Diagenesis of the Amposta offshore oil reservoir (Amposta Marino C2 well,Lower Cretaceous,Valencia Trough,Spain)

Samples from the Amposta Marino C2 well (Amposta oil field) have been investigated in order to understand the origin of fractures and porosity and to reconstruct the fluid flow history of the basin prior, during and after oil migration. Three main types of fracture systems and four types of calcite cements have been identified. Fracture types A and B are totally filled by calcite cement 1 (CC1) and 2 (CC2), respectively; fracture type A corresponds to pre‐Alpine structures, while type B is attributed to fractures developed during the Alpine compression (late Eocene‐early Oligocene). The oxygen, carbon and strontium isotope compositions of CC2 are close to those of the host‐rock, suggesting a high degree of fluid‐rock interaction, and therefore a relatively closed palaeohydrogeological system. Fracture type C, developed during the Neogene extension and enlarged by subaerial exposure, tend to be filled with reddish (CS3r) and greenish (CS3g) microspar calcite sediment and blocky calcite cement type 4 (CC4), and postdated by kaolinite, pyrite, barite and oil. The CS3 generation records lower oxygen and carbon isotopic compositions and higher ⁸⁷Sr/⁸⁶Sr ratios than the host‐limestones. These CS3 karstic infillings recrystallized early within evolved‐meteoric waters having very little interaction with the host‐rock. Blocky calcite cement type 4 (CC4 generation) has the lowest oxygen isotope ratio and the most radiogenic ⁸⁷Sr/⁸⁶Sr values, indicating low fluid‐rock interaction. The increasingly open palaeohydrogeological system was dominated by migration of hot brines with elevated oxygen isotope ratios into the buried karstic system. The main oil emplacement in the Amposta reservoir occurred after the CC4 event, closely related to the Neogene extensional fractures. Corrosion of CC4 (blocky calcite cement type 4) occurred prior to (or during) petroleum charge, possibly related to kaolinite precipitation from relatively acidic fluids. Barite and pyrite precipitation occurred after this corrosion. The sulphur source associated with the late precipitation of pyrite was likely related to isotopically light sulphur expelled, e.g. as sulphide, from the petroleum source rock (Ascla Fm). Geofluids (2010) 10 , 314–333     

Origin of palaeo‐waters in the Ordovician carbonates in Tahe oilfield,Tarim Basin: constraints from fluid inclusions and Sr,C and O isotopes

Petrographic features, isotopes, and trace elements were determined, and fluid inclusions were analyzed on fracture‐filling, karst‐filling and interparticle calcite cement from the Ordovician carbonates in Tahe oilfield, Tarim basin, NW China. The aim was to assess the origin and evolution of palaeo‐waters in the carbonates. The initial water was seawater diluted by meteoric water, as indicated by bright cathodoluminescence (CL) in low‐temperature calcite. The palaeoseawater was further buried to temperatures from 57 to 110°C, nonluminescent calcite precipitated during the Silurian to middle Devonian. Infiltration of meteoric water of late Devonian age into the carbonate rocks was recorded in the first generation of fracture‐ and karst‐filling dull red CL calcite with temperatures from <50°C to 83°C, low salinities (<9.0 wt%), high Mn contents and high ⁸⁶Sr/⁸⁷Sr ratios from 0.7090 to 0.7099. During the early Permian, ⁸⁷Sr‐rich hydrothermal water may have entered the carbonate rocks, from which precipitated a second generation of fracture‐filling and interparticle calcite and barite cements with salinities greater than 22.4 wt%, and temperatures from 120°C to 180°C. The hydrothermal water may have collected isotopically light CO₂ (possibly of TSR‐origin) during upward migration, resulting in hydrothermal calcite and the present‐day oilfield water having δ¹³C values from ?4.3 to ?13.8‰ and showing negative relationships of ⁸⁷Sr/⁸⁶Sr ratios to δ¹³C and δ¹⁸O values. However, higher temperatures (up to 187°C) and much lower salinities (down to 0.5 wt%) measured from some karst‐filling, giant, nonluminescent calcite crystals may suggest that hydrothermal water was deeply recycled, reduced (Fe‐bearing) meteoric water heated in deeper strata, or water generated from TSR during hydrothermal water activity. Mixing of hydrothermal and local basinal water (or diagenetically altered connate water) with meteoric waters of late Permian age and/or later may have resulted in large variations in salinity of the present oilfield waters with the lowest salinity formation waters in the palaeohighs.     

Bleached mudstone,iron concretions,and calcite veins: a natural analogue for the effects of reducing CO2‐bearing fluids on migration and mineralization of iron,sealing properties,and composition of mudstone cap rocks

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.     

Fracture mineralization and fluid flow evolution: an example from Ordovician–Devonian carbonates,southwestern Ontario,Canada

Petrography, geochemistry (stable and radiogenic isotopes), and fluid inclusion microthermometry of matrix dolomite, fracture‐filling calcite, and saddle dolomite in Ordovician to Devonian carbonates from southwestern Ontario, Canada, provide useful insights into fluid flow evolution during diagenesis. The calculated δ¹⁸O_fluid, ΣREE, and REE_SN patterns of matrix and saddle dolomite suggest diverse fluids were involved in dolomitization and/or recrystallization of dolomite. The ⁸⁷Sr/⁸⁶Sr ratios of dolomite of each succession vary from values in the range of coeval seawater to values more radiogenic than corresponding seawater, which indicate diagenetic fluids were influenced by significant water/rock interaction. High salinities (22.4–26.3 wt. % NaCl + CaCl₂) of Silurian and Ordovician dolomite–hosted fluid inclusions indicate involvement of saline waters from dissolution of Silurian evaporites. High fluid inclusion homogenization temperatures (>100°C) in all samples from Devonian to Ordovician show temperatures higher than maximum burial (60–90°C) of their host strata and suggest involvement of hydrothermal fluids in precipitation and/or recrystallization of dolomite. A thermal anomaly over the mid‐continent rift during Devonian to Mississippian time likely was the source of excess heat in the basin. Thermal buoyancy resulting from this anomaly was the driving force for migration of hydrothermal fluids through regional aquifers from the center of the Michigan Basin toward its margin. The decreasing trend of homogenization temperatures from the basin center toward its margin further supports the interpreted migration of hydrothermal fluids from the basin center toward its margin. Hydrocarbon‐bearing fluid inclusions in late‐stage Devonian to Ordovician calcite cements with high homogenization temperatures (>80°C) and their ¹³C‐depleted values (approaching ?32‰ PDB) indicate the close relationship between hydrothermal fluids and hydrocarbon migration.     

Fluid mixing induced by hydrothermal activity in the ordovician carbonates in Tarim Basin,China

Permian hydrothermal activity in the Tarim Basin may have been responsible for the invasion of hot brines into Ordovician carbonate reservoirs. Studies have been undertaken to explain the origin and geochemical characteristics of the diagenetic fluid present during this hydrothermal event although there is no consensus on it. We present a genetic model resulting from the study of δ¹³C, δ¹⁸O, δ³⁴S, and ⁸⁷Sr/⁸⁶Sr isotope values and fluid inclusions (FIs) from fracture‐ and vug‐filling calcite, saddle dolomite, fluorite, barite, quartz, and anhydrite from Ordovician outcrops in northwest (NW) Tarim Basin and subsurface cores in Central Tarim Basin. The presence of hydrothermal fluid was confirmed by minerals with fluid inclusion homogenization temperatures being >10°C higher than the paleo‐formation burial temperatures both in the NW Tarim and in the Central Tarim areas. The mixing of hot (>200°C), high‐salinity (>24 wt% NaCl), ⁸⁷Sr‐rich (up to 0.7104) hydrothermal fluid with cool (60–100°C), low‐salinity (0 to 3.5 wt% NaCl), also ⁸⁷Sr‐rich (up to 0.7010) meteoric water in the Ordovician unit was supported by the salinity of fluid inclusions, and δ¹³C, δ¹⁸O, and ⁸⁷Sr/⁸⁶Sr isotopic values of the diagenetic minerals. Up‐migrated hydrothermal fluids from the deeper Cambrian strata may have contributed to the hot brine with high sulfate concentrations which promoted thermochemical sulfate reduction (TSR) in the Ordovician, resulting in the formation of ¹²C‐rich (δ¹³C as low as ?13.8‰) calcite and ³⁴S‐rich (δ³⁴S values from 21.4‰ to 29.7‰) H₂S, pyrite, and elemental sulfur. Hydrothermal fluid mixing with fresh water in Ordovician strata in Tarim Basin was facilitated by deep‐seated faults and up‐reaching faults due to the pervasive Permian magmatic activity. Collectively, fluid mixing, hydrothermal dolomitization, TSR, and faulting may have locally dissolved the host carbonates and increased the reservoir porosity and permeability, which has significant implications for hydrocarbon exploration.     

The relevance of vein texture in understanding the past hydraulic behaviour of a crystalline rock mass: reconstruction of the palaeohydrology of the Mecsekalja Zone,south Hungary

This study reconstructs the palaeohydrogeologic evolution of the shallow‐to‐moderate Mesozoic subsidence history for the Mecsekalja Zone (MZ), a narrow metamorphic belt in the eastern Mecsek Mountains, Hungary. Brittle deformation of the MZ produced a vein system with a cement history consisting of five sequential carbonate generations and one quartz phase. Vein textures suggest different fluid‐flow mechanisms for the parent fluids of subsequent cement generations. Combined microthermometric and stable‐isotope measurements permit reconstruction of the character of subsequent fluid generations with different flow types, as defined by vein textures, yielding new information regarding the hydraulic behaviour of a metamorphic crystalline complex. Textural observations and geochemical data suggest that fracture‐controlled flow pathways and externally derived fluids were typical of some flow events, while percolation through the rock matrix and the relationship to the Cretaceous volcanism and dyke emplacement were typical of others. The difference in the mode of calcite deposition from pervasive fluids (i.e. pervasive carbonatisation along grain boundaries versus deposition in antitaxial veins) between two calcite generations related to the volcanism inspired a stress‐dependent model of antitaxial vein growth. Textural and isotope variations in a vein generation produced by the same parent fluid indicate rock‐dependent hydraulic behaviour for different rock types, distinct action of the contemporaneous fracture systems and different extents of fluid–rock interaction. Cathodoluminescence microscopy and fluid‐inclusion microthermometry shed light on the possible role of hydraulic fracturing in the formation of massive calcite. The time of formation was estimated from the isotope composition of the oldest calcite generation and its presumptive relationship with the sedimentary sequences to the north, whereas microthermometry permitted conciliation of the reconstructed flow sequence with the Mesozoic subsidence history of the Mórágy Block (including the MZ).     

Hydrothermal dolomitization,mixing corrosion and deep burial porosity formation: numerical results from 1‐D reactive transport models

Major corrosion has been found at depth in carbonate hydrocarbon reservoirs from different geologic provinces. Fluid inclusion microthermometry and stable isotopic compositions of carbonate cements, predating major corrosion, constrain the interpretation of the evolution of parental fluids during progressive burial and prior to the major corrosion event. Post‐major corrosion mineral paragenesis includes pyrite (‐marcasite), anhydrite, kaolinite (dickite) and fluorite. Although the post‐corrosion mineral paragenesis represents minor volumes of rock, it may provide valuable insights into the post‐corrosion brine chemistry. Using reactive transport numerical models, the roles of cooling and/or mixing of brines on corrosion have been evaluated as controls for dolomitization, deep burial corrosion and precipitation of the post‐corrosion mineral paragenesis. Modelling results show that cooling of deep‐seated fluids moving upward along a fracture may cause minor calcite dissolution and porosity generation. Significant dolomitization along a fracture zone and nearby host‐rock only occurs when deep‐seated fluids have high salinities (4 mol Cl kg^?1 of solution) and Darcian flow rates are relatively high (1 m³ m^?2 year^?1). Only minor volumes of quartz and fluorite precipitate in the newly formed porosity. Moreover, modelling results cannot reproduce the authigenic precipitation of kaolinite (dickite at high temperatures) by cooling. As an alternative to cooling as a cause of corrosion, mixing between two brines of different compositions and salinities is represented by two main cases. One case consists of the flow up along a fracture of deep‐seated fluids with higher salinities than the fluid in the wall rock. Dolomite does not precipitate at a fracture zone. Nevertheless, minor volumes of dolomite are formed away from the fracture. The post‐corrosion mineral paragenesis can be partly reproduced, and the results are comparable to those obtained from cooling calculations. Minor volumes of quartz and fluorite are formed, and kaolinite‐dickite does not precipitate. The major outputs of this scenario are calcite dissolution and slight net increase in porosity. A second case corresponds to the mixing of low salinity deep‐seated fluids, flowing up along fractures, with high salinity brines within the wall rock. Calculations predict major dissolution of calcite and precipitation of dolomite. The post‐corrosion mineral paragenesis can be reproduced. High volumes of quartz, fluorite and kaolinite‐dickite precipitate and may even completely occlude newly formed porosity.     

Role of compressive tectonics on gas charging into the Ordovician sandstone reservoirs in the Sbaa Basin,Algeria: constrained by fluid inclusions and mineralogical data

Structure‐ and tectonic‐related gas migration into Ordovician sandstone reservoirs and its impact on diagenesis history were reconstructed in two gas fields in the Sbaa Basin, in SW Algeria. This was accomplished by petrographical observations, fluid inclusion microthermometry and stable isotope geochemistry on quartz, dickite and carbonate cements and veins. Two successive phases of quartz cementation (CQ1 and CQ2) occurred in the reservoirs. Two phase aqueous inclusions show an increase in temperatures and salinities from the first CQ1 diagenetic phase toward CQ2 in both fields. Microthermometric data on gas inclusions in quartz veins reveal the presence of an average of 92 ± 5 mole% of CH₄ considering a CH₄‐CO₂ system, which is similar to the present‐day gas composition in the reservoirs. The presence of primary methane inclusions in early quartz overgrowths and in quartz and calcite veins suggests that hydrocarbon migration into the reservoir occurred synchronically with early quartz cementation in the sandstones located near the contact with the Silurian gas source rock at 100–140°C during the Late Carboniferous period and the late Hercynian episode fracturing at temperatures between 117 and 185°C, which increased in the NW‐direction of the basin. During the fracture filling, three main types of fluids were identified with different salinities and formation temperatures. A supplementary phase of higher fluid temperature (up to 226°C) recorded in late quartz, and calcite veins is related to a Jurassic thermal event. The occurrence of dickite cements close to the Silurian base near the main fault areas in both fields is mainly correlated with the sandstones where the early gas was charged. It implies that dickite precipitation is related to acidic influx. Late carbonate cements and veins (calcite – siderite – ankerite and strontianite) occurred at the same depths resulting from the same groundwater precipitation. The absence of methane inclusions in calcite cements result from methane flushing by saline waters.     

Fluid effect on hydraulic fracture propagation behavior: a comparison between water and supercritical CO2‐like fluid

The initiation of hydraulic fractures during fluid injection in deep formations can be either engineered or induced unintentionally. Upon injection of CO₂, the pore fluids in deep formations can be changed from oil/saline water to CO₂ or CO₂ dominated. The type of fluid is important not only because the fluid must fracture the rock, but also because rocks saturated with different pore fluids behave differently. We investigated the influence of fluid properties on fracture propagation behavior by using the cohesive zone model in conjunction with a poroelasticity model. Simulation results indicate that the pore pressure fields are very different for different pore fluids even when the initial field conditions and injection schemes (rate and time) are kept the same. Low viscosity fluids with properties of supercritical CO₂ will create relatively thin and much shorter fractures in comparison with fluids exhibiting properties of water under similar injection schemes. Two significant times are recognized during fracture propagation: the time at which a crack ceases opening and the later time point at which a crack ceases propagating. These times are very different for different fluids. Both fluid compressibility and viscosity influence fracture propagation, with viscosity being the more important property. Viscosity can greatly affect hydraulic conductivity and the leak‐off coefficient. This analysis assumes the in‐situ pore fluid and injected fluid are the same and the pore space is 100% saturated by that fluid at the beginning of the simulation.     

Textural and isotopic evidence on the fluid source and transport mechanism of antitaxial fibrous microstructures from the Alps and the Appalachians

The source of fluid‐forming veins is of great importance in order to understand the hydraulic system acting in the earth's crust. The study of syntectonic antitaxial veins is one of the few methods by which the opening history can be deduced from rocks, and thus these veins are of primary importance in determining rock kinematics. Antitaxial veins were taken from black shales in two different tectonic settings in the Helvetic Alps, Switzerland, and the Taconic Appalachians, New York State. These syntectonic extension veins are regularly spaced and are oriented sub‐normal to bedding. The vein microstructure displays a symmetry around the median line in the centre of the vein, and a symmetry in cathodoluminescence banding parallel to the vein–wall interface, which suggests transport along bedding‐parallel dissolution planes from both vein‐walls. Antitaxial veins nucleated in transgranular fractures, but evidence for ongoing multiple crack‐seal increments is lacking; rather, veins grew continuously keeping close contact to the vein‐wall. Radiogenic ⁸⁷Sr/⁸⁶Sr ratios are higher in the surrounding matrix than in the vein, and higher than the corresponding seawater data in all samples. Variations are small and calcite in both the vein and the host rock were derived from the same source of fluid in the Helvetic samples. Mass balance of Sr suggests that the amount of calcite is too small in the surrounding host rock to be derived locally. Stable oxygen compositions are heavier in the host rock than in the veins, with overall low variation in both δ¹⁸O and δ¹³C values in the Mesozoic Helvetic samples. Data point to a rock‐buffered system, the precipitate most likely derived from an external source. The lower Palaeozoic Appalachian veins have lesser δ¹⁸O values than the host rock, similar to the Helvetic veins. Radiogenic ⁸⁷Sr/⁸⁶Sr data and a large heterogeneity in stable isotope values indicate an open system. Microstructural and isotopic evidence suggests that the antitaxial veins were formed by pervasive fluid flow, with the solute at least partly derived from an external source.     

Infiltration of basinal fluids into high-grade basement, South Norway: sources and behaviour of waters and brines

Quartz veins hosted by the high‐grade crystalline rocks of the Modum complex, Southern Norway, formed when basinal fluids from an overlying Palaeozoic foreland basin infiltrated the basement at temperatures of c. 220°C (higher in the southernmost part of the area). This infiltration resulted in the formation of veins containing both two‐phase and halite‐bearing aqueous fluid inclusions, sometimes with bitumen and hydrocarbon inclusions. Microthermometric results demonstrate a very wide range of salinities of aqueous fluids preserved in these veins, ranging from c. 0 to 40 wt% NaCl equivalent. The range in homogenization temperatures is also very large (99–322°C for the entire dataset) and shows little or no correlation with salinity. A combination of aqueous fluid microthermometry, halogen geochemistry and oxygen isotope studies suggest that fluids from a range of separate aquifers were responsible for the quartz growth, but all have chemistries comparable to sedimentary formation waters. The bulk of the quartz grew from relatively low δ¹⁸O fluids derived directly from the basin or equilibrated in the upper part of the basement (T < 200°C). Nevertheless, some fluids acquired higher salinities due to deep wall‐rock hydration reactions leading to salt saturation at high temperatures (>300°C). The range in fluid inclusion homogenization temperatures and densities, combined with estimates of the ambient temperature of the basement rocks suggests that at different times veins acted as conduits for influx of both hotter and colder fluids, as well as experiencing fluctuations in fluid pressure. This is interpreted to reflect episodic flow linked to seismicity, with hotter dry basement rocks acting as a sink for cooler fluids from the overlying basin, while detailed flow paths reflected local effects of opening and closing of individual fractures as well as reaction with wall rocks. Thermal considerations suggest that the duration of some flow events was very short, possibly in the order of days. As a result of the complex pattern of fracturing and flow in the Modum basement, it was possible for shallow fluids to penetrate basement rocks at significantly higher temperatures, and this demonstrates the potential for hydrolytic weakening of continental crust by sedimentary fluids.     

Contrasting paleofluid systems in the continental basement: a fluid inclusion and stable isotope study of hydrothermal vein mineralization,Schwarzwald district,Germany

An integrated fluid inclusion and stable isotope study was carried out on hydrothermal veins (Sb‐bearing quartz veins, metal‐bearing fluorite–barite–quartz veins) from the Schwarzwald district, Germany. A total number of 106 Variscan (quartz veins related to Variscan orogenic processes) and post‐Variscan deposits were studied by microthermometry, Raman spectroscopy, and stable isotope analysis. The fluid inclusions in Variscan quartz veins are of the H₂O–NaCl–(KCl) type, have low salinities (0–10 wt.% eqv. NaCl) and high T_h values (150–350°C). Oxygen isotope data for quartz range from +2.8‰ to +12.2‰ and calculated δ¹⁸O_H2O values of the fluid are between ?12.5‰ and +4.4‰. The δD values of water extracted from fluid inclusions vary between ?49‰ and +4‰. The geological framework, fluid inclusion and stable isotope characteristics of the Variscan veins suggest an origin from regional metamorphic devolatilization processes. By contrast, the fluid inclusions in post‐Variscan fluorite, calcite, barite, quartz, and sphalerite belong to the H₂O–NaCl–CaCl₂ type, have high salinities (22–25 wt.% eqv. NaCl) and lower T_h values of 90–200°C. A low‐salinity fluid (0–15 wt.% eqv. NaCl) was observed in late‐stage fluorite, calcite, and quartz, which was trapped at similar temperatures. The δ¹⁸O values of quartz range between +11.1‰ and +20.9‰, which translates into calculated δ¹⁸O_H2O values between ?11.0‰ and +4.4‰. This range is consistent with δ¹⁸O_H2O values of fluid inclusion water extracted from fluorite (?11.6‰ to +1.1‰). The δD values of directly measured fluid inclusion water range between ?29‰ and ?1‰, ?26‰ and ?15‰, and ?63‰ and +9‰ for fluorite, quartz, and calcite, respectively. Calculations using the fluid inclusion and isotope data point to formation of the fluorite–barite–quartz veins under near‐hydrostatic conditions. The δ¹⁸O_H2O and δD data, particularly the observed wide range in δD, indicate that the mineralization formed through large‐scale mixing of a basement‐derived saline NaCl–CaCl₂ brine with meteoric water. Our comprehensive study provides evidence for two fundamentally different fluid systems in the crystalline basement. The Variscan fluid regime is dominated by fluids generated through metamorphic devolatilization and fluid expulsion driven by compressional nappe tectonics. The onset of post‐Variscan extensional tectonics resulted in replacement of the orogenic fluid regime by fluids which have distinct compositional characteristics and are related to a change in the principal fluid sources and the general fluid flow patterns. This younger system shows remarkably persistent geochemical and isotopic features over a prolonged period of more than 100 Ma.     

Basin‐scale fluid movement patterns revealed by veins: Wessex Basin,UK

Calcite veins at outcrop in the Mesozoic, oil‐bearing Wessex Basin, UK, have been studied using field characterization, petrography, fluid inclusions and stable isotopes to help address the extent, timing and spatial and stratigraphic variability of basin‐scale fluid flow. The absence of quartz shows that veins formed at low temperature without an influence of hydrothermal fluids. Carbon isotopes suggest that the majority of vein calcite was derived locally from the host rock but up to one quarter of the carbon in the vein calcite came from CO₂ from petroleum source rocks. Veins become progressively enriched in source‐rock‐derived CO₂ from the outer margin towards the middle, indicating a growing influence of external CO₂. The carbon isotope data suggest large‐scale migration of substantial amounts of CO₂ around the whole basin. Fluid inclusion salinity data and interpreted water‐δ¹⁸O data show that meteoric water penetrated deep into the western part of the basin after interacting with halite‐rich evaporites in the Triassic section before entering fractured Lower and Middle Jurassic rocks. This large‐scale meteoric invasion of the basin probably happened during early Cenozoic uplift. A similar approach was used to reveal that, in the eastern part of the basin close to the area that underwent most uplift, uppermost Jurassic and Cretaceous rocks underwent vein formation in the presence of marine connate water suggesting a closed system. Stratigraphically underlying Upper Jurassic mudstone and Lower Cretaceous sandstone, in the most uplifted part of the basin, contain veins that resulted from intermediate behaviour with input from saline meteoric water and marine connate waters. Thus, while source‐rock‐derived CO₂ seems to have permeated the entire section, water movement has been more restricted. Oil‐filled inclusions in vein calcite have been found within dominant E‐W trending normal faults, suggesting that these may have facilitated oil migration.     

Long‐term stability of fracture systems and their behaviour as flow paths in uplifting granitic rocks from the Japanese orogenic field

In granitic rocks, fracture networks typically provide pathways for groundwater flow and solute transport that need to be understood to assess the long‐term performance of deep underground storage or disposal facilities such as radioactive waste repositories. However, relatively little is known about the long‐term processes of fracturing and/or the longevity of flow paths (FP) in granitic rocks distributed within orogenic belts. To clarify these issues, Japanese plutons of different ages and in situ fractures in granite at the Mizunami Underground Research Laboratory (MIU) located in central Japan were studied. Detailed structural characterization and geochemical analysis of in situ fracture fillings sampled from a depth of 300 m were carried out to clarify the relationship between fracturing and mineral infilling processes. Different plutons show identical episodes of fracturing and fracture filling, consisting of: brittle tensile fracturing, due to decreasing temperature through the ductile–brittle transition after plutonic intrusion (Stage I); relatively rapid uplifting (ca. a few mm/year) accompanied by hydrothermal water circulation, which produced uncrushed layered mineral fillings (Stage II); and a period of low‐temperature meteoric water circulation following exposure after uplift (Stage III). The parageneses of carbonate mineral fracture fillings and their carbon isotopic compositions (¹⁴C, δ¹³C) show that there were distinct episodes of carbonate mineral precipitation during the rapid uplifting of a pluton. The carbonate minerals that formed during each episode incorporated carbon from a distinct source. The evolution of fillings identified here enables development of a specific model of fracturing and persistence of fluid‐conducting systems in the plutons of the orogenic field.     

Fracture-fill calcite as a record of microbial methanogenesis and fluid migration: a case study from the Devonian Antrim Shale, Michigan Basin

The Devonian Antrim Shale is an organic‐rich, naturally fractured black shale in the Michigan Basin that serves as both a source and reservoir for natural gas. A well‐developed network of major, through‐going vertical fractures controls reservoir‐scale permeability in the Antrim Shale. Many fractures are open, but some are partially sealed by calcite cements that retain isotopic evidence of widespread microbial methanogenesis. Fracture filling calcite displays an unusually broad spectrum of δ¹³C values (+34 to ?41‰ PDB), suggesting that both aerobic and anaerobic bacterial processes were active in the reservoir. Calcites with high δ¹³C values (>+15‰) record cementation of fractures from dissolved inorganic carbon (DIC) generated during bacterial methanogenesis. Calcites with low δ¹³C values (13C values between ?10 and ?30‰ can be attributed to variable organic matter oxidation pathways, methane oxidation, and carbonate rock buffering. Identification of ¹³C‐rich calcite provides unambiguous evidence of biogenic methane generation and may be used to identify gas deposits in other sedimentary basins. It is likely that repeated glacial advances and retreats exposed the Antrim Shale at the basin margin, enhanced meteoric recharge into the shallow part of the fractured reservoir, and initiated multiple episodes of bacterial methanogenesis and methanotrophic activity that were recorded in fracture‐fill cements. The δ¹⁸O values in both formation waters and calcite cements increase with depth in the basin (?12 to ?4‰ SMOW, and +21 to +27‰ PDB, respectively). Most fracture‐fill cements from outcrop samples have δ¹³C values between ?41 and ?15‰ PDB. In contrast, most cement in cores have δ¹³C values between +15 and +34‰ PDB. Radiocarbon and ²³⁰Th dating of fracture‐fill calcite indicates that the calcite formed between 33 and 390 ka, well within the Pleistocene Epoch.     

Metamorphic and basin fluids in quartz–carbonate–sulphide veins in the SW Scottish Highlands: a stable isotope and fluid inclusion study

Metalliferous (Fe–Cu–Pb–Zn) quartz–carbonate–sulphide veins cut greenschist to epidote–amphibolite facies metamorphic rocks of the Dalradian, SW Scottish Highlands, with NE–SW to NW–SE trends, approximately parallel or perpendicular to regional structures. Early quartz was followed by pyrite, chalcopyrite, sphalerite, galena, barite, late dolomite–ankerite and clays. Both quartz–sulphide and carbonate vein mineralisation is associated with brecciation, indicating rapid release of fluid overpressure and hydraulic fracturing. Two distinct mineralising fluids were identified from fluid inclusion and stable isotope studies. High temperature (>350°C) quartz‐precipitating fluids were moderately saline (4.0–12.7 wt.% NaCl equivalent) with low (approximately 0.05). Quartz δ¹⁸O (+11.7 to +16.5‰) and sulphide δ³⁴S (?13.6 to ?1.1‰) indicate isotopic equilibrium with host metasediments (rock buffering) and a local metasedimentary source of sulphur. Later, low‐temperature (T_H = 120–200°C) fluids, probably associated with secondary carbonate, barite and clay formation, were also moderately saline (3.8–9.1 wt.% NaCl equivalent), but were strongly enriched in ¹⁸O relative to host Dalradian lithologies, as indicated by secondary dolomite–ankerite (δ¹⁸O = +17.0 to +29.0‰, δ¹³C = ?1.0 to ?3.0‰). Compositions of carbonate–forming fluids were externally buffered. The veins record the fluid–rock interaction history of metamorphic host rocks during cooling, uplift and later extension. Early vein quartz precipitated under retrograde greenschist facies conditions from fluids probably derived by syn‐metamorphic dehydration of deeper, higher‐grade rocks during uplift and cooling of the Caledonian metamorphic complex. Veins are similar to those of mesothermal veins in younger Phanerozoic metamorphic belts, but are rare in the Scottish Dalradian. Early quartz veins were reactivated by deep penetration of low‐temperature basin fluids that precipitated carbonate and clays in veins and adjacent Dalradian metasediments throughout the SW Highlands, probably in the Permo‐Carboniferous. This event is consistent with paragenetically ambiguous barite with δ³⁴S characteristic of late Palaeozoic basinal brines.     

Hydrothermal palaeofluid circulation in the fracture network of the Baksa Gneiss Complex of SW Pannonian Basin,Hungary

A well‐developed fracture‐filling network is filled by dominantly Ca‐Al‐silicate minerals that can be found in the polymetamorphic rock body of the Baksa Gneiss Complex (SW Hungary). Detailed investigation of this vein network revealed a characteristic diopside→epidote→sphalerite→albite ± kfeldspar→chlorite1 ± prehnite ± adularia→chlorite2→chlorite3→pyrite→calcite1→calcite2→calcite3 fracture‐filling mineral succession. Thermobarometric calculations (two feldspar: 230–336°C; chlorites: approximately 130–300°C) indicate low‐temperature vein formation conditions. The relative succession of chlorites in the mineral sequence combined with the calculated formation temperatures reveals a cooling trend during precipitation of the different chlorite phases (T_chlorite1: 260 ± 32°C →T_chlorite2: 222 ± 20°C →T_chlorite3: 154 ± 13°C). This cooling trend can be supported by the microthermometry data of primary fluid inclusions in diopside (T_h: 276–362°C) and epidote (T_h: 181–359°C) phases. The identical chemical character (0.2–1.5 eq. wt% NaCl) of these inclusions mean that vein mineralization occurred in a same fluid environment. The high trace element content (e.g. As, Cu, Zn, Mn) and Co/Ni ratio approximately 1–5 of pyrite grains support the postmagmatic hydrothermal origin of the veins. The vein microstructure and identical fluid composition indicate that vein mineralization occurred in an interconnected fracture system where crystals grew in fluid filled cracks. Vein system formed at approximately <200 MPa pressure conditions during cooling from approximately 480°C to around 150°C. The rather different fluid characteristics (T_h: 75–124°C; 17.5–22.6 eq. wt% CaCl₂) of primary inclusions of calcite1 combining with the special δ¹⁸O signature of fluid from which this mineral phase precipitated refer to hydrological connection between the crystalline basement and the sedimentary cover.     

Fluids and halogens at the diagenetic–metamorphic boundary: evidence from veins in continental basins, western Norway

Seven vein types are recognized in three continental Devonian molasse basins (the Hornelen, Kvamshesten and Solund basins) in western Norway. These include calcite‐, quartz‐ and epidote‐dominated veins. The salinities of fluid inclusions from quartz‐dominated veins in the Hornelen and Kvamshesten basins are close to or slightly higher than those for modern seawater, whereas the fluids from quartz‐ and calcite‐dominated veins in the Solund basin range from seawater values to 20 wt % NaCl equivalent. Minerals such as biotite, amphibole, titanite, chlorite and epidote are abundant in the latter veins, and are important constituents of the authigenic mineral assemblages. A combination of fluid inclusion and petrological data suggest that at least some of the veins formed at depths around 12–14 km. The Cl/Br ratios and the salinity of the fluid inclusions can be explained by interactions with evaporites, implying that the sedimentary environment forming the basin fill had the strongest influence upon low‐grade metamorphic fluid Cl and Br contents. Differences in the Cl/I and Na/Br ratios between the Solund basin and the Hornelen and Kvamshesten basins are best explained by local mass transfer between pore fluids and the surrounding rock matrix during burial and increasing temperatures.     

Effects of pore fluid flow and chemistry on compaction creep of calcite by pressure solution at 150°C

Uni‐axial compaction creep experiments were performed on crushed limestone and analytical grade calcite powders at 150°C, a pore fluid pressure of 20 MPa, and effective axial stresses of 30 and 40 MPa. Previous experiments have shown that compaction under these conditions is dominated by intergranular pressure solution (IPS). The aim of the present tests was to determine the inter‐relationship between pore fluid chemistry, compaction rate and the rate‐controlling process of IPS. Intermittent flow‐through runs conducted using CaCO₃ solution showed no effect on creep rate at low strains (<4–5%) but a major acceleration at high strains (5–10%). Measurements of the Ca concentration present in fluid samples revealed the build‐up of a high super‐saturation of CaCO₃ during compaction under zero flow conditions, especially at high strains. Active flow‐through led to a drop in Ca concentration, which corresponded with creep acceleration. Addition of NaCl to the pore fluid, at a concentration of 0.5 m , increased the creep rate of the analytical grade calcite samples roughly in proportion to the enhancement of calcite solubility. Addition of Mg²⁺ and to the pore fluid, in concentrations of 0.05 and 0.001 m, respectively, caused major retardation of compaction creep. Integrating our mechanical, flow‐through and chemical data points strongly to diffusion‐controlled IPS being the dominant deformation mechanism in the calcite‐water system under closed‐system (zero flow) conditions at low strains (<4–5%), giving way to precipitation control at higher strains. Our fluid composition data suggest that this transition is because of accumulation of impurities in the pore fluid. As Mg²⁺ and phosphate ions are common in natural pore fluids, it is likely that retarded precipitation will be the rate‐limiting step of IPS in carbonates in nature. To quantify diagenetic compaction and porosity‐permeability reduction rates by IPS in carbonates needs to account for this.     

Fracture spacing during hydro‐fracturing of cap‐rocks

Layered low permeability rock units, like shales, represent seals or ‘cap‐rocks’ in a variety of geological settings. A continuous increase in the fluid pressure gradients across a virtually impermeable rock layer will ultimately lead to hydro‐fracturing. Depending on the boundary conditions, such fracturing may lead to the formation of a set of sub‐parallel cracks oriented more or less perpendicular to the cap‐rock layer. In this article, we propose a new numerical model that describes interactions between multiple cross‐cutting fractures in an elastic low permeability rock layer. The width of each fracture and the spacing between them are modeled as a force balance between the fluid pressure and the elastic forces in the cap‐rock and between each fracture. The model indicates that the system of fractures evolves toward a spatially periodic steady‐state distribution with a fixed fracture spacing and aperture. The results are similar for incompressible and compressible fluids. The steady‐state conditions depend on only two dimensionless parameters, and the fracture spacing is only weakly dependent on the cap‐rock thickness. This is in contrast to fracturing produced by simple extension of an elastic rock layer beyond the fracture strength, in which case fracture spacing is proportional to layer thickness.