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The preservation of natural gas can be easily dismissed by late crustal deformations, which are often concealed by fracture-filling calcite vein networks. This study introduces a novel approach to confine the timeframe of natural gas-leakage processes by employing coupled in situ U–Pb dating and clumped isotope (Δ 47) analysis of late-fracture calcite veins associated with brittle deformation in caprock/unconventional reservoirs during basin uplift. Specifically, Δ 47 and U–Pb data were acquired for fracture calcite veins that formed in high-angle or vertical fractures within the early Silurian shale reservoirs of the Southeastern Sichuan Basin in China. Absolute U–Pb ages revealed two distinct calcite precipitation events at 6.89 ± 0.18 and 3.55 ± 0.11 Ma, respectively. The primary mechanism for shale gas escape from early Silurian shale reservoirs was the leakage of natural gas through activated high-angle faults that had remained unsealed since at least 6.89 Ma. At approximately 3.55 Ma, the overlying weathered layer was lifted to the surface of the Earth, facilitating the connection between meteoric water and shale reservoirs through activated fractures. Natural gas leakage occurred concurrently with meteoric water infiltration, and a non-linear mixing effect between methanogenesis and meteoric water, as well as intense ventilation along the water-bearing fracture surface, is identified as the cause of delayed late-fracture calcite precipitation, leading to an abnormal δ13C of calcite vein (up to +22.1‰). This approach holds significant implications for assessing natural-gas preservation in uplifted basins worldwide. • Late crustal movements can mask natural gas risks, challenging preservation assessments. • Combined U–Pb dating and clumped isotope analysis on calcite veins show timing of natural gas leakage events at 6.89 Ma and 3.55 Ma. • Lifting of weathered layer connected meteoric water to shale reservoirs, causing natural gas leakage and abnormal calcite precipitation. [ABSTRACT FROM AUTHOR]

The lack of effective paleogeothermometers restricts the study of the thermal history of carbonate strata. Carbonate clumped isotope thermometry and in-situ U–Pb dating are appropriate for reconstructing the thermal history of carbonate strata. Herein, this study analyzed clumped isotope compositions and U–Pb ages of calcite fabrics, including micrite matrix and veins, from Permian carbonate strata in the eastern Sichuan Basin. U–Pb dating of the micrite matrix was performed to rule out the influence of recrystallization and determine the diagenetic age of the samples. Clumped isotope temperatures of the four micrite matrix samples ranged from 102 °C to 137 °C. The maximum paleotemperature derived from a solid-state reordering model was between 210 °C and 220 °C. From all four samples, three calcite veins yielded crystallization temperatures corresponding to 160 ± 10 °C, 60 ± 10 °C, and 29 ± 4 °C, as determined by clumped isotope and solid-state reordering model analyses. In-situ U–Pb dating provided corresponding crystallization ages of 159 ± 17 Ma, 256.6 ± 5.5 Ma, and 17.86 ± 0.8 Ma. The crystallization ages and temperatures of the three calcite veins align well with the thermal history of the Permian strata, indicating a high degree of accuracy. This study introduces a new strategy for reconstructing the thermal history of carbonate strata. The thermal history can be quantified using the constraints of the crystallization ages and temperatures from different stages of calcite veins or cements. This approach is suitable for determining the maturation progress of the source rock and conducting oil and gas exploration in petroliferous basins. • Permian strata's maximum paleotemperature simulated by clumped isotope is 210–220 °C. • Solid-state reordering model can get the calcite vein's crystallization temperature. • Calcite veins are essential to reconstructing the thermal history of carbonate strata. [ABSTRACT FROM AUTHOR]

Many low-permeability, hydrocarbon-bearing rocks are productive for petroleum because natural fractures enhance hydrocarbon delivery to wellbores. Exploration and development decisions often must be made in the face of uncertainty about the presence and properties of natural fractures at scales of beds, field and plays, and how they contribute to production. Fractures patterns in subsurface rocks are challenging to predict or characterize owing to the difficulty in obtaining representative samples of fractures and the sparse set of tools for reconstructing how and when fractures grow during sedimentary basin evolution. In this study, we investigate bed-parallel fracture diagenesis in the Vaca Muerta shales (Argentina) based on a set of calcite vein collected from core across a thermal maturity sequence that ranges from the oil to dry gas window. We characterize these samples using secondary ions mass spectrometry (SIMS) measurements of stable isotope ratios, carbonate clumped isotope geothermometry and fluid inclusion analysis. These measurements precisely reconstruct the evolution of fluid geochemistry, temperature and pore fluid pressure during fracture opening. Here we show that within each studied site, all bedding parallel fibrous veins calcite formed at similar ranges of temperature. The fracture mineralization temperature increases substantially at the basin scale, from 90 °C to 120 °C, correlated with host-rock thermal maturity. We compare our constraints on carbonate and fluid temperature with apatite fission track thermal models and find that most of the fractures of the Vaca Muerta shales have formed in a relatively early stage of subsidence of the basin (c.a. 90 to 70 Ma). These ages estimations are consistent with recently published U–Pb ages on calcite veins from the outcropping area (117–61 Ma). The presence of petroleum inclusions (oil, condensate, and gas) in most of the vein's calcite fibers from cores also provides clear evidence that fracture formation was contemporaneous or posterior with petroleum generation, constraining the timing of catagenesis in the Vaca Muerta. Reconstructed paleo pressures conditions of veins formation from fluid inclusion analysis highlight significant fluid overpressure in the shale interval, which likely contributed to fracture initiation and growth. Preserved apparent Δ 47 temperatures in fibrous veins carbonates mineralization evolves gradually from 130 °C to 175 °C with increasing host rock thermal maturity. These clumped-isotope-based temperatures are systematically higher than trapping temperatures of associated fluid inclusions; one interpretation of this observation is that carbonate clumped isotope compositions re-equilibrated to near maximum burial conditions by solid-state isotopic re-ordering, yielding a record of the maximum burial temperature the shale unit experienced. The combination of paleothermometry techniques used in this study illustrates the importance of simultaneously exploring several independent geothermometers, particularly the informative combination of fluid inclusion and clumped isotope constraints, which provides a means of understanding the complex diagenesis of old sedimentary rocks that have experienced burial temperatures in the oil and gas thermal windows. • Paired clumped isotope paleothermometry and fluid inclusion analyses for time–temperature evaluation for natural fractures in the VMF. • Direct relationship between hydraulic fracturing intensity and the level of organic catagenesis (maturity) • PVT modelling of aqueous-HC fluid inclusions indicate that calcite precipitation under fluid overpressure. • Combination of FI and Δ 47 geothermometry indicate altered (reset) Δ 47 compositions during burial. [ABSTRACT FROM AUTHOR]

Abstract The dissolution of marine shales and its influence on reservoir physical properties was revealed based on shale samples from the Ordovician-Silurian Wufeng and Longmaxi Formations in the southeast of the Sichuan Basin, China. Three kinds of gas storage spaces were formed by dissolution, including intergranular dissolution (InterD) pores, intragranular dissolution (IntraD) pores and dissolution microfractures (DmF). The diameters of the InterD and IntraD are 10.2–72.4 μm and 0.13–1.52 μm, respectively. The apertures of the DmF are between 0.5 and 11.2 μm. The dissolution effect resulted in a diagenetic environment for quartz cementation and a material basis for calcite veins. The main controlling factors of the dissolution effect are the mineral constituent content in the matrix, the amount of organic matter and the strata burial rate. Dissolution pores and fractures play important roles in shale gas enrichment and migration. The porosity of shale reservoirs with DmF is twice the porosity of reservoirs without DmF, and the spaces formed by dissolution pores and fractures are well connected. A new model for shale gas flow in shale reservoirs was proposed, which provides a new understanding for the connection between organic material pores, and the results can be used for shale reservoirs worldwide. Highlights • Calcite veins are effective channels for shale flow. • Dissolution pores and dissolution fractures play an important role in the transfusion of shale gas. • Three main controlling factors of dissolution in shale reservoirs are given. • A new model for shale gas flow in shale reservoirs is proposed. [ABSTRACT FROM AUTHOR]

The Jabal Qusaybah Anticline, in north Oman, is affected by syn-folding strike-slip and extensional fault zones developed during foreland deformation ahead of the Northern Oman Mountains thrust wedge, in Cenozoic times. Migration of fluids in fault-damage zones is recorded in complex calcite vein networks. By employing the microthermometric and compositional microanalysis of the fluid inclusions (crush-leach), two distinct generations of veins have been studied. The aim was to determine the source of elevated salinity in fluids involved in their cementation and explain their compositional evolution through fluid-rock interactions. The ionic ratios (Na/Br and Cl/Br) obtained from crush-leach analysis give supporting evidence that the elevated salinity of fluid inclusions in both vein groups originated from an evaporated seawater beyond the onset of halite precipitation (residual brines). The results reveal a gradual increase in salinity of the fluids, F/Cl molar ratios, as well as Li/Cl molar ratios. These results imply the progressively increasing contribution of evaporitic residual brines and fluids that interacted with, or were derived from siliciclastic rocks. We suggest that the most likely origin of the former fluids is provided by residual brines associated with precipitation of the Ara evaporites (Cambrian). The regional driving mechanism for such a significant fluid migration is believed to be compaction-driven upward flow that was channeled into faults and fractures during major deformational events. [ABSTRACT FROM AUTHOR]

This study integrates field structural data, petrographic and geochemical (δ18O, δ13C, Δ 47 , 87Sr/86Sr, and elemental composition) analyses and U–Pb dating of calcite veins cutting the Bóixols-Sant Corneli anticline (Southern Pyrenees) in order to date and to investigate the spatio-temporal relationships between fluid flow and fold evolution. This E-W trending anticline grew from Late Cretaceous to Paleocene at the front of the Bóixols thrust sheet deforming pre-growth and growth sedimentary sequences. U–Pb dating reveals Late Cretaceous to late Miocene deformation ages, which agree with the age of growth strata deposition and the sequence of deformation interpreted from field and microstructural data. Dates coeval (71.2 ± 6.4 to 56.9 ± 1.4 Ma) and postdating (55.5 ± 1.2 to 27.4 ± 0.9 Ma) Upper Cretaceous to Paleocene growth strata are interpreted to record: (i) the growth of the Bóixols-Sant Corneli anticline during the Bóixols thrust emplacement, and (ii) the tightening of the anticline during the southern tectonic transport of the South-Central Pyrenean Unit. Other ages (20.8 ± 1.2 to 9.0 ± 4.6 Ma) postdate the folding event and have been associated with the collapse of the Bóixols-Sant Corneli anticline. The geochemistry of calcite veins indicates that the fluid flow behavior varied across the Bóixols-Sant Corneli anticline through its growth, showing a compartmentalized fluid system. In the hinge of the anticline and in the upper Santonian to middle Campanian syn-orogenic sequence along the footwall of the Bóixols thrust, the similar petrographic and geochemical features between all calcite cements and host rocks point towards a locally-derived or well-equilibrated fluid system. Contrarily, along large faults such as the Bóixols thrust, and in the anticline limbs, the geochemistry of vein cements indicates a different scenario. Cements in large faults yielded the lightest δ18O values, from −8 to −14 ‰VPDB, and variable enrichment in δ13C, 87Sr/86Sr, elemental composition and δ18O fluid. This is interpreted as the migration of fluids, through fault zones, that evolved from distinct fluid origins. Cements in the fold limbs exhibit δ18O and δ13C between −8 and −6 ‰VPDB and between −10 and + 2 ‰VPDB, respectively, the lowest Sr contents and the lowest precipitation temperatures, suggesting that the anticline limbs recorded the infiltration and evolution of meteoric waters. The paleohydrological system in the Bóixols-Sant Corneli anticline was restricted to the Bóixols thrust sheet. The Upper Triassic evaporitic basal detachment likely acted as a lower fluid barrier, preventing the input of fluids from deeper parts of the Pyrenean crustal thrust system. This study provides a well-constrained absolute timing of fracturing and fluid flow during basin inversion and folding evolution and highlights the suitability of U–Pb geochronology to refine the age of fractures and veins and their sequential evolution in orogenic belts. • Twenty-three new U-Pb dates provide a well-constrained timing of deformation in an anticline that preserves pre-, syn- and post-folding fractures that do not exhibit a symmetrical orientation with respect to the fold axis, which have important implications for the interpretation of fold-fracture systems. • We further constrain the age and duration of fold evolution in the Bóixols-Sant Corneli anticline. • The geochemistry of calcite veins reveals that the fluid flow behavior varied across the different structural positions of the Bóixols-Sant Corneli anticline, evidencing a compartmentalized fluid system. • The paleohydrological system in the Bóixols-Sant Corneli anticline only involves fluids sourced above the detachment levels, which may act as a lower boundary for the fluid system, preventing the input of fluids from deeper parts of the belt. [ABSTRACT FROM AUTHOR]

Abstract: In fold-and-thrust belts, shortening is mainly accommodated by thrust faults which are preferential zones for recrystallisation and mass transfer. This study focuses on a detachment fault related to the emplacement of the Monte Perdido thrust unit in the southern Pyrenees. The studied fault zone consists of a 10 m thick intensively foliated phyllonite developed within the Millaris marls, of Eocene age. The lithological homogeneity of the hanging wall and footwall allows us to compare the Millaris marls outside the fault zone with the highly deformed marls located in the fault zone and to quantify the chemical, mineralogical and volumetric changes related to deformation processes along the fault. The Millaris marls are composed of detrital quartz, illite, chlorite, minor albite and pyrite, in a micritic calcite matrix. In the fault zone, the cleavage planes are marked by clay minerals and calcite ± chlorite veins attest to fluid–mineral interactions during deformation. The mineral proportions in all samples from both the fault zone and Millaris marls have been quantified by two methods: (1) X-ray diffraction and Rietveld refinement, and (2) bulk chemical analyses as well as microprobe analyses to calculate modal composition. The excellent agreement between the results of these two methods allows us to estimate mineralogical variations using a modification of the Gresens' equation. During fault activation, up to 45 wt% of calcite was lost while the amounts of quartz and chlorite remained unchanged. Illite content remained constant to slightly enriched. The mineralogical variations were coupled with a significant volume loss (up to 45%) mostly due to the dissolution of micritic calcite grains. Deformation was accompanied by pressure solution and phyllosilicates recrystallisation. These processes accommodated slip along the fault. They required fluids as catalyst, but they did not necessitate major chemical transfers. [Copyright &y& Elsevier]

The Paraná Basin, southern Brazil, has an atypical thermal and fluid history due to the occurrence of an episodic continental flood volcanism during the Early Cretaceous. So far, there are few data about the influence of this volcanic event on the paleotemperatures and paleofluids of the Paraná Basin sedimentary rocks. The Teresina Formation in the northern flank of the Ponta Grossa dyke swarm hosts high concentration of subsurface igneous rock bodies (sills and dykes), besides its covering by a hundreds meter thick volcanic rock cap. In this study, we used fluid inclusion analysis performed in horizontal and vertical calcite veins from the Teresina Formation and from a Late Cretaceous basic dyke to estimate paleotemperatures and to characterize the composition of diagenetic paleofluids. Homogenization temperatures of requilibrated fluid inclusions show that the Teresina Formation reached temperatures above 200 °C. Horizontal parallel bedding calcite veins from the Teresina Formation record low to high salinity (2–26 wt.% NaCl eq.) aqueous paleofluids. The prevalence of high salinity fluid inclusions associated with light hydrocarbon fluid inclusions indicates deep buried fluids. Fluid inclusions in vertical calcite vein from basic dyke comprise only low salinity aqueous fluids (0–3 wt.% eq.NaCl) interpreted as dominated by meteoric water. The recorded paleotemperatures are attributed to the heating by the Paraná volcanic event during the Early Cretaceous, with the thermal effect of the volcanic rock cap surpassing the effect of nearby sills and dykes. Estimated paleotemperatures higher than 200 °C would allow the generation of light liquid and gaseous hydrocarbons. Overpressured compartments in the Teresina Formation allowed the expulsion of buried pore fluids (high salinity aqueous fluids and hydrocarbons) to fracture systems, where they mixed with meteoric water. The input of meteoric water through fracture systems connected with the surface favored hydrocarbons degradation in the early stages of source rock maturation during the Early Cretaceous. [ABSTRACT FROM AUTHOR]

Abstract: During compressive events, deformation in sedimentary basins is mainly accommodated by thrust faulting and related fold growth. Thrust faults are generally rooted in the basement and may act as conduits or barriers for crustal fluid flow. Most of recent studies suggest that fluid flow through such discontinuities is not apparent and depends on the structural levels of the thrust within the fold-and-thrust belt. In order to constrain the paleofluid flow through the Jaca thrust-sheet-top basin (Paleogene southwest-Pyrenean fold-and-thrust belt) this study compares on different thrust faults located at different structural levels. The microstructures in the different fault zones studied are similar and consist of pervasive cleavage, calcite shear veins (SV1), extension veins (EV1) and late dilatation veins (EV3). In order to constrain the nature and the source of fluids involved in fluid-rock interactions within fault zones, a geochemical approach, based on oxygen and carbon stable isotope and trace element compositions of calcite from different vein generations and host rocks was adopted. The results suggest a high complexity in the paleohydrological behaviors of thrust faults providing evidence for a fluid-flow compartmentalization within the basin. Previous studies in the southern part of the Axial Zone (North of the Jaca basin) indicates a circulation of deep metamorphic water, probably derived from the Paleozoic basement, along fault zones related to the major basement Gavarnie thrust. In contrast, in northern part of the Jaca basin, the Monte Perdido thrust fault is affected by a closed hydrological fluid system involving formation water during its activity. The Jaca and Cotiella thrust faults, in turn, both located more to the south in the basin, are characterized by a composite fluid flow system. Indeed, stable isotope and trace element compositions of the first generations of calcite veins suggest a relatively closed paleohydrological system, whereas the late calcite vein generations, which are probably associated with the late tectonic activity of the basin, support a contribution of both meteoric and marine waters. Based on these results, a schematic fluid-flow model is presented. This model allows visualization of three main fluid flow compartments along a N–S transect. [Copyright &y& Elsevier]