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Middle Permian Qixia Formation in the southwestern region of Sichuan (SW China) has experienced multiphase fluidisation, resulting in an unclear understanding of the reservoir reconstruction effect. In this study, a systematic analysis of the Qi2 member in Wulong Town was carried out by combining field outcrop petrology and geochemistry. The results demonstrated that multiple sets of crystalline dolomite-bioclastic limestone cycles were stacked vertically in the Qi2 member, accompanied by the development of fractures and karst channels. The dolomite was mainly composed of silty-fine dolomite (D1) and recrystallised dolomite (D2). Furthermore, obvious multiphase dolomitic cements (Cd1-Cd2) were present in the fractures and pores. Early karst is known to have lithologic mutation surface development and karst channel development at the top of several secondary cycles. The vadose silt dolomites (Cd1) having karst channels developed dull luminescence under cathode luminescence (CL). Both the geochemical indicators of elements and rare earth element (REE) content indicated dysoxic-oxic environmental conditions. The hydrothermal solution displayed tectonic carniole characteristics in the strata burial stage. Fractures and pores were filled with hydrothermal minerals such as coarse dolomites-saddle dolomites (Cd2, with some caused by recrystallisation of the Cd1 hydrothermal solution) and fluorites. Coarse dolomites-saddle dolomites developed dull-red luminescence with a bright-red rim under CL and their δ18OVPDB values were more negative than those of middle Permian limestone samples. Both the geochemical indicators of elements and REE content indicated the suboxic-anoxic environmental conditions. Karstification had minor constructive impact on the reservoir of the Qi2 member in Baoxing in southwestern Sichuan. Most products of karstification were distributed as fillings in channels. Aside from creating certain networked fractures, the hydrothermal solution was mainly filled with hydrothermal minerals along the fractures, pores and early karst channels. Karst and the hydrothermal solution mainly damaged the middle and upper parts of the middle Permian Qixia Formation in Southwest Sichuan. The impact of episodic fluid on the restoration of the carbonate reservoir was mainly restricted by channels for fluid migration and thickness differences among the reservoir. However, certain thick-layered and massive crystalline dolomite may hold promise for exploration. [ABSTRACT FROM AUTHOR]

To elaborate genesis of the Ordovician Ma55 Sub-member dolomites in the Ordos Basin, this study implemented a series of analyses, including petrological investigation and geochemical (constant and trace elements, and isotopes) characterization. Three main types of dolomites were identified, namely micritic dolomite, micritic silty dolomite and silty dolomite. Specifically, the micritic dolomite is featured by dull red luminescence, low degree of order, high Sr isotope value, positive linear MgO–CaO correlation, high contents of Na and Sr elements, low contents of Fe and Mn elements, obvious negative δ13C excursion and slightly positive δ18O value, all of which indicate a syngenetic–penesyngenetic closed, high salinity environment. In comparison, the micritic silty dolomite is characterized by no or extremely dull red luminescence, moderate-to-high degree of order, higher δ13C value, slightly lower 87Sr/86Sr ratio, and sharp decrease in contents of K, Al, Fe and Mn elements, all of which indicate this type of dolomite to be the product of seepage–reflux dolomitization. Moreover, difference in geochemical characteristics between micritic silty dolomites in the depression and those in the platform flat suggests the Mg-rich brine from the tidal flat to be the dolomitization fluid for the platform flat while the Mg-rich brine from the overlying Ma54 Sub-member to be the dolomitization fluid for the depression. Meanwhile, the silty dolomite is observed to have no or extremely dull red luminescence, high degree of order, heavy δ18O value, high Sr isotope, lower Na content, and higher Fe and Mn contents, which are attributed to burial dolomitization in the fresh water environment. The distribution of dolomite was demonstrated to be controlled by sedimentary microfacies, and the dolomite in the West Jingbian dolomitic flat area turns out to have the largest distribution area and thickness as well as the best reservoir capacity. Massive fractures and vugs were formed in the dolomitization process, which were the basis of high-quality reservoir formation, involving generation of a relatively small amount of intercrystalline micropores during the syngenetic–penesyngenetic dolomitization, intercrystalline pores after the penecontemporaneous dolomitization, and intercrystalline and dissolution pores during the burial dolomitization. Moreover, dissolution during hypergene and burial periods contributed to most of the reservoir space, while filling effect during the burial period significantly reduced porosity. [ABSTRACT FROM AUTHOR]

In this paper, the Cambrian crude oil and source-rock geochemistry in the ZS well field of Tazhong area, Tarim Basin, was analyzed using the gas chromatography-mass spectrometry (GC-MC). Moreover, the hydrocarbon source in the Tazhong was identified through oil–source correlation. The analysis of light hydrocarbons and biomarkers indicates that the Cambrian and Ordovician protokerogens are of marine sapropelic type formed in a strongly reducing environment. The result of oil–source correlation shows that both Cambrian crude oil in the ZS well field and Ordovician crude oil in the Tazhong area originated in the Cambrian–Lower Ordovician source rocks. [ABSTRACT FROM AUTHOR]

The RePu area is located in the Tarim Basin, China. Based on the exploration and development, this paper analyzed the characteristics of the Ordovician crude oil and the source rocks by gas chromatography-mass spectrometry. The Ordovician carbonatite reservoir oil in this area exhibited light component, rich chain hydrocarbon and poor aromatic hydrocarbon. The composition of C7 compound showed the advantage of n-heptane, which belonged to type I generative kerogen, and it was deep buried highly mature crude oil. The crude oil biomarker compounds all exhibited high content of long-chain tricyclic terpanes and high Ts/Tm values. The large presence of alkane aromatization compounds indicated strong reducing environmental conditions during crude oil formation. By comprehensive judgment that the main oil source of the Ordovician reservoirs in the RePu area was the Cambrian source rock. [ABSTRACT FROM AUTHOR]