Homogenization Temperatures Of Inclusions Research Articles

Late Miocene magmatic‐hydrothermal system and related Cu mineralization of the Arakawa area, Akita, Japan

AbstractThe Northeast Japan arc hosts a number of hydrothermal vein‐type copper deposits associated with Neogene felsic intrusions. The Arakawa area is underlain by Cretaceous granites and Tertiary sedimentary rocks, which were intruded by the Miocene Ushizawamata dacite. Zircon grains from the dacite intrusion yield a 206Pb/238U intercept age (laser ablation inductively coupled plasma mass spectrometry) of 8.10 ± 0.30 Ma, consistent with a previously reported K‐Ar illite age (8.1 ± 0.4 Ma) of the Ushizawamata lead and zinc prospect in the Arakawa area. The dacite intrusion and the surrounding Miocene sedimentary rocks were altered by hydrothermal activity on the surface, classified into four alteration zones: (1) biotite‐chlorite, (2) illite, (3) chlorite and (4) smectite, centered on the intrusion. About 20 major vertical sub‐parallel copper‐bearing quartz veins occur in the chlorite alteration zone on the west side of the dacite. The first vein stage is composed of chalcopyrite and chamosite with a minor amount of quartz in brecciated wall rocks, and the second‐stage is characterized by the presence of hematite in addition to the first‐stage mineral assemblage. The third‐stage consists of comb‐shaped quartz veins with a minor amount of chalcopyrite, sphalerite, galena and pyrite, and the fourth‐stage of barite and apatite present in druse in the third‐stage veins. Primary fluid inclusions in quartz of the first‐ and third‐stages are all liquid‐rich and two‐phase. Homogenization temperature and salinity of first‐stage quartz are 263–277°C and 5.7–7.5 wt% NaCl equivalent (eq.); in quartz of the third‐stage, 251–270°C and 2.7–4.2 wt% in the inner zone and 207–250°C and 2.7–3.7 wt% in comb‐shaped quartz on the vein margin. The fluid inclusions in quartz phenocrysts of the Ushizawamata dacite show two distinct assemblages, halite‐bearing polyphase inclusions that coexist with vapor‐phase inclusions and/or vapor‐rich two‐phase inclusions, and liquid‐rich two‐phase inclusions. Homogenization temperature and salinity of the polyphase inclusions are higher than 401°C and 46.7 wt%, respectively, and those of the vapor‐rich two‐phase inclusions report 393–419°C and 2.6–3.7 wt% NaCl, whereas the liquid‐rich two‐phase inclusions returned 344–403°C and 8.0–9.3 wt%, respectively. These results indicate that the ore forming fluid was slightly cooler and lower in salinity than the late‐stage hydrothermal fluid in the Ushizawamata intrusion. The spatial and temporal proximity between the Ushizawamata dacite and the hydrothermal veins indicates that the dacitic magma was genetically related to the vein copper mineralization in the Arakawa area.

Developmental characteristics and formational stages of natural fractures in the Wufeng - Longmaxi Formation in the Sangzhi Block, Hunan Province, China: Insights from fracture cements and fluid inclusions studies

The developmental characteristics and formational stages of natural fractures are of great significance for shale oil and gas accumulation in complex structural areas. We determined the developmental characteristics and formational stages of fractures in the Wufeng - Longmaxi Formation in the Sangzhi Block, Hunan Province, China based on outcrops, cores, optical microscopy, cathodoluminescence, scanning electron microscopy, energy spectrum analysis, fluid inclusion homogenization temperature, micro laser Raman spectrum analysis, and acoustic emission analysis. Furthermore, we deduced the possible fracture formational depth, filling time, and stress magnitude of structural fractures in rocks, and established the development model of structural fractures of time series. The results show that the Wufeng - Longmaxi Formation fractures can be divided into tensile fractures, compression-torsion fractures, and strike-slip fractures in terms of the combination characteristics of fracture plane and the roughness of fracture plane. The fractures mainly occurred in the late Yanshanian - Himalayanian period after the violent uplift. In the process of burial-hydrocarbon generation, it experienced at least six stages of repeated process of splitting, healing, re-splitting, which reflected the periodic release of local stress caused by the continuous westward push of Xuefeng uplift. The microcracks of different periods have obvious differences in orthogonal polarization and cathodoluminescence reflection characteristics. The cathodoluminescence spectrum ranges from orange red to bright yellow, and some even do not glow, which is a comprehensive performance of mineral filling composition, crystal size, crystallization degree and alteration degree. The gas generated in the early stage are easy to enter the early opened fractures. With the continuous opening and healing, the shale gas has a continuous dynamic process of enrichment, dispersion, re-enrichment, and re-dispersion, which affects the distribution and preservation of shale gas.

Hydrothermal ore mineralization from the Polish part of the Tatra Mts., Central Western Carpathians

Several areas with hydrothermal ore mineralization are present in the Polish part of the Western Tatra Mts. Massive and disseminated sulfides, mainly minerals from the tetrahedrite group and chalcopyrite, fill quartz veins accompanied by siderite, dolomite and baryte – many of these were mined between the 16th and 18th century. This paper presents information on the mineralogy of the common sulfides and the preliminary studies of both the isotopic composition of sulfur in tetrahedrite as well as the origin of hydrothermal fluids. The most common primary sulfide minerals in the ores are tetrahedrite-(Zn) and tetrahedrite-(Fe) corresponding to Zn ranges from 1.83 to 5.87 wt.% (0.47–1.44 apfu), and Fe from 1.63 to 5.05 wt.% (0.48–1.52 apfu). The substitution of As for Sb shows maximum As content of 7.25 wt.% (1.588 apfu) which corresponds to the Sb/(Sb+As) = 0.60. Both varieties show substitutions of Bi and Hg, up 0.6 wt.% (0.049 apfu) and 0.96 wt.% (0.081 apfu), respectively, and content of trace elements: Co (max. 976 ppm), Cd (max. 735 ppm), In (max. 14 ppm). Chalcopyrite, pyrite, and galena show compositions close to the ideal formula. Differences in the content of trace elements in the studied tetrahedrite and chalcopyrite were explained by element partitioning between these minerals. The distributions of In, Ga and Sn in the studied minerals indicate that the mineralization was formed at low temperature and pressure. The homogenization temperature of the primary inclusions in quartz in the range of 120–174°C indicate the origin of mineralization in low-temperature stage from low salinity fluids (up to 17.92 wt.% NaCl eq.). Stable δ34S isotopes in minerals of the tetrahedrite group suggest that the igneous fluids might be one of the sources from which ores were crystallized.

PT formation conditions of polymetallic ores from the Pb-Zn-Fe Aktash skarn deposit (Western Karamazar, Tajikistan)

Fluid inclusions are studied in calcite from magnetite ores and sulfde-carbonate veins of the Aktash sulfde-magnetite deposit (Western Karanazar, Tajikistan) to identify their formation conditions. The deposit is confned to a contact zone between carbonate (Upper Devonian–Lower Carboniferous dolomite and limestones) and intrusive rocks (Middle Carboniferous–Early Triassic granodiorites and porphyry granodiorites) of the Kansai ore feld. The fuid inclusion study showed that calcite of ore veins formed from moderately saline (4.4–10.8 wt. % NaCl-equiv.) aqueous Na-K ± Mg chloride fuids at a decreasing temperature from 300 to 160 °C. The homogenization temperatures of fuid inclusions are consistent with thermometric data for chlorite, which formed together with calcite (176–295 °C). Keywords: calcite, chlorite, formation conditions, fuid inclusions, polymetallic ores, magnetite ores, Aktash deposit, Western Karamazar.

Reservoir characterization of the Paka geothermal system in Kenya: Insights from borehole PK-01

Trachytic and basaltic volcanic material erupted from ∼390 ka to Holocene coalesced to form the 136 km2 active volcanic edifice of Paka on top of an older ∼580 ka plateau eruptive sequence. The latter constrains the maximum age for the Paka geothermal system manifested mainly as surface fumaroles and hot altered grounds. To date, three exploration wells have been drilled to >2500 m depth. Results from analyses of well PK-01 show that the upper part of the well (0–1050 m) is exclusively a trachyte lava sequence while the lower part (1050–2552 m) penetrates sequences of trachyte, intermediate, basaltic and syenite intrusive rocks. Temperature attained in the well based on hydrothermal alteration assemblage vary from ∼180 °C at 400 m depth where quartz was first observed in the drill cuttings to >220 °C within the chlorite-illite zone at 700 m and 280 °C below 1450 m in the actinolite-epidote zone. Fluid inclusion homogenization temperatures (Th) range widely between 211 and 286 °C at 1390 m and 209–271 °C at 2550 m. Fluid inclusion indicate that the reservoir fluid from the paleo system was bicarbonate-rich and of very low salinity according to ice-melting temperature, which is comparable to aqueous fluid discharge from the well. Stabilised well temperature increases with depth to a maximum of 245 °C at 1800 m depth where a reversal occurs and becomes nearly isothermal at 230 °C, then increases slightly below 2200 m before an inversion to about 220 °C. The main high-temperature geothermal system is hosted within the lower part of the well (1050–2552 m). It is probably steam-dominated above 900 m depth where calculated formation temperatures intersect the boiling point curve for pure water, and liquid-dominated below 900 m. Suitability modelling from surface data integration indicates that the most suitable area (P90) for geothermal development is within an area of 12 km2 in the summit area and Northeast of the Paka caldera.

Fluid Histories of Middle Ordovician fault–fracture hydrothermal dolomite oil fields in the southern Michigan Basin, U.S.A.

ABSTRACTDolomitized fault–fracture structures in the Trenton and Black River formations (TBR) are the type example for “hydrothermal” petroleum reservoirs world-wide. However, fluid histories of these structures are only partially understood. Trenton and Black River reservoirs in the southern Michigan Basin are composed of fault-associated, vertical dolomite bodies that are highly fractured and brecciated. Open spaces are partially to completely filled by saddle dolomite and less frequently by calcite cement. Cathodoluminescence microstratigraphies of void-filling carbonate cements are not correlatable between oil fields. Fluid inclusion homogenization temperatures (Th) measured in carbonate cements indicate two fluid endmembers: a warm fluid (∼ 80° to 180° C) and a hot fluid (180° to ∼ 260° C). Increasing Th proximal to the underlying Proterozoic Mid-Michigan Rift (MMR) suggest that the hot fluids emanated from the rift area. Included fluids are saline (16.1–49.4 wt. % NaCl equivalent), and salinity likely is sourced from overlying Silurian Salina Group evaporites. First melting temperatures (Tfm), interpreted as eutectic temperatures (Te), of fluids range from –112° C to –50° C, indicating a complex Na–Ca–KCl brine; the expected composition of dissolved Salina salts. Lower Te proximal to the MMR suggest the rift as a source of additional complexing ions. C and O isotope values for carbonate cements are depleted with respect to δ18O (–6.59 to –12.46‰ VPDB) relative to Ordovician seawaters, and somewhat depleted with respect to δ13C (–1.22 to +1.18‰ VPDB). Equilibrium calculations from δ18O and Th values indicate that cement precipitating waters were highly evolved (+1.3 to +14.4‰ δ18O‰ VSMOW) compared to Ordovician and Silurian seawaters (–5.5‰ δ18O‰ VSMOW). Strontium isotope values indicate two fluid sources: Proterozoic basement and Late Silurian evaporites. Values of 87Sr/86Sr for cements in the Freedom, Napoleon, Reading, and Scipio fields (0.7086–0.7088) are influenced by warm water sourced from Silurian strata, and values for cements in the Albion, Branch County, and Northville fields (0.7091–0.7110) record continental basement signatures. Cement precipitating fluids in TBR oil fields likely have similar sources and timing. However, water–rock interactions along fault pathways modified source waters, giving each oil field a unique petrographic and geochemical signature. Fluid movement in TBR oil fields likely were initiated by reactivation of basement faulting during Silurian–Devonian tectonism.

Magmatic hydrothermal fluid genesis of zeolite in the Paleogene Kongdian Formation of Zaoyuan oilfield, Bohai Bay Basin, China

Electronic probe, fluid inclusion homogenization temperature, Raman spectroscopy and laser ablation inductively coupled plasma mass spectrometry were utilized to identify the hydrothermal fluid-rock interactions in the second member of the Paleogene Kongdian Formation of Zaoyuan oilfield in Bohai Bay Basin (Kong 2 Member for short) of Well Z56 to find out the relationship between zeolite and hydrothermal fluid. The experimental results show that: (1) Pyrobitumen coexists with hydrothermal fluid characteristic minerals such as chlorite, barite, chalcopyrite, pyrite, natrolite and analcime in mudstone fractures. (2) The temperatures calculated from laser Raman spectrum of pyrobitumen, from the chlorite geothermometer and from measured homogenization temperature of natrolite inclusions are 324–354 °C, 124–166 °C and 89–196 °C, respectively; although vary widely, all the temperatures are obviously higher than the normal geothermal temperature. (3) The positive Eu anomaly of chlorite and barite, and the similar distribution pattern in rare earth elements between natrolite and basalt indicate they are from magmatic hydrothermal fluid. Moreover, drilling data shows that the Kong 2 Member in Well Z56 has several sets of basalt interlayers, suggesting there was geologic base of magmatic hydrothermal fluid activity. The magmatic hydrothermal fluid-rock interaction may be one of the reasons for the abnormal enrichment of zeolite in Kong 2 Member of the Cangdong Sag.

Deformation Bands and Associated FIP Characteristics From High-Porous Triassic Reservoirs in the Tarim Basin, NW China: A Multiscale Analysis

Deformation bands are widely formed and distributed in Triassic high-porous rocks as a result of multistage tectonic movement. In this research, core observation, the rock thin section (fluorescence and casting thin section), FIB-SEM, X-ray diffraction, Raman laser, and thermometry of fluid inclusions were employed to describe the macro- and micro characteristics of deformation bands and their associated relationship with microfractures. Results indicate that the main types of deformation bands formed in the Lunnan Triassic high-porosity sandstone during the Yanshanian and Himalayan periods under different temperature and pressure conditions are compaction shear bands, and their quantity increases evidently with the distance of thrust faults. The density of deformation bands near the fault is about 15/m; porosity and permeability decrease sharply compared with those of the host rock. Microscopically, two obtained fluid-inclusion planes (FIPs) can be distinguished as 51 samples collected from 12 wells by the cutting relationship and mechanical characteristics. The homogenization temperature of associated aqueous inclusion is generally characterized by two peaks, mainly 70–80°C and 110–120°C, which were formed in the Late Yanshanian and Late Himalayan periods. The formation period of deformation bands induced by the intragranular microfractures improved the reservoir seepage capacity. In the later stage, as the interlayer and barrier with low porosity and low permeability affects the distribution of oil and gas, which is an important factor in this study of the local fluid dynamic field and high-quality reservoir evolution distribution.

The Accumulation Characteristics of the Paleozoic Reservoir in the Central-Southern Ordos Basin Recorded by Organic Inclusions

The Permian tight clastic reservoir and Ordovician carbonate reservoir were developed in the central-southern Ordos Basin. This study investigated the fluid inclusion petrography, diagenetic fluid characteristics, formation process of natural gas reservoir, source rock characteristics, and reservoir accumulation characteristics of these Paleozoic strata by petrographic observations, scanning electron microscope imaging, fluid inclusion homogenization temperature, salinity, laser Raman spectrum, and gas chromatograph analyses. The results have suggested two phases of fluid inclusions in both the Permian sandstone and the Ordovician Majiagou Formation dolomite reservoirs, and the fluid inclusions recorded the history from the early thermal evolution of hydrocarbon generation to the formation, migration, and accumulation of natural gas. The early-phase inclusions show weak yellow fluorescence and recorded the early formation of liquid hydrocarbons, while the late-phase inclusions are nonfluorescent natural gas inclusions distributed in the late tectonic fractures and recorded the late accumulation of natural gas. The brine systems of the Permian and Ordovician fluid inclusions are, respectively, dominated by CaCl2-H2O and MgCl2-NaCl-H2O. The diagenetic fluids were in the ranges of medium-low temperature and moderate-low salinity. The natural gas hydrocarbon source rocks in the Ordos Basin include both the Permian coal-bearing rocks and the Ordovician carbonates. The process of the early-phase liquid hydrocarbon formation and migration into the reservoir corresponded to 220 Ma (Late Triassic). The late large-scale migration and accumulation of natural gas occurred at 100 Ma (early Late Cretaceous), which was close to the inclusion Rb/Sr isochron age of 89.18 Ma, indicating that the natural gas accumulation was related to the Yanshanian tectonic movement.

Fluid origin and evolution of the Ruanjiawan W-Cu-(Mo) deposit from the Edong District in the Middle-Lower Yangtze River metallogenic belt of China: Constraints from fluid inclusions and H-O-C-S isotopes

Ruanjiawan W-Cu-(Mo) deposit, located in the Edong District of the Middle-Lower Yangtze River metallogenic belt, is one of the largest W deposits (40348 tons WO3 at an average grade of 0.32 wt%) in this ore district. The orebodies mainly occur in the contact zone between the granitoid and Ordovician carbonate strata. A detailed field geology and petrographic observation indicates four stages of ore formation: (1) prograde skarn stage, (2) retrograde skarn stage, (3) quartz-sulfide stage, and (4) quartz-carbonate stage. The δ18O values of the ore fluids vary from 3.0‰ to 11.2‰, and the δD values vary from −88‰ to −105‰, suggesting a dominant magmatic water origin, with a small amount of meteoric water mixing during the late stages. The δ13C values of fluid inclusions vary from −15.0‰ to −7.1‰, also indicating a major magmatic carbon source but with some mixing of organic carbon component in the sedimentary strata that probably leached by circulated meteoric water. The sulfides show a large δ34S variation from −4.8‰ to +16.3‰, and the sulfur may have two different sources, with a majority magmatic source (with δ34S around 0), and some marine sulphate source (with high positive δ34S value) in sedimentary strata in the region leached by meteoric water. In the prograde skarn stage, garnet shows a fluid inclusion homogenization temperature above 517 °C. The estimated salinity of the vapour-liquid two-phase inclusions ranges from 15.7 to 21.4 wt% NaCl equiv., and the salinity of the daughter mineral-bearing inclusions ranges from 39.8 to 57.1 wt% NaCl equiv. In the retrograde skarn stage, the fluid inclusions in epidote show a homogenization temperature of 385 °C to 463 °C, and the salinity estimated from the vapour-liquid two-phase inclusions ranges from 9.7 to 16.4 wt% NaCl equiv. The fluid inclusions in coarse-grained garnet from the retrograde skarn stage show similar homogenization temperature of 339 °C to 398 °C, and similar salinity of vapour-liquid two-phase inclusions of 7.4 to 15.3 wt% NaCl equiv., however, the salinity estimated from the daughter mineral-bearing inclusions ranges from 35.2 to 42.0 wt% NaCl equiv. In the quartz-sulfide stage, fluid inclusions in quartz show a homogenization temperature of 274 °C to 344 °C. The salinity varies from 6.5 to 14.0 wt% NaCl equiv. for the vapour-liquid two-phase inclusions, from 5.1 to 8.0 wt% NaCl equiv. for the CO2-bearing three phase inclusions, and from 35.3 to 39.8 wt% NaCl equiv. for the daughter-mineral-bearing inclusions. In the quartz-carbonate stage, the fluid inclusions have homogenization temperatures from 137 °C to 261 °C, and the salinity for the vapour-liquid two-phase inclusions shows a range from 3.9 to 1.0 wt% NaCl equiv. Fluid boiling and water-rock reaction in the retrograde skarn stage lead to the first precipitation of scheelite. The precipitation of scheelite, chalcopyrite and molybdenite in the quartz-sulfide stage was mostly triggered by the mixing of meteoric water and magmatic fluids, and fluid immiscibility also plays an important role. According to petrographic evidence and all available geological and geochemical data, we conclude that the Ruanjiawan deposit is an oxidized skarn-type W-Cu-(Mo) deposit.

Tungsten Ores of the Dzhida W-Mo Ore Field (Southwestern Transbaikalia, Russia): Mineral Composition and Physical-Chemical Conditions of Formation

This article discusses the peculiarities of mineral composition and a fluid inclusions (FIs further in the text) study of the Kholtoson W and Inkur W deposits located within the Dzhida W-Mo ore field (Southwestern Transbaikalia, Russia). The Mo mineralization spatially coincides with the apical part of the Pervomaisky stock (Pervomaisky deposit), and the W mineralization forms numerous quartz veins in the western part of the ore field (Kholtoson vein deposit) and the stockwork in the central part (Inkur stockwork deposit). The ore mineral composition is similar at both deposits. Quartz is the main gangue mineral; there are also present muscovite, K-feldspar, and carbonates. The main ore mineral of both deposits is hubnerite. In addition to hubnerite, at both deposits, more than 20 mineral species were identified; they include sulfides (pyrite, chalcopyrite, galena, sphalerite, bornite, etc.), sulfosalts (tetrahedrite, aikinite, stannite, etc.), oxides (scheelite, cassiterite), and tellurides (hessite). The results of mineralogical and fluid inclusions studies allowed us to conclude that the Inkur W and the Kholtoson W deposits were formed by the same hydrothermal fluids, related to the same ore-forming system. For both deposits, the fluid inclusion homogenization temperatures varied within the range ~195–344 °C. The presence of cogenetic liquid- and vapor-dominated inclusions in the quartz from the ores of the Kholtoson deposit allowed us to estimate the true temperature range of mineral formation as 413–350 °C. Ore deposition occurred under similar physical-chemical conditions, differing only in pressures of mineral formation. The main factors of hubnerite deposition from hydrothermal fluids were decreases in temperature.

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).

Eocene magmatism and associated Fe-Cu mineralization in northeastern Turkey: a case study of the Karadağ skarn

ABSTRACT The metallogenic belt containing numerous skarn mineralized systems starting from Eastern Europe continues through Turkey and Iran. The study area in the Eastern Pontides is within this metallogenic belt. The 44.39 ± 0.53 Ma Karadağ Intrusion sourced skarn mineralization is located along the granitoid-to-limestone contact. The Karadağ Intrusion is an arc-like pluton with medium–high K calc-alkaline and peraluminous–metaluminous affinity. The Karadağ Fe–Cu skarn mineralization (Gümüşhane, Turkey), an exoskarn, is hosted by Late Cretaceous limestone and consists of early garnet, clinopyroxene, and orthoclase with late epidote and chlorite. Primary ore minerals are magnetite and haematite, accompanied by minor amounts of chalcopyrite and pyrite. Pyroxene comprises mostly diopside and less hedenbergitic diopside and johannsenite. Wollastonite is very rare. Garnet is andradite and grossular–andradite. Thus, all these results imply that the Karadağ Fe–Cu skarn mineralization formed under relatively oxidized and Fe-rich conditions. The homogenization temperatures of the fluid inclusions vary from 189°C to 405°C for quartz and 186°C to 430°C for calcite. The salinity contents in the fluid inclusions are within 0.2–9 wt% NaCl equivalent for quartz and 0.4–14.8 wt% NaCl equivalent for calcite. First melting temperatures between −36.5 and −47.4°C were attained with the presence of NaCl–FeCl2± CaCl2 in the initial phase of fluid inclusions. The first melting in the later stage of fluid inclusions is between −3.8 and −17.2°C, thereby, supporting the existence of NaHCO3-Na2CO3-KCl-type salt composition. The δ18O values of minerals are 1.7–10.3‰ for garnet, 11.1–14.6‰ for quartz, 6.4 to 6.8‰ for epidote, and 2.8–3.8‰ for magnetite in the Karadağ skarn mineralization. The δ18O value of epidote indicates magmatic fluid, whereas those of garnet, magnetite, and quartz suggest magmatic fluids together with less meteoric effect. The δ18O values in limestone (18.7 to 22.3‰), marble (8.7‰), and calcite (10.5 to 12.2‰) were depleted relative to typical δ18O contents of marine carbonates (20 to 30‰). Moreover, the C isotope values of calcite (−5.4 to −1.4‰), marble (−2.1‰), and limestone (1.0 to 1.2‰) reflect those of metamorphic rocks and skarns (−4.0 to 3‰ and −18 to 2‰). Mineral paragenesis, fluid inclusion, and stable isotopic signatures are consistent with ore mineralization caused by the interaction of magmatic fluids with carbonate host rocks. Karadağ Fe-Cu skarn mineralization, associated with Karadağ postcollisional magmatism, is an example of skarn mineralized system in the Eastern Pontides.

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.

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.

Origin and accumulation of natural gas in the Ningbo Tectonic Zone in the Xihu Sag

In this study, the geochemical characteristics of the oil and gas and the related source rocks in the Ningbo Tectonic Zone in the Xihu Sag are analysed to determine the source of the natural gas. The natural gas in the Ningbo Tectonic Zone is dry gas. It has a high content of methylcyclohexane and low contents of n-heptane and dimethylcyclopentane in the C7 light hydrocarbons and a heavy ethane-carbon isotope composition, indicating that the natural gas in the study area was derived from humic parent material, making it a coal-type gas. The ratios of the light hydrocarbon parameters of the NB-A and NB-B natural gas are significantly different from those of crude oil and the Pinghu Formation source rocks, indicating that there is no pronounced relationship between the natural gas and the Pinghu Formation source rocks, and the natural gas may be from the source rocks in the deeper horizons of the Pinghu Formation. The homogenization temperatures of the fluid inclusions in the upper member of the Huagang Formation (NB-A1) are mainly 140–150°C, which correspond to 3.5–0 Ma (Late Cenozoic) according to the paleogeothermal history, indicating that in this area, the natural gas in the upper member of the Huagang Formation has charged since the sedimentary period of the Santan Formation.

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.

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.

Determination of Formation Time of Calcareous Cements in Marine Sandstone and Their Influence on Hydrocarbon Accumulation: A Case Study of the Carboniferous Donghe Sandstone in the Hadexun Oilfield, Tarim Basin

Based on core observations, the microheterogeneity, diagenetic features, diagenetic mineral compositions, and stable isotopes of cements in the calcareous interlayers in the Donghe sandstone were studied by polarizing microscopy, cathodoluminescence microscopy, X-ray diffractometry, isotope ratio mass spectrometry, and other techniques. By determining the proportions of cements of two phases by a statistical method and their clumped isotope values by an end-member method, the multiphase calcareous cementation was shown to be the major contributor to densification. Cluster isotopes revealed that the average formation temperatures of calcareous cements in phases II and III of cementation were 45–50°C and 80–90°C, indicating that they were products of the A and B phases during early diagenesis, respectively. According to the homogenization temperatures of coeval salt-water inclusions associated with hydrocarbon inclusions, which range from 100°C to 130°C, basin modeling revealed that the basin underwent mainly one stage of hydrocarbon charging during 8–5 Ma in the Miocene period. The cements of the two phases in the oil-free calcareous interlayers in the Donghe sandstone, which are the main controlling factor of the oil-water distribution in the reservoir at present, formed much earlier than the oil filling in the oil-bearing sandstone.

Characteristics of paleogeothermal field and its influence on hydrocarbon accumulation in Qingyang area

Geothermal field has an important influence on hydrocarbon accumulation. The geotemperature field in Qingyang area is studied by vitrinite reflectance and homogenization temperature of inclusions. The results show that the average geothermal gradient in Qingyang area is about 2.8 °C/100M, which is close to the median value of global average heat flow. It belongs to the medium temperature type geothermal field. In Qingyang area, the geothermal gradient is low from Paleozoic to early Cenozoic, ranging from 2.2 °C to 3.0 °C/100M, rising to 3.3-4.3 °C/100m in the late Mesozoic, and gradually decreasing to 2.8 °C/100M since Cenozoic. Generally speaking, high temperature field is favorable to the formation of petroleum system, and vice versa.