Fluid Inclusion Analysis Research Articles

PT conditions of deformation of the Gol-e-Gohar shear Zone, SW Iran: Insights from analysis of quartz c-axis fabrics, recrystallization mechanisms and syndeformational fluid inclusions

An investigation into the correlation between microstructural characteristics and deformation temperatures of quartz in the middle crust is of utmost importance in yielding substantial knowledge of the nature of deformation zones. Mylonitic rocks from the Gol-e-Gohar Shear Zone were investigated to infer innovative insights into the ductile deformation pressure–temperature (PT) conditions of the Sanandaj–Sirjan metamorphic rocks within the Zagros, southwest Iran. This approach used a multi-method strategy including quartz c-axis fabrics, recrystallization mechanisms and fluid inclusions (Fls) data. Geothermobarometric analysis based on these data highlight deformational PT conditions ranging from ∼400 to ∼500 °C and P < 5 kbar that correspond to ductile deformation with greenschist to lower-amphibolite conditions at depth of ∼ 12–19 km. Measured opening angles of quartz c-axis fabric and dominance of basal < a > slip system indicate crystal plastic deformation occurred in T > 400 °C. Microstructures highlighting the occurrence of Grain Boundary Migration (GBM) confirm temperature of 500–600 °C. The deduced P–T–salinity Fls history of deformation-related fluid inclusions is highlighted by Fls (B-type) with H2O-CO2–rich metamorphic fluids. These fluids were trapped during ductile deformation in temperatures ranging from ∼ 355 to ∼ 535 °C and pressure of P < 5 kbar. The petrofabric analyses show that the mylonitic rock in the Gol-e-Gohar shear zone have undergone metamorphism at the amphibolite facies condition, and then followed by a retrograde greenschist facies metamorphism. These results emphasize the utility of microstructural and fluid inclusion techniques to constrain the history of mylonitic rocks exhumation in the deformation zones.

Reliability of Raman analyses of CO2-rich fluid inclusions as a geobarometer at Kīlauea

Reliability of Raman analyses of CO2-rich fluid inclusions as a geobarometer at Kīlauea

Mineralogical characteristics and fluid inclusions of the Dashuixiabei crystalline graphite deposit in the Dunhuang block

AbstractGraphite is a strategic mineral resource of the world. The Dashuixiabei (DSXB) crystalline graphite deposit is located in the Dongbatu area in the Altyn crystalline graphite metallogenic belt. This study presents data from x‐ray diffraction (XRD), scanning electron microscopy (SEM), laser Raman spectroscopy, and fluid inclusions analysis conducted on the DSXB crystalline graphite deposit. The graphite is flake shaped, with a diameter of 0.1–0.4 mm. A group of extremely complete cleavage was observed under SEM, and the overall distribution was a flake. The C content in graphite is greater than 90 wt%, followed by N, O, and so on. The results of XRD analysis show that the lattice parameters of the DSXB crystalline graphite is a = 0.246–0.2465 nm, c = 0.6714–0.6715 nm, and V = 0.03519–0.03532 nm3. The degree of graphitization ranges from 0.8490 to 0.8519, indicating that the graphite crystal structure is relatively intact, the average metamorphic temperature is 552°C, and the content of 3R polytypes ranges from 13.16% to 13.25%. It is concluded that the metamorphic rocks of the DSXB crystalline graphite deposit have undergone medium‐grade metamorphism. The Raman spectral peaks show that DSXB crystalline graphite has a high degree of crystallinity. The DSXB crystalline graphite ore is dominated by hydrothermal fluids with medium‐low temperature and medium‐low salinity, and the vapor phase of the fluid inclusions is composed of CH4. There may be a superposition of the main and secondary mineralization stages. The crystalline graphite from the DSXB deposit shows high quality and stable ore quality, which has good metallogenic conditions and development and utilization prospects.

Multiple geochronological evidences revealing oil charging and redistribution processes in the Tazhong-4 Oilfield, Tazhong Uplift (Tarim Basin): Implications for quantitative reconstruction of petroleum accumulation history in superimposed basins

Re-migration within a hydrocarbon reservoir is an important yet poorly constraint process in petroleum system analysis. Here we investigated a Paleozoic petroleum reservoir in the Tarim Basin and quantitatively reconstructed the oil re-migration process using fluid inclusions and multiple isotopic dating techniques including the calcite U–Pb dating, the oil Re–Os dating, and the illite K–Ar dating. The Re–Os dating for the Carboniferous oil samples demonstrated an oil generation process during the Middle Caledonian (479 ± 110Ma). Two episodes of oil charging were revealed by each represented oil inclusion fluorescing yellowish and blue color in the Cambrian and Ordovician intervals. Calcite U–Pb ages suggest the Cambrian and Ordovician paleo-reservoirs represented oil inclusion fluorescing yellowish color were charged by mature oils during the late Caledonian orogeny (420-384Ma), and illite K–Ar ages show the oils with similar maturity charged into the Carboniferous reservoir during the late Hercynian orogeny (278.4–246.0Ma). This contradiction in absolute ages reveals an oil re-migration process with oils that originally was generated during the Middle Caledonian and charged in the Cambrian and Ordovician reservoirs and then migrated vertically to the Carboniferous reservoir ∼200 Myrs later possibly due to fault reactivation. The successful application of fluid inclusion analysis and multiple absolute dating techniques in the Tarim Basin provides a paradigm for quantitative reconstruction of petroleum (re-) accumulation history in similar tectonic superimposed basins.

Genesis and supergene weathering of tetrahedrite-(Hg) in meta-carbonate rocks: Bearing on differential mobility of priority pollutant metals

The Cu-Sb-Hg mineralization of San Giuliano Terme (Monti Pisani, Tuscany, Italy) is characterized by the widespread occurrence of deeply altered tetrahedrite-(Hg) hosted within joints and faults in the Liassic “Calcari ceroidi” Formation. Through a multi-technique approach (optical and scanning electron microscopy, electron microprobe analysis, single-crystal and powder X-ray diffraction, micro-Raman spectroscopy, X-ray fluorescence, fluid inclusion analysis), tetrahedrite-(Hg) and its alteration products have been characterized and data about its genesis and successive supergene weathering have been collected. Tetrahedrite-(Hg) from San Giuliano Terme has the empirical formula Cu9.92Ag0.02(Hg1.64Fe0.30Zn0.04)Σ1.98(Sb3.53As0.56)Σ4.09S12.95, and is found with scarce gangue minerals represented by calcite, baryte, and very rare fluorite. The mineralization formed from hydrothermal fluids with moderately high salinity (∼17.5 wt% NaClequiv.) of dominantly metamorphic origin at T ∼ 285 °C. Tetrahedrite-(Hg) is usually fully replaced by a mixture of cinnabar, roméite-group minerals, malachite, and rarely azurite. Based on the modal abundance of the mineralogical constituents and their micro-textures, four stages of supergene weathering of tetrahedrite-(Hg) have been recognized. The alteration stages reflect the progressive ingression of meteoric water into the system, with consequent oxidation of tetrahedrite-(Hg) and the introduction of Ca2+, HCO3– and H2O, as well as the progressive removal of S, Sb, and Cu. The relative proportions of the supergene minerals in the four alteration stages can be taken as a proxy for the mobility of the main chemical constituents of tetrahedrite-(Hg), three of which (Hg, Sb, and Cu) are priority pollutant metals. Mercury was found to be the least mobile element in the reconstructed geochemical process. In fact, the widespread precipitation of cinnabar in the space formerly occupied by tetrahedrite-(Hg) hinders the dispersion of Hg in the environment, sequestering it in a solid matrix.

Raman spectroscopic characterization of the CO2-N2 gaseous system at 24–300°C and 2–40 MPa and applications

ABSTRACT CO2 and N2 are important gas components of deep-sourced geological fluids. To establish a quantitative method for the study of these fluids, we used a high-pressure optical cell, a heating–cooling stage, and a laser Raman spectrometer to examine pure CO2 and N2 gases and nine CO2-N2 gaseous mixtures at 24–300°C and 2–40 MPa. The results show that the CO2 Fermi diad split and the peak area ratios of the ν 1 band of N2 to the upper band of CO2 are considered to be the best for determining pressure and composition, respectively. The quantitative Raman analysis method developed in this study for the determination of the C N2/C CO2 molar ratio was verified by measuring those in synthetic N2-CO2-H2O fluid inclusions. The developed method has also been applied to the analysis of natural fluid inclusions in quartz from Donghai for the determination of both the C N2/C CO2 molar ratio and pressure.

Genesis of Pb-Zn deposits in northwestern Guizhou province of China: Constraints from the in situ analyses of fluid inclusions and sulfur isotopes

The northwestern Guizhou ore concentration area on the southwest margin of the Yangtze Block is an important part of the Sichuan-Yunnan-Guizhou Pb-Zn metallogenic province. Several large Pb-Zn deposits (e.g., Shanshulin, Shaojiwan, Yadu, and Tianqiao) are found in the area with similar mineral assemblages, ore-controlling structures, ore-hosting rocks, and genetic types. In this study, fluid inclusion microthermometry and in situ geochemical analysis were employed to determine the nature and compositions of the fluid inclusions trapped in calcite and sphalerite and the sulfur isotopic compositions of sphalerite and pyrite. Microthermometric results show that the ore-forming fluids are moderate to low temperature and salinity. The fluid inclusions in calcite and sphalerite share similar homogeneous temperatures (120 to 200 °C) and salinities (7–14 wt% NaCl equiv.). The in situ LA-ICP-MS analyses reveal that the fluid inclusions have high concentrations of elements such as Na, K, Mg, Ca, Rb, Sr, Ba, Cs, and Li. The ratios of K/Na, Rb/Na, Cs/Na, Ca/Na, and Ba/Na could be comparable to the basinal brines. The trend towards metamorphic water is possibly attributed to the involvement of basement-derived fluids. The in situ δ34S values of sphalerite and pyrite range from 11.3 to 20.2 ‰ and 12.0 to 18.9 ‰, respectively, which indicate that the reduced sulfur is most likely related to the thermochemical reduction of sulfate with multiple sources either from the ore-hosting strata or from Precambrian-Cambrian basement rocks which have high sulfur isotopic compositions. We propose that the fluid mixing is the main mechanism for the formation of Pb-Zn deposits in NW Guizhou.

Diagenesis of the Permian Fengcheng Formation in the Mahu Sag, Junggar Basin, China

The Fengcheng Formation in the Mahu sag of the Junggar Basin was primarily composed of detritus, pyroclastic material, carbonates, and evaporites. In order to establish the diagenesis pathways of the Fengcheng Formation, some methods of polarized light microscope, SEM, CL, EPMA, LR, and fluid inclusion analysis were applied to discuss the diagenesis process. The results showed the following: (a) The formation of an alkaline lake was the result of the influence of a high concentration of sodium-rich sources, and it led to the preservation of alkaline minerals in the stratum. (b) After the sediments were buried, three mineral assemblages were formed in the Fengcheng Formation, which are carbonate mineral assemblages (i.e., calcite + ferreous dolomite), reedmergnerite and carbonate mineral assemblages (i.e., reedmergnerite + calcite + ferreous dolomite), and reedmergnerite and alkaline mineral assemblages (i.e., reedmergnerite + shortite + trona), respectively. (c) According to the homogenization temperature of reedmergnerite primary fluid inclusions, the alkaline diagenesis of Fengcheng Formation was divided into an early stage (≤100 °C) and a middle stage (&gt;100 °C), respectively. The earlier stage is marked by the formation of ferrous saddle dolomite, quartz dissolution, and the agglutination of laumontite. These processes occurred under normal burial conditions. The latter is marked by the reedmergnerite’s appearance, which is correlated with the deep hydrothermal activity controlled by faults. (d) Based on sedimentary and diagenetic factors, including climate, provenance, diagenetic surroundings, and the action of subsurface fluid, the alkaline deposition-diagenesis model for shale series in four stages of the Fengcheng Formation was established.

Magmatic‐hydrothermal evolution and mineralization mechanisms of the Wangjiazhuang Cu (‐Mo) Deposit in Zouping Volcanic Basin, Shandong Province, China: Constraints from Fluid Inclusions

AbstractWangjiazhuang Cu (‐Mo) deposit, located within the Zouping volcanic basin in western Shandong Province, China is the only one with economic value found in the area so far. In order to expound the metallogenetic characteristics of this porphyry‐like hydrothermal deposit a detailed fluid inclusion study has been conducted employing the techniques of representative sampling, fluid inclusion petrography, microthermometry, Raman spectroscopy, La‐ICP‐Ms analysis of single fluid inclusions, as well as cathode fluorescence spectrometer analysis of host mineral quartz. The deposit contains mainly two types of orebodies, i.e. veinlet‐dissemination‐stockwork orebodies in the K‐Si alteration zone and pegmatitic‐quartz sulfide veins above them. In addition, minor breccia ore occurs locally. Four types of fluid inclusions in the deposit and altered quartz monzonite are identified: L‐type one‐ or two‐phase aqueous inclusions, V‐type vapor‐rich inclusions with V/L ratios greater than 50‐90%, D‐type multiphase fluid inclusions containing daughter minerals or solids and S‐type silicate‐bearing fluid inclusions containing mainly muscovite and biotite. Ore petrography and fluid inclusion study has revealed a three‐stage mineralization process driven by magmatic‐hydrothermal fluid activity as follows. Initially a hydrothermal fluid separated from the parent magma infiltrated into the quartz monzonite, resulting in its extensive K‐Si alteration as indicated by silicate‐bearing fluid inclusions trapped in altered quartz monzonite. This is followed by the early mineralization, the formation of quartz veinlets and dissemination‐stockwork ores. During the main mineralization stage due to the participation and mixing of meteoric groundwater with the magmatic‐sourced hydrothermal fluid the cooling and phase separation caused deposition of metals from the hydrothermal fluids. As a result the pegmatitic‐quartz sulfide‐vein ores formed. In the late mineralization stage, decreasing in fluid salinity has led to the formation of L‐type aqueous inclusions and chalcopyrite‐sulfosalt ore. Coexistence of V‐type and D‐type inclusions and their similar homogenization temperatures but different homogenization modes suggest that phase separation or boiling of the ore‐forming fluids once took place during the early and the main mineralization stages. The formation P‐T conditions of S‐type inclusions, and the early and the main mineralization stages are estimated as ca. 156 ∼ 182 MPa and 450 ∼ 650 °C, 350‐450 °C, 18‐35 MPa and 280‐380 °C, 8‐15 MPa, respectively based on the microthermometric data of the fluid inclusions formed at the individual stages.

Restoration of reservoir diagenesis and hydrocarbon accumulation process by calcite in-situ U-Pb dating and fluid inclusion analysis: A case study on Cretaceous Qingshuihe Formation in Gaoquan Structure, southern Junggar Basin, NW China

The complexity of diagenesis and hydrocarbon accumulation in the deep reservoirs in southern Junggar Basin restricts hydrocarbon exploration in the lower reservoir assemblage. The lithofacies and diagenesis of reservoirs in the Cretaceous Qingshuihe Formation in the Gaoquan structure of the Sikeshu Sag, southern Junggar Basin were analyzed. On this basis, the thermal history was calibrated using calcite in-situ U-Pb dating and fluid inclusion analysis to depict the hydrocarbon accumulation process in the Gaoquan structure. The results show that the Qingshuihe reservoir experienced two phases of calcite cementation and three phases of hydrocarbon charging. The calcite cements are dated to be (122.1±6.4) Ma, (14.4±1.0) Ma – (14.2±0.3) Ma. The hydrocarbon charging events occurred at around 14.2–30.0 Ma (low-mature oil), 14.2 Ma (mature oil), and 2 Ma (high-mature gas). The latter two phases of hydrocarbon charging contributed dominantly to the formation of reservoir. Due to the S-N compressive thrust activity during the late Himalayan period since 2 Ma, the traps in the Gaoquan structure were reshaped, especially the effective traps which developed in the main reservoir-forming period were decreased significantly in scale, resulting in weak hydrocarbon shows in the middle-lower part of the structure. This indicates that the effective traps in key reservoir-forming period controlled hydrocarbon enrichment and distribution in the lower reservoir assemblage. Calcite U-Pb dating combined with fluid inclusion analysis can help effectively describe the complex diagenesis and hydrocarbon accumulation process in the central-west part of the basin.

Tracing metal source of copper-rich tungsten mineralization: Evidence from individual fluid inclusion analysis of the Huangsha Cu-rich W deposit in South China

The Huangsha Cu-rich W deposit, located in the northeast part of the Nanling region, is a typical large-size wolframite-quartz vein-type W deposit with considerable reserves of copper. Wolframite and chalcopyrite usually distribute in the same quartz veins, while field and petrographic observation show that wolframite commonly precipitates earlier than coexisting quartz and sulfides. Veined quartz can be divided into three successive generations (Q1, Q2, and Q3) based on SEM-CL imaging, and only Q2 is strictly coeval with chalcopyrite and other sulfides. Primary two-phase aqueous (type Lw) fluid inclusions are identified from wolframite with homogenization temperatures of 292 to 327 ℃ and salinities of 6.9 to 9.9 wt% NaCl equiv. Pseudosecondary two-phase aqueous (type Lq) and vapor-rich (type Vq) fluid inclusions are recognized from Q2, supporting the occurrence of fluid immiscibility. Homogenization temperatures of type Lq inclusions in Q2 range from 200 to 258 ℃ with salinities of 4.3 to 7.7 wt% NaCl equiv. LA-ICP-MS analysis shows that all inclusions contain Na and K as the major cations. For all the detected elements, type Lw inclusions in wolframite have higher concentration ratios X/Na than type Lq inclusions to varying degrees. The ore-forming fluid in both W and Cu mineralization stages share the same characteristics of high B concentrations, consistent Rb/Cs ratios and evolving nature of ore metals, indicating that W and Cu were sourced from the same magmatic fluid but precipitated in different mineralization stages. The variation of fluid Sr concentration likely indicates fluid-rock interaction during precipitation of wolframite, whereas the deposition of chalcopyrite is possibly influenced by fluid cooling and immiscibility. Compared with typical porphyry Cu systems and W/Sn mineralized systems worldwide, the ore-forming fluid in the W mineralization stage at Huangsha shows considerably higher mineralization potential of Cu, which is consistent with the geological fact. The Cu contents in initial W ore-forming fluids may serve as an indicator in identifying Cu-rich W deposits.

Diagenetic history of calcite fractures in Vaca Muerta shales (Argentina) inferred from paired Δ47 and fluid inclusion geothermometry

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 Δ47 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.

An integrated approach for quantifying fluid inclusion data combining microthermometry, LA-ICP-MS, and thermodynamic modeling

The development of LA-ICP-MS and other microanalytical techniques for in situ analysis of single fluid inclusions has made it possible to quantify the elemental concentration of major elements, trace metals, sulfur and halogens in fluid inclusions from a wide range of geological environments. The established quantification methods typically interpret the LA-ICP-MS and microthermometric data (melting points of ice, salt and hydrate phases) in terms of simple binary or ternary model systems (e.g., H2O-NaCl and H2O-NaCl-CaCl2), which can be inaccurate for complex natural geofluids such as multi-solute basinal brines, magmatic-hydrothermal fluids or pegmatitic fluids. Another approach uses mass or charge balance constraints along with the elemental ratios obtained from LA-ICP-MS. This improves the quantification of more complex fluids to some extent, but because it is based on the equivalent concentration of NaCl from the NaCl-H2O binary system, it typically introduces systematic uncertainty in the concentrations of all elements analyzed. These limitations can be fundamentally overcome by combining microthermometric and LA-ICP-MS data with thermodynamic modeling. In this approach, elemental ratios (element/Na) are directly calculated from the LA-ICP-MS data and then the unknown Na concentration is calculated by modeling the chemical equilibrium between the aqueous solution and the last melting solid phase (e.g., ice, hydrohalite, sylvite) of the fluid inclusion using a Pitzer model for the aqueous solution. Comparison between modeled and experimental data of chloride systems (including salt-solution equilibria and synthetic fluid inclusion data) demonstrates excellent agreement over wide ranges of temperature and composition. Our new model, accompanied by a new approach for quantifying uncertainty, is more general than previous methods and makes it possible to quantify complex multi-element fluid inclusions from diverse geological environments including magmatic, metamorphic, sedimentary and various ore-forming hydrothermal fluids.

Fracturing timing of Jurassic reservoirs in the Dibei-Tuziluoke gas field, Kuqa foreland basin: Evidence from petrography, fluid inclusions, and clumped isotopes

Determining the timing of fracturing is crucial for understanding reservoir evolution and hydrocarbon accumulation in foreland basins. Using fracturing data from cores, borehole images, and outcrops, combined with the clumped isotope (Δ47) and fluid inclusion analyses of carbonate minerals filled in pores and fractures, this study ascertained the fracturing timing of the Jurassic reservoirs in the Dibei-Tuziluoke Gas Field, Kuqa Foreland Basin. Data from outcrops and borehole images show two dominant fracture sets in the study area: W-E and NE-SW striking fractures. Some W-E striking fractures are carbonate-filled, while NE-SW striking fractures lack mineral fillings. Bitumen veins, not easy to be identified in borehole images, are prevalent in cores. The petrographic analysis reveals that these bitumen veins formed before the calcite cementation in pores and display high viscosity and low maturity. Homogenization temperatures (Th) from primary fluid inclusion assemblages in two representative calcite vein samples were notably lower than TΔ47 values from corresponding samples. This suggests the Δ47 signature underwent alteration due to partial reordering during burial. Thus, Δ47-derived temperatures (apparent temperatures) may not faithfully represent the mineral precipitation temperatures. When plotting these apparent temperatures vs. the burial history, only the possible latest ages of fracturing emerged. These ages were further refined by considering petroleum charging, tectonic evolution, and stress orientation. Bitumen-filled fractures likely resulted from the Late Cretaceous uplift, marking the migration of low-maturity hydrocarbons in the study area. Carbonate-filled E-W striking fractures emerged during the late Miocene (∼13–6.5 Ma) alongside fold development. NE-striking fractures that crosscut W-E ones possibly formed recently due to stress reorientation.

Utility of fluid inclusion paleo-temperature in petroleum system modelling: A case study from western offshore, India

The paleo-temperature (Th) data from fluid inclusions are utilized for thermal history modelling using PetroMod software. Generally, bottom hole temperature (BHT) and vitrinite reflectance (Ro) measurements are widely used in petroleum system modelling (PSM) in the oil industry for calibration purposes. Th representing the minimum temperature of fluid entrapment estimated from fluid-inclusion study provides extra support to build the thermal models for PSM. Fluid inclusion parameters along with Rock-Eval pyrolysis analysis have been used to predict the maturity of oil in terms of API gravity as well as the maturity of source rocks respectively. Two exploratory wells RV-1 (Mumbai Offshore Basin) and KK4C-A-1 (Kerala-Konkan Offshore Basin), India were examined and the Th from most of the fluid inclusions of wells RV-1 and KK4C-A-1 fell in the oil window range of 60–140 °C suggesting thermal conditions favourable for oil generation in both of the wells. Th of coeval aqueous inclusions along with the Hydrocarbon Fluid inclusions (HCFIs) was used to calibrate PSM. Vital parameters show that source rocks of well RV-1 are mature and that of well KK4C-A-1 are immature. Two sets of PSM are created in terms of generation and expulsion for the dry wells RV-1 and KK4C-A-1 and calibrated each well using fluid inclusion Th and BHT. From the fluid inclusion analysis method, it is evident that hydrocarbon generation happened in both wells and the paleo-temperature indicates that the formations of both wells were subjected to temperatures in the oil window range, even though it was designated as dry wells in the present scenario. The present study highlights the application of fluid inclusion paleo-temperature (Th) during calibration instead of commonly used methods. We could obtain desirable and accurate data output from PSM using Th calibration.

The evolution of the majiang paleoreservoir, west margin of xuefeng uplift: Insights from in situ calcite U–Pb dating and bitumen and shale trace element analysis

The Majiang Reservoir, situated within the Yangtze Block, is the largest paleoreservoir in the region. Employing a comprehensive methodology involving the analysis of trace and rare earth elements in bitumen and shale, in situ U–Pb dating of hydrocarbon-associated calcite, and incorporating previously published data on oil-source correlation and significant stages of reservoir development, we successfully reconstructed the complete evolution of the Majiang Reservoir quantitatively. The bitumen trace elements data show the ratio of V/(V + Ni) ranging from 0.64 to 0.82, Ni/Co exceeding 7.0 and Zr/Cr below 1.0. PAAS-normalized rare earth element distribution patterns of both bitumen and Cambrian shales present zig-zags distributions. In addition, biomarker distribution acquired by catalytic hydropyrolysis and bulk δ13C values of both bitumen and Cambrian shales (−29.1‰ to −34.7‰ vs −30.2‰ to −32.3‰) also show similarity. All the geochemical parameters together indicate that the primary source of bitumen stems from Early Cambrian shales. Integrating fluid inclusion analysis, specifically examining two Th groups with temperatures of 97.2 °C and 142.6 °C, and U–Pb dating on hydrocarbon-bearing calcite (∼300 Ma), referencing prior Re–Os dates on bitumen (∼430 Ma) and pyrobitumen (∼80 Ma) and quartz fluid inclusion Ar–Ar dating (∼225 Ma), we successfully reconstructed the entire hydrocarbon evolution. The findings reveal that initial oil generation occurred during the Caledonian event, followed by secondary continuous oil and gas generation and migration, influenced by the Indosinian event. The combination of fluid inclusion analysis and isotopic geochronology data exemplifies immense potential in precisely determining the progression of petroleum evolution.

Formation of natural fractures and their impact on shale oil accumulation in the Mahu Sag, Junggar Basin, NW China

Lacustrine shale oil reservoirs of the Fengcheng Formation in the Mahu Sag, Junggar Bain, NW China contain abundant oil resources, with the highest daily oil production of a single well reaching over 700 barrels. Based on core and thin section observation along with borehole image interpretation, this study integrated paleotectonic stress regime, fluid inclusion analysis, and calcite UPb dating to clarify fracturing periods and discuss the influence of fractures on shale oil accumulation. Results show that natural opening-mode fractures are widespread in the Fengcheng Formation, which play an important role in oil accumulation. Fractures are developed in several stages and filled by three minerals: reedmergnerite, quartz and calcite. Reedmergnerite and quartz fillings show similar fluid inclusions and petrographic features, with numerous hydrocarbon inclusions compared to aqueous ones, yet, such inclusions were less observed in calcite fillings. Fractures in the study area were formed in two separate stages. In the first period, due to the NW-SE compression of the Hercynian movement, the WNW-ESE and NNW-SSE striking fractures were formed in the Late Permian, with Th values of 95–115 °C. The first stage of fractures coincided with hydrocarbon generation, providing migration pathways for the oil. In the second period, due to the nearly SN compression of the Indosinian movement, the NNW-SSE and NNE-SSW striking fractures were formed in the late Triassic, with Th values of 120–140 °C. The second stage of fractures was formed subsequent to major hydrocarbon generation and expulsion period, hence their contribution to hydrocarbon migration and accumulation was found relatively minor. Moreover, fractures with the nearly E-W, ENE-WSW and WNW-ESE strikes intersect with the present-day maximum horizontal principal stress at a small angle, with large apertures and good connectivity, which should have a positive impact on shale oil accumulation.

Widespread development of bedding-parallel calcite veins in medium–high maturity organic-rich lacustrine shales (Upper Cretaceous Qingshankou Formation, Northern Songliao Basin, NE China): Implications for hydrocarbon generation and horizontal compression

Bedding-parallel calcite veins (BPCVs) occur widely in Upper Cretaceous lacustrine shales of the Qingshankou Formation in the Northern Songliao Basin, China. Research on the formation processes and geological significance of BPCVs has focused on the sources of fluid, timing of formation, initiation, dilation and growth mechanisms, and on relationships with hydrocarbon generation. In this study, we addressed these issues through a comprehensive study of drill cores containing BPCVs using petrographic observations, isotope geochemistry, rare earth element (REE) analysis, fluid inclusion analysis, in situ U–Pb dating, and basin modeling. BPCVs were observed in medium–high maturity shales with high organic matter contents with heterogeneous compositions. Similarities in δ13C, δ18O, 87Sr/86Sr, and REEs contents between BPCVs and the host shale indicate that locally derived formation fluids driven by chemical gradients were involved in BPCV formation. The in situ U–Pb age dating (70 ± 8.9 Ma and 70.6 ± 9.7 Ma) constrained BPCVs formation to the late Cretaceous, corresponding to a burial depth of ∼1600 m, indicating that the BPCVs formed in the oil window. The latter is also supported by fluid inclusions-derived temperatures (∼76–109 °C). BPCV formation included vein initiation and dilation, and the microstructures, e.g., domal and pincer structures, sigmoidal calcite veins, conjugate en-échelon calcite vein arrays, as well as mechanical crystal twins of the BPCVs, all indicate that tectonic compression influenced fracture development as also indicated in other basins where beef occur. Fluid overpressure caused by hydrocarbon generation, tectonic compression, and clay mineral transformation (mainly smectite to illite conversion) was responsible for vein dilatation. Antitaxial fibrous crystals with smooth crystal boundaries and high aspect ratios, containing sinusoidal solid inclusions as well as median planes, indicate that crystallization force promoted vein growth. Thus, BPCVs can be used as a good indicator of hydrocarbon generation, and horizontal tectonic compression.

Discovery, geological setting and mineralization of the Shuvuutai molybdenum deposit, central-southern Mongolia

We document the newly discovered Shuvuutai Molybdenum (Mo) porphyry deposit located in the central-east Mongolian volcanic/-plutonic belt of the southern Altaids. The deposit is characterized by quartz vein- and veinlet-disseminated-type Mo ore bodies mainly hosted in a Late Permian-Early Triassic granitic intrusion. In this study, we provide a detailed study that integrates detailed geology with petrographic, mineralogic, geochemical, and geochronologic date to determine the tectonic setting of the deposit. Fluid inclusion analysis is used to reconstruct the fluid evolution history of the Shuvuutai Mo mineralization system and to understand the origin of the deposit. The ore bodies occur mainly as veins, lenses, and pods in positions from inner intrusions through contact zones to the host rocks distal to causative intrusions. The host rocks are variable in lithology, including granites, porphyries, volcanic breccias and tuffs, and sedimentary rocks. Outward from ore bodies to host rocks, the wall rock alteration is zoned from potassic (K-feldspar-quartz-mica), through phyllic (quartz-sericite-chlorite-epidote), to propylitic or argillic alterations. Two samples from porphyry stock intrusions at Shuvuutai were selected for U–Pb zircon geochronology and trace element composition studies, including the ore-hosting monzodiorite porphyry and quartz syenite porphyry. Our new ages are consistent with, or slightly postdate published zircon U-Pb ages of early phase of monzodiorite porphyry (268–232 Ma) and the main phase of quartz syenite porphyry (251.6 ± 1.7 Ma). Hydrothermal mineralization generally includes four stages, from early to late, represented by (1) potassic feldspar-quartz veins or veinlets, (2) quartz-molybdenite stockworks, (3) quartz-sulfide stockworks, and (4) quartz ± carbonate veins or veinlets. The ore-forming fluids were initially magmatic in origin and show high-temperature and high-salinity, containing daughter mineral and CO2-bearing fluid inclusions; and eventually evolved to low-temperature, low-pressure, low-salinity and CO2-poor meteoric water. S isotopic compositions of sulfides from the Shuvuutai Mo porphyry deposit indicate that the source of the ore-forming material was derived from the felsic magma. The Central-Eastern Mongolian volcanic-plutonic belt continues from Mongolia into northeast China, where similar Mo deposits are not yet recognized, suggesting that a new exploration strategy, based on the characteristics and history of the Shuvuutai discovery, is needed for the Chinese segment of the Altaids.

Evolution characteristics and exploration targets of Permian clastic rock reservoirs in Bohai Bay Basin, East China

Based on core observation, thin section examination, fluid inclusions analysis, carbon and oxygen isotopic composition analysis, and other approaches, the structural and burial evolution histories were investigated, and the diagenetic evolution process and genetic/development models were systematically discussed of the Upper Paleozoic Permian clastic rock reservoirs in the Bohai Bay Basin, East China. The Bohai Bay Basin underwent three stages of burial and two stages of uplifting in the Upper Paleozoic. Consequently, three stages of acid dissolution generated by the thermal evolution of kerogen, and two stages of meteoric freshwater leaching occurred. Dissolution in deeply buried, nearly closed diagenetic system was associated with the precipitation of authigenic clay and quartz, leading to a limited increase in storage space. Different structural uplifting–subsidence processes of tectonic zones resulted in varying diagenetic–reservoir-forming processes of the Permian clastic reservoirs. Three genetic models of reservoirs are recognized. The Model I reservoirs with pores formed in shallow strata and buried in shallow to medium strata underwent two stages of exposure to long-term open environment and two stages of meteoric freshwater leaching to enhance pores near the surface, and were shallowly buried in the late stage, exhibiting the dominance of secondary pores and the best physical properties. The Model II reservoirs with pores formed in shallow strata and preserved due to modification after deep burial experienced an early exposure-open to late burial-closed environment, where pore types were modified due to dissolution, exhibiting the dominance of numerous secondary solution pores in feldspar and the physical properties inferior to Model I. The Model III reservoirs with pores formed after being regulated after multiple periods of burial and dissolution experienced a dissolution of acidic fluids of organic origin under a near-closed to closed environment, exhibiting the dominance of intercrystalline micropores in kaolinite and the poorest physical properties. The target reservoirs lied in the waterflood area in the geological period of meteoric freshwater leaching, and are now the Model II deep reservoirs in the slope zone–depression zone. They are determined as favorable options for subsequent exploration.