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The Gucheng area, located in the east of the Tarim Basin, is one of the important gas exploration targets, and the Upper Cambrian dolostone is the main pay layer. The dolostone is considered to be altered by hydrothermal fluids, but the hydrothermal altering degrees are still unclear. In this study we report the petrography and in situ trace elemental data, and try to discuss the hydrothermal fluids and their impact on the Upper Cambrian dolostones. Based on the petrography and CL properties, five types of dolomites are identified. MD1 and MD2 are the matrix dolomites. The crystal size of MD1 range from approximately 30 to 100 μm, while the MD2 is larger than 200 μm. Cement dolomite (CD) occurring only in vugs and fractures is subdivided into three types depending on the CL properties. CD‐zone1 is the core of CD with dull CL colours. CD‐zone2 comprises a bright red CL colour zone and a dull CL colour zone, and CD‐zone3 is the rim of CD dolomite with bright red CL colours. MD2 and CD‐zone1 show light rare earth elements (LREE) depletion, middle rare earth elements (MREE) enrichment and negative Eu anomalies. The REEY patterns of CD‐zone3 dolomite are marked by LREE enrichment and positive Eu anomalies. CD‐zone2 is the transitional mineral phase that shows slightly LREE depletion and negative Eu anomalies. The REEY patterns of CD‐zone1 suggest that CD‐zone1 is likely formed by local redistribution of older dolomites in a closed system. The REEY patterns of CD‐zone3 may indicate a magmatic associated hydrothermal fluid. Based on the petrography and in situ trace elemental data, the Permian magmatic fluid, which is recorded in CD‐zone2 and CD‐zone3 dolomites, may be only limited within fractures, while unlikely significantly altered the matrix dolomite in the Upper Cambrian. [ABSTRACT FROM AUTHOR]

The exploration of deeply buried hydrocarbon is still a challenge for the petroleum geology. The Shunbei area is a newly discovered oil fields, located in the center of the Tarim Basin. The oil is mainly yielded from the Middle–Lower Ordovician carbonate reservoirs with depth > 7000 m in the Shunbei No. 1 and No. 5 fault zones. Calcite cements filled in vugs (v-calcite) and fractures (f-calcite) are identified in limestones and dolostones of the carbonate reservoirs. F-calcites in the Shunbei No. 1 fault zone trap secondary inclusions in trails, which comprise liquid-dominated biphase aqueous inclusions, liquid-dominated biphase oil inclusions, and/or oil-bearing triphase inclusions. F-calcite and v-calcite in the No. 5 fault zone trap secondary inclusions in trails, which consist of liquid-only monophase aqueous inclusions, liquid-dominated biphase aqueous inclusions, liquid-dominated biphase oil inclusions, liquid-only monophase oil inclusions, and/or oil-bearing triphase inclusions. The ranges of the homogenization temperature ( T h ) and ice-melting temperature ( T m − ice ) in the Shunbei No. 1 fault zone are, respectively, 130–150°C and -2.1–-1.5°C. The coexistence of liquid-only and liquid-dominated aqueous inclusions in the Shunbei No. 5 fault zone indicates that the aqueous inclusions are trapped at low temperatures. The aqueous inclusions in the Shunbei No. 5 fault zone show a range from -0.4 to -0.2°C in T m − ice which is very close to the meteoric fluid. In the context of the burial-thermal history and the Cambrian source rock evolution, the charging process of hydrocarbon in the Shunbei No. 1 and No. 5 fault zones corresponds to the Silurian and Middle Ordovician, respectively. Results of fluid inclusions indicate a tightly coupling relationship between the hydrocarbon charging process and fault system evolution in the Shunbei area. This study reveals the application of fluid inclusion under the systemically petrographic constraints to decipher the charging history of hydrocarbon, especially for the deeply buried reservoirs. [ABSTRACT FROM AUTHOR]

The Lower Triassic Jialingjiang Formation, characterised by a carbonate–evaporite succession, is one of the important pay layers in the eastern Sichuan Basin. Several diagenetic events were identified in this study, including meteoric dissolution, dolomitisation, anhydrite/gypsum replacement/cementation, TSR, BSR, and multiple calcite cementation (C1–C3). Dolomitisation and anhydrite/gypsum replacement/cementation are the major diagenetic events affecting reservoir porosity. The dolostone is composed of D1, D2, and D3 dolomite. D1 dolomite is very finely crystalline, dominating in the low-energy, restricted depositional facies. D2 dolomite is finely crystalline, constituting a major portion of the dolostones. D3 dolomite is depositional fabric retentive dolomite, including dolo-wacke-/packstone, dolo-grainstone, and dolo-boundstone. Four types of anhydrites (A1–A4) were identified in the core and thin sections. A1 is stratabound anhydrite. A2 is the anhydrite in dolostones, filling in the interparticle spaces in dolo-grainstone and the intraparticle spaces in dolo-wackestone. A3 is the anhydrite in limestones, which occurs non-fabric-selectively. A4 is the anhydrite filling in the fractures. The diagenetic anhydrites (A2–A4) are interpreted to postdate to major dolomitisation. D1 and D2 dolomites are characterised by δ13C VPDB values of −3.4–+7.3 ‰ and δ18O VPDB values of −4.7−-2.4 ‰. The REE patterns of D1–D3 dolomites are characterised by light rare earth element (LREE) depletion relative to heavy rare earth elements (HREE) ((Pr/Tm) N of 0.54–1.16, 0.81 ± 0.20), middle rare earth elements (MREE) enrichment (BSI values of 1.06–1.38, 1.26 ± 0.10), and negative Ce anomalies (Ce/Ce* ratios of 0.74–0.94, 0.83 ± 0.07). The diagenetic anhydrites (A2–A4) show δ34S V-CDT values of +25.9–+45.7 ‰, similar to those of the stratabound anhydrite (+29.6–+43.0 ‰). The large variations in the δ13C VPDB values of D1 and D2 dolomites are attributed to methanogenesis and BSR. The δ18O VPDB values and REE characteristics of D1–D3 dolomites suggest that the carbonates are dolomitised by the mesohaline fluid. The δ34S V-CDT values of A1–A4 indicate that the diagenetic anhydrites are formed by redistribution in a closed system. The petrographical and geochemical data suggest that the Jialingjiang carbonate evolved in a closed system during burial diagenesis. Primary pores dominate in the Jialingjiang carbonate reservoirs, and the boundstone and grainstone show the best porosities based on the porosityversus lithofacies data. Therefore, we propose that the lithofacies, instead of the diagenetic events, are the key factors of the Jialingjiang carbonate. The resisting compaction of dolomitisation helps preserve the reservoir porosity, and anhydrite is the major diagenetic event destroying the reservoir porosity. • Lithofacies are the key factor of the reservoir qualities. • Petrography and geochemical work suggest the closed diagenetic systems. • Jialingjiang carbonate is dolomitized by mesohaline fluids predating diagenetic anhydrite. • Anhydrite selectively replace calcitic part in Jialingjiang carbonate. [ABSTRACT FROM AUTHOR]

Various degrees of dolomitization are widely present in the Yingshan Formation in the Gucheng area of the Tarim Basin. To determine the petrological characteristics and origin of the massive dolomites present, a rich combination of tools was well used, including previous research results, core and thin-section samples, cathodoluminescence, carbon (C-) and oxygen (O-) isotopes, strontium (Sr-) isotopes and content, and burial-thermal history data. The results of this analysis successfully reveal three primary matrix dolomites and three special cements, as follows: (1) microcrystalline to very fine crystalline nonplanar-a dolomite (MD1); (2) very fine to fine crystalline planar-s to planar-e dolomite (MD2); (3) medium to coarse crystalline nonplanar-a to planar-s dolomite (MD3); (4) coarse crystalline nonplanar-a saddle dolomite cement (SD) and (5) medium to coarse crystalline calcite cements (C1 and C2). Among them, MD1 precipitated from the reflux of mildly evaporated seawater, and MD2 was formed by the first significant recrystallization of MD1 in shallow burial environments. In particular, some MD3 was formed by the second significant recrystallization of MD1 in medium-burial environments. The others, even SD, were likely the products of the third significant recrystallization and cementation in deep-burial environments that precipitate under the action of a hydrothermal fluid mixture composed of mantle-derived fluid and Cambrian thermal brine. In addition, C1 and C2 would form close to the times in which the Himalayan and Late Caledonian–Early Hercynian hydrocarbon charging events occurred, respectively. • The origin of the dolomites of the Yingshan Formation in the study area was analysed systematically for the first time. • MD had undergone dolomitization and varying degrees of recrystallization by two different sources of fluids. • C1 and C2 formed during the Himalayan and Late Caledonian–Early Hercynian orogenic periods respectively. [ABSTRACT FROM AUTHOR]