Homogenization Temperatures Of Fluid Inclusions Research Articles

Unraveling the early petroleum migration of the Potiguar Basin, Brazil: Constraints from fluid inclusions of the Ubarana and Lorena oilfields

A coupled study of fluid inclusions and one-dimensional basin modeling allowed the reconstruction of the early hydrocarbon filling history of the main petroleum systems of the Potiguar Basin, Brazil. Confocal laser scanning microscopy, microthermometry and fluorescence spectra measurements were used to model PVTx properties of petroleum inclusions in samples from the onshore Lorena and the offshore Ubarana oilfields. The fluid inclusion data and basin modeling suggest that the initial filling of the Lorena reservoir (Pendência Formation) occurred during the rift stage in two scenarios at 62.7–80.6 °C (11–16.8 MPa): (i) the late rift phase in the Barremian–Early Aptian (ca. 129–124 Ma) or during the beginning of the uplifting phase in the Aptian (ca. 118–117 Ma). In the Ubarana Oilfield, hydrothermal fluids migrated alongside petroleum during the reservoir initial filling. Fluid inclusion homogenization temperatures for both the Açu (mode interval of homogenization temperatures, Th(LV→L)mode interval = 124–128 °C) and Ponta do Mel (Th(LV→L)mode interval = 115–130 °C) formations are higher than the present-day reservoir temperature (110 °C). Isochoric modeling of saddle dolomite-hosted fluid inclusions from the Ponta do Mel Formation indicates trapping temperatures between 128.9 and 133.1 °C and pressures between 10.6 and 12.9 MPa. Temperatures and salinities (16 and 20 mass % equivalent in NaCl + CaCl2) of aqueous inclusions hosted in saddle dolomite are similar to those of the fluid inclusions trapped in hydrothermal minerals in Mississippi Valley-Type Pb–Zn deposits. No specific timing can be suggested for the hydrothermal event in Ubarana Oilfield. However, this atypical (hydrothermal) petroleum system could be linked to two possible scenarios: (i) the onset of petroleum migration of the Alagamar Formation during the Cenozoic Macau (ca. 48–8.8 Ma) magmatic event; or (ii) an earlier activity of the petroleum system, prior to the peak oil generation (50–10 Ma).

Evolution Mechanism of Differential Diagenesis Combination and Its Effect on the Reservoir Quality in the Tight Sandstone: A Case from the Lower Shihezi Formation in the Hangjinqi Area of Ordos Basin, China

Abstract The physical property heterogeneity of tight sandstones was mainly caused by complex alteration of various diagenesis combinations during burial process. However, diagenetic evolution of different diagenesis combinations which generally result in the strong difference and heterogeneity of physical property and pore structure is rarely well understood. The Middle Permian lower Shihezi Formation is one of the most important tight gas sandstone reservoirs in the Hangjinqi area of Ordos Basin, China. The reservoir heterogeneity of lower Shihezi Formation, which was caused by the differential diagenesis combination, is crucial to efficient exploration and development. Evolution mechanism of differential diagenesis combination and its effect on the reservoir quality in the tight lower Shihezi Formation sandstone in the Hangjinqi area of Ordos Basin was investigated by means of thin-section description, cathodoluminescence (CL) imaging, X-ray diffraction (XRD), scanning electron microscopy (SEM), and homogenization temperature of fluid inclusions. The lower Shihezi Formation sandstones can be divided into four diagenesis combination types according to the reservoir characteristics and diagenetic relationship. The main diagenetic sequence was mechanical compaction-chlorite rim-early pore-filling calcite cementation-dissolution-authigenic kaolinite-quartz cementation-late calcite cementation. Differential diagenesis combination was mainly controlled by the petrological characteristics, microfacies, and fault. Low content of rock fragment and high content of detrital quartz were beneficial to the compaction resistance and cementation. The moderate content of pore-filling calcite was conducive to pore space protection and feldspar dissolution. The faults control dissolution and differential diagenesis combination by influencing the migration of acid fluids. Moderate compaction-moderate cementation-moderate dissolution type (BBB type) and weak compaction-moderate cementation-strong dissolution type (CBA type) were in favour of high-quality reservoir development.

Reflux dolomitization and subsequent hydrothermal dolomitization induced by the Alkaline Igneous Province in the Middle Devonian Nowshera Formation (Peshawar Basin, NW Pakistan)

Dolomitized strata are potential exploration targets because they host economic mineral and hydrocarbon deposits around the globe. Establishing a petrogenetic history for dolomite is thus very vital. In this study, dolomitic bodies present in the Devonian carbonates of Nowshera Formation in Peshawar Basin, North-West Lesser Himalayas in north Pakistan are investigated through field observations, petrographic studies, and geochemical analysis. The carbonates of Nowshera Formation show evidence for multistage dolomitization and a complex diagenetic history. In a first stage the succession was completely dolomitized probably by the reflux of penecontemporaneous mesohaline seawater. This process resulted in both fabric-retentive and fabric-destructive dolomite types under near surface to shallow burial realms. In the subsequent stage saddle dolomites (matrix & cement) were formed under intermediate and/or deep burial realm. This later saddle dolomite phase likely resulted from the circulation of exotic fluids at comparatively high temperature. This local hydrothermal event, evidenced from the fluid inclusion homogenization temperatures, and stable isotopic signatures, also resulted in vuggy, fracture, and dissolution enhanced porosity. Microthermometric analysis of primary fluid inclusions in saddle dolomite confirm the presence of hot (125–178 °C) and highly saline brines (17–25 mass% NaCl equivalent). The δ18Owater (+2 to +9.2‰V-SMOW) calculated from fluid-inclusion homogenization temperatures in conjunction with the comparatively high salinity values is compatible with a magmatic origin of the hydrothermal fluid. It is suggested that the investigated hydrothermal dolomites in the Nowshera Formation formed in Carboniferous-Permian time. During this time, thermal convection heated by the Peshawar Plain Alkaline Igneous Province (PPAIP) may have provided the high temperature and sufficient magnesium (Mg) flux for several millions of years (~40 M.y.), ultimately leading to the formation of hydrothermal dolomite bodies. The pore spaces resulted from the leaching of these hydrothermal fluids were subsequently occluded by meteoric calcite related to the Eocene to Middle Miocene Himalayan exhumation of the studied area. This study provides important implications for potential source of Mg in understanding the genesis and timing of dolomitization.

Fluid inclusion and C-O isotopic constrains on the origin and evolution of ore-forming fluids of the Badaguan Cu-Mo deposit, Inner Mongolia

The Badaguan porphyry Cu–Mo deposit is located in the Derbugan metallogenic belt. Aqueous (W-type), CO2-rich (WC-type), vapor-rich (V-type), and daughter mineral–bearing inclusions (S-type) were recognized in the hydrothermal quartz. Fluid inclusions in the early stage show high homogenization temperatures (247–374 ℃) and high pressures (66–191 MPa), with salinities of 0.95–12.15 wt% NaCl equiv. The presence of anhydrite in veins and phenocrysts suggests high oxygen fugacity conditions. The veins in the Mo- and Cu mineralization stages were formed by immiscible fluids at temperatures of 244–366 ℃ and 183–342 ℃, and pressures of 39–137 MPa and 30–98 MPa, respectively. They yielded salinities of 1.69–10.52 and 0.35–6.85 wt% NaCl equiv, respectively. Homogenization temperatures of fluid inclusions in late stage range from 194 to 269 ℃ with a salinity of 0.6–5.88 wt% NaCl equiv. It is concluded that the ore-forming fluids in the early stage were magmatic in origin, then gradually diluted and cooled by meteoric water. During this process, fluid immiscibility and CO2 release induced by pressure drop and oxygen fugacity decrease resulted in Mo mineralization, while temperature drop was the main trigger for the precipitation of chalcopyrite. Laser-Raman analyses suggest that the fluid inclusions of the Badaguan porphyry Cu-Mo deposit in Inner Mongolia are rich in CO2. Minor CH4 occurs in the Mo- and Cu-mineralization stage, which may be generated during water–rock interaction. The hybrid carbon isotope compositions of the fluid inclusions, mainly representing CO2, show a very depleted δ13CPDB (−21‰ to −28.3‰), which can be explained by the joint influence of ~ 0.5% subducted oceanic sedimentary contamination in the mantle and carbon isotope fractionation induced by 35–80% CO2 degassing. Petrographic observation, previous studies on the geochemical and isotopic characteristics of ore-causative rocks rule out the possibility of contamination of ascending magma with reducing wall rocks. The large variations in δ18Owater (−6.6‰ to +2.4‰) for quartz and calcite suggest the participation of meteoric water during water–rock interaction.

Geochemical characteristics comparison of primary gas with producing gas and its significance for depleted ethane carbon isotopic values in the Southeastern gas field of the Ordos Basin, China

To identify the reasons for more depleted ethane carbon isotopic value (MD-δ13C2) in the Southeastern gas field (SEGF), fluid inclusions samples from northern and the SEGF were collected. Homogenization temperature and salinity of fluid inclusions, components and carbon isotopic values of gases released from the crushed inclusions were tested. Geochemical fingerprints information of producing gas in wells and primary gas in inclusions were contrasted. The comparison suggests that geochemical parameters of primary and producing gas in the NGF are similar, whereas those in the SEGF are completely different. The ethane carbon isotopic values of producing gas became more depleted compared with the primary gas, whereas the methane carbon isotopic values are not much different from the primary gas. This indicates that the secondary effect caused the more depleted ethane carbon isotopic values. There was one more period of high temperature fluid filling (140–160 °C) in the SEGF in comparison with the NGF. Higher thermal maturity is the key inducement of the abnormal MD-δ13C2.

Stable isotope and fluid inclusion constraints on the source and evolution of ore fluids in the Xiuwacu W-Mo granite-related quartz-vein deposit, Yunnan Province, China

The Xiuwacu granite-related quartz-vein type W-Mo deposit (9.16 million metric tons at 0.26% WO3 and 0.27% Mo) is situated in the South Yidun terrane (SYT) in SW China. Orebodies, together with aplite veins, occur mainly along the NW-SE-trending fault systems within the Xiuwacu complex intrusions. However, the metallogenic mechanism of this deposit is still ambiguous. In this study, three stages for the ore-bearing quartz veins have been recognized based on the field and petrographic observations: (1) pre-ore fluorite-quartz; (2) syn-ore scheelite-quartz; and (3) post-ore calcite-quartz assemblage. The average δ34S value is 3.84‰, combination with the molybdenite Re values (4.3–18.6 × 10−6), indicating that the ore-forming fluid derived from lower crust magmatic fluid. The δD and δ18Ofluid values of the ore-forming fluids in the Xiuwacu deposit vary broadly from −104.9‰ to −75.0‰ and from −2.5‰ to +5.9‰, respectively, indicating a mixture of magmatic and meteoric water or seawater during the ore-forming process. The coexisting quartz with scheelite contains two-phase aqueous inclusions (type Ia), minor two-phase vapor inclusions (type Ib) and two- or three-phase CO2-bearing inclusions (type II), while scheelite only contains two-phase aqueous inclusions (type Ia). The homogenization temperatures for fluid inclusions in scheelite are very similar in both mineralized zones, which are slightly higher than those in quartz in the East mineralized zone, but are slightly lower than those in quartz in the West mineralized zone. These indicate that fluid cooling is predominant in the formation of scheelite. As the homogenized temperatures in the fluid inclusions of scheelites have a normal distribution, together with the medium-coarse grained signatures of scheelite, it could be concluded that Ca supply was insufficient during the tungsten precipitation process. Thus, the ore-forming mechanism of the Xiuwacu deposit is controlled by both the simple fluid cooling and addition of Ca during fluid mixing and/or fluid-rock interaction.

Oil Accumulation Model and Its Main Controlling Factors in Lower Yanchang Formation, Wuqi-Dingbianarea, Ordos Basin, China

Based on the studies of sedimentary facies, oil-source correlation, formation pressure structure, homogenization temperature of fluid inclusions, etc., the oil and gas accumulation model and main controlling factors in lower Yanchang Formation in Wuqi-Dibian area have been discussed. It is believed that two sets of source rocks, C7 and C9, are developed in study area, and hydrocarbon produced from layers of C8 and C9 is mainly from C7 source rock, followed by C9, according to oil-source correlation; hydrocarbon-generating pressurization of C7 source rock is the main driving force for the downward migration of oil. The high value area formed by the low value of overpressure difference between C7 and C8 is the main hydrocarbon accumulation area; deltaic front subaqueous distributary channel and mouth bar in lower Yanchang formation are the main accumulation spaces due to their good porosity and permeability; besides, C8 reservoir shows the characteristics of “episodic filling and continuous accumulation,” and they both are the undersource reservoir-forming combination; it is believed that the distribution of oil reservoir in Triassic series is controlled by the factors of “near source, low pressure, superior facies.”

Mapping solid reservoir bitumen by deconvoluting formation mechanism

Solid reservoir bitumen is a geologic risk that controls reservoir quality and fluid distribution and has a profound impact on field development and near-field exploration. Understanding the exact mechanism responsible for the development of solid bitumen is therefore a prerequisite toward its proper quantification and mapping. A sandstone reservoir of an anticlinal gas condensate field was investigated geochemically, and data integrated with reservoir diagenetic history to explain abundant solid bitumen encountered within several meters immediately below a regional unconformity in crestal wells, as opposed to deeper sections and downdip wells. The reservoir is generally characterized by a similar sedimentological and diagenetic history across the field, with diagenetic illite grain coats and quartz cement occurring in both updip and downdip wells. The influx of gas condensates into the upper part of the preexisting oil reservoir, possibly facilitated by a major fault, appears to have caused thermodynamic disequilibrium, leading to de-asphalting and subsequent deposition of solid bitumen. Textural and radiometric age dating, integrated with fluid inclusion homogenization temperatures and basin modeling, suggests the initiation of bitumen deposition 129 Ma, immediately after the arrival of gas condensates into the reservoir. As the process was restricted to the uppermost part of the reservoir, it is possible that the solid bitumen, once developed, has improved sealing efficiency along the unconformity and the trapping of subsequent hydrocarbon charges in the underlying reservoir. Better reservoir quality can be inferred farther downdip in areas inaccessible to gas incursions and hence not affected by gas de-asphalting. Other processes, such as regional uplift-induced phase separation, in-reservoir thermal cracking, water-washing, and biodegradation, were also examined and found irrelevant to the formation of solid bitumen in question.

Accumulation characteristics of tight sandstone gas reservoirs in the Upper Paleozoic in the Shenfu block of the Ordos Basin

The Shenfu gas field on the eastern margin of the Ordos Basin is one of the key blocks of CNOOC’s land tight sandstone gas field development in recent years. In order to further analyze the physical properties and accumulation and evolution characteristics of the Upper Paleozoic tight sandstone gas reservoirs in the Shenfu block, we characterized of the reservoir pore structure, diagenetic evolution, and natural gas accumulation stages, combined with fluid inclusions, the types of tight sandstone gas reservoirs in the Upper Paleozoic are studied, revealing the main controlling factors for the natural gas accumulation in the study area. Studies have shown that the core porosity of tight sandstone reservoirs in the Upper Paleozoic in the Shenfu block is less than 10%, and the permeability is less than 1mD. Compaction and cementation are the main factors for reservoir tightness. The results of porosity evolution and homogenization temperature of fluid inclusions indicate that the Upper Paleozoic sandstone tightness is formed during the Middle-Late Triassic-Early Jurassic, which is earlier than the Late Jurassic-Early Cretaceous period of large-scale natural gas accumulation.

Mid-Eocene sea surface cooling in the easternmost proto-Paratethys sea: constraints from quantitative temperatures in halite fluid inclusions

The temperature fluctuations in the Eocene climate are one of the most mysterious phenomena in Cenozoic climate dynamics, and they severely affected the marine and terrestrial biospheres. However, quantitative and comprehensive temperature reconstructions for middle Eocene climatic variations remain limited due to fragmentary paleotemperature records, especially in the easternmost proto-Paratethys sea (Kuqa Depression), where traditional paleoclimatic proxies are restricted by the occurrence of extensive and thick evaporite deposits. In this study, we reconstruct middle Eocene sea surface temperatures (SST) and even air temperatures in the Kuqa Depression based on 584 homogenization temperatures (Th) for fluid inclusions in cumulate and chevron halite crystals from cores DW1 and KL4. The Th values from core DW1 have ranges of 10.5–45.7 °C, the Th values from core KL4 have ranges of 12.6–35.2 °C. The measured Th values are mainly concentrated within 20–30 °C, and an average Th value of 25.6 °Crepresents the SST and is consistent with previous coeval research results. The highest recorded Th is 45.7 °C, clearly representing local hot climate conditions. The average and maximum SST in the middle Eocene Kuqa Depression are cooled by 5.2–11.3 °C and 6.5–14.1 °C, respectively. The cooling trend in the Kuqa Depression occurred in response to global and Tethyan realm cooling in the middle Eocene, but the cooling amplitude in the study area was greater than that of the global climate. The Th data for primary halite fluid inclusions can effectively and quantitatively track past environmental and climatic variations, especially in evaporitic basins where traditional paleoclimatic proxies are often not available.

Basement aquifer evolution and the formation of unconformity-related hydrothermal vein deposits: LA-ICP-MS analyses of single fluid inclusions in fluorite from SW Germany

The main ore stage of three similar unconformity-related vein systems in the Schwarzwald, SW Germany spanning a period of activity of ~150 Ma, was investigated to understand the details of fluid penetration from overlying sediments/marine environment into the basement, their evolution as well as the processes involved in vein formation. To investigate temporal and spatial variations of the hydrothermal fluids responsible for mineralization, over 1650 fluid inclusions were analyzed by microthermometry. Of these, a total of 108 fluid inclusions (mainly in fluorite) were successfully analyzed by LA-ICP-MS. The fluid inclusions reveal a binary mixing trend between a CaCl2- and a NaCl-rich endmember. Independent of major element composition, the fluids are metal-bearing (e.g., up to ~100 mg/kg Ba, Pb, Zn, Ni and up to 10 mg/kg Ag), show high As (up to 1000 mg/kg) and low S (below the detection limit in most analyses). Over time, the mixed fluid shows a gradual decrease in CaCl2 and increase in NaCl with slightly decreasing total salinity.Based on earlier studies and geochemical arguments, the veins formed by anisothermal binary fluid mixing of two fluids, which both were originally derived from seawater and chemically modified through interaction with the basement and sedimentary rocks in different ways. This produced a gradual stratified basement fluid reservoir comprising a modified bittern/halite dissolution brine. The fluids involved in the vein formation are sourced from different depths of this modified bittern/halite dissolution basement brine reservoir: fluid A, a CaCl2-dominated, KCl-poor, deeper seated brine with a salinity of ~25 wt% CaCl2 + NaCl, and fluid B, an NaCl-dominated and KCl-richer brine situated at shallower depths in the crystalline basement with salinities of ~22 wt% NaCl+CaCl2. Based on the Na-Ca-K and NaK thermometers and on Rb/Cs systematics, fluid A records alteration of the Na-, K- and Ca-bearing feldspars of the host rocks; progressive alteration led to consumption of mainly Ca-rich plagioclase in contact with these basement brines. Accordingly, fluid B that subsequently entered the basement was only in equilibrium with alkali feldspars and clay minerals. This scenario produced a gradual change of fluid composition with depth that was pushed to greater depth over time.The source temperatures are estimated to ~250 °C while vein formation occurred at 100–170 °C, based on fluid inclusion homogenization temperatures. Thus, significant fluid cooling without abundant fluid mixing (and without major mineralization) must have occurred during fluid ascent (3–7 km, depending on the assumed geothermal gradient). Fluid mixing then resulted in the formation of the major gangue minerals fluorite, quartz, barite and calcite, while the locally confined sulfides must have formed due to an influx of sulfide into the binary mixed fluid. The process of fluid mixing is a rapid and turbulent process. This is recorded by a great diversity of fluid compositions (including ones close to the mixing endmembers) trapped within the same crystal. Compositional variations are even visible within individual fluid inclusion trails.

Fluid evolution and ore genesis of the Chaobuleng skarn Fe[sbnd]Zn polymetallic deposit, Northeast China: Evidence from fluid inclusions, C[sbnd]O[sbnd]S[sbnd]Pb isotopes, and geochronology

The Chaobuleng skarn deposit, located in the Erlian–East Ujimqin region, Northeast China, is a medium-sized FeZn polymetallic deposit. Iron-polymetallic mineralization occurs mainly as stratiform, lenticular, and vein types hosted in the contact zone between the Chaobuleng syenogranite and the Devonian Taerbagete Formation. The Chaobuleng deposit is a calcic skarn deposit dominated by a grossular-rhodonite-allanite-epidote assemblage. The ore minerals mainly consist of magnetite, pyrite, sphalerite, chalcopyrite, and bismuthinite, with lesser amounts of molybdenite, arsenopyrite, and pyrrhotine. Four mineralization stages have been identified: (I) a prograde skarn stage; (II) a retrograde skarn stage; (III) a quartz-minor calcite-polymetallic sulfide stage; and (IV) a quartz-calcite-minor pyrite stage. Three types of fluid inclusions (FIs) in garnet, quartz and calcite were classified: vapor-rich two-phase FIs (V-type), liquid-rich two-phase FIs (L-type), and daughter mineral-bearing three-phase FIs (S-type). The homogenization temperatures of FIs from stages I, II, III, and IV are 446–547, 411–519, 279–415, and 147–217 °C, respectively, with corresponding salinities of 0.87–59.47, 0.52–51.52, 0.70–13.25, and 3.05–7.85 wt% NaCl equiv., respectively. The results suggest that mineralization in the Chaobuleng deposit occurred at a depth of 1.0–2.8 km, with fluid boiling and mixing with meteoric water likely being responsible for ore precipitation. The CO isotope data suggest the early ore-forming fluids were mainly related to felsic rocks and were accompanied by intense water/rock reactions during the fluid migration. The SPb isotopic compositions indicate that ore materials were derived from the mixing between magma and wall rocks. Zircon UPb dating yields a weighted mean 206Pb/238U age of 135.9 ± 0.8 Ma (MSWD = 1.03), which represents the crystallization time of the syenogranite. Together, these data suggest that the Chaobuleng deposit is a skarn ore system that is spatially, temporally, and genetically related to syenogranite and formed in an extensional environment resulted from the roll back of the subducted Paleo-Pacific Plate.

Petroleum charge history of deeply buried carbonate reservoirs in the Shuntuoguole Low Uplift, Tarim Basin, west China

The newly discovered Shunbei Oilfield marks a major breakthrough in deep marine carbonate oil and gas exploration in the Tarim Basin. Understanding the petroleum charge history of the deep reservoirs will greatly benefit further petroleum exploration and development in this area. In this paper, we reconstruct the petroleum charge history of the reservoirs by integrating molecular geochemical correlations of reservoir oils and oil components (free oils and quasi-inclusion oils) from reservoir rocks, fluid inclusion analysis and basin modelling results. Molecular parameters derived from steranes, terpanes and aromatic compounds indicate that the reservoir oils, free oils and quasi-inclusion oils were all derived from the same source rock, deposited in a highly reduced marine environment with organic matters being mainly contributed by algae and bacteria. Utilizing the correlation between methylated aromatic compounds and vitrinite reflectance, the calculated vitrinite reflectance (Rc) values for the reservoir oils, free oils and quasi-inclusion oils fall into two distinct maturity ranges, with one spanning from 0.80%Rc to 0.96%Rc, and the other ranging from 1.15%Rc to 1.24%Rc, respectively. The timing of oil charge in the Shunbei Oilfield is constrained by combining fluid inclusion homogenization temperature (Th) with burial history modelling. Two oil charge events occurred in the reservoirs with the first marked by the relatively “low-maturity” oil, occurring in the latest Silurian around 426 Ma; whereas the second represented by the relatively “high-maturity” oil, occurring during the early Carboniferous around 330 Ma. This study highlights the significance of stable tectonic setting and excellent sealing capability of cap rocks for reservoir preservation. The finding unravels the petroleum charge and accumulation history of the Shunbei oil reservoirs and provides new insights for future exploration.

The influence of magmatic-hydrothermal activities on porosity and permeability of sandstone reservoirs in the Linxing area, Ordos Basin, Northern China

The eastern margin of the Ordos Basin in the Linxing area has great potential for exploration of natural gas. However, the intrusion of the Zijinshan igneous pluton and associated hydrothermal activities have affected the sandstone reservoirs. Through the data analyses of rock thin sections, cathodoluminescence, X-ray diffraction, vitrinite reflectance, fluid inclusions, and reservoir physical property, this paper aims to clarify the influences of magmatic-hydrothermal activity and to explore the response characteristics of reservoir porosity and permeability. From the outer zone to the inner zone, the closer to the pluton, the closer the contact between the reservoir clastic particles, which confirms the compression caused by magma intrusion. The differences in feldspar dissolution and alteration, siliceous cementation, clay mineral content, vitrinite reflectivity, homogenization temperature and salinity of fluid inclusions, confirmed that magmatic-hydrothermal activity affected fluid properties and evolution of strata temperature. Combined with the vertical distribution characteristics of feldspar dissolution-alteration and the abnormal conversion of clay minerals, it is considered that the influence of hydrothermal fluid was limited to the reservoir under the thick mudstone of the Xiashihezi Formation. Magma intrusion causes the development of reservoir fractures, thus the permeability of the reservoir decreases from the inner zone to the outer zone, and the porosity presents the distribution pattern of “high-low-slightly high”. The reservoir porosity and permeability genetic model was established to provide references about favorable areas for oil and gas exploration and development in the Linxing area and areas with similar geological history.

Origin of the Tudui–Shawang gold deposit, Jiaodong Peninsula, north China Craton: Constraints from fluid inclusion and H-O-He-Ar-S-Pb isotopic compositions

We suggest deep-seated, most likely mafic dike magmatic and mantle-derived, sources for ore-forming fluids, materials, and heat for ore formation. • Deep-seated, most likely mantle-derived, sources for gold ore formation. • Gold deposits throughout the Jiaodong share similar geodynamic setting. • Gold mineralization is related to destruction of the North China Craton. Recent mineral exploration has identified a new gold metallogenic belt which contains reserves of >120 tonnes Au along the northeastern margin of the Jiaolai Basin, Jiaodong Province. However, the origin of these deposits remains a subject of considerable debate. This paper presents a combination of fluid inclusion and systematic H–O–He–Ar–S-Pb isotopic data of the Tudui–Shawang deposit which is characterized by sulfide-rich ores. Homogenization temperatures of fluid inclusions in the three recognized stages are 245 to 426 °C, 210 to 410 °C, and 160 to 270 °C, respectively, all of which exhibit salinities lower than 15 wt% NaCl equivalent. The δ 18 O H2O values of quartz in ore stages 1 and 2 range from + 3.4‰ to + 6.3‰ and + 2.3‰ to + 4.5‰, whereas the δD SMOW from − 93‰ to − 86‰ and − 97‰ to − 83‰, respectively. The data suggest that ore fluids are derived from magma gradually mixing with more meteoric water late. The measured 3 He/ 4 He ( 40 Ar/ 36 Ar) of hydrothermal fluids in pyrites are 1.53 to 3.21 R a (732 to 1783) for stage 1, and 0.38 to 2.58 R a (360 to 1544) for stage 2. The linear correlation between He and Ar isotopes is the result from a mixture between mantle components and crustal fluid. The estimated 3 He/heat ratios range from 0.13 to 2.55 × 10 −12 cm 3 STP J −1 , reflecting the hydrothermal fluids are derived by convection rather than conduction. The in-situ δ 34 S values of three types of pyrite (Py1, Py2-1, and Py 2–2) from two stages have a narrow and similar range from + 7.1 to + 9.7‰, indicating common deep sulfur sources. Lead isotopic compositions of pyrites ( 206 Pb/ 204 Pb = 17.102 to 18.248; 207 Pb/ 204 Pb = 15.435 to 15.590; 208 Pb/ 204 Pb = 37.659 to 38.691) show a consistent origin as mafic dikes. Thus, we propose deep-seated, most likely mafic dike magmatic and mantle-derived sources for gold ore formation. When compared to other Jiaodong gold deposits, they exhibit similar geochemical signatures and thus consistent geodynamic setting. They are considered to associate with the subduction of the Paleo-Pacific Plate and the destruction of the North China Craton during the Early Cretaceous.

Diagenesis and fluid evolution in the third member of the Eocene Shahejie Formation, Bonan Sag, Bohai Bay Basin, China

Diagenesis significantly influences sandstone reservoir quality, so that a detailed understanding of diagenesis is essential for the identification and prediction of favorable reservoirs. Several studies have been conducted on individual sandstone reservoirs in the Bonan Sag, a typical half-graben, oil-producing sag in eastern China, but few have explicitly considered the potential impact of the asymmetric shape of the sag, which is steep in the north and gentle in the south. This research takes full consideration of the tectonic setting and distribution of source rocks in the sag to compare the style, magnitude and timing of diagenesis in the northern Steep Slope and Sag Zone (SSSZ) and the southern Gentle Slope Zone (GSZ) by means of drill core observations, thin section petrography, high resolution scanning electron microscopy, fluid inclusion microthermometry, as well as cathodoluminescence and carbonate stable isotope analysis. Petrographic observations identify the wide-spread occurrence of authigenic quartz and clay minerals in association with dissolved feldspar, as well as abundant carbonate cements occurring as several distinct generations due to evolving pore fluid composition. The SSSZ has variable carbonate mineralogy while ankerite dominates in the GSZ. The homogenization temperature of fluid inclusions in authigenic quartz in the SSSZ has a wider range (93°C–157.9 °C) and lower starting temperature (93 °C) compared with the GSZ (106.2 °C) and is characterized by more lighter carbonate δ13C and δ18O values (δ18O: −14.8‰~-11.3‰, δ13C: 1.1‰–2.7‰ for the SSSZ; δ18O: −11.9‰~-5.8‰, δ13C: 2.9‰–5.5‰ for the GSZ). The results indicate that (1) Diagenetic conditions in the Bonan Sag evolved from alkaline to acidic to alkaline in general, but with distinct late stage carbonate cementation in SSSZ vs GSZ; (2) the diagenetic processes as well as the nature of pore waters varied across different zones and also temporally. Acidic conditions in the SSSZ started earlier and persisted for longer than in the GSZ. (3) The regional differentiation of the diagenetic processes and pore fluid evolution result from the source rock distribution and burial histories which differ across the Bonan Sag because of the asymmetric shape of the half-graben. We propose a model of fluid evolution over space and time which may provide a new framework for understanding the diagenetic processes and the fluid evolution of oil-producing sags and also contribute to the prediction of favorable reservoirs.

Generation and structural modification of the giant Kengdenongshe VMS-type Au-Ag-Pb-Zn polymetallic deposit in the East Kunlun Orogen, East Tethys: Constraints from geology, fluid inclusions, noble gas and stable isotopes

The Kengdenongshe giant Au-Ag-Pb-Zn polymetallic deposit is located in the East Kunlun Orogen (EKO). It contains about 42.2 t of Au, 608.6 t of Ag and 1.05 Mt of Pb and Zn with an average grade of Au 2.31 g/t, Ag 19.29 g/t and Pb + Zn 3.49 wt% (Pb: 1.23 wt%, Zn: 2.26 wt%). The NWW-trending ore bodies are predominantly hosted in Late Permian to Triassic rhyolitic tuff, which formed during Late Permian back-arc extension to Triassic arc-continental collision. The ore bodies are subdivided into Pb-Zn rich ore bodies on the top with high grades of Pb and Zn and low grades of Au and Ag, and Au-Ag rich ore bodies below with high grades of Au and Ag and low grades of Pb and Zn. The Pb-Zn rich ore bodies occur as vein, stockwork, and in breccia, and comprise quartz, pyrite, galena, sphalerite, and small amounts of chalcopyrite. The Au-Ag rich ore bodies consist of auriferous barite-sulfide-oxide veins and contain barite, pyrite (early strawberry and oolitic pyrite and later eu- to subhedral pyrite), galena, sphalerite, chalcopyrite, tetrahedrite and covellite. Gold is present as electrum, kustelite and native gold and silver is present as polybasite, pearceite, kongsbergite, and as minor native silver in microfractures in sulfides. The hydrothermal alteration minerals include, from bottom to top, quartz + barite + calcite around the Au-Ag rich orebodies, quartz + chlorite + epidote around the Pb-Zn rich orebodies, and quartz + K-feldspar within the tuff. Fluid inclusions from both the Pb-Zn rich and the Au-Ag rich orebodies consist of two phases (V–L-type) fluid inclusions of which the vapor phase has a size of 10–40 vol%. Fluid inclusions microthermometry reveal homogenization temperatures of fluid inclusions in Pb-Zn rich and Au-Ag rich ore bodies of 128–230 °C and 110–320 °C, with corresponding salinities of 0.7–9.9 and 0.2–18.3 wt% NaCl equivalent, respectively. H-O-S-Pb stable isotope and He-Ar noble gas isotope data indicate a mixed magmatic water-seawater source for both the Pb-Zn and Au-Ag rich ore bodies, and an additional meteoric water component for the Au-Ag rich ore bodies. The Pb-Zn and Au-Ag rich ore bodies share the same sulfur and lead sources, i.e. sulfur is derived from crustal magma and seawater/marine sulfate, and the lead originated from a mixed magmatic-ancient crustal sedimentary source. Collectively, the regional geology, mineralogy, alteration, and geochemistry indicate that the Kengdenongshe Au-Ag-Pb-Zn polymetallic deposit can be characterized as a VMS-type (volcanic-associated massive sulfide) deposit. Formation of the ore-hosting rhyolite tuff and mineralization are associated with Late Permian to Triassic marine volcanic exhalation. Middle to Late Triassic basin closure and arc-continent collision modified the deposit and resulted in the location inversion of the Pb-Zn and Au-Ag rich orebodies.

Geological, Fluid Inclusion, H-O-S-Pb Isotope Constraints on the Genesis of the Erdaogou Gold Deposit, Liaoning Province

The Erdaogou gold deposit is located in the conjuncture of North China Carton and Xingmeng orogenic belt. The ore-forming process of Erdaogou gold deposit is divided into three stages (I to III), which are quartz-pyrite stage (stage I), quartz-polymetallic sulfide stage (stage II) and quartz-calcite stage (stage III). Two types of fluid inclusions is distinguished in the ore-forming stage, i.e., aqueous type (W-type) and aqueous-carbonic type (C-type) inclusions. From the stage I to III, the homogenization temperature of fluid inclusions are respectively 334–395, 214–364 and 172–272 °C, with salinities of 7.72 wt.%–11.23 wt.% NaCl equiv., 0.20 wt.%–23.18 wt.% NaCl equiv., and 0.35 wt.% to 5.25 wt.% NaCl equiv. The ore-forming fluids of the Erdaogou deposit have the characteristics of medium-low temperatures, moderate salinities and low densities. And the fluids belong to the CO2-H2O-NaCl system. The values of δ34SV-CDT in sulphide samples at different stages (stages I to III) are between −2.2‰ to 2.3‰, indicating sulfur source from magma volatiles or subvolcanic rock leaching. The lead isotopes suggest that ore-forming metals may be derived from a mixture of lower crust and mantle materials. Oxygen and hydrogen isotope data at Erdaogou indicate that magmatic fluid and meteoric water may both be involved in the hydrothermal system. Based on the geological characteristics, fluid inclusion results, stable and radiogenic isotope results of Erdaogou gold deposit, we believe that the temperature decrease, fluid boiling are the key factors leading to the ore precipitation and the genetic type of Erdaogou gold deposit is low-sulfidation epithermal deposit.

Mineralogy and fluid characteristics of the Waschleithe Zn skarn – a distal part of the Schwarzenberg mineral system, Erzgebirge, Germany

The Waschleithe skarn is situated in the northern sector of the Schwarzenberg district in the western Erzgebirge (Germany), a district which hosts several large polymetallic skarn deposits. The Waschleithe is a Zn-dominant skarn comprising abundant dark Mn-rich hedenbergite, minor grandite garnet and magnetite related to the prograde stage as well as epidote, quartz, fluorite, amphibole, chlorite, ilvaite and Mn-rich pyroxenoids related to the retrograde stage of skarn formation. Ore minerals are related to the early retrograde stage and include sphalerite, galena, chalcopyrite, pyrite as well as minor amounts of scheelite and cassiterite. Fluid inclusion homogenization temperatures range from ~385 to 360 °C and from ~320 to 285 °C for the prograde and retrograde skarn stage, respectively. Thus, late-stage cooling is proposed as the major controlling factor for sulfide mineral precipitation. Fluid salinities for both stages are very low (<3% eq. w[NaCl]) and all observed fluid inclusion assemblages show homogeneous liquid–vapor ratios. Formation pressures >25 MPa suggest a minimum depth of formation of approximately 3 km. Oxygen fugacity during prograde skarn formation was initially low and increased towards the late prograde stage. The late-Variscan Eibenstock granite is identified as the most likely source of the skarn-forming fluids. The recognition of the Waschleithe skarn as a distal Zn skarn has major implications for the understanding of the Schwarzenberg district mineral system and provides valuable information for exploration targeting.

Genesis of the Cuonadong tin polymetallic deposit in the Tethyan Himalaya: Evidence from geology, geochronology, fluid inclusions and multiple isotopes

The Cuonadong deposit is located in southern Tibet and represents the first large-scale tin polymetallic deposit in the Himalayan region. Sn-(W) skarn mineralization is spatially related to leucogranites, whereas Be-Nb-Ta mineralization mainly develops in pegmatites. Alteration and veining in this deposit can be divided in to five stages: prograde skarn (stage I), retrograde skarn (stage II), cassiterite-quartz vein (stage III), cassiterite-sulfide vein (stage IV) and fluorite quartz vein (stage V). 40Ar39Ar dating of muscovite from the skarn and phlogopite from the cassiterite-sulfide vein yield isochron ages of 15.4 ± 0.3 Ma and 15.0 ± 0.3 Ma, respectively, whereas UPb dating of cassiterite from the skarn yields a Tera-Wasserburg low-intercept precise age of 14.2 ± 0.2 Ma. Zircon and monazite UPb ages of the causative stanniferous leucogranites are 15.3 ± 0.1 Ma and 14.9 ± 0.2 Ma, respectively. Mineralization ages are consistent with the emplaced ages of the granites within the analytical uncertainties, which indicates that the tin polymetallic mineralization is genetically related to the Miocene leucogranites. Sn-bearing leucogranites have zircon εHf(t) values that vary from −13.3 to −8.5 (-10.7 on average), with an average TDM2 value of 1.56 Ga, which reveals that the leucogranites are derived from partial melting of ancient metasedimentary rocks. According to the zircon trace elements, the Miocene leucogranites are highly differentiated reduced S-type granites that formed at relatively high temperature. The average homogenization temperatures of fluid inclusions in different minerals that formed at stages II, III, IV and V are 351, 315, 240 and 175 °C, respectively, whereas their salinities are 8.3, 4.9, 9.5 and 3.8 wt% NaCl equiv., respectively. The C-H-O-S-Pb isotopes indicate that the ore-forming fluid and materials mainly originated from Miocene leucogranite. Dehydration and partial melting of mica in the Greater Himalayan crystalline complex due to east-west extension at 18–14 Ma developed the stanniferous leucogranite and ore-controlling fault system. Because of the pervasive occurrence of gneiss domes and Miocene Sn-bearing leucogranites similar to Cuonadong, the Himalaya has strong potential to be a new globally important Sn-(W) rare metal metallogenic belt.