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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]

The study of extraordinarily high organic matter content (EHOMC) is beneficial to promote the fine evaluation of shale oil and gas, but so far, there have been few studies on its mechanism. This paper carried out a comprehensive lithological and geochemical analysis of the black shales of the Wujiaping Formation in the Kaijiang-Liangping Trough. The results showed that the black shales of the Wujiaping Formation can be divided into two units (the upper and lower parts), and EHOMC occurs in its upper part. The redox-sensitive trace elements (RSTEs), MoEF-UEF covariation, and Ni/Co correlation showed that the lower part of Wujiaping Formation was a weakly restricted oxidation environment, while the upper part evolved into a sulfidation reducing environment. The productivity indicator elements (BaXS, NiXS, CuXS, P, and Mo) indicated that the upper Wujiaping Formation had higher primary productivity than the lower part. However, rare earth elements ((La/Yb)N) indicated that the deposition rate in the lower part of the Wujiaping Formation was higher than that in the upper part. In general, although the oxygenated water in the lower part of the Wujiaping Formation is not conducive to the preservation of organic matter, the high input of nutrients from land sources and the high deposition rate inhibit the decomposition of organic matter, so the lower part has a certain degree of organic matter accumulation. The sulfidation reducing environment and high paleoproductivity are the main reasons for the enrichment of organic matter in the upper part of the Wujiaping Formation. In addition, the sulfidation reducing environment and high paleoproductivity occurred during the violent upwelling at the end of Guadeloupe, a period of high sea level, these factors have jointly contributed to the enrichment of OM in the upper the Wujiaping Formation. [ABSTRACT FROM AUTHOR]

Organic matter pores (OMP) provide significant storage space for hydrocarbons in lower Silurian Longmaxi shales in the Dingshan field of southern Sichuan, China. The distributions of organic matter and the different OMP structure parameters were characterized through Ar-ion polishing, scanning electron microscopy (SEM), and image analysis software for shale samples of different wells. The research results indicated that organic matter has been divided into two categories based on its occurrence, location, and its relationship with authigenic minerals: organic matter in situ and migrated organic matter. OMP for organic matter in situ are mainly micropores mostly arranged isolatedly, while in migrated organic matter pores show larger sizes and higher roundness. The development of OMP in samples is predominantly controlled by the formation pressure. The existence of overpressure alleviated the stress on the rock skeleton, causing the compaction of some migrated organic matters to lag or decrease. This played a positive role in protecting the development of pores in the interior and edge of the rock skeleton, and it can also induce the development of microfractures in shale. The protective effect of formation pressure on organic pores was provided for understanding the exploration and exploitation of Longmaxi shales in the study area. [ABSTRACT FROM AUTHOR]

Cast thin section observation, scanning electron microscopy (SEM), high‐pressure mercury injection (HPMI), and nuclear magnetic resonance (NMR) were used to examine the microstructure of tight carbonate reservoirs in the Lower Jurassic Da'anzhai Member, the central Sichuan Basin. The pore space in the Da'anzhai Member is classified into 2 types and 17 subtypes, with nano‐scale pore throats of 'O', 'S', 'Z', and 'I' shapes. Poorly sorted pore throats vary greatly in diameter; thus, it is difficult for fluid flow to pass through these pore throats. There are three classes of pore throats in carbonate reservoirs, i.e. isolated pores, pores coexisting with fractures, and large pores and fractures. Isolated pores may provide some pore space, but the permeability is low. Pores and fractures coexisting in the reservoir may have a great impact on porosity and permeability; they are the major pore space in the reservoir. Large pores and fractures have a great impact on reservoir properties, but they only account for a limited proportion of total pore space. The microstructure of Da'anzhai reservoirs, which dominates fluid mobility, is dependent on sedimentary environment, diagenesis, and tectonic process. Pore structure is related to sedimentary environment. The occurrence of microfractures, which may improve reservoir properties, is dependent on tectonic process. Diageneses are of utmost importance to pore evolution, cementation and growth of minerals have played an important role in destroying reservoir microstructure. [ABSTRACT FROM AUTHOR]

Shale gas mainly exists in organic-rich shale in the form of free gas and adsorbed gas. Shale gas content has a direct impact on the productivity and life of the gas well, and is also an important indicator for areal selection and reservoir evaluation. Through the study of gas content experiments of shale gas, we can better guide the exploration and development of shale gas. According to the isothermal adsorption and desorption experiments of Wufeng-Longmaxi Formations in Jiaoshiba block of the Fuling shale gas field in Sichuan basin, China, 7 core samples from isothermal adsorption experiment and 30 core samples from desorption experiment were analyzed. The experimental results show that the isothermal adsorption experiment is an effective way to obtain Langmuir Volume (V L) and Langmuir Pressure (P L). The total gas content obtained from shale desorption experiment in the drilling site is mainly adsorption gas content, and the free gas content is relatively small. The desorption rate is different in different desorption stages, and the samples with higher total organic carbon (TOC) content have faster desorption rate and higher total desorption gas content. The carbon isotope value of methane gradually increases with the increase of desorption time. The results of methane isotope analysis in the desorption experiment are compared with the results of carbon isotope monitoring in the producing process. The findings reveal that after years of development in Jiaoshiba block of the Fuling shale gas field, the methane isotope in gas wells has basically remained unchanged, and the current gas production should be dominated by free gas. • Compare the analysis results of isothermal adsorption and desorption experiment. • The change law of carbon isotope with time in desorption process is studied. • The producing status is determined for the first time by using this law. [ABSTRACT FROM AUTHOR]

The Dengying Formation is an important stratum of exploration in the Gaoshiti-Moxi structure belt in the Sichuan Basin. Therefore, this study combines core, thin section, mud logging data and geochemical analysis (C, O and Sr isotopes) with seismic section data to investigate the basic characteristics and genesis of the Dengying Formation reservoir. Observations indicate that the reservoir rocks mainly consist of sand crumb dolomites, algae laminated dolomite and breccia. The reservoir space is dominated by porous dissolved holes, dissolved pores (or holes) along the direction of algal laminae, intra-grit holes and residual “grape lace” holes. The reservoir has the characteristics of low porosity and medium-permeability. There are three types of reservoir: porous reservoir, fractured porous reservoir and cave reservoir. The reservoir was formed by the common action of deposition, diagenesis and tectogenesis. Sedimentation is the basis and presupposition for reservoir development, but the effective reservoir is mainly controlled by the algal flat microfacies; Furthermore, compaction, cementation and filling are the main causes of reservoir density; Recrystallization is the basis for the formation of porous reservoir; Supergene dissolution is the key to reservoir formation; Buried dissolution also promotes the formation of a high quality reservoir. The fractures formed by tectogenesis significantly improve the permeability of the reservoir and contribute very little to porosity. The reservoir gradually evolved and formed under the control of various geological and diagenetic activities. This study provides important information and references for oil and gas exploitation and development in central Sichuan. [ABSTRACT FROM AUTHOR]

The article focuses on a study which examines the color, mineral composition, texture, structure, and biological characteristics of rock in Lower Silurian Longmaxi Formation in Eastern Sichuan Basin, China. It mentions two subfacies with six microfacies in the study area of Longmaxi formation, such as Calcareous Silty Shelf, Muddy Silty Shelf, Muddy Limy Shelf in the Inner Shelf. Information about the organic-rich shale in the area is presented.

The marine carbonate rock in the Changxing-Feixianguan Formation of Xuanhan-Daxian area, northeastern Sichuan, the Sichuan Basin holds abundant natural gas resource, where the dolomite includes two types, the one with original texture (grain dolomite, reef dolomite) and the one without original texture (idiomorphic to anhedral-crystal dolomite, porphyritic dolomite). The analysis on the isotopes of C, O, Sr and other geochemical features shows that the dolomitizing process happened in fluid rich in Mg2+ and lean in Ca2+ and brought in Fe2+, Mn2+ and took out Sr2+; with the rise of recrystallization, the Fe2+, Mn2+ contents tend to increase while δ18O PDB and Srcontent tend to decrease. According to the variation of 87Sr/86Sr and other geochemical features, it is considered that the dolomitizing fluid participating in the reservoir diagenesisis the high salinity sea water later than the end of Feixianguan stage, and the dolomitization takes place mainly through the multiple thermal convective cycle effect. The micritic dolomite is of evaporative dolomitization origin with the characteristics of high Sr2+, Na+ and low Mn2+ which reflects the open environment of the surface exposed in the quasi syngenetic period, while the reef dolomite is of high Mg/Ca dolomitizing fluid origin with the characteristics of lowSr2+, Na+ and Mn2+, whose diagenetic environment changed from near surface to shallow burial environment; crystalline dolomite is formed by metasomatism of low Mg/Ca dolomitizing fluid at deep burial depth and high temperature, with the increase of burial depth,in the high temperature and high pressure deep sealing system, its Fe2+ and Mn2+ elements increased while Sr2+ decreased. Through reconstruction of dynamic diagenesis-pore evolution, it is concluded that the low porosity of shallow beach grain dolomite is resulted from early compaction and medium-term cementation, yet the reservoir quality has been apparently improved by the later hydrocarbon dissolution; compared with shallow beach grain dolomite,the reef dolomite has smaller initial porosity, but stronger compressive capacity. The early compaction and medium-term cementation have less impact on the reef dolomite, and the later dissolution strength is slightly weaker than that of the granular dolomite, thus the change range of porosity is smaller than that of the grain dolomite; the micrite-silt dolomite mainly suffered porosity reduction caused by compaction, and the later burial dissolution and recrystallization are key to the quality improvement of this kind of dolomite reservoir. • There are two types of dolomite in the study area: the original fabric and without original fabric. • The high salinity marine source fluid of late period is involved in the dolomitization process of dolomite reservoir. • Dissolution is the key to the formation of high quality reservoir. • Compaction and cementation are the key factors to reduce the porosity of dolomite reservoir. [ABSTRACT FROM AUTHOR]

In order to effectively evaluate shale gas reservoirs with low porosity, extra-low permeability, and no natural productivity, dipole array acoustic logging, which can provide various types of information including P-wave slowness (DTC) and S-wave slowness (DTS), is widely used. As the dipole array acoustic logging tool has a larger investigation depth and is suitable for complex borehole environments, such as those with a high wellbore temperature, high drilling fluid column pressure, or irregular borehole wall, it has been mainly applied to the evaluation of lithology, gas potential, fractures, and stimulation potential in shale gas reservoirs. The findings from a case study of the Sichuan Basin in China reveal that the acoustic slowness, S-P wave slowness ratio (RMSC), and S-wave anisotropy of the dipole array acoustic logging can be used to qualitatively identify reservoir lithology, gas potential, and fractures. Using the relationship between DTC and the total porosity of shale gas reservoirs, and combined with the compensated neutron (CNL) and shale content (Vsh) of the reservoir, a mathematical model for accurately calculating the total porosity of the shale gas reservoir can be established. By using the relationship between the RMSC and gas saturation in shale gas reservoirs and tied with density log (DEN), a mathematical model of gas saturation can be established, and the determination of gas saturation by the non-resistivity method can be achieved, delivering a solution to the issue that the electric model is not applicable under low resistivity conditions. The DTS, DTC, and DEN of shale can be used to calculate rock mechanic parameters such as the Poisson's ratio (POIS) and Young's modulus (YMOD), which can be used to evaluate the shale stimulation potential. [ABSTRACT FROM AUTHOR]

The Wufeng-Longmaxi Formations are the focus of shale gas exploration in China. Their sedimentation is affected by global and regional geological events, and the resulting heterogeneity hinders the expansion of exploration results. This study combines logging, mineralogy, and geochemistry to reconstruct the sedimentary mechanisms of shale in the southern Sichuan Basin and discusses its implications for exploration. The shale can be divided into third-order sequences, in which the Wufeng Formation and lower Long-1 Member are transgressive systems tracts (TST1 and TST2), and the Guanyinqiao Member and the middle-upper Long-1 Member are highstand systems tracts (HST1 and HST2). Volcanic eruption and glaciation evolution are the driving factors of the sedimentary environment during the TST. The area is in a cold-warm climate, characterized by anoxic conditions, slow sedimentation, booming productivity, and low terrigenous influx, thereby depositing four types of siliceous shale. The siliceous shale is rich in biogenic microcrystalline quartz and high TOC, which are the keys to becoming the "sweet spot" of shale gas. Microcrystalline quartz fills the intergranular pores to support the entire pore network framework. The organic matter undergoes thermal evolution to produce abundant organic matter pores, which constitute the main place for adsorbed gas and free gas. For the highstand systems tract, HST1 is the product of the peak of glaciation, mainly composed of calcareous shale and carbonate rocks. The sedimentation of HST2 is affected by turbidity currents and violent orogeny. The area is in a warm climate, characterized by high terrigenous influx, dysoxic-oxic conditions, medium productivity and rapid sedimentation rates. The lithofacies gradually changed from siliceous shale to mixed shale and argillaceous shale. Excessive clay minerals and low TOC make it difficult for HST2 to realize the commercial development of shale gas. • The Wufeng Formation-Long 1 Member was divided into two 3rd sequences, and four geological events were identified. • Volcanism and postglaciation created conditions for the enrichment of organic matter and microcrystalline quartz. • Kwangsian Orogeny and turbidity currents led to the formation of organic-poor shale. [ABSTRACT FROM AUTHOR]

The Leikoupo Formation, which was deposited during the Anisian stage in the central Sichuan Basin, has produced significant amounts of conventional gas but there have been few studies on its unconventional resources. Organic-rich shale in the Leikoupo Formation was discovered for the first time in the wells of CT1 and JY1 in the central Sichuan region, but information on its sedimentary environment and climate evolution is scarce. A series of lithofacies, organic geochemistry and inorganic geochemistry analyses were used to analyse core samples of CT1 and JY1 to estimate their vertical changes. The organic matter characteristics, the sedimentary environment and the factors controlling the Leikoupo Formation shale in the Sichuan basin were revealed. The organic-rich shale of the Lei3-2 sub-member was formed in a semi-restricted lagoon environment. The sedimentary environment was dysoxic-anoxic, with high palaeoproductivity, a warm and humid to semi-arid and semi-humid climate. The vertical sea level decreased for a short period, led to an increase in oxygen content in the mixed shale, clayey shale and calcareous shale that were sequentially deposited from bottom to top. In the vertical direction, the palaeo-productivity and aridity decreased, and there was an increase in the salinity and terrigenous debris input. All of these corresponded well with TOC. According to Mo-TOC and Mo–U patterns, the mixed shale was characterised by the highest values of Mo EF , U EF and Mo EF /U EF , followed by clayey shale and calcareous shale, which indicated mixed shale was deposited in a moderately restricted water condition, which progressively became more restricted during the deposition of clayey shale and calcareous shale with the fall of relative sea level. Biomarker compounds (steranes, terpanes), Tmax and other pyrolysis parameters indicated that the organic matter had reached the stage of high-to over-maturity. The gammacerane to C 30 hopane in the ratio indicated that it was the bottom water in which it was deposited was sratified. • The first discovery of Lei3-2 sub-member shale in the Sichuan Basin. • The shale has good original hydrocarbon generation potential. • The shale was formed in a semi-confined environment. • Lithological changes reflect variations in sedimentary environment and sea level. [ABSTRACT FROM AUTHOR]