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Based on the observation and analysis of cores and thin sections, and combined with cathodoluminescence, laser Raman, fluid inclusions, and in-situ LA-ICP-MS U-Pb dating, the genetic mechanism and petroleum geological significance of calcite veins in shales of the Cretaceous Qingshankou Formation in the Songliao Basin were investigated. Macroscopically, the calcite veins are bedding parallel, and show lenticular, S-shaped, cone-in-cone and pinnate structures. Microscopically, they can be divided into syntaxial blocky or columnar calcite veins and antitaxial fibrous calcite veins. The aqueous fluid inclusions in blocky calcite veins have a homogenization temperature of 132.5–145.1 °C, the in-situ U-Pb dating age of blocky calcite veins is (69.9±5.2) Ma, suggesting that the middle maturity period of source rocks and the conventional oil formation period in the Qingshankou Formation are the sedimentary period of Mingshui Formation in Late Cretaceous. The aqueous fluid inclusions in fibrous calcite veins with the homogenization temperature of 141.2–157.4 °C, yields the U-Pb age of (44.7±6.9) Ma, indicating that the middle-high maturity period of source rocks and the Gulong shale oil formation period in the Qingshankou Formation are the sedimentary period of Paleocene Yi'an Formaiton. The syntaxial blocky or columnar calcite veins were formed sensitively to the diagenetic evolution and hydrocarbon generation, mainly in three stages (fracture opening, vein-forming fluid filling, and vein growth). Tectonic extrusion activities and fluid overpressure are induction factors for the formation of fractures, and vein-forming fluid flows mainly as diffusion in a short distance. These veins generally follow a competitive growth mode. The antitaxial fibrous calcite veins were formed under the driving of the force of crystallization in a non-competitive growth environment. It is considered that the calcite veins in organic-rich shale of the Qingshankou Formation in the study area has important implications for local tectonic activities, fluid overpressure, hydrocarbon generation and expulsion, and diagenesis-hydrocarbon accumulation dating of the Songliao Basin.

Based on the observation and analysis of cores and thin sections, and combined with cathodoluminescence, laser Raman, fluid inclusions, and in-situ LA-ICP-MS U-Pb dating, the genetic mechanism and petroleum geological significance of calcite veins in shales of the Cretaceous Qingshankou Formation in the Songliao Basin were investigated. Macroscopically, the calcite veins are bedding parallel, and show lenticular, S-shaped, cone-in-cone and pinnate structures. Microscopically, they can be divided into syntaxial blocky or columnar calcite veins and antitaxial fibrous calcite veins. The aqueous fluid inclusions in blocky calcite veins have a homogenization temperature of 132.5-145.1 °C, the in-situ U-Pb dating age of blocky calcite veins is (69.9±5.2) Ma, suggesting that the middle maturity period of source rocks and the conventional oil formation period in the Qingshankou Formation are the sedimentary period of Mingshui Formation in Late Cretaceous. The aqueous fluid inclusions in fibrous calcite veins with the homogenization temperature of 141.2-157.4 °C, yields the U-Pb age of (44.7±6.9) Ma, indicating that the middle-high maturity period of source rocks and the Gulong shale oil formation period in the Qingshankou Formation are the sedimentary period of Paleocene Yi'an Formaiton. The syntaxial blocky or columnar calcite veins were formed sensitively to the diagenetic evolution and hydrocarbon generation, mainly in three stages (fracture opening, vein-forming fluid filling, and vein growth). Tectonic extrusion activities and fluid overpressure are induction factors for the formation of fractures, and vein-forming fluid flows mainly as diffusion in a short distance. These veins generally follow a competitive growth mode. The antitaxial fibrous calcite veins were formed under the driving of the force of crystallization in a non-competitive growth environment. It is considered that the calcite veins in organic-rich shale of the Qingshankou Formation in the study area has important implications for local tectonic activities, fluid overpressure, hydrocarbon generation and expulsion, and diagenesis-hydrocarbon accumulation dating of the Songliao Basin. [ABSTRACT FROM AUTHOR]

The Middle Atlas hosts calcite veins of considerable economic value, being found in the Mahdi and Bou Naceur ridges in the eastern part of the Moroccan Middle Atlas. In this study, we aim to identify the fundamental factors controlling mineralization, which could be essential for the exploration of calcite minerals. Jurassic dolomites and limestones host calcite deposits. Mineralization is controlled by the NE-SW sinistral fault system of the Mahdi Ridge as well as by the NW-SE dextral fault system of the Bou Naceur Ridge. These veins exhibit a Riedel shear system. The edges of the veins display different textures, such as banded and brecciated calcite. At the heart of the veins are deposits of massive, automorphic, pure crystalline calcite. Geochemical analyses revealed carbonate rock dissolution and carbonate fluid infiltration, indicating the presence of a low-temperature hydrothermal system. These mineralizations are a response to the evolution of the geodynamic uplift of the Middle Atlas during the Neogene, which occurred during the Alpine orogeny. [ABSTRACT FROM AUTHOR]

Objective Calcite veins are the products of tectonic diagenesis. Information about fracture opening and fluid activity was recorded by calcite veins. Methods Multiple approaches, consisting of thin section observation, cathodoluminescence, U-Pb isotope dating, and REE, C, O and Sr isotope analyses of calcite veins from the Triassic Qinglong carbonate reservoirs in the Huangqiao area, Subei Basin, are used to analyse the origin of vein-forming fluid. Results Four stages of calcite veins were identified in the Qinglong Formation and successively formed at (115.30±0.42), (97.03±0.43), (85.29±0.25), (45.5±19.0) Ma. In the first stage, the calcite veins were derived from deep hydrothermal fluids.And in the second stage, the calcite veins were derived from mixed fluids of atmospheric fresh water and seawater. Formation water and deep shell-source hydrothermal fluid mixed in the vein-forming fluid of the third-stage calcite veins. In the fourth stage of calcite veins formation, deep mantle-source hydrothermal fluid mixed with the formation water, resulting in the precipitation of calcite veins. The formation timing of the four stages of calcite veins corresponds to multistage tectonic movements during different periods, indicating that fluid evolution in the Triassic Qinglong carbonate reservoirs was controlled by multiple stages of tectonic movements. Conclusion Importantly, three stages of deep hydrothermal fluid injectionin the reservoir were the result of deep fault opening caused by tectonic movements, which possibly indicate multistage hydrocarbon accumulation in the Triassic Qinglong Formation in the Huangqiao area.

The relationship between fracture calcite veins and shale gas enrichment in the deep Ordovician Wufeng Formation–Silurian Longmaxi Formation (Wufeng–Longmaxi) shales in southern Sichuan Basin was investigated through core and thin section observations, cathodoluminescence analysis, isotopic geochemistry analysis, fluid inclusion testing, and basin simulation. Tectonic fracture calcite veins mainly in the undulating part of the structure and non-tectonic fracture calcite veins are mainly formed in the gentle part of the structure. The latter, mainly induced by hydrocarbon generation, occurred at the stage of peak oil and gas generation, while the former turned up with the formation of Luzhou paleouplift during the Indosinian. Under the influence of hydrocarbon generation pressurization process, fractures were opened and closed frequently, and oil and gas episodic activities are recorded by veins. The formation pressure coefficient at the maximum paleodepth exceeds 2.0. The formation uplift stage after the Late Yanshanian is the key period for shale gas migration. Shale gas migrates along the bedding to the high part of the structure. The greater the structural fluctuation is, the more intense the shale gas migration activity is, and the loss is more. The gentler the formation is, the weaker the shale gas migration activity is, and the loss is less. The shale gas enrichment in the core of gentle anticlines and gentle synclines is relatively higher.

The relationship between fracture calcite veins and shale gas enrichment in the deep Ordovician Wufeng Formation–Silurian Longmaxi Formation (Wufeng–Longmaxi) shales in southern Sichuan Basin was investigated through core and thin section observations, cathodoluminescence analysis, isotopic geochemistry analysis, fluid inclusion testing, and basin simulation. Tectonic fracture calcite veins mainly in the undulating part of the structure and non-tectonic fracture calcite veins are mainly formed in the gentle part of the structure. The latter, mainly induced by hydrocarbon generation, occurred at the stage of peak oil and gas generation, while the former turned up with the formation of Luzhou paleouplift during the Indosinian. Under the influence of hydrocarbon generation pressurization process, fractures were opened and closed frequently, and oil and gas episodic activities are recorded by veins. The formation pressure coefficient at the maximum paleodepth exceeds 2.0. The formation uplift stage after the Late Yanshanian is the key period for shale gas migration. Shale gas migrates along the bedding to the high part of the structure. The greater the structural fluctuation is, the more intense the shale gas migration activity is, and the loss is more. The gentler the formation is, the weaker the shale gas migration activity is, and the loss is less. The shale gas enrichment in the core of gentle anticlines and gentle synclines is relatively higher. [ABSTRACT FROM AUTHOR]

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Calcite veins are widely developed in organic-rich shale, and their petrological characteristics and genetic mechanism are the focus of research. The Electron Back Scatter Diffraction (EBSD) can characterize the micro-structure and orientation of mineral crystals in situ, which has been widely used in the field of material science and has been rapidly developed in the field of geology. In order to clarify the crystal characte-ristics of fibrous calcite veins in organic-rich shale of the Longmaxi Formation in the Sichuan Basin, EBSD was used to characterize the mineralogical and crystallographic characteristics of calcite veins. The calcite veins are mainly composed of calcite and quartz. Calcite is the main body, with an average grain size of 372 μm, while quartz is mainly distributed at the interface of calcite lamina. The calcite crystals in calcite veins belong to trigonal[JP] or rhombohedral system, and the corresponding unit cell is trigonal or rhombohedral. The lattice parameters are a0=b0=4.99 Å, c0=17.061 Å, α=β=90°, γ=120°, respectively. Calcite veins have a certain preferred orientation on longitudinal section, which is due to the development of polysynthetic twin crystals in calcite grains. The adjacent twin crystal stripes have different crystal orientations, and the crystal misorientation is 75° while the alternate twin stripes have the same crystal orientation, and the crystal orientation of the same twin stripe is the same. In calcite grains, perfect cleavage occurs in groups with sharp angle with twin crystal stripes. Both cleavage and twin crystal stripes are formed by tectonic compression and shearing during the crystallization of calcite, and the maximum principal stress direction is parallel to the twin crystal stripes.

Copyright of Journal of China University of Petroleum is the property of China University of Petroleum and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Calcite veins are common in organic-rich mudrocks, but their genesis and ability to transmit fluids are debated. A combined microstructural and isotopic investigation of an array of calcite veins recovered in core from the Marcellus Formation reveals that the veins grew via a combination of continuous fibrous growth and punctuated fracture-opening increments. Continuous opening is the result of pressure-solution creep and involves no mechanical fracturing, but rather the growth of a pressure fringe around a pre-existing, sealed fracture. In contrast, incremental opening is accomplished by overpressured, mineral-saturated fluid, which repeatedly ruptures the rock at the cement / host-rock interface. Punctuated growth increments occurred repeatedly throughout an otherwise protracted, continuous growth history, indicating that the present structures preserve hybrid deformation conditions between brittle, fluid-assisted cracking and plastic strain. Stable isotopic signatures match those of a regional opening-mode fracture set that formed in response to catagenetic fluid overpressures amid a tectonically imposed (Alleghanian) stress field. It is concluded that calcite veins form as opening-mode hydraulic fractures and are susceptible to increments of brittle reactivation, even while inelastic growth processes widen and fill the veins with fibrous cement. [ABSTRACT FROM AUTHOR]

The characteristics and genesis of the calcite veins in Carboniferous basalt in the east slope of Mahu Sag, Junggar Basin are investigated based on observation of cores and thin sections; analyses of X-ray fluorescence, in situ major, trace and rare earth elements (REE), carbon, oxygen and strontium isotopes, fluid inclusions, as well as basin modeling. There are three periods of calcite fillings. The Period I calcite is characterized by low Mn content, flat REE pattern, strong negative cerium (Ce) anomaly, weak to moderate negative Eu anomaly, and light carbon isotopic composition, indicating the formation of the calcite was affected by meteoric water. The Period II calcite shows higher Mn and light REE contents, weak positive Ce anomaly and slight positive europium (Eu) anomaly, and a little heavier carbon isotopic composition and slightly lower strontium isotope ratio than the Period I calcite, suggesting that deep diagenetic fluids affected the formation of the Period II calcite to some extent. The Period III calcite is rich in iron and manganese and has REE pattern similar to that of the Period II calcite, but the cerium and europium enomalies vary significantly. The Period I and II calcites were formed in shallow diagenetic environment at approximately 250–260 Ma, corresponding to Late Hercynian orogeny at Late Permian. The Period III calcite was probably formed in the Indo-China movement during Late Triassic. It is believed that the precipitation of calcite in basalt fractures near unconformity was related to leaching and dissolution of carbonates in the overlying Lower Permian Fengcheng Formation by meteoric water, which destructed the Carboniferous weathering crust reservoirs in early stage. Relatively high quality reservoirs could be developed in positions with weak filling and strong late dissolution, such as structural high parts with Fengcheng Formation missing, distant strata from Fengcheng Formation vertically, buried hills inside lake basin, etc.

• Rock carbonation occurs along an upward migration pathway of slab-derived COH fluid. • About 20%–50% of slab-released C is sequestered within the metasedimentary veneer. • Subducted metasediments act as a "carbon filter", hindering slab-to-arc C transfer. • The apparent excess of C release from slabs is reconciled with a smaller arc C flux. The movement of carbon in subduction zones plays a crucial role in regulating the global carbon cycle, controlling Earth's climate, and maintaining its habitability. Recent work suggests that only a fraction of the carbon released from subducting slabs at sub-arc depths is ultimately released from volcanic arcs, necessitating the existence of hidden carbon reservoirs within the slab-to-arc pathways. However, the precise location of these reservoirs remains enigmatic. Slab fluid serves as the primary medium for carbon transport in subduction zones; thus, a comprehensive understanding of fluid-rock interaction during slab fluid migration is essential for reconciling the carbon flux imbalance between the slab and the arc. In this study, we explore rock carbonation along a fluid conduit in the Southwestern Tianshan HP metamorphic belt in northwest China. Field evidence and petrologic observation reveal significant carbonation of a siliciclastic metasediment at its contact with a high-pressure garnet-bearing calcite (formerly aragonite) vein. We find that rock carbonation (by progressive Fe-bearing magnesite, dolomite, then aragonite precipitation) occurred when slab-derived carbonic fluids migrated through the metasedimentary sequence at approximately 80 km depth. Furthermore, modeling demonstrates that the metasedimentary layer atop the slab has the capacity to sequester 20%–50% of the fluid carbon from the ascending slab devolatilization flux. We propose that the metasedimentary veneer at the plate interface functions as a "carbon filter", hindering the transfer of carbon from the slab to the arc and helping to reconcile the carbon flux imbalance between the amount released by the slab and that emitted by the arc. This study also provides insights into decarbonation efficiency and mechanisms, carbon-transfer pathways, and temporal aspects of the subduction zone carbon cycle. [ABSTRACT FROM AUTHOR]

Wells with low gas content and low resistivity in the Changning area, southern Sichuan Basin were selected for this study. The burial-thermal history was reconstructed and the characteristics of multi-phase fluid activity were clarified using microscopic observation and testing of fluid inclusions in the Longmaxi shale fracture veins. Compared with wells with a high gas content and high resistivity, the influence of fluid activity on resistivity was analyzed. The results showed that the thermal evolution of the bituminous inclusions trapped in the veins has reached the stage of carbonaceous-metamorphic bitumen, and the organic matter is fully cracked for gas generation, with some organic matter exhibiting the phenomenon of “graphitization.” The synchronous fluid with bitumen was existed due to shallow burial with a middle-low maturity stage of about 280 and 292 Ma. Two phases of fluids existed in the deep burial stage, thus maturing for about 103 Ma, and the uplift stage at about 28 and 32 Ma, with high homogenization temperatures (Th) (varying from 185 to 195°C and 165–180°C). The corresponding pressure coefficients varied between 1.67 and 2.09, 1.56 and 1.92 in a moderate-strong high-pressure state. The last two phases of fluid formation in the late uplift stage for about 4 to 19 Ma and 6 to 10 Ma were characterized by low salinity at medium-low Th (varied from 140 to 155°C and 120–135°C), with pressures of 57.47–74.50 MPa and 51.44–59.41 MPa (pressure coefficients of 1.09–1.41 and 1.18–1.37), in an atmospheric-weak overpressure state. In the initial uplift stage after deep burial, the fluid closure in the Changning area was good. In contrast, the wells are filled with low gas content because of the strong tectonic forces causing the shale gas to be released and the multi-phase fluid activity that happens during the late uplift stage. New evidence indicates that the emergence of low resistance in the localized Changning area is not only related to the high degree of evolution of organic matter but is also affected by the multi-phase fluid modification in the late uplift stage.

Coal rocks often contain calcite, which has a significant effect on the mechanical properties of coal and the energy evolution during rupture damage. In this study, the meso-scale of rock is considered, and the spatial distribution of the internal structure of coal is characterized by digital image technology. Uniaxial compression tests were conducted using RFPA on coal rocks containing calcite veins with diverse dip angles. The research results show that the different azimuth angles of the calcite veins change the internal stress distribution of the coal, resulting in higher coal compressive strength at low dip angles (0°, 15° and 30°). Under high dip angles (45°, 60°, 75° and 90°), coal has lower compressive strength. The fracture mode of coal is significantly affected by calcite. At low dip angle, the fracture mode of coal and rock is complex, which are inclined Z-type (0°), V-type (15°) and inverted V-type (30°), respectively. At high dip angle, the fracture mode of coal and rock is single, which is type I failure mode. The destruction process of coal rocks is influenced by calcite veins. Under low dip angle, the internal stress distribution of coal is relatively uniform, the weak cementation between matrix and calcite vein in coal is not easy to be damaged, the stress required for coal failure is large and the input energy, accumulated elastic energy and impact energy index are large. Under high dip angle, the internal stress distribution of coal is uneven, the weak cementitious material between matrix and calcite vein in coal is easy to be damaged and the input energy, accumulated elastic energy and impact energy index are small. [ABSTRACT FROM AUTHOR]

Palaeostress analysis based on multiple inversion of fault-slip data in 9-dimensional space in the area of the SE margin of the Elbe Fault Zone (E part of the Bohemian Cretaceous Basin, the Czech Republic) reflects five palaeostress phases. Formation of syntectonic calcite veins mostly took place at 500-1000 m depth. Homogenisation temperature of fluid inclusions (~80 °C) and only subtle variations in fluid salinity point to syntectonic calcite vein formation from a single fluid source and one-step crystallisation. Fluid trapping occurred at 82-84 °C and 4.84-9.67 MPa. Both oxygen isotope composition of the fluid (δ18Ofluid = 2 ‰ V-SMOW) and fluid salinity (~3 wt. % eq. NaCl) reflect formation of calcite veins from formation waters expelled from the Cretaceous strata. Compressional phase D1 (σ1 trending ~N-S) is documented in a regime of reverse faults and strike-slip faults which indicate Europe-Africa coupling and N-directed thrusting of the Alps in the Palaeogene-Early Oligocene. Extensional phase D2 (σ3 generally trending N-S to NE-SW) is indicated by the formation of oblique to normal faults and accompanied by volcanic intrusions in the Middle Oligocene-Early Miocene. Palaeostress phase D3 (σ1 trending ~NE-SW) reflects Early Miocene compression in the Eastern Alps. Palaeostress phase D4 (σ3 in the direction WNW-ESE) is a result of crustal extension in the Eastern Alps and the Western Carpathians. Compressional palaeostress phase D5 (σ1 generally trending NW-SE to WNW-ESE) is evidenced by reverse faults and strike-slip faults from the Pliocene. [ABSTRACT FROM AUTHOR]

Coal rocks often contain calcite, which has a significant effect on the mechanical properties of coal and the energy evolution during rupture damage. In this study, the meso-scale of rock is considered, and the spatial distribution of the internal structure of coal is characterized by digital image technology. Uniaxial compression tests were conducted using RFPA on coal rocks containing calcite veins with diverse dip angles. The research results show that the different azimuth angles of the calcite veins change the internal stress distribution of the coal, resulting in higher coal compressive strength at low dip angles (0°, 15° and 30°). Under high dip angles (45°, 60°, 75° and 90°), coal has lower compressive strength. The fracture mode of coal is significantly affected by calcite. At low dip angle, the fracture mode of coal and rock is complex, which are inclined Z-type (0°), V-type (15°) and inverted V-type (30°), respectively. At high dip angle, the fracture mode of coal and rock is single, which is type I failure mode. The destruction process of coal rocks is influenced by calcite veins. Under low dip angle, the internal stress distribution of coal is relatively uniform, the weak cementation between matrix and calcite vein in coal is not easy to be damaged, the stress required for coal failure is large and the input energy, accumulated elastic energy and impact energy index are large. Under high dip angle, the internal stress distribution of coal is uneven, the weak cementitious material between matrix and calcite vein in coal is easy to be damaged and the input energy, accumulated elastic energy and impact energy index are small.

Shale oil has been discovered and produced in the Cretaceous Qingshankou Formation (K 2 qn) of the Gulong sag in the Songliao Basin (the Gulong shale oil). However, the selection of favorable layers for shale oil development remains uncertain due to significant variations in oil production of different intervals in the Formation (Q 1 –Q 9). In this study, the geochemical features of shale, the content and chemical features of extracted oils in different shale intervals are analyzed in detail, and the favorable intervals and mechanism for shale oil accumulation are also addressed. It is shown that the Gulong shale has total oil contents of 200–400 mg/g TOC with considerable contents of light hydrocarbons in extracted oils. In the shale, there is a "W" trend for the evolution of chemical and carbon isotopic compositions (δ13C) of shale oil with depth. It is deduced that this evolution trend of shale oil is governed by the variation of organic matter precursor associated with the deposition environment. The distributions of δ13C of mud gas and individual n -alkanes with depth reveal that the Gulong shale oil can be separated as two independent petroleum systems at the Middle–Lower Q 7. Mass balance calculations imply Q 2 –Q 3 and Q 8 –Q 9 are two intervals with lower oil expulsion efficiency. Combined with the morphology of calcite cements and veins as well as the homogenization temperature of inclusions, an intralayer accumulation model of the Gulong shale oil was established. In addition, the statistical analysis of the oil content, lithological properties, and oil production capacity of different layers from 95 wells, demonstrates that Q 2 –Q 3 and Q 8 –Q 9 can be selected as the preferential target layers for future development of the Gulong shale oil. More importantly, this study may provide new insights into the enrichment of shale oil with tight shale type in China and around the world. • OM precursors govern the vertical "W" trend for group compositions and δ13C of Gulong shale oil. • The Q 2 –Q 3 and Q 8 –Q 9 layers have oil expulsion efficiency < 50% and are preferential target intervals. • The late micro-sealing by calcite veins or cements is responsible for the accumulation of Gulong shale oil. • Oil content and brittleness are the two key geological parameters determining Gulong shale oil production. [ABSTRACT FROM AUTHOR]

Highlights • Multiphase flow in the fractured medium was investigated using the method of "multiple interacting continua" (MINC). • The formation processes of calcite vein and its impact on the sealing efficiency of caprock were discussed. • The simulation results were validated using the field observations of the natural CO 2 gas fields. • The formation of calcite vein eventually prevented the CO 2 from leaking through the fracture at a long time scale. Abstract For the long-term CO 2 geological storage, the evolution of the caprock sealing efficiency has received increasing attention. In this paper, the Huangqiao CO 2 gas field in Jiangsu Province, China, where the calcite veins have been found in the lower part of mudstone caprock, is considered as a natural analogue site for CO 2 geological sequestration (CCS). To ascertain the dynamic formation process of calcite vein and its impact on the evolution of caprock sealing efficiency, a one-dimensional model was designed to represent the fractured reservoir-caprock system. Numerical simulations were performed using the multiphase reactive transport program TOUGHREACT. Sensitivity analyses of fracture permeability and calcite reaction rate were made. The simulation results illustrate that calcium bicarbonate decomposes to form calcium carbonate and release CO 2 from solution due to the pressure decay as the CO 2 -rich fluids migrate upwards along the fracture. The formation of calcite vein decreases the porosity and permeability significantly, which can enhance the integrity and sealing efficiency of caprock. Sensitivity analyses indicate that the formation of calcite vein is facilitated by the fast fluid flow, and calcite vein tends to form in the vicinity of reservoir-caprock interface under the slow-flow condition as the decrease of fracture permeability. Information currently available in Huangqiao CO 2 gas field and some other area describing the formation of calcite vein shows good agreement with our simulation results. The mudstone caprock with fractures and faults is able to keep CO 2 from leaking for a long time as the case of Huangqiao area. The methods and analyses presented here may be useful for CO 2 storage sites with similar conditions. [ABSTRACT FROM AUTHOR]

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Dispersed geoheritage points, which are small-sized and low-unique localities, manifest local geoheritage and, thus, may be useful for its comprehensive understanding. Field studies in the Lagonaki Highland (Western Caucasus, southwestern Russia) have permitted to find four geoheritage points that are described with argumentation of their importance to scientists, educators, and/or tourists. These points, none of which can be defined as a ‘traditional’ geosite, provide some precious sedimentary, palaeontological, and mineralogical information that improves perception of the local geoheritage landscape dominated by the Late Jurassic carbonate platform deposits. The studied localities are constituents of the local geoheritage resource because of their utility to three main categories of potential users. Scientists may be interested in these dispersed geoheritage points because of the need to collect massive geological data from big areas. Educators and tourists may appreciate these points because of their location along principal roads, i.e., their good accessibility.

Linking the architecture of structural conduits with the hydrothermal fluids migrating from the reservoir up to the surface is a key-factor in geothermal research. A contribution to this achievement derives from the study of spring-related travertine deposits, but although travertine depositional systems occur widely, their feeding conduits are only rarely exposed. The integrated study carried out in the geothermal Gölemezli area, nearby the well-known Pamukkale area (Denizli Basin, western Anatolia, Turkey), focused on onyx-like calcite veins (banded travertine) and bedded travertine well exposed in a natural cross-section allowing the reconstruction of the shallower part of a geothermal system. The onyx-like veins represent the thickest vein network (> 150 m) so far known. New field mapping and structural/kinematic analyses allowed to document a partially dismantled travertine complex (bedded travertine) formed by proximal fissure ridges and distal terraced/pools depositional systems. The banded calcite veins, WNW-trending and up to 12 m thick, developed within a > 200 m thick damaged rock volume produced by parallel fault zones. Th/U dating indicates a long lasting (middle-late Pleistocene) fluids circulation in a palaeo-geothermal system that, due to its location and chemical characteristics, can be considered the analogue of the nearby, still active, Pamukkale system. The isotopic characteristics of the calcite veins together with data from fluid inclusions analyses, allow the reconstruction of some properties (i.e. temperature, salinity and isotopic composition) and processes (i.e. temperature variation and intensity of degassing) that characterized the parent fluids and the relation between degassing intensity and specific microfabric of calcite crystals (elongated/microsparite-micrite bands), controlled by changes/fluctuations of the physico-chemical fluid characteristics. [ABSTRACT FROM AUTHOR]

This study investigates the Wangfu Depression of the Songliao Basin, China, as a natural analogue site for Fe migration (bleaching) and mineralization (formation of iron concretions) caused by reducing CO2-bearing fluids that leak along fractures after carbon capture, utilization, and storage. We also examined the origin of fracture-filling calcite veins, the properties of self-sealing fluids, the influence of fluids on the compositions of mudstone and established a bleaching model for the study area. Our results show that iron concretions are the oxidative products of precursor minerals (pyrite and siderite) during uplift and are linked to H2S and CO2 present in early stage fluids. The precipitation of calcite veins is the result of CO2 degassing and is related to CO2, CH4, and minor heavy hydrocarbons in the main bleaching fluids. In our model, fluids preferentially enter high-permeability fracture systems and result in the bleaching of surrounding rocks and precipitation of calcite veins. The infilling of calcite veins significantly decreases the permeability of fractures and forces the fluids to slowly enter and bleach the mudstone rocks. The Fe2+ released during bleaching migrates to elsewhere with the solutions or is reprecipitated in the calcite veins and iron concretions. The formation of calcite veins reduces the fracture space and effectively prevents fluid flow. The fluids have an insignificant effect on minerals within the mudstone. In terms of the chemistry of the mudstone, only the contents of Fe2O3, U, and Mo change significantly, with the content of U increasing in the mudstone and the contents of Fe2O3 and Mo decreasing during bleaching. [ABSTRACT FROM AUTHOR]

Beef structures (bedding-parallel veins of fibrous calcite) are widespread within the Lower Triassic carbonate rocks in the Sichuan Basin of China, especially within clay-rich strata of low permeability. In the veins, fibrous calcite occurs in the outer zones, and coarse equant calcite in the inner zones. At least two generations of calcite crystallization took place during aqueous alteration, at the same time as deformation recorded by the calcite. The first-generation calcite fibers are at steep angles to the hydrocarbon-bearing host beds, and they grew vertically against the force of gravity at a time when the source rocks were maturing. Second-generation calcite occurs as coarse equant grains that sealed pores via localized fluid flow during horizontal tectonic compression, so that shear stresses acted at the fracture margins. Shearing of the host rock was accommodated in part by dissolution-precipitation creep (DPC), grain rotation, and grain slippage, recorded in crystallographic preferred orientations (CPOs) of the host calcite grains beside the crack walls. The beef veins formed during high pore-fluid overpressures along hydrofractures. We propose that the bedding-parallel veins with beef structures are evidence of a 'crack-seal slip' fault valve process during hydrocarbon generation. The hydrocarbon-bearing calcite beef structure may be a good indicator of oil or gas migration, and of the flow direction of aqueous solutions. [ABSTRACT FROM AUTHOR]

Numerous calcium carbonate veins were recovered from the igneous basement of the Early Cretaceous Shatsky Rise during Integrated Ocean Drilling Program (IODP) Expedition 324. The chemical (Sr/Ca, Mg/Ca) and isotopic ( 87 Sr/ 86 Sr, 143 Nd/ 144 Nd, δ 18 O, δ 13 C) compositions of these veins were determined to constrain the timing of vein formation. A dominant control by seawater chemistry on calcite composition is evident for most vein samples with variable contributions from the basaltic basement. Slightly elevated precipitation temperatures (as inferred from oxygen isotope ratios), indicative of hydrothermal vein formation, are only observed at Site U1350 in the central part of Shatsky Rise. The highest 87 Sr/ 86 Sr ratios (least basement influence) of vein samples at each drill site range from 0.70726 to 0.70755 and are believed to reflect the contemporaneous seawater composition during the time of calcite precipitation. In principle, age information can be deduced by correlating these ratios with the global seawater Sr isotope evolution. Since the Sr isotopic composition of seawater has fluctuated three times between the early and mid Cretaceous, no unambiguous precipitation ages can be constrained by this method and vein precipitation could have occurred at any time between ~ 80 and 140 Ma. However, based on combined chemical and isotopic data and correlations of vein composition with formation depth and inferred temperature, we argue for a rather early precipitation of the veins shortly after basement formation at each respective drill site. [ABSTRACT FROM AUTHOR]

Calcite veins in the sedimentary rocks seal the natural fractures by crystal growth and play a very important role for rocks under compression. The interfaces between the veins and host rock are generally considered as structures determining the mechanical response and fracture of the rocks. Furthermore, thickness variation of the calcite vein also can affect the mechanical behavior and failure of the sedimentary rocks significantly. In this work, carboniferous limestone containing calcite veins were obtained from crop in field, and multiple rock specimens with varying thicknesses of calcite veins were prepared for a uniaxial compression experiment. The mineral identification and investigation were conducted on the rock slices before and after water saturation for analysis of the mineralogical composition. The effect of water saturation on the mechanical behavior was considered due to the presence of clay minerals. Complete stress-strain relationships were obtained through the uniaxial compression experiment. Comuted tomography scanning was employed for the observation of fracture configuration in the failed rock specimens. The mechanical behavior and fractures of the limestone containing calcite veins were analyzed, in which more plasticity and lower strength are generated as the thickness of the calcite veins increase. Depending on the thickness, calcite veins not only can act as the geological structures in the rock but also can join themselves with the host rock to resist compression. • Calcite veins in the limestone control its mechanical behavior and fracture propagation. • Calcite veins can join together with the host rock to resist loading besides its structural effect. • Transformation from elastic to plastic deformation is shown as the vein's thickness increase. [ABSTRACT FROM AUTHOR]

Normal fault zones affecting the pre-rift limestones in the Corinth Rift comprise breccia corridors separated by slip surfaces, but also numerous stylolites and calcite veins attesting the importance of fluid-driven mass transfer during a(inter)seismic episodes of fault activity. Cathodoluminescence microscopy shows that external (meteoric?) waters circulated in the fault zones, whereas mass transfer in the surrounding rocks implied a closed system with water chemically equilibrated with the host limestone. From these observations, we propose preliminary qualitative models of structural development and fluid flow in active normal fault zones in superficial conditions, with a tendency to concentration of deformation and fluid-flow-related to fault-tip propagation. To cite this article: P. Labaume et al., C. R. Geoscience 336 (2004). [Copyright &y& Elsevier]

Fluid infiltration into active faults and shear zones is usually studied in middle to upper crustal environment. Circulating fluids deposit fine-grained sediments like clay and carbonate material into cracks within the fault zone. Such crack-fill fine-grained material, calcite veins, and oxidized/weathered open cracks are well observed in the drill cores, from near-surface to a depth of 1800 m, in the Nojima fault zone, Japan, which triggered the 1995 M7.2 Kobe earthquake. Powder X-ray diffraction analysis indicates that the crack-fill fine-grained materials are mainly composed of siderite, calcite, laumontite, and fine-grained clasts of granitic rock. Isotopic analyses of carbonate material within the fine-grained materials and calcite veins reveal variable δ13C (SMOW) values ranging from 19.3‰ to 27.7‰, and δ13C (PDB) values of −4.6‰ to −18.5‰, which are consistent with those of typical meteoric and seawater. 14C dates of 10 calcite vein samples range from 35.0 to 58.4 kyr B.P.Geological, petrological, stable isotopic, and 14C data suggest that these crack-fill fine-grained materials and calcite veins and brown open cracks were developed by the repeated infiltration of O2- and CO2-bearing meteoric and seawater downward into the deep Nojima fault zone during the last 35–60 kyr. We propose a seismic fault suction-pumping model to interpret the infiltration of subsurface waters being carried down into the deep fault zone by rapid potential change during episodes of seismic faulting. [Copyright &y& Elsevier]

Petrology, geochemistry, fluid inclusions and U Pb dating of a wide (30 cm) calcite vein in Upper Ordovician (Sandbian) limestone characterize a hydrochemical microcosm of changing fluid sources and carbon pathways related to burial, then uplift during the Phanerozoic along the Ottawa-Bonnechere graben, central Canada. U Pb dating of the host limestone produces an age younger than its chronostratigraphic age suggesting the impact of diagenesis, otherwise well-defined petographically. An interred calcite fragment dated as Cambrian may highlight a platform source in this part of the graben hitherto unrecognized stratigraphically. The vein was initiated with extension across a high-angle narrow (2 cm) fault gouge allowing incursion of hot (80–125 °C) basin-derived (Mg, Ca)-Cl 2 brine recorded by a saddle dolomite–dedolomite–calcite succession, not isotopically datable. Microbial methanogenesis occurred in initial narrow structural recesses giving way to recycled sedimentary bicarbonate with fracture widening. The oldest datable vein calcite (122 ± 5 Ma) consists of microcrystalline geopetal cumulates of vadose meteoric origin demarcating renewed carbonate formation following a period of near-surface alteration, dissolution, and microfracturing of the prior vein calcite during uplift. Initial 207Pb/206Pb ratios are consistent with average crust (0.819 ± 0.003), and δ18O signatures identify a meteoric signature similar to the host rock. Subsequent interlayering of isopachous calcite and pyrite, with an age of 105 ± 3 Ma for the calcite, defines onset of a meteoric phreatic environment. The role of microbial sulphate reduction is characterized isotopically. Initial 207Pb/206Pb ratios scatter below the crustal average (0.84–0.76) that, along with a meteoric δ18O signature, suggest incursion of a new fluid reservoir. A subsequent Sr, Mn, Ba-bearing–magnesium calcite marks the beginning of sustained phreatic meteoric dilution for the remaining vein history. Stable (C, O) isotopes document varying temperature and bicarbonate mixtures of sedimentary and bacterial (sulphate reduction) origins associated with a meteoric fluid. This calcite has a mean age of 94 ± 2 Ma and displays the widest range of initial 207Pb/206Pb ratios, which scatter down to 0.65. Stable and radiogenic isotopic compositions suggest a meteoric setting largely influenced by a crustal fluid reservoir of long residence time and-or transport resulting in leaching of radiogenic minerals. The final vein calcite, also of meteoric origin, has an 87Sr/86Sr ratio similar to that of the initial basin brine, yet with 13C signatures that characterize a source of oxidized methane. U Th series analysis delimits an age older than 0.5 Ma. This stage of vein calcite likely arises through meteoric fracking of Ordovician source rocks during continued basin inversion. In summary, initial brine influx followed by a shallow-crustal meteoric realm document structurally focused fluid migration with and following early Mesozoic graben exhumation. The phreatic meteoric regime of Early Cretaceous age spans a ~ 20 Ma period coincident with transient magmatism arising from near passage of the Great Meteor Hotspot. Regional paleohydrology appears to have been influenced for ~10 Ma after plume passage. [Display omitted] [ABSTRACT FROM AUTHOR]

Dispersed geoheritage points, which are small-sized and low-unique localities, manifest local geoheritage and, thus, may be useful for its comprehensive understanding. Field studies in the Lagonaki Highland (Western Caucasus, southwestern Russia) have permitted to find four geoheritage points that are described with argumentation of their importance to scientists, educators, and/or tourists. These points, none of which can be defined as a 'traditional' geosite, provide some precious sedimentary, palaeontological, and mineralogical information that improves perception of the local geoheritage landscape dominated by the Late Jurassic carbonate platform deposits. The studied localities are constituents of the local geoheritage resource because of their utility to three main categories of potential users. Scientists may be interested in these dispersed geoheritage points because of the need to collect massive geological data from big areas. Educators and tourists may appreciate these points because of their location along principal roads, i.e., their good accessibility. [ABSTRACT FROM AUTHOR]