Fluid Inclusions Research Articles

Formational stages of natural fractures revealed by U-Pb dating and C-O-Sr-Nd isotopes of dolomites in the Ediacaran Dengying Formation, Sichuan Basin, southwest China

Deep core (>4.9 km) from Ediacaran Deng IV Member algal dolomites in the Gaoshiti-Moxi block in the Sichuan Basin, southwest China, reveals multiple generations of dolomite-lined and dolomite-filled opening-mode fractures. Three progressive stages of fracture formation are marked by crosscutting relations visible in the core, by acoustic emission experiments revealing evidence of past stress directions, and by fluid inclusions, U-Pb ages, C-O-Sr-Nd isotope patterns, and rare earth element data for dolomite cements in fractures, which document ages and differing thermal conditions and fluid compositions during fracture. In calcite-filled fractures, U-Pb ages and carbon and oxygen isotope signatures vary greatly, indicating that fractures developed with intensified tectonic activity marked by regional structures and with enhanced diagenetic alteration. In stage I, WNW-striking opening-mode fractures formed that contain dolomite deposits precipitated from basinal fluids between ca. 549 Ma and ca. 532 Ma. At this time, the Sichuan Basin experienced Xingkai taphrogenesis (rifting) from the late Neoproterozoic to early Cambrian. The central Sichuan paleo-uplift was undergoing ENE extension, and preexisting ESE- and nearly E-W−striking faults were oblique to the ENE principal stress orientation. This led to a local stress field favoring dextral shear near fault zones accommodated by the fractures. In stage II, ENE-striking fractures that are younger based on crosscutting relations contain dolomite deposits from basinal fluids with ages from ca. 423 Ma to ca. 411 Ma. Contemporaneous with Xuefeng thrusting, the central Sichuan paleo-uplift was in a NNE-striking transpressional stress field, which likely further generated ENE-striking fractures. In stage III, nearly N-S−striking fractures formed in the Gaoshiti-Moxi block. High-temperature fluids related to the Permian Emeishan large igneous province invaded these fractures from ca. 260 Ma to ca. 256 Ma. At this time, the Sichuan Basin was uplifted under the influence of the Emei taphrogenesis in the late Permian, and the central Sichuan paleo-uplift was subjected to E-W−striking extension. In fractures in these carbonate rocks, micro-computed tomography imaging reveals that macropores (>10 μm, 12.1%−21.8%) and small pores (2−10 μm, 76.6%−86.1%) dominate the dolomite mineral deposits, and that there are few (1.6%−1.8%) micropores or nanopores (<2 μm). Medium-sized throats (1−3 μm) are the main connecting channels. We infer that fractures served as conduits for fluid migration, leading to the dissolution of matrix pores adjacent to the fractures. This secondary porosity not only enhances reservoir storage capacity but also augments reservoir connectivity. Our study shows that in situ U-Pb dating and full-diameter rock acoustic emission data can effectively constrain the timing of fractures. By integrating this information with regional tectonic sequences and fracture diagenetic sequences from combined relative timing, geochemistry, and rock mechanics evidence, we clarify the factors controlling fracture formation.

Raman spectral quantification of Li2SO4, Na2SO4, K2SO4 and Cs2SO4 at 25 to 250 °C and its potential implications for the determination of sulfate contents in natural fluid inclusions

The quantitative study of the sulfate content in natural fluid inclusions (FIs) will help our understanding of the processes of metal transport and related mineralization. The sulfate-rich FIs typically contain several types of sulfates with diverse cations such as Na+, K+, and so on. In order to get the sulfates contents in natural FIs, in situ Raman spectral quantitative calibration method is used to calibrate the Li2SO4, Na2SO4, K2SO4 and Cs2SO4 solutions at temperatures ranging from 25 °C to 250 °C in our study. The results show that the A(SO42−)/A(H2O) ratio increases linearly with increasing mSO42− in sulfate solutions containing single cation of Li, Na, K or Cs at the set temperatures, where A is Raman peak area and m is molality (mol/kg H2O), and a negative linear relationship exists between k (quantitative calibration coefficient) and temperature for sulfate solutions with specific cations up to 250 °C. Meanwhile, the slopes and the intercepts of the k-T correlation curves are obviously different among sulfate solutions with different cations, and the coefficients k of these sulfate solutions at certain temperature decrease with the increase of atomic number of the cation, in accordance with the following order: Li+ > Na+ > K+ > Cs+. As Na+ and K+ are the main cations in the natural sulfate-bearing FIs, the quantification of the multi-cation (Na ± K ± Li) sulfate solutions was evaluated by using the Raman quantitative calibration curves of Li2SO4, Na2SO4, K2SO4 and Cs2SO4 separately, the errors exist due to the use of calibration curves with different cations; the average concentration errors derived from the use of Na2SO4 or K2SO4 Raman quantitative calibration curves are within about 5 %, while those from the use of Li2SO4 or Cs2SO4 Raman quantitative calibration curves are within about 15 %. Therefore, the acceptable errors suggest that the calibration curves of individual sulfates can be applied to the determination of sulfate content of natural sulfate-bearing FIs.

Fluid inclusion evidence for overpressure-induced self-sealing and accumulation of deep shale gas

Shale oil and gas exploration and development are extending to greater depths, and it is important to understand the accumulation mechanisms of shale oil and gas. Here we conducted a case study in the Upper Ordovician Wufeng Formation–Lower Silurian Longmaxi Formation in the Ningxi area, Sichuan Basin, southwest China, by using petrological and fluid inclusion investigation to determine the processes involved during the overpressure-induced self-sealing accumulation of deep shale gas. Results show that fibrous quartz–calcite veins in the source rocks contain abundant gas inclusions and coeval two-phase gas–liquid aqueous inclusions. The gas inclusions are relatively uniform, translucent to transparent, single-phase inclusions with a high density, indicative of a closed shale system. The gas inclusions record varying levels of fluid overpressure (115.23–169.46 MPa), with a paleo-pressure coefficient of 2.12–2.98. Fluid overpressure and self-sealing resulted in shale gas accumulation, with bedding-parallel and laminar fractures acting as important migration pathways, which improved the horizontal porosity and permeability of the shale. These processes are common and important phenomena in petroleum systems. Unconventional oil and gas development commonly requires horizontal fracturing of wells to form a fracture network, which can improve the production capacity.

Determination of multistage oil charge processes in the Ediacaran Dengying gas reservoirs of the southwestern Sichuan basin, SW China

The Sichuan Basin has experienced multi periods of tectonic uplift event, accompanied with multiple hydrocarbon accumulation and phase transitions processes in the deep and ultra-deep reservoirs. The formation of paleo-oil reservoir plays an important role in the distribution of present-day gas fields, yet many records of early oil reservoirs have been destroyed. A series of methods including, petrography, cathodoluminescence, fluid inclusion, in-situ U–Pb geochronology, and basin modeling, were used to determine the multistage oil charge process in the Ediacaran Dengying Formation of the Weiyuan area, SW Sichuan Basin. The lace-like dolomite lamination and three stages of dolomite cement were petrographically, and geochronologically distinguished in the Dengying Formation, dated at c. 542 Ma, 486–482 Ma, 410 Ma, and 270 Ma, respectively. Three stages of bitumen in the Dengying reservoir indicated three episodes of oil charge. The first and second oil-charging events occurred at 500–486 Ma and 410–400 Ma, by combining the modeled timing of oil generation with mineral ages of the two generations before and after the solid bitumen. The modification of the Weiyuan paleo-oil pool occurred during the Caledonian tectonic uplift, after the second oil-charging event. The timing of the third oil charge was at 270–230 Ma, according to the Th value of aqueous inclusions coeval with secondary bitumen inclusions in the stage 3 dolomite cement (CD-3), close to the timing of main oil generation for the Lower Cambrian source rocks. The first oil charge may be a contribution from the Doushantuo source rock, and the Lower Cambrian source rocks provide the major contribution for the third oil charge. This research reveals the timing of multistage oil charging events in the southwest Sichuan basin, and provides a further method for determining the timing of oil charge in multi-cyclonic composite basins.

Fluid inclusions in quartz of the gold-ore occurrences and gold-quartz intergrowths from placers in the Sololi uplift of the Olenyok arch (Yakutia)

Fluid inclusions have been studied in vein quartz with gold sulfide mineralization from metamorphosed sandstones of the Eekite series and metarhyolites of the Early Proterozoic, in quartz breccia from the zone of overlapping gold mineralization on the Early and Middle Permian sandstones, as well as in the gold quartz intergrowths from the Sololi River placer. It has been revealed that formation of quartz breccias occurred within a wide temperature interval from 230 to 425 ºC, with predominance of carbon dioxide and nitrogen in the vapor phase. It is suggested that the increased nitrogen content may be associated with a chemical reaction between the fluid and ammonium-containing silicates of host rocks, in which nitrogen in the form of NH4+ isomorphically replaces potassium at the regressive stage of metamorphism. At the same time, it is possible that mantle nitrogen, which was transported along the Anabar-Eekite deep fault, participated in formation of the studied breccias. The close homogenization temperatures and similar nature of the water-salt composition for the fluid inclusions of quartz veins that inject the Eekite series meta-rocks and meta-rhyolites indicate the synchronism of their formation and attribute them to the common stage of ore formation. Quartz veins with gold sulfide mineralization were the primary sources of pebbles with gold-quartz intergrowths from the Sololi River, this is evidenced by similarity of principal characteristics of fluid inclusions. Oxidizing conditions of the mineralization serve as favorable factor for the Au deposition, it is indicated by the predominant CO2 content in fluid inclusions, keeping role of a geochemical barrier and leading to an elevated gold content in quartz veins.

Coupling of source rock gas expulsion intensity and reservoir quality in time and space-a key to determine gas accumulation in Upper Paleozoic tight sandstones of the Mugua gas field, Ordos Basin, China

Research on gas accumulation mechanisms and variations in gas saturation is limited in the newly established Mugua gas field in the Ordos Basin. Understanding these aspects is vital for reduce drilling uncertainty and exploration risk. This study employed a variety of methods, including: (1) the use of organic geochemical examinations to determine the gas expulsion intensity of hydrocarbon source rocks; (2) fluid inclusion analyses to determine the time of gas accumulation; (3) a combination of optical microscopy, scanning electron microscopy observations, pore-simulation and permeability tests, mercury-intrusion capillary pressure experiments, and nuclear magnetic resonance studies to assess the properties of the reservoir quality and identify the tight sandstone rock type; (4) wireline logging and interpretive data analyses to research the gas saturations features; and (5) establish reservoir quality evolution model to systematically study this problem. Integrating fluid inclusion homogenization temperatures with burial and thermal histories indicates that gas accumulation in the Mugua gas fields occurred throughout the Early Jurassic to Late Cretaceous period. Five diagenetic facies have been identified: dissolution, quartz-cemented, clay mineral filling, tightly compacted, and carbonate-cemented facies. The reservoir quality evolution model reveals that the five diagenetic facies experienced densification before the period of gas accumulation. Therefore, for gas accumulation in tight sandstones, it is essential that mature and gas-generating source rocks, provide sufficient expulsion forces to drive gas into the rocks. The mechanism of gas accumulation in the Mugua gas field's tight gas sandstone reservoirs involves the process occurring in sandstones that are either near or interbedded with source rocks, under high pore-fluid pressure generated by gas production. Compared to other Upper Paleozoic gas fields in the Ordos Basin, the gas expulsion intensity in the Mugua gas field is lower, ranging from 6.12–8.39 × 108 m3/km2. Due to the low gas expulsion intensity, the Shihezi 8 Member's tight sandstone reservoirs, situated away from the source rock, typically show low gas saturation. Conversely, the dissolution and quartz-cemented facies within the Benxi, Taiyuan, and Shanxi Formations, intimately interbedded with hydrocarbon source rocks, exhibit good reservoir quality. They offer ideal conditions for gas accumulation, making them prime candidates for exploration and development.

Dynamic coupling between cementation and porosity evolution of the Chang 8 tight sandstone reservoir, Ordos Basin: Insights from in-situ microanalysis

Understanding cement paragenesis is crucial to the study of the porosity evolution in low-permeability sandstone reservoirs. Carbon and oxygen isotope analysis with micro-drilling sampling, electron probe microanalysis, and laser Raman spectroscopy microanalysis was used to clarify the origin of cements and their relationship with reservoir quality of the Chang 8 tight sandstone reservoir in the Longdong area of the Ordos Basin. The predominant cements are chlorite, illite, kaolinite, quartz, and carbonate, which occur in 3 forms: grain rim, overgrowth, and filling of intergranular pore and intragranular dissolved pore. The quantitative paragenesis of the cements is established based on the formation temperature of cements obtained through fluid inclusion, oxygen isotope, chemical composition and Raman parameters. The main source of carbon for carbonate cements is the decarboxylation of organic matter in the adjacent mudstone interlayers. The conversion of smectite to illite and the dissolution of feldspar and previous carbonates provide Ca2+, Mg2+, and Fe3+ ions for carbonate cements and authigenic chlorite, and the authigenic chlorite rim continuously grows from the grain surface outward. The porosity reduction by the cements mainly occurs during the main hydrocarbon filling period (80–120 °C), and the reservoir belongs to the type of “hydrocarbon accumulation prior to significant porosity reduction”. This study provides a new approach for studying the temporal and spatial evolution of clastic rock diagenesis.

Magmatic−hydrothermal evolutionary processes in highly evolved granitic systems: Insights from zircons of the Baishitouquan pluton, NW China

As a robust accessory mineral in igneous rocks, the mineralogical and geochemical characteristics of zircon can record the lithological differentiation and magmatic−hydrothermal evolution of highly evolved granitic systems. The F-Rb−rich, highly evolved Baishitouquan pluton of NW China exhibits gradual lithological changes from leucogranite, amazonite-bearing granite, and amazonite granite in the lower levels of the pluton to topaz-bearing amazonite granite, topaz albite granite, and pegmatite in the upper levels. In this study, three types of zircon grains were identified in five lithological zones based on textural and chemical characteristics. Type I zircon, which mostly occurs in leucogranite and amazonite-bearing granite, exhibits oscillatory zoning in cathodoluminescence images and experienced low degrees of radiation damage (0.21−0.68 × 1015 α-decay events/mg), which is indicative of its magmatic origin. Type II zircon, which mostly occurs in amazonite granite and amazonite pegmatite, exhibits textures that are indicative of hydrothermal alteration (e.g., spongy texture, porosity, and microcracks), and has elevated concentrations of some cations, such as Ca and Al. Type II zircon contains a higher concentration of non-formula elements, including rare earth elements (REEs), and Hf, Th, and U, than Type I and III zircons. Additionally, Type II zircon exhibits a significant M-type lanthanide tetrad effect and experienced varying levels of radiation damage (3.75−11.72 × 1015 α-decay events/mg). These characteristics suggest that Type II zircon has a hydrothermally altered origin. Type III zircon, which is restricted to the topaz-albite granite, has the smallest crystal size among all types of zircon grains, shows a euhedral to anhedral mottled appearance, and is characterized by patchy, cloudy, or irregular zoning, with numerous fluid inclusions. This type of zircon contains higher concentrations of Ti (110−1030 μg/g) than other types of zircon grains. Additionally, this type of zircon experienced limited radiation damage (2.18−3.69 × 1015 α-decay events/mg), and has a smooth surface and homogeneous internal textures. These characteristics suggest that Type III zircon is the product of fluid interaction with hydrothermally altered Type II zircon. Accordingly, this type of zircon crystallized directly from a Zr-saturated hydrothermal fluid during the later stages of magmatic−hydrothermal evolution. These contrasting textural and compositional features of the three types of zircon grains are indicative of three stages of magmatic−hydrothermal evolution of the Baishitouquan pluton: magmatic, magmatic−hydrothermal transition, and hydrothermal. These magmatic and hydrothermal processes were involved in the enrichment, transport, and precipitation of rare metals, such as Rb. Accordingly, this contribution demonstrates that the textures and chemistry of zircon grains can serve as petrogenetic indicators for assessing magmatic−hydrothermal evolution and rare-metal mineralization in highly evolved granitic systems. Furthermore, this study presents a model of the magmatic−hydrothermal evolution of F-rich, highly evolved granitic systems through the lens of zircon.

Charging dynamic, resistance and accumulation process of tight sandstone oil of the paleogene Es3 member in linnan sag, jiyang depression

The study of the coupling relationship between the dynamic and resistance of oil charging is the key to revealing the process of tight oil accumulation. The Paleogene Es3 member of Linnan sag, Jiyang Depression was taken as an example, basin simulation, fluid inclusion analysis, and porosity inversion were comprehensively utilized to recovery the residual fluid pressure and charging resistance during the critical oil accumulation periods. Combined with the physical modeling of tight oil charging, the charging process and characteristics of tight oil under dynamic and resistance coupling were reproduced and the effects of dynamic and resistance on the oiliness of tight reservoirs were analyzed. The comprehensive analysis indicates that there were early and late periods of oil accumulation in early Dongying period and late Guantao-Quaternary period. The reservoirs had not been densified during the early oil accumulation period. The residual pressure of the formation was between 1.0–4.4 MPa and the reservoir charging resistance was generally small, so that the dynamic was generally greater than the resistance. But the oil saturation of the reservoirs was low due to the small amount of hydrocarbon generation. During the late oil accumulation period, the source rocks entered the peak period of hydrocarbon generation and expulsion and the residual pressure of the formation was between 4.2–12.1 MPa. The reservoirs had been close to or completely densified. The dynamic, resistance and dynamic and resistance difference all increased significantly. Under the stronger dynamic, the oil saturation of reservoirs rapidly increased, ultimately achieving tight oil enrichment. The results of this study provide insights into the dynamic process of tight oil accumulation and the distribution of oil in tight reservoirs.

Hydrothermal alteration of the surface volcanic rocks at the Acoculco geothermal field, Mexico: a multi-parametric approach

AbstractThe studies on hydrothermal alteration-induced effects in surface and subsurface rocks provide useful information in the characterization and exploitation of a geothermal reservoir. Generally, these studies are based on traditional, and reliable methods like petrography (primary and secondary minerals, and grade of alteration), and geochemistry (mobility of elements, changes in mass and concentration of elements, and fluid inclusions). Recently, apart from these established methods, some methods based on the geochemical (Chemical Index of Alteration, CIA; Weathering Index of Parkar, WIP; Loss on Ignition, LOI; and Sulfur, S) and rock magnetic properties (magnetic susceptibility, χlf; and percentage frequency-dependent susceptibility, χfd%) are also being applied in the identification of whether a rock is an altered or a fresh one. The Acoculco Geothermal Field (AGF), Mexico, is characterized by high temperature and very low permeability, and it is considered a promissory Enhanced Geothermal System. The following changes are observed in the rocks as a result of an increase in hydrothermal alteration: (1) an increase in CIA, LOI, and S values, and a decrease in WIP; (2) an increase in quartz and quartz polymorph minerals (silicification), and clay minerals (argillization); and (3) decrease in χlf values. At AGF, the most altered surface acid rocks are characterized by entirely quartz and its polymorphs, and clay minerals. The present study also indicates the applicability of the binary plots of major elements (felsic vs mafic component) and rock magnetic parameters (χlf vs. χfd%). The rock with χfd% value of 2–10 and χlf value < 0.5 × 10–6 m3 kg−1 indicate the presence of single domain and stable single domain grains, which in turn suggests that it is an altered rock. These methods are simple to apply, rapid, reliable, and have the potential to become effective tools for the identification of hydrothermally altered rocks during the initial stage of geothermal exploration.

High-precision microcline 40Ar/39Ar dating by combined techniques

K-feldspar is one of the most common minerals applied for 40Ar/39Ar dating because of its high K content and ubiquitous presence. Excess 40Ar in K-feldspar resulting in unreliable ages has long been reported, however, its occurrence and trapping time are still inconclusive. Combined argon extraction techniques, including laser step heating, and step crushing, as well as succedent furnace incremental heating, have been applied to six microcline samples from two hydrothermal ore deposits in South China, to address these issues. Two microcline samples (09SZ37 & 39) measured by all the above three gas extraction techniques yield flat age spectra and concordant inverse isochron ages respectively. However, one sample (09NL10) produces a flat spectrum but with abnormally low radiogenic 40Ar, and the other three samples obtain uneven age spectra, by laser heating. All six samples measured by step crushing yield quite similar age spectra characterized with declining staircases for the first several steps, and followed with very flat plateaus of high-precision plateau ages for the subsequent later crushing steps. The declining sections with abnormally old ages indicate that excess 40Ar (and obvious atmospheric argon) was trapped in the microcracks and the secondary fluid inclusions. The data points corresponding to age plateaus well define inverse isochron ages which are in consistence with their plateau ages respectively. Furthermore, the crushed powders of all six samples by further furnace step heating also yield very flat age spectra, whose plateau ages are well concordant with those obtained by the foregoing step crushing experiments. The trapped excess 40Ar caused the total fusion ages of laser heating 0.7–17.8% older than their true ages. Mineral grains of ca. 0.10 mm may effectively remove excess 40Ar in microcracks and secondary fluid inclusions, proposed for direct 40Ar/39Ar step heating. Stepwise crushing, combined with furnace heating, is not only very useful to date fluid inclusions, but also robust to remove excess 40Ar and obtain more precise true ages for K-bearing minerals, e.g., K-feldspar, amphibole, tourmaline, and so on.

Three-stage hydrocarbon accumulations in the Middle Permian in the Central Sichuan Basin

The Middle Permain Maokou Formation (P2m) is a new region of natural gas exploration in the Sichuan Basin, is characterized by bioclastic limestone with localized dolomitization, and karst fractured-vuggy reservoirs. Currently, on the gas source, hydrocarbon accumulation process and control factors in the Sichuan Basin during the Permian are lacking. To bridge this gap, herein, we identified the filling sequence minerals inside the pores/vugs, along with the oil charge of the Maokou Formation using drill cores, thin sections, oil inclusion analysis, and U-Pb dating of calcite cements. The results showed that the reservoir space of the Maokou Formation was predominated by the residual dissolved pores/vugs, fractures, and dissolved fractures. The pores/vugs underwent four stages of mineral filling by very fine-fine (-crystalline, CC1) calcite → fine-medium calcite (CC2: from 256.4 ± 1.7 to 244.1 ± 6.3 Ma) → fibrous calcite (FC; ∼183.9 ± 8.2 Ma) → coarse-macro calcite (CC3; ∼171.5 ± 5.3 Ma). Combined with the homogenization temperature and salty of fluid inclusion, we considered that three stages of oil charge were present in the Maokou Formation reservoirs. The first stage involved the formation of paleo-oil reservoirs during the Late Permian to Early Triassic, corresponding to the high-maturity aqueous inclusions in CC2, with a homogenization temperature of 106.7°C–137.8°C. At that time, the oil generation from the Lower Cambrian Qiongzhusi Formation rocks peaked, and the generated hydrocarbons migrated upward into the Maokou Formation through the strike-slip faults in the basin center. The second stage involved the formation of paleo-oil reservoirs during the Early Jurassic. The Permian source rocks reached the oil generation window with hydrocarbon expulsion, which was consistent with the oil inclusions in FC. The third stage involved the formation of paleo-gas reservoirs during the Middle Jurassic to Early Cretaceous, corresponding to the high-density methane inclusions and bitumen inclusions occurring in CC3, with the homogenization temperature peaking at 151.9°C–178°C. The natural gas in the Middle Permian of the Central Sichuan Basin is predominantly sourced from the Lower Cambrian Qiongzhusi Formation mudstone and partially from the source rocks of the Middle Permian, indicating a significant source-reservoir conduit of the strike-slip faults in the basin center. The findings provide considerable baseline data to advance further research in the Sichuan Basin.

Diagenetic evolution and origin of cements of the Cretaceous carbonates in the Akal Block, southeast Gulf of Mexico

The Cretaceous succession of deep marine facies (580 m-thick) of the Akal Block, southeast of the Gulf of Mexico, is dominated by lime mudstones that have been extensively affected by dolomitization. Petrographic examination, including cathodoluminescence (CL), and microthermometry reveal three phases of dolomitization that occurred as replacement and pore-/fracture-filling cements. These are (1) an early replacement dolomicrite (D1, <50 μm) with dull orange CL, (2) eu-to subhedral dolomite (D2, 60 μm–150 μm) with distinctive cloudy cores (dull CL) and clean edges (non-luminescent to dull CL), and intercrystalline porosity (6 %), and (3) anhedral vug-/fracture-filling dolomite (D3, 200 μm–500 μm) exhibiting CL zonation. The mean δ18O values of those dolomites show a trend of slight decrease from D1 to D3 (D1 = 0.0 ± 1.3‰ VPDB, D2 = −0.2 ± 1.1‰ VPDB, D3 = −1.2 ± 1.6‰ VPDB), which reflects the influence of progressive burial. The low Sr contents (101 ± 29 ppm) of D1, along with its near-micritic grain size and estimated δ18OD1fluid of parent dolomitizing fluid (−3.6‰–1.9‰ SMOW), support early-stage dolomitization at low temperature of near-surface conditions from solutions of mixed meteoric and seawaters. The higher estimated δ18Ofluid values of D2 and D3 (3.0–13.9‰ SMOW) are expected for fluids in burial settings of high temperatures. The homogenization temperatures of the primary two-phase fluid inclusions of D2 (Th = 99.1 ± 7.9 °C) and D3 (Th = 89.5 ± 11.2 °C) suggest precipitation at deeper burial settings, which is also consistent with their lower Sr contents (80 ± 14 ppm and 98 ± 33 ppm, respectively). The lower mean Th value of D3 than that of D2 likely reflects the influence of tectonic uplift that affected the Akal Block. The shale-normalized (REESN) patterns of the dolomites mimic that of modern seawater and those of D1 and D2, in particular, coincide with that of the lime mudstones (C1), thus suggesting that C1 was their precursor and also D3 originated almost from the same parent fluid that its geochemical composition evolved with progressive burial. The latest fracture-filling calcite cement (C2) postdated D3. The investigation of the products of burial diagenesis, such as fracturing, dissolution, and dolomitization events and their relationship with the tectonic uplift of the Akal Block allows a better understanding of the factors controlling the quality of Cretaceous reservoir rocks.

Genesis of the Yu'erya gold deposit, East Hebei, China: Insights from geology and fluid inclusions

The Yu'erya gold deposit is a large‐scale gold deposit in eastern Hebei. However, the ore genesis is controversial because the gold orebody is closely related to biotite granite in time and space. The hydrothermal processes of the Yu'erya gold deposit can be divided into early quartz (Q1)‐pyrite stage, middle quartz (Q2)‐sulphide stage and late quartz (Q3)‐calcite stage. The Q1 is subdivided into Q1‐1 and Q1‐2 generations by SEM‐CL images, and Q2 is subdivided into Q2‐1 and Q2‐2 generations. Three types of fluid inclusions (FIs), including aqueous‐carbonic type (C‐type), aqueous‐rich type (W‐type) and vapour‐rich type (V‐type), were observed in the Yu'erya gold deposit. The Q1‐1 is dominated by C‐type inclusions, and the other quartz is dominated by W‐type inclusions. Laser Raman analysis shows that the initial ore‐forming fluids mainly belong to the CO2‐H2O‐NaCl system and gradually transferred to the H2O‐NaCl system. The C‐type inclusions of Q1‐1 show the salinity of 3.1–10.1 (av. 6.9 ± 1.7) wt.% NaCl eqv., and the total homogenization temperature (Th) of 305–391°C. The homogenization temperature decreases gradually from the early to the late stage, with insignificant change in salinity, indicating that fluid cooling is the main precipitation mechanism. Trapping pressures of C‐type FIs are estimated at 206–284 MPa and 203–211 MPa for the early and middle stages, corresponding to the mineralization depths of 7.2–9.9 km and 7.1–7.4 km, respectively. The FIs in the early stage indicate that the initial ore‐forming fluids are characterized by low salinity and CO2 richness, which are different from those of the intrusion‐related gold deposits, but consistent with the orogenic gold deposits.

Conditions of Formation of the Pavlik Gold–Sulfide–Quartz Deposit (Northeast of Russia) According to a Study of Fluid Inclusions

The stockwork gold–sulfide–quartz ores of the Pavlik deposit (northeast of Russia) are formed mainly by homogeneous, low saline (9.4–4.3 wt %-eq. NaCl), essentially water–chloride fluid at temperatures of 275–330°C and a fluid pressure of 600–1840 bar. The fluid is characterized by a rather high CO2 content and a reduced methane content, with the ratio of CO2 : CH4 being 17–37.3. In the fluid, among the cations, the main role is played by Na and Ca, and K and Mg are in a subordinate amount. In addition, many trace elements were detected in the fluid composition: As, Li, Rb, Cs, Mo, Ag, Sb, Cu, Zn, Cd, Pb, U, Ga, Ge, Ti, Mn, Fe, Co, Ni, V, Cr, Y, Zr, Sn, Ba, W, Au, Hg, and rare-earth elements. The obtained data allow us to suppose that there was a decrease in the depth of ore formation during the formation of the deposit by 4–5 km due to the rise of the ore-bearing block. Attention is drawn to the low fluid pressures for a number of samples (330–140 bar), which may be associated with a separate stage of mineral formation. The ore-forming fluid of the Pavlik deposit in composition, temperatures and pressures shows similarities with the fluids of the neighboring Natalka and Rodionovsky deposits and has a great similarity with typical fluids of orogenic gold deposits. The information given in the article is of great practical importance for regional predictive metallogenic constructions, as well as prospecting and evaluation of gold deposits.

Geochemistry of hydrothermal illitizations in Eocene Kösedağ magmatic rocks, Zara-Suşehri area, NE Sivas, East-Central Anatolia: Origin and age of alteration

The study area located at the periphery of the collision zone between the Eurasian plate (i.e. Pontides) and Tauride-Anatolide platform, NE of Sivas in the east-central Turkey, which is part of the Tethyan Metallogenic Belt. Mixed-layer illite-smectite (IS) and illite minerals are derived within the hydrothermal alteration zones with a few km2 surface areas (up to 30 km2) in Eocene volcanic and plutonic rocks. The representative IS and illite samples taken from plutonic- and volcanic-hosted alteration zones are investigated by optical and scanning electron microscopy (SEM), X-ray diffraction (XRD), major and trace elements and OH isotope geochemistry and KAr dating methods. Different types of hydrothermal alterations, such as propylitic and phyllic alteration in the plutonic rocks and argillic alteration in the volcanic rocks were developed as a result of intrusion of Kösedağ Pluton (syenite) into Karataş Volcanics (basaltic trachy-andesite and trachyte) with relations of hot-hot contact. The main phyllosilicate/clay minerals are characterized by kaolinite and IS in volcanic-hosted argillic alteration zones, whereas IS and illite in plutonic-hosted phyllic zones. The ordering types (Reichweite) of IS and illites are represented by R1 IS (I = 65–80 % in IS) + R3 IS (I = 90 % in IS) in the volcanic-hosted rocks, and R3 IS (I = 90 % in IS) and illite (S = 3–5 %). Dioctahedral (d060 ≤ 1.500 Å) R3 IS and illites have 1 Md + 1 M and 1 Md + 1 M + 2 M1 polytypes, respectively. The major and trace elements such as TiO2, Fe2O3, MgO, Na2O, P2O5, Sc, V, Cu, Ge, Sr, Hf, Zr and Y increase in the volcanic-hosted IS, whereas SiO2, Al2O3, K2O, Pb, W, Mo, As, Sb, Rb and U in the plutonic-hosted IS and illites. The chondrite-normalized distributions of IS and illites present a great similarity to those of host rocks. The chondrite-normalized rare earth element (REE) concentrations are more enriched in the volcanic-hosted IS in comparison with the plutonic-hosted IS and illites having distinctive Eu negative anomaly, which indicate deriving from volcanic matrix and K-feldspar, respectively. Oxygen and hydrogen isotope data of illitic clays indicate that the hydrothermal fluids are originated from magmatic water. According to stable isotopes and fluid inclusion data, IS and illites were formed at the temperature conditions ~150 °C in volcanic-hosted argillic zone, whereas ~250 °C in plutonic-hosted phyllic zones. KAr dating of alunite, IS and illite minerals indicate that the hydrothermal alteration was started at 40.45 ± 1.28 Ma, almost 2 Ma after the Q-syenite intrusion, within the plutonic body as phyllic alteration stage, and continued up to 35.27 ± 2.81 Ma, with a duration of ~5 Ma, and finalized before the exhumation of the Kösedağ Pluton (28–30 Ma). The geochemical characteristics of IS and illites were controlled by host-rock, condition, origin, and ages of alterations and they can be used as an important tool for magmatic-hydrothermal systems.

Influences of burial process on diagenesis and high-quality reservoir development of deep–ultra-deep clastic rocks: A case study of Lower Cretaceous Qingshuihe Formation in southern margin of Junggar Basin, NW China

Taking the Lower Cretaceous Qingshuihe Formation in the southern margin of Junggar Basin as an example, the influences of the burial process in a foreland basin on the diagenesis and the development of high-quality reservoirs of deep and ultra-deep clastic rocks were investigated using thin section, scanning electron microscope, electron probe, stable isotopic composition and fluid inclusion data. The Qingshuihe Formation went through four burial stages of slow shallow burial, tectonic uplift, progressive deep burial and rapid deep burial successively. The stages of slow shallow burial and tectonic uplift not only can alleviate the mechanical compaction of grains, but also can maintain an open diagenetic system in the reservoirs for a long time, which promotes the dissolution of soluble components by meteoric freshwater and inhibits the precipitation of dissolution products in the reservoirs. The late rapid deep burial process contributed to the development of fluid overpressure, which effectively inhibits the destruction of primary pores by compaction and cementation. The fluid overpressure promotes the development of microfractures in the reservoir, which enhances the dissolution effect of organic acids. Based on the quantitative reconstruction of porosity evolution history, it is found that the long-term slow shallow burial and tectonic uplift processes make the greatest contribution to the development of deep–ultra-deep high-quality clastic rock reservoirs, followed by the late rapid deep burial process, and the progressive deep burial process has little contribution.

Physico-chemical constraints associated with hypogene hydrothermal alteration and supergene oxidation at the Farallón Negro intermediate-sulfidation epithermal Au-Ag deposit, NW Argentina

The Farallón Negro deposit is located in the Farallón Negro Volcanic Complex (FNVC; ∼9.40 to 4.88 Ma) in NW Argentina and includes intermediate-sulfidation, epithermal Au-Ag mineralization in the Alto de la Blenda deposit. Detailed mineralogical and geochemical investigations of the Esperanza-Esperanza Sureste, Cecilia veins, both hosted in the Alto de la Blenda monzonite, and the Laboreo vein, hosted in the volcaniclastic Morada Central andesitic breccia, reveal four hypogene vein stages. Each of these four stages are comprised of a texturally early quartz, followed by a sulfide-sulfosalt and a late Mn-carbonate sub-stage. Stage 1 is characterized by a quartz, base metal and Mn-carbonate sub-stage showing high δ18OVSMOW (δ18OQuartz = 13.7 ± 0.4‰ and δ18OMn-Calcite = 11.6 ± 0.2‰) and low δ13CVPDB values (δ13CMn-Calcite = –4.7 ± 0.2‰). Stage 2 includes most gold and silver minerals in a tennantite-tetrahedrite-sphalerite-pyrite-galena±electrum sub-stage. Fluid inclusions trapped in vein-anhydrite, only associated with stage 2, record the highest temperatures (TEntrapment = 327–335°C) and highest salinity (3.8NaCleq.wt.%) when compared to all other stages. Stage 2 quartz (δ18OQuartz = 9.9 to 11.9‰) and carbonate display lower δ18OVSMOW (δ18OMn-Calcite = 7.9 to 10.3‰), as well as lower δ13CVPDB values (δ13CMn-Calcite = -5.2 to 4.7‰) than all other stages. Stages 3 and 4 consist of barren quartz and carbonate sub-stages that show lower temperatures (TEntrapment = 187–262°C) and higher δ18OVSMOW (δ18OQuartz = 13.4 to 13.9‰ and δ18OMn-Calcite = 10.7 to 11.8‰) and δ13CVPDB values (δ13CMn-Calcite = –3.5 to −3.9‰) when compared to earlier stages.Petrographic, sulfide mineral chemistry, microthermometric, and stable isotope data combined with distinct alteration mineral assemblages allow the reconstruction of a model for the Alto de la Blenda deposit. During the hypogene hydrothermal fluid flow stage, around 7.16 Ma, at ∼2.2 km depth (PHYDROSTATIC = 220 bars), the high temperature alteration mineralogy, low fluid salinities and 18O and 13C isotope compositions indicate cooling of ascending magmatic-hydrothermal fluids. These fluids were derived from a contracted magmatic vapor phase that exchanged with unexposed sedimentary rocks of the Sierras Pampeanas. Hypogene quartz-R3 illite/smectite-chlorite-calcite-pyrite alteration in the andesitic-breccia and quartz-illite-R3 illite/smectite-chlorite-calcite-pyrite alteration in the monzonite are temporally and spatially to the hydrothermal vein minerals deposited between 187 and 335°C. During the post-hydrothermal supergene weathering event, around 2.70 Ma, sulfuric acids generated by meteoric fluids and supergene oxidation of hypogene sulfides, particularly pyrite, descended along normal fault zones and related vein and fracture networks to at least 450 m depth below the surface level, forming the supergene alteration assemblage of dioctahedral smectite-kaolinite-Mn-/Fe-oxides/-hydroxides-gypsum±interstratified chlorite/smectite±interstratified R1 illite/smectite in both wall rock types at temperatures <50°C.The dominant hypogene alteration assemblage, present in the monzonite and volcaniclastic andesitic breccia, may be indicative of deeper-seated, transitional magmatic-hydrothermal systems (>1 km depth), generally controlled by fluid cooling, as is the case at Farallón Negro. Pervasive supergene alteration can conceal these systems but the identified supergene alteration assemblage of dioctahedral smectite-kaolinite-interstratified chlorite/smectite-interstratified R1 illite/smectite that replaces the hypogene alteration assemblage along the permeable fault, vein and fracture zone network may present a useful vectoring tool towards concealed, deeper-seated, epithermal Au-Ag mineralization in arid climates.

Fluid inclusion records of oil-cracked wet gas in Permian carbonate reservoirs from the Eastern Sichuan Basin, China

Oil-cracked gas is an important gas resource in petroliferous basins. In theory, oil-cracked gas should be rich in C2+ (i.e., wet gas). However, gas reservoirs of oil cracking origin in the Sichuan Basin (South China) are mainly composed of dry gas. Fluid inclusions record hydrocarbon generation and evolution. Obviously, identifying wet gas inclusions is the key to solving the above controversy. Here, we calibrate the Raman spectroscopic detection limits of ethane and propane (0.007 and 0.016 MPa, respectively) and demonstrate that Raman spectroscopy is a powerful method to identify C2+-bearing inclusions. Based on this technique, C1–C2–C3-bearing wet gas inclusions (C1/ƩCi = 0.90–0.93) are identified in calcite cements of the Permian Changxing Formation carbonate reservoir in the Eastern Sichuan Basin. These inclusions are direct geological evidence of oil-cracked gas because the calcite cement precipitated after the formation of pyrobitumen. Furthermore, reconstruction of the temperature-pressure-composition evolutions of calcite-hosted (Changxing Formation) and quartz-hosted gas inclusions in the Ordovician Wufeng–Silurian Longmaxi Formation black shale indicates that high pressure can effectively retard the cracking of C2+ gas hydrocarbons. Differences in gas composition and δ13CCH4 signature between reservoir gas and inclusion gas lead us to conclude that large-scale oil cracking occurred during the Early–Middle Jurassic in the Permian Changxing carbonate reservoir. Nevertheless, oil-cracked gas escaped during the evolution of the paleo-gas reservoir. Formation of the current dry gas reservoir can be attributed to the continuous mixing of late-stage kerogen pyrolysis gas.

Diagenesis and pressure evolution in Ediacaran carbonate rocks in the North Slope (Penglai area) of the central Sichuan Basin (SW China)

The Penglai area is an emerging giant gas field in the Sichuan Basin supporting the carbon neutrality goal. To accelerate the exploration, this study combines petrographic observations using optical and cold cathodoluminescence microscopy, fluid inclusion microthermometry, Raman spectroscopy, 1D basin modeling and fluid inclusion PVTX modeling techniques to reveal the paleo-fluid P-T evolution in the second member of the Dengying Formation (Ediacaran age). The results firstly showed a diagenetic sequence which includes three phases of dolomite (1–3), bitumen, quartz (core and overgrowth) and fluorite. Then, ten paleo-fluid episodes were identified based on microthermometric histograms of different mineral phases. Finally, by integrating the Raman results, the hydrostatic and lithostatic gradients calculated by the constructed 1D basin model, and the existing equation of states, the homogenization pressure of fluid inclusions from each fluid episode was determined. By assuming the system was saturated by CH4 as suggested by Raman results, the homogenization pressure equals to the trapping pressure. The P-T trajectory indicates that the second member of the Dengying Formation experienced two sever pressure accumulation stages. One is between fluid episode #3 and #4 recorded by dolomite 3, due to oil generation, and the other is from fluid episode #5 in dolomite 3 to episode #7 in quartz overgrowth to episode #8 in fluorite due to oil cracking to gas. Also, two pressure releasing periods were recorded by fluid episode #5 in dolomite 3 and #9 and #10 in fluorite, caused by both basin uplifting and fluid pressure reaching the lithostatic pressure. This study provides insights into fluid pressure evolution in deeply buried carbonate reservoirs. The understandings can support further geoenergy utilization and reduce exploration risks.