Shale Research Articles

Gold in phosphorites: Accumulation forms and practical significance

Research subject. The gold-bearing potential of modern (oceanic) and ancient industrial types of phosphorite ores, including aphanitic, microgranular, granular, nodular, shell and karst. Aim. To assess the ability of phosphorites to accumulate gold to industrial concentrations. This issue has been actively discussed in the geological community in recent years due to the advent of high-precision methods for studying matter. Methods. Russian and foreign publications, unpublished geological data, as well as the authors’ data collected over a 30-year period of studying phosphorites. Results and discussion. In the section, 14 packs of rocks were identified, which are represented by 6 lithological types of secondary dolomites. Their distribution along the section, along with the presence of characteristic faunal remains, made it possible to establish three members in the section, which can be correlated with the Katian, Hirnantian, and Ruddanian stages of the General Stratigraphic Scale. Conclusion. Conditions for the possible joint accumulation of phosphates and gold are manifested in black shale basins, as well as in shelf shallow waters with glauconite-sand sediments. However, due to significant differences in the natural features of gold and phosphates, their industrial clusters are formed separately from each other. This is evidenced by the insignificant accumulation of phosphorus in the ores of gold deposits of black shale formations,as well as its low-quality concentrations in industrial phosphorite beds of phosphorite-bearing basins. For gold, phosphorites are only temporary collectors, the productivity of which depends on the combination of many, frequently random and slightly related factors.

The marine conservation deposits of Monte San Giorgio (Switzerland, Italy): the prototype of Triassic black shale Lagerstätten

Marine conservation deposits (‘Konservat-Lagerstätten’) are characterized by their mode of fossil preservation, faunal composition and sedimentary facies. Here, we review these characteristics with respect to the famous conservation deposit of the Besano Formation (formerly Grenzbitumenzone; including the Anisian–Ladinian boundary), and the successively younger fossil-bearing units Cava inferiore, Cava superiore, Cassina beds and the Kalkschieferzone of Monte San Giorgio (Switzerland and Italy). We compare these units to a selection of important black shale-type Lagerstätten of the global Phanerozoic plus the Ediacaran in order to detect commonalities in their facies, genesis, and fossil content using principal component and hierarchical cluster analyses. Further, we put the Monte San Giorgio type Fossillagerstätten into the context of other comparable Triassic deposits worldwide based on their fossil content. The results of the principal component and cluster analyses allow a subdivision of the 45 analysed Lagerstätten into four groups, for which we suggest the use of the corresponding pioneering localities: Burgess type for the early Palaeozoic black shales, Monte San Giorgio type for the Triassic black shales, Holzmaden type for the pyrite-rich black shales and Solnhofen type for platy limestones.

The Cambrian microfossil Qingjiangonema reveals the co-evolution of sulfate-reducing bacteria and the oxygenation of Earth’s surface

Sulfate reduction is an essential metabolism that maintains biogeochemical cycles in marine and terrestrial ecosystems. Sulfate reducers are exclusively prokaryotic, phylogenetically diverse, and may have evolved early in Earth’s history. However, their origin is elusive and unequivocal fossils are lacking. Here we report a new microfossil, Qingjiangonema cambria, from ∼518-million-year-old black shales that yield the Qingjiang biota. Qingjiangonema is a long filamentous form comprising hundreds of cells filled by equimorphic and equidimensional pyrite microcrystals with a light sulfur isotope composition. Multiple lines of evidence indicate Qingjiangonema was a sulfate-reducing bacterium that exhibits similar patterns of cell organization to filamentous forms within the phylum Desulfobacterota, including the sulfate-reducing Desulfonema and sulfide-oxidizing cable bacteria. Phylogenomic analyses confirm separate, independent origins of multicellularity in Desulfonema and in cable bacteria. Molecular clock analyses infer that the Desulfobacterota, which encompass a majority of sulfate-reducing taxa, diverged ∼2.41 billion years ago during the Paleoproterozoic Great Oxygenation Event, while cable bacteria diverged ∼0.56 billion years ago during or immediately after the Neoproterozoic Oxygenation Event. Taken together, we interpret Qingjiangonema as a multicellular sulfate-reducing microfossil and propose that cable bacteria evolved from a multicellular filamentous sulfate-reducing ancestor. We infer that the diversification of the Desulfobacterota and the origin of cable bacteria may have been responses to oxygenation events in Earth’s history.

Study of CO2 Injection into Sumatran Shale Layers to increase Hydrocarbon Gas Productivity of The Shale Gas Reservoir

Injecting CO2 into a reservoir has some important reasons. CO2 injection can enhance oil and gas recovery by reducing the capillary pressure, increasing the pressure gradient, and changing the phase behavior of the fluids. It can reduce greenhouse gas emissions by capturing and storing CO2 underground. It can also create economic benefits by utilizing CO2 as a valuable resource and generating revenue from carbon credits. Therefore, injecting CO2 into a reservoir benefits the environment and the industry. We can inject CO2 in shale gas reservoirs to increase productivity because CO2 has a stronger adsorption capacity on shale surfaces than hydrocarbon gas. When CO2 is injected into shale reservoirs, it can displace the adsorbed CH4 flow out of the micropores and free up more space for gas flow. Injecting CO2 can also reduce the viscosity and density of shale gas, improving its mobility and transport. Moreover, injecting CO2 can provide environmental benefits by reducing carbon emissions and storing CO2 underground. Therefore, CO2 injection is a promising technique for enhancing shale gas recovery and mitigating climate change. We characterized several types of shale from Sumatra using XRD to determine the mineral content. We injected the shale sample with the inert gas and CO2 gas. The characteristics of pressure build up after CO2 injection seem lower than one after inert gas injection. The volumetric of released gas after injection shows the same phenomena as pressure build up’s phenomena which shows clearly that shale rock released less of CO2 gas than the one of inert gas (CH4). These phenomena show that the CH4 can be released easier than the CO2 in the shale rock. Therefore, CO2 can be utilized as material for enhancing the gas recovery in shale reservoir.

Sedimentary paleoenvironment and its control on organic matter enrichment in the Mesoproterozoic hydrocarbon source rocks in the Ordos Basin, southern margin of the North China Craton

The black shale of the Mesoproterozoic Cuizhuang Formation in the Changcheng System in Yongji city, North China Craton, is a potential source rock. Understanding the organic matter enrichment mechanism is crucial for evaluating source rock resources and understanding oil and gas accumulation mechanisms. In this study, we evaluated the sedimentary paleoenvironment and organic matter enrichment mechanisms of shale using thin section observations, mineral composition analysis, organic geochemistry, and elemental geochemistry. We found significant differences in the sedimentary paleoenvironment and organic matter enrichment mechanisms between the lower Cuizhuang Formation and the Beidajian Formation shale. The Cuizhuang Formation was deposited in a late-stage, restricted basin environment during the rift phase, and elemental and geochemical indicators showed that the Cuizhuang Formation was in a suboxic–anoxic water environment, that was influenced by a warm and humid paleoclimate and submarine hydrothermal activities, which promoted the accumulation of organic matter. However, the enrichment of organic matter in the Cuizhuang Formation was mainly controlled by redox conditions. The formation of suboxic-anoxic water environments may be closely related to the warm and humid paleoclimate and submarine hydrothermal activities. Warm conditions promote continental weathering and increase marine productivity, thereby consuming oxygen in the bottom water. Moreover, acidic hydrothermal activity also helps to establish an anoxic environment. Our results reveal the effects controlling various coupled mechanisms dominated by redox conditions, which may explain the development of source rocks in the Cuizhuang Formation.

Identification of shale gas existence based on 2-dimensional magnetotelluric data in the Kutai basin, East Kalimantan, Indonesia

Research on unconventional methods in the shale gas exploration process has begun to be developed to meet the demand for oil and natural gas sustainability. The research area is located in the Kutai Basin, East Kalimantan, Indonesia. The current study from the Geological Survey Center shows that the Kutai Basin has 46.79 TCF of shale gas potential. The magnetotelluric (MT) method was applied in this study to describe the distribution of subsurface resistivity values. The inversion schemes performed in the MT method are 1-dimensional and 2-dimensional inversions. In this study, there are 8 measurement stations in one line. The presence of low resistivity values or conductive zones identifies the potential for black shale layers. The potential for shale gas is suspected to be detected at station KT29, which is included in the Pamaluan Formation, and at stations KT12, KT31, KT13, and KT15, which are included in the Pulaubalang Formation, because it is associated with source rock from petroleum in the Kutai Basin system, as supported by previous research data.

Authigenic Carbonate Burial Within the Late Devonian Western Canada Sedimentary Basin and Its Impact on the Global Carbon Cycle

AbstractStable carbon isotope ratios (δ13C values) of marine carbonates are widely used to infer the relative burial rates of organic carbon, a source of oxygen to the ocean‐atmosphere system. This inference, however, is based on the assumption that ocean‐atmospheric carbon is buried either as organic carbon or as marine carbonate minerals. The burial of authigenic carbonate minerals formed within sediments after deposition, with low δ13C values (i.e., similar to organic carbon), has been proposed to explain high δ13C values in marine carbonates without the need for high burial fluxes of organic carbon. To test this hypothesis, we focus on the Late Devonian, a time period with both pervasive ocean anoxia and a severe reduction in shallow‐water carbonate deposition—conditions hypothesized to promote authigenic carbonate formation. We present sedimentological and geochemical data from limestones and black shales of the Wabamun Group, Besa River and Exshaw formations of the Western Canada Sedimentary Basin. These data are compared to inorganic and organic weight percent measurements of North American shales acquired from the USGS National Geochemical Database (N = 4,437). Results show that basinal shale lack authigenic carbonate with low δ13C values and that the mean δ13C value of carbonate in these shales (−0.3‰) do not differ substantially from mean δ13C of carbonates in platform carbonates of a similar age (0.6‰). Furthermore, inorganic carbon content in Late Devonian shales (mean weight percent = 0.55%, N = 54) is lower than average Phanerozoic North American shale (mean of 1.95%, N = 4,055). Lastly, organic carbon‐to‐inorganic carbon ratios (OC:IC) of North American shales are well above 1 (mean = 3.72 for Late Devonian shales (N = 374), 2.25 for shales (N = 3,653) of all other ages). Therefore, even if the burial of fine‐grained siliciclastic formations carrying authigenic carbonates were to increase, the concomitant increase in organic carbon burial would be even larger. Together, data from this study do not provide evidence that the burial of authigenic carbonate would have a significant effect on global carbon isotope mass balance.

Depositional Model of Early-Middle Turonian Deep Water Gulneri Formation, in Selected Outcrop and Subsurface Sections in Northern Iraq

The Gulneri Formation (Early-Middle Turonian) Deep-water depositional model was constructed using detailed microfacies analysis from the northern Iraqi wells of Kirkuk (K-116) and Bai Hassan (BH-81) as well as the Dokan outcropped area. In the outcropped section, the formation consists of 2 m of thin friable marly limestone with high organic matter and thin-bedded black shale in addition to boulder and gravel-like limestone masses in the lower part. In the K-116 and BH-81 wells, the formation is 4.3 m and 9 m thick, respectively. It is composed mainly of black bituminous, pyritic calcareous shale, and shaley limestone with scattered glauconites. Petrographic studies of seventeen thin sections of the Gulneri rocks reveal that the pelagic/deep marine faunas are the dominant skeletal grain in the micritic groundmass. Three main microfacies were recognized in the studied rocks of the Gulneri Formation, namely lime mudstone, lime wackestone, and lime packstone. However, the latter is absent in the Dokan section. According to their environmental interpretation, these microfacies were grouped into two facies associations (basinal deep water and deep shelf). Based on the results of petrographic and microfacies analyses, it is concluded that the Gulneri Formation was deposited in a basinal deep marine environment with quiet and reducing conditions in the Dokan section and a basinal deep marine environment in the K-116 and BH-81 wells which changed at intervals to a shallower deep shelf setting toward the upper part with semi reducing conditions. The euxinic deep basin that formed at the beginning of the Kurdistan foreland basin's formation was quite anoxic and deeper from the northeast to the relatively shallower basin with semi-reduced conditions towards the southwest. This is how the depositional model of the Gulneri Formation in northern Iraq changed.

Features of determining organometallic compounds in organic matters of black shale using diffuse reflectance infrared Fourier transform spectroscopy

Relevance. The problem of studying organometallic compounds in carbonized and carbonaceous substances is part of the global problem of the structure of natural materials from plant remains and manifestation of ore genesis in organic sedimentary deposits. This problem covers a number of issues. The most pressing one among them is the form of occurrence of finely dispersed gold in the form of organometallic compounds in metal-bearing coals and shales. This is especially true for large gold deposits of black shale strata. Aim. To study the relationship between sedimentation of organic formations and ore genesis in black shale deposits using the example of the Verninskoe deposit (Patom Highlands, Yakutia). Using the SKAUFV hardware and software complex together with ICP-MS and INAA methods, which allow one to assess the degree of Au concentration in organic matter, to substantiate the possibility of determining organometallic compounds of gold that are significant in the processes of sedimentogenesis, lithogenesis and epigenesis. Methods. IR spectroscopy, instrumental neutron activation analysis (INAA) and inductively coupled plasma mass spectrometry (ICP-MS). Results. The authors have determined that the SKAUFV hardware and software complex, together with the ICP-MS and INAA methods, makes it possible to establish the role of organic matter in metasomatism and to identify temperature zonality and the associated gold concentration in the ore zone. To determine the organometallic compounds of Au, structural and genetic indicators (Pm, Ko, PVt+L, PI) were selected. These indicators reflect the significance of sedimentation, diagenesis, catagenesis and epigenesis on Au concentration of in organic matter. These indicators made it possible to assess the level of temperature impact on sedimentary deposits of regional metamorphism and near-ore metasomatism. In this regard, subzones of Au concentration were established in the ore zone, determined by temperature zoning in the form of a manifestation of local metasomatism. The latter was influenced by the protein-fat composition of plants that form organic matter.

Marine eutrophication within the Tarim Platform in sync with Middle to Late Ordovician climatic cooling

Previous work has proposed climatic cooling and atmosphere–ocean oxygenation as potential triggers for the Great Ordovician Biodiversification Event, with the suggestion of better oxygenated oceans during the Middle to Late Ordovician. However, recent studies have argued for spatial and temporal heterogeneity in marine redox state on several continents. Here we investigate a black-shale succession accumulated within the Tarim Platform via a combination of geochemical proxies to address these debates. Negative shifts in bulk nitrogen isotopes and synchronous increases in excess phosphorus suggest moderate-high marine primary production coinciding with the development of bottom-water anoxia, as indicated by enrichments in highly reactive iron and modest concentrations of redox-sensitive trace metals (Mo, U). Moreover, the occurrence of black shale correlates well with equivalent successions formed in deep-water marginal basins along several continents, including South China, North China, Laurentia and Baltica. This may suggest an expansion of marine anoxia in low-latitude zones of the late Darriwilian to early Sandbian oceans, probably as a result of enhanced upwelling in sync with climatic cooling. The extent and ultimate cause of marine anoxia requires further quantifying constraints at a global scale, which will enable potential links between global oceanic redox conditions and concurrent biotic changes to be evaluated in more detail. Supplementary material: Geochemical dataset for the Dawangou section (Table S1) is available at https://doi.org/10.6084/m9.figshare.c.7036552 Thematic collection: This article is part of the Chemical Evolution of the Mid-Paleozoic Earth System and Biotic Response collection available at: https://www.lyellcollection.org/topic/collections/chemical-evolution-of-the-mid-paleozoic-earth-system

Study on CO2 and CH4 Competitive Adsorption in Shale Organic and Clay Porous Media from Molecular- to Pore-Scale Simulation

Summary The elucidation of the competitive adsorption behaviors between CO2 and CH4 holds great importance in the context of improving natural gas recovery in shale reservoirs. Shale rock, as a complex porous medium, exhibits a highly interconnected multiscale pore network with pore size spanning from several to tens of nanometers. Nevertheless, accurately capturing the adsorption effects and studying the CO2/CH4 competitive adsorption within a large-scale, realistic, 3D nanoporous matrix remains a significant challenge. In this paper, we proposed a novel lattice Boltzmann method (LBM) coupled molecular simulation to investigate CO2/CH4 competitive adsorption in 3D shale nanoporous media. The initial step involves conducting Grand Canonical Monte Carlo (GCMC) simulations to simulate the competitive adsorption behaviors of CO2 and CH4 in kerogen and illite slit pores, with the aim of obtaining the atomic density distribution. Subsequently, a Shan-Chen-based lattice Boltzmann (LB) simulation is used under identical conditions. By coupling the molecular simulation results, the fluid-solid interaction parameters are determined. Finally, LB simulations are performed in designed 3D porous media, utilizing the fluid-solid interaction parameters. The effects of mineral type, CO2 concentration, and pore structure on competitive adsorption behaviors are discussed carefully. Our research offers significant contributions to the improvement of gas recovery and carbon geological sequestration through the examination of CO2/CH4 competitive adsorption in nanoporous media. Additionally, it serves as a link between molecular and pore-scale phenomena by leveraging the benefits of both molecular simulations and pore-scale simulations.

Numerical Simulation Study on the Effect of Preinjected CO2 on the Hydraulic Fracturing Behavior of Shale Oil Reservoirs.

Due to the low permeability and apparent mechanical anisotropy of shale reservoirs, shale oil production highly depends on the performance of hydraulic fracturing of the tight reservoir. Pre-injection of CO2 before hydraulic fracturing treatment has been proven beneficial to enhance shale oil production. A comprehensive understanding of the effect of changes in the properties of shale reservoirs after preinjected CO2 on the hydraulic fracturing behavior of shale reservoirs is essential to improve the stimulated reservoir volume (SRV) of shale oil reservoirs. In this study, comprehensive evaluating parameters were proposed to specify the variation of mechanical properties of shale rock at different soaking times of CO2 based on published testing parameters of shale. Accordingly, a three-dimensional (3D) numerical model was established and three groups of horizontal well fracturing cases with different cluster spacings were conducted based on the adaptive FE-DE method to simulate and compare the hydraulic fracturing behavior in the reservoirs with different mechanical properties. We established a quantitative relationship between the alterations in reservoir properties and the stimulated reservoir volume. The results indicate that both the brittleness and conductivity properties of shale rock are dramatically improved as the increment of soaking time of CO2. It is beneficial to improve the SRV, and the initiation pressure is reduced with the increment of soaking time of CO2. However, as the stress shadow effect is involved in the horizontal well fracturing, the complexity of the hydrofracturing crack is significantly enhanced to restrain the development of hydrofracturing crack. When the cluster spacing is larger, the stress shadow effect is weakened, and the weakening effect of CO2 soaking on reservoir is more obvious.

Molecular Insights of Excessive Water Cut during Cyclic Gas Injection in Liquid-Rich Shale Reservoirs: Contributions of Hydrocarbon Condensation and Water Trapping

Summary In a recent pilot test of cyclic gas injection (huff ‘n’ puff) in a Permian shale reservoir, excessive water product was observed, the reason for which remains unclear. In this work, we analyze the mechanisms of gas huff ‘n’ puff processes using molecular dynamics (MD) simulations and explain the reason for the high water-cut phenomenon. We aim to investigate the hydrocarbon-water-rock interactions during the gas injection as well as production within a shale rock in the pore scale. To mimic the heterogeneous pore structure of the shale rock, we have designed a pore system, including a bulk pore, a pore throat, and a dead-end pore. We simulate the distribution of different fluids during the initial equilibrium stage, the primary depletion stage, and the huff ’n’ puff stage. The results show that an excessive amount of water is trapped by the condensation mechanism in the larger pores during the primary depletion stage. The water is then recovered with the injection of working (lean) gases. Moreover, we have analyzed the effect of different injection gases (IGs) and found that carbon dioxide (CO2) yields a higher water cut compared with methane (C1). Moreover, our findings have revealed the trapping mechanisms of hydrocarbon-water mixtures in shale rocks and have highlighted the impacts of pore structures on the recovery of shale reservoirs. As such, we have provided a potential explanation of the observed phenomenon.

A New Epistemological Insight of the Coniacian-Santonian Oceanic Anoxic Event (OAE3)

Eighteen planktic and eleven benthic foraminiferal species were recorded from the dark grey to black shale facies of the Matulla Formation in Abu Zeneima area, West-Central Sinai, Egypt. The faunal assemblage is dominated by cosmopolitan whiteinellids, marginotruncanids, Dicarinellids, Contusotruncanids and Heterohelicids. The planktic species with high taxonomic diversity were used to zone the Coniacian and Santonian stages, as well as define the Coniacian/ Santonian boundary, while benthic foraminifera is of minor contribution in age assignment. The stratigraphic analysis of the relations and ranges of these fauna led to the recognition of five biozones<i>; Dicarinella primitiva</i> or <i>Huberella huberi</i> or <i>Marginotruncana</i> <i>sinuosa</i> for the Coniacian, while <i>Dicarinella concavata</i> and <i>Dicarinella asymetrica</i> for the Santonian. Also, the Coniacian/Santonian boundary was delineated, considering the appearance of <i>Dicarinella</i> <i>concavata</i> and disappearance of <i>Huberella huberi</i>, as well as the<i> </i>increase of Marginotruncanids (<i>M. renzi, M. sigali, M. marginata, M. pseudolinneiana</i>….etc.). Furthermore, the black shales found in the middle part of the Matulla Formation were attributed to the Coniacian-Santonian Oceanic Anoxic Event (OAE3). The occurrence of black shales with planktic foraminifera during the Coniacian–Santonian interval in several countries belonging to five continents, was the main impetus to render this event a global event.

Pedostratigraphic age model for Devonian red beds of NW China

Detrital zircon geochronology has been used to determine provenance, but also gives maximum depositional ages that can constrain other forms of geochronology, such as magnetostratigraphy and pedostratigraphy. Pedostratigraphy and magnetostratigraphy correlate a series of global events by spacing or intensity in a fashion comparable with a commercial barcode. The Late Devonian Shaliushui Formation of NW China is poorly fossiliferous, but shows spikes in magnetic susceptibility, Al/Si ratios, Rb/Sr ratios and the depth to carbonate nodules (Bk horizon) corresponding with the timing of named Late Devonian black shale horizons. By contrast, the Ti/Al ratios show only small variations, indicating little long-term change in the source terrain or in the hydraulic sorting of heavy minerals. Most of the detrital zircon ages are older than an age model from the pedostratigraphic correlation of deep calcic horizons corresponding to global atmospheric CO 2 spikes at times of black shale expansion, but one detrital zircon age converges with the pedostratigraphic depositional ages near the top of the section. Pedostratigraphy is widely used in Quaternary sequences, but can also be a basis for age models of rocks in deep time if supported by detrital zircon ages. Supplementary material: Paleosol and geochemical data, and youngest zircon ages for a section of Devonian rocks near Ciyao, Gansu, China are available at https://doi.org/10.6084/m9.figshare.c.7008192

Performance Study of Black Shale Modified Soil for Road Use Based on Eshelby–Mori–Tanaka Theory

Black shale, as a type of soft rock, exhibits high strength when freshly exposed. However, it easily disintegrates upon contact with water, making it unsuitable for direct use in roadbed construction. Using it as discarded material not only increases construction costs but also pollutes the environment. Therefore, the reuse and modification of black shale have become particularly important. Based on the theory of composite material equivalent inclusions, this study investigates the strength and water stability characteristics of black shale gravel after being mixed with cement and compacted with clay. The results show that the strength of cemented soil increases linearly with the cement content. The water absorption properties of the modified soils with different amounts of black shale added are similar, with an average water absorption rate of about 2.53%. The strength of black shale modified soil is generally positively correlated with the cement content, although the linear correlation is not significant. The modified black shale soil used in the experiment is suitable for the subgrade of medium- and light-grade secondary roads and below. The recommended mass ratio is Mshale:Mclay:Mcement = 70:21:9. The unconfined compressive strength of the material under 7-day curing is 1.36 MPa. The relationship between the strength of modified soil, clay strength, cement content, and gravel addition has been established, clarifying the physical significance of each parameter. The “drying and soaking” cycle can accelerate the strength degradation of modified soil. It is recommended to strengthen the construction of roadbed drainage facilities during construction to maintain a stable and dry environment for the modified soil as a roadbed filling material. The research results not only provide clear technical indicators for the reuse of discarded black shale in engineering but also serve as a basis for proportion of crushed stone discarded material as roadbed fill.

Submarine hydrothermal fluids facilitated the accumulation of cadmium in Lower Cambrian black shales, South China

The effects of Cd-rich venting fluids on the Cd contents and isotopic compositions of continental margin sediments remain poorly understood. Characterization of the Cd isotopic composition of sediment overprinted by venting fluids is critical, as this may provide insights into how mixing between the venting fluids and bottom waters affects the Cd isotopic composition. Our study determined the Cd isotopic compositions of Lower Cambrian metalliferous sediment deposited on the passive continental margin of the southeastern Yangtze Block, South China. The Cd is predominantly hosted in sphalerite of sulfide-bearing barite sediments, V deposits, and Ni-Mo−platinum group element (PGE) sulfide layers. The Cd isotopic compositions of samples from these three units indicate that the distribution of Cd at this type of continental margin was controlled by submarine hydrothermal activity. Following the venting of hydrothermal fluids onto the seafloor, Cd2+ reacted with a limited amount of H2S generated mainly by thermochemical sulfate reduction near the vents. This resulted in isotopically lighter Cd in the sulfide-bearing barite sediments and V deposits, which have δ114/110Cd values of 0.00‰ ± 0.13‰ and 0.00‰ ± 0.14‰, respectively. The residual dissolved Cd in the fluids migrated with the upwelling seawater and ultimately precipitated almost entirely as sphalerite in the sulfidic wedge, which resulted in Ni-Mo−PGE sulfide layers with a narrow range of high δ114/110Cd values (0.35‰ ± 0.03‰). The presence of Cd-rich venting fluids can alter the Cd contents and isotopic compositions of passive margin sediments, particularly those located within long-term stratified seawater and fault depressions. Previous studies may have underestimated the migration distance and affected areas of hydrothermal activity involving Cd-rich fluids.

Geochemistry and petrology of Early Permian lacustrine shales in the Lodève Basin, Southern France: Depositional history, organic matter accumulation and thermal maturity

The lacustrine shales in the Lodève Basin, southern France, serve as excellent archives of paleo-lake development as well as climatic evolution during the Early Permian. In this study, an extensive dataset is presented encompassing organic petrographic data, major and trace element quantification, bulk and molecular organic geochemical proxies, as well as compound-specific stable carbon isotope data derived from the analysis of 36 black shale outcrop samples originating from the Usclas-St. Privat Formation (USPF), Tuilières-Loiras Formation (TLF), and Viala Formation (VF). All sample are thermally oil-mature, as evident from a vitrinite reflectance (VRr) of around 0.8%. The lower section of the USPF, characterized by notably higher total sulfur (TS) concentrations, displays total organic carbon (TOC) and TS contents spanning from 2.69 to 7.83 (avg. 4.28) wt% and 0.42–1.55 (avg. 1.09) wt%, respectively. In contrast, the upper section of the USPF (average TOC of 2.59 wt%), TLF (average TOC of 2.66 wt%), and VF (average TOC of 3.17 wt%) exhibit considerably lower TS contents of 0.26 wt%, 0.22 wt%, and 0.17 wt%, respectively. The lower section of the USPF, characterized by the lowest pristane/phytane (Pr/Ph) ratio and the highest TS/TOC ratios and chemical index of alteration (CIA) values, was deposited in oxygen-depleted and saline lacustrine environments. These conditions prevailed under more humid climatic conditions and were probably related to marine incursions. In contrast, the upper section of the USPF, the TLF, and the VF display elevated Pr/Ph ratios but reduced TS/TOC and CIA values, signifying deposition within oxic-dysoxic and freshwater-brackish bottom water conditions with a significant change towards arid conditions. All samples are characterized by low vitrinite and inertinite contents together with consistently similar average values of Al2O3 and TiO2. The biomarker analysis suggests that the organic matter (OM) in most samples mainly originates from planktonic/algal biomass with additional microbial OM. Only the lower section of the USPF displays a slightly enhanced contribution of terrestrial OM input and also more detrital elements. It is concluded that the structural evolution of the basin from narrow deep towards wider and shallower settings as well as the postulated marine transgressive events during early stages played a crucial role in shaping the deposition environments of the two distinct sets of lacustrine shales, thereby influencing the OM accumulation mechanisms. In contrast, the substantial climatic aridification seems to have a relatively minor impact on the sources of OM and the conditions prevailing within the water column.

Vanadium isotope evidence for seawater contribution to V enrichment/mineralization in early Cambrian metalliferous black shales

Vanadium isotope evidence for seawater contribution to V enrichment/mineralization in early Cambrian metalliferous black shales

Shale brittleness evaluation under high temperature and high confining stress based on energy evolution

Deep and ultra-deep shale gas reservoirs are crucial targets for future natural gas development. Extensive research has shown the shale brittleness is closely associated with the hydraulic fracture network quality. However, the shale brittleness may be altered in deep reservoirs due to the high temperature and high geo-stress. At present, commonly used shale brittleness evaluation methods cannot distinguish the differences in energy evolution caused by different mineral components in shale rock. In order to distinguish this difference, this study innovatively proposes a shale brittleness evaluation method that considers mineral composition based on energy evolution. A high-temperature triaxial compression experiment was carried out on the Longmaxi Formation shale, its fracture mode was analyzed, and the brittleness of the shale was evaluated based on this innovative method. The results indicate that with increasing confining stress, the shale fracture mode transitions from brittle tensile-shear multi-crack penetrating fracture to brittle-plastic shear single-crack fracture, and the brittleness of shale shows a decreasing trend. Under low confining stress, the shale brittleness decreases with temperature. Under high confining stress, temperature is the non-main controlling factor of shale brittleness. This study provides valuable guidance for the design and optimization of hydraulic fracturing in deep shale reservoirs.