing mainly clayey-organic- and carbonate-rich shales (Kupferschiefer) but also other members of different ore sections, including hangingwall dolomites (Z1 Werra) and footwall Weissliegend sandstone (Lower Permian), have been collected in different mines of the Lubin–Głogów mining district, mainly near the contact (transitional zone) between the copper-mineralized zone and secondarily oxidized (Rote Fäule=RF) zone. In general, the Polish Kupferschiefer shales are enriched in MREE in comparison to NASC. In a typical copper-rich ore section the REE amounts and patterns depend on lithologies, being generally similar in shales and dolomite. ∑REE vary among sandstones, shales and dolomites (average 73, 143 and 85ppm, respectively), probably reflecting mainly their clay contents. Sandstones have strongly convex REE patterns with positive Eu and negative Gd anomalies and depletion in LREE and enrichment in MREE relative to HREE. The REE patterns of shale and dolomite are similar to one another and rather flat, with strong negative Eu anomalies, and a small positive Gd anomaly in the case of shales.The REE patterns of shales from the mineralized Cu-zone are generally convex (MREE enriched) and have negative Eu anomalies. However, in a section with Cu-, Zn- and Pb-shales the REE pattern of Pb-bearing shales shows a positive Eu anomaly, in contrast to other shales and overlying dolomite. More oxidizing conditions of deposition can be assumed for Pb-shales.No significant differences between REE distributions in transitional and oxidized zones have been observed. Their REE patterns are more convex and are much higher (av. 247ppm) than those in the mineralized zone and they do not show Eu anomalies. The strongly convex pattern may suggest either enrichment in MREE or relative depletion in LREE due to localized precipitation of light REE minerals, both in shales and in the uppermost part of the sandstones.Two unique sections, one Cu-rich and one Cu-lean (partly oxidized), comprising three shale beds interbedded with dolomites have been compared. Generally ∑REE contents are similar in these two sections. Also similar are changes in contents of REE between beds in both sections, which decrease significantly upwards (from 157–171ppm to 54–60ppm). The REE patterns of the lowermost beds (directly overlying sandstones) are ramp-like, with LREE enrichments. The upper beds have concave REE patterns. Comparison between sections shows generally stronger negative Eu and positive Gd anomalies in the highly-mineralized section.There is a highly significant positive relationship between Cu and ∑REE contents in Cu-rich shales and slightly less significant negative relationship for their concentration in oxidized and transitional shales. There is a moderate significant positive correlation between P2O5 and ∑REE contents in Cu-rich shales.The observed differences in REE contents cannot be provenance dependent but have been caused by diagenetic processes, possibly related to mineralization and oxidation processes. Europium anomalies, generally reflecting different Eh conditions in the deposit, can be eliminated by the prolonged oxidation. Strong enrichment of the RF zones in REE may result from their desorption from large volumes of oxidizing, including mineralizing, solutions which probably emerged from the underlying molasse lithologies into the Rote Fäule areas. Higher contents of REE in the lowermost shales suggest upward movement of solutions from the underlying sandstones also far away from the RF areas.

Methods and key parameters for shale gas resource evaluation

Methods and key parameters of shale gas resources evaluation are studied so as to accurately assess the potential of shale gas resources. In much-explored areas, the volume method, followed by the analogy method, is recommended for evaluation; in less-explored areas, the analogy method is recommended. The concept of “gas-bearing shale system” is put forward and taken as an evaluation unit to calculate the thickness of shale. It refers to the lithologic assemblage sealed by upper and lower tight layers in one pressure system, composed of organic carbon-rich shale and the interbeds of sandstones and carbonates, featuring the logging responses of gas-bearing shale, with tight rocks at top and bottom, and the continuous thickness of gas-bearing interval being more than 30 m and no obvious water layer. According to exploration degree and available data, different methods may be selected to calculate the thickness of gas-bearing shale system. The logging data and the content of organic carbon are used in the areas with high degree of exploration and abundant data, while field geological profile, seismic profile, depositional characteristics and other data are considered in areas with low or ultra-low degree of exploration.

A model for uranium mobility in groundwater in the Grand Pré region, Nova Scotia, Canada

Elevated levels of uranium that surpass maximum allowable concentrations have been recognized in some well water samples throughout Nova Scotia. In this study we examine the influence of redox conditions and regional geology on the mobility of uranium in groundwater in the Grand Pré region, located in the eastern end of the Annapolis-Cornwallis Valley in southwestern Nova Scotia. The study site is underlain by carbon-rich shale and arkosic sandstone of the Late Mississippian Horton Bluff Formation and the arenaceous sandstone of the Triassic Wolfville Formation. Water samples from drilled wells were analyzed on site for various water quality parameters (pH, conductivity, temperature, dissolved oxygen) and in the laboratory to determine alkalinity and cation (72) and major anion (9) concentrations.
 
 Several samples exceeded Health Canada (2011) limits for uranium (20 μg/L) with values up to 50.8 μg/L and aesthetic objectives (AO) for iron (up to 605 μg/L), manganese (up to 2.28 mg/L) and other cations. Data show that different redox conditions occur in the study area. The geospatial arrangement of the chemical data suggests that the lithology of the underlying aquifer strongly influences the availability of uranium in groundwater. Reduced groundwater in Horton Bluff sandstone typically contains elevated concentrations of Fe 2+ and Mn 2+ , whereas groundwater containing high concentrations of U is found primary in oxidizing environments in quartz arenite sandstone units in the Wolfville Formation.
 
 Consumption of groundwater in the Grand Pré region may pose a health risk; however, the level of risk remains unknown. This study emphasizes that in the area of Grand Pré, water that is considered aesthetically undesirable (containing high Fe and Mn) may pose less of a health risk (low U) than waters that are aesthetically pleasing.

Caprock interaction with CO2: A laboratory study of reactivity of shale with supercritical CO2 and brine

Abstract Crushed rock from two caprock samples, a carbonate-rich shale and a clay-rich shale, were reacted with a mixture of brine and supercritical CO 2 (CO 2 –brine) in a laboratory batch reactor, at different temperature and pressure conditions. The samples were cored from a proposed underground CO 2 storage site near the town of Longyearbyen in Svalbard. The reacting fluid was a mixture of 1 M NaCl solution and CO 2 (110 bar) and the water/rock ratio was 20:1. Carbon dioxide was injected into the reactors after the solution had been bubbled with N 2 , in order to mimic O 2 -depleted natural storage conditions. A control reaction was also run on the clay-rich shale sample, where the crushed rock was reacted with brine (CO 2 -free brine) at the same experimental conditions. A total of 8 batch reaction experiments were run at temperatures ranging from 80 to 250 °C and total pressures of 110 bar (∼40 bar for the control experiment). The experiments lasted 1–5 weeks. Fluid analysis showed that the aqueous concentration of major elements (i.e. Ca, Mg, Fe, K, Al) and SiO 2 increased in all experiments. Release rates of Fe and SiO 2 were more pronounced in solutions reacted with CO 2 –brine as compared to those reacted with CO 2 -free brine. For samples reacted with the CO 2 –brine, lower temperature reactions (80 °C) released much more Fe and SiO 2 than higher temperature reactions (150–250 °C). Analysis by SEM and XRD of reacted solids also revealed changes in mineralogical compositions. The carbonate-rich shale was more reactive at 250 °C, as revealed by the dissolution of plagioclase and clay minerals (illite and chlorite), dissolution and re-precipitation of carbonates, and the formation of smectite. Carbon dioxide was also permanently sequestered as calcite in the same sample. The clay-rich shale reacted with CO 2 –brine did not show major mineralogical alteration. However, a significant amount of analcime was formed in the clay-rich shale reacted with CO 2 -free brine; while no trace of analcime was observed in either of the samples reacted with CO 2 –brine.

Drilling of Early Cretaceous Oceanic Anoxic Event 1a in Southern France

No abstract available. doi:10.2204/iodp.sd.9.03.2010

Petrology of the Paleocene Picrites and Flood Basalts on Disko and Nuussuaq, West Greenland

The c. 62^60 Ma rift-related Paleocene volcanic succession in West Greenland comprises voluminous picrites (the Vaigat Formation) overlain by flood basalts (the Maliga“ t Formation). A detailed stratigraphy for the c. 3 km combined succession on Disko and Nuussuaq has been established that provides evidence of the evolution of the volcanic systems, processes and sources through time. Picrites constitute about one-third of the total erupted volume. The primary magmas for the whole succession were highly magnesian; the weighted average of 16� 6 wt % MgO for the uncontaminated Vaigat Formation is a lower limit for the primary magmas. During the emplacement of the Vaigat Formation, the magmas fractionated olivine in the feeder channels before eruption but generally did not stop in magma chambers. Only minor magma batches stopped in high-level magma chambers, where many became crustally contaminated. At the transition to the Maliga“ t Formation magma chambers developed, presumably in the lower crust. From then on, the main eruption products were basalts with compositions buffered by refilling^fractionation^tapping processes. Large-scale cyclicity can be seen, with more and less fractionated steady-state compositions. The upper part of the Maliga“ t Formation is contaminated throughout, except for the three youngest flows. Repeated pulses of new picritic magmas can be seen in both formations. The asthenospheric mantle source was heterogeneous and the dominant component was depleted, mid-ocean ridge basalt (MORB)-like mantle with Nb/ La51. An additional, less depleted mantle component akin to the Iceland source is evident in the upper Vaigat Formation but disappeared again at the transition to the Maliga“ t Formation. A third mantle component similar to the source for the E-type lavas on Baffin Island may be present in the earliest rocks. Melting took place in the garnet peridotite facies because the melting column was curtailed by the 100 km thick lithosphere. The picrites show coupled isotopic and elemental variations even within single units, indicating that the accumulated primary melts were not homogenized when they left the melting column. An incompatible-element-rich lithospheric mantle component is seen in the ManaŒ tdlat Member of alkaline picrites and basalts. Single scattered tholeiitic lavas somewhat enriched in the same trace elements are considered to be slightly contaminated with this component.The volume of alkaline and enriched rocks is very small. The crustal contaminants are the sediments in the Nuussuaq Basin. The contamination processes included bulk assimilation, inmixing of partial melts of the sediments, assimilation^fractional crystallization processes, and exchange reactions. When carbon-rich shales were assimilated severe reduction led to formation of graphite or metallic iron. All rocks more evolved than basalt arose by contamination processes. The units of contaminated rocks constitute 5^9% of the Vaigat Formation and around 30% of the Maliga“ t Formation.

Geochemical environment of the Coniacian–Santonian western tropical Atlantic at Demerara Rise

Organic carbon-rich shales deposited during the Coniacian–Santonian Oceanic Anoxic Event 3 were drilled during ODP Leg 207 at Demerara Rise. We present integrated high-resolution geochemical records of core intervals from ODP Sites 1259 and 1261 both from nannofossil biozone CC14. Our results reveal systematic variations in marine and detrital sediment contribution, depositional processes, and bottom water redox conditions during black shale formation at two locations on Demerara Rise in different paleo-water depths. A combination of redox proxies (Fe/S, P/Al, C/P, redox-sensitive/sulfide-forming trace metals Mn, Cd, Mo, Ni, V, Zn) and other analytical approaches (bulk sediment composition, P speciation, electron microscopy, X-ray diffraction) evidence anoxic to sulfidic bottom water and sediment conditions throughout the deposition of black shale. These extreme redox conditions persisted and were periodically punctuated by short-termed periods with less reducing bottom waters irrespective of paleo-water depth. Sediment supply at both sites was generally dominated by marine material (carbonate, organic matter, opal) although relationships of detrital proxies as well as glauconitic horizons support some influence of turbidites, winnowing bottom currents and/or variable detritus sources, along with less reducing bottom water at the proposed shallower location (ODP Site 1259). At Site 1261, located at greater paleo-depth, redox fluctuations were more regular, and steady hemipelagic sedimentation sustained the development of mostly undisturbed lamination in the sedimentary record. Strong similarities of the studied deposits exist with the stratigraphic older Cenomanian–Turonian OAE2 black shale sections at Demerara Rise, suggesting that the primary mechanisms controlling continental supply and ocean redox state were time-invariant and kept the western equatorial Atlantic margin widely anoxic over millions of years.

Genesis of fibrous calcite and emerald by amagmatic processes in the southwestern Uinta Mountains, Utah

Large bodies of fibrous calcite, up to 400 m long and up to 20 m wide, are hosted by Mississippian carbonate units in the Uinta Mountains, Utah. Subjacent to the carbonate rocks, emerald mineralization has been recognized recently in the Neoproterozoic Red Pine Shale. Two types of fibrous calcites are recognized on the basis of appearance and geochemical characteristics. Fibrous orange calcite is translucent, similar to “Mexican calcite” commonly sold at gem-and-mineral shows, and is being mined and used in carvings and for other decorative purposes. Recently recognized fibrous brown calcite is also present in the same general vicinity. Calcite veins hosted with Mississippian carbonate rocks are located a few kilometers south of the South Flank fault zone (SFFZ) in the vicinity of a swarm of north-trending faults. Parts of the Red Pine Shale along the SFFZ contain abundant secondary pyrite. Altered shale also contains quartz and calcite veins as well as albitized feldspar in some arkosic units and is variably enriched in Cu and Zn. At least three emeralds have been discovered in altered Red Pine Shale. b 18 O VSMOW values of calcite (-10 to -13‰) would be in equilibrium with basin brines (assumed to be –3 to +5‰), suggesting precipitation in the range of 100°–300° C. Our work suggests that sulfate-bearing basinal brines from the Uinta Basin may have migrated upward along the SFFZ and interacted with the carbon-rich shale. As sulfate was reduced by organic carbon to form sulfides, emerald was produced and a CO 2 -rich fluid migrated upward and dissolved parts of the Mississippian carbonates. The fluid experienced intermittent loss of CO 2 gas, which allowed fibrous calcite with cone-in-cone textures to precipitate rapidly. This model and geologic setting have broad similarities to both Mississippi Valley and Colombian emerald deposits. The genesis of fibrous brown calcite, however, is less certain. These calcites lack critical geochemical characteristics that tie orange calcite to precipitation from hot brines; rather stable isotope data are permissive of origin from low-temperature ground water.

Numerical modeling of self-limiting and self-enhancing caprock alteration induced by CO 2 storage in a depleted gas reservoir

This paper presents numerical simulations of reactive transport which may be induced in the caprock of an on-shore depleted gas reservoir by the geological sequestration of carbon dioxide. The objective is to verify that CO 2 geological disposal activities currently being planned for the study area are safe and do not induce any undesired environmental impact. In our model, fluid flow, mass transport and mineral alteration are induced in the caprock by penetration of high CO 2 concentrations from the underlying reservoir, where it was assumed that large amounts of CO 2 have already been injected at depth. The main focus is on the potential effect of precipitation and dissolution processes on the sealing efficiency of caprock formations. Concerns that some leakage may occur in the investigated system arise because the seal is made up of potentially highly-reactive rocks, consisting of carbonate-rich shales (calcite + dolomite averaging up to more than 30% of solid volume fraction). Batch simulations and multi-dimensional 1D and 2D modeling have been used to investigate multicomponent geochemical processes. Numerical simulations account for multiphase advection, aqueous diffusion, fracture–matrix interactions (advective and diffusive exchange of species between fractures and matrix rock), gas phase participation in multiphase fluid flow and geochemical reactions, and kinetics of fluid–rock interactions. The sensitivity of CO 2 concentrations to geochemical processes and parameters is investigated by conceptualizing different mass transport mechanisms (i.e. diffusion and mixed advection + diffusion). The most relevant mineralogical transformations occurring in the caprock are described, and the feedback of these geochemical processes on physical properties such as porosity is examined to evaluate how the sealing capacity of the caprock could evolve in time. The simulations demonstrate that the occurrence of some gas leakage from the reservoir may have a strong influence on the geochemical evolution of the caprock. In fact, when a free CO 2-dominated phase migrates into the caprock through pre-existing fractures, or through zones with high initial porosity acting as preferential flow paths for reservoir fluids, low pH values are predicted, accompanied by significant calcite dissolution and porosity enhancement. In contrast, when fluid–rock interactions occur under fully liquid-saturated conditions and a diffusion-controlled regime, pH will be buffered at higher values, and some calcite precipitation is predicted which leads to further sealing of the storage reservoir.

Rapid heating of carbonaceous matter by igneous intrusions in carbon-rich shale, Isle of Skye, Scotland: an analogue for heating of carbon in impact craters?

The response of organic matter to high-temperature events is important to astrobiology, as it governs the survival of carbon during several processes that may be critical to the origin and spread of life. Impact cratering is a widespread high-temperature process. The behaviour of carbon during impact events is not well understood. But there is the potential to examine other examples of the response of organic matter to high-temperature processes in the terrestrial geological record. In this study, we report on the interaction of Tertiary igneous intrusions (dolerite sills) and carbon-rich Jurassic mudrocks on the Isle of Skye, Scotland. Despite the high temperatures of the igneous intrusion, carbon has been preserved at the dolerite–shale contact and in shale enclaves where partial melting of the shale has occurred. Even though the temperatures achieved by igneous intrusion are much lower than during impact events, it is a valuable analogue, because it represents a rapid introduction of high temperatures to a carbon-rich rock.

An analysis of horizontal microcracking during catagenesis: Example from the Catskill delta complex

Horizontal bitumen-filled microcracks are common within clay laminae of the finely laminated organic carbon-rich shale in the lower half of the heavily jointed Upper Devonian Dunkirk Shale, western New York state. Such cracks are not found higher in the Dunkirk Shale, where moderate bioturbation resulted in a relatively porous and permeable microfabric. Horizontal microcracks in a hydrocarbon source rock that carries regional vertical joints indicating a horizontal least principal stress owe their presence to material properties of the fractured shale and the magnitude and orientation of the crack-driving stress during kerogen maturation. Three material properties favored the horizontal initiation of microcracks in the Dunkirk Shale: (1) the abundance of flat kerogen grains oriented parallel to layering; (2) a marked strength anisotropy in large part caused by the laminated nature of the rock; and (3) the tight, strongly oriented planar clay-grain fabric produced by gravitational compaction of flocculated clay at shallow-burial depth. The latter was especially important to sustaining elevated pore pressure, the crack-driving stress, which was generated by the conversion of kerogen to bitumen. Poroelastic deformation of the low-permeability laminated shale pressurized by catagenesis, perhaps enhanced by compaction disequilibrium prior to kerogen conversion, elevated the in-situ horizontal stress in excess of the vertical stress, which remained constant during pore-pressure buildup, thereby favoring the propagation of microcracks in the horizontal plane.