Cherts have been thought to originate from biosilicification, terrestrial inputs and hydrothermal activity. The study of cherts is helpful in understanding the paleo-ocean environment and tectonic–sedimentary processes. Large amounts of cherts occur widely in the Maokou Formation in the Sichuan Basin, which may be largely connected to the Permian Chert Event (PCE). However, the source of silica and the formation process of cherts remain debated. Here, we analyze the petrographic and geochemical features of the cherts from the Guadalupian Maokou Formation (~268–259 Ma) in six sections in the Sichuan Basin. Two main types of cherts, nodular and bedded, are recognized in the Maokou Formation. The formation of nodular cherts was mainly affected by hydrothermal fluids, whereas the bedded cherts are mainly of biogenetic origin. The Emeishan large igneous province (ELIP) caused the activation of deep faults, accompanied by intense hydrothermal activities. Correspondingly, the cherts of significant hydrothermal origin developed near the active deep faults. The intensified hydrothermal activities may provide extra silica supplies and flourish the silica-secreting organisms by the associated volcanogenic upwellings that facilitated the enrichment of cherts. The study of Maokou cherts can help to record the volcanic- and silicon-related biological activities in the eastern Paleo-Tethys Ocean and can provide significant implications for chert enrichment in analogous settings.

In recent years, abundant natural gas has been found in microbial carbonates in the fourth member of the Leikoupo Formation in Western Sichuan Basin. In this study, from the observation of 626 microbial thin sections, four types of microbial carbonates are classified based on the differences of mesostructures. Among them, thrombolites and stromatolites are subdivided into eight types based on the differences of microstructures. Six types of microbial microstructure association (MSA) are identified, and are mainly developed in microbial mounds. The energy of sedimentary environment and hydrodynamic conditions of them from low to high is MSA-5, MSA-1, MSA-6, MSA-3, MSA-4 and MSA-2. Because of the arid climate in the Annie Period, a restricted platform are developed in the upper sub-member of the Leikoupo Formation in Western Sichuan Basin, and the sedimentary facies are lagoon (gypsiferous lagoon or salt lake in evaporate conditions), microbial mounds, shoals, inner platform shoals and open sea from the east to the west. Microbial microstructures not only affect the pore evolution of microbial carbonate reservoirs, but also affect the diagenesis of microbial carbonate reservoirs of the fourth member of the Leikoupo Formation.

Lacustrine shale has garnered significant attention due to its significant resource potential. The Jurassic shale in the Sichuan Basin is an important target for lacustrine shale exploration in China. However, previous studies have predominantly focused on the Da’anzhai member of the Ziliujing Formation, and little attention has been paid to the shale of other strata. This paper aims to address this gap by investigating the Jurassic Dongyuemiao member in the Sichuan Basin. The study systematically characterizes the geological properties of the Dongyuemiao shale system, reconstructs the paleosedimentary environment, identifies the key factors influencing organic matter (OM) enrichment and physical properties, and assesses its resource potential through comparative analysis. Our results show that the Dongyuemiao shale was deposited in an oxic and semi-humid sedimentary environment characterized by intense weathering conditions. The enrichment of OM in the shale is primarily controlled by redox conditions and salinity, with redox conditions playing the most crucial role in OM accumulation. In terms of pore system characterization, clay mineral-associated pores dominate the pore types in the Dongyuemiao shale, while two types of organic matter-associated pores are also widely observed. The development of framework grain-associated pores is limited and only present in certain siliceous and carbonate minerals. The shales of the Dongyuemiao member and the Da’anzhai member exhibit slight differences in TOC content. However, the kerogen in the Dongyuemiao member displays higher hydrocarbon generation potential, and the Dongyuemiao shale exhibits more favorable pore structure parameters. Overall, the Dongyuemiao shale does not exhibit any significant disadvantages compared to the Da’anzhai shale. Therefore, it deserves greater attention in future exploration endeavors. The research findings presented in this paper provide a solid theoretical foundation for expanding the exploration scope of lacustrine shale in the Sichuan Basin.

In recent years, Ordos Basin has been the largest petroleum field in China and represents significant potential for gas exploration in the Lower Paleozoic carbonate reservoirs. Taking the central-eastern Ordos Basin as an example, this work studies the reservoir characteristics, origin, and controlling factors of the crystalline dolomites of the Sanshanzi Formation by means of petrological and geochemical studies. The Sanshanzi Formation dolostones comprise three kinds: crystalline dolostone with grain-shadow (Rd1), very fine–fine crystalline dolostone (Rd2), and medium-coarse crystalline dolostone (Rd3). The pore spaces include intercrystalline pores and intercrystalline solution pores, residual intergranular pores, vugs, and fractures. The medium-coarse crystalline dolostone is the best reservoir. The average porosity is 4.61%, and the average permeability is 0.91 mD. The sedimentary environment in the upper Cambrian was a large area of tidal flats. Under the influence of penecontemporaneous meteoric water leaching, dissolution developed at the top of the dolomite flat–lagoon depositional cycle, with vugs developing in the very fine–fine crystalline dolostone. Penecontemporaneous dolomitization enhanced the compaction resistance and effectively preserved the pore space of the reservoir. In addition, recrystallization improved the crystalline texture, turning the intergranular pores into residual intergranular pores and intercrystalline micropores into intercrystalline pores. In this case, the very fine–fine crystalline dolostone was transformed into medium-coarse crystalline dolostone. The results of this study provide a case for the in-depth understanding of the development and preservation mechanism of ancient dolomite reservoirs and also point to the research pathways for oil and gas exploration in the Cambrian in the Ordos Basin in the future.

The Late Permian strata of the Xuanwei Formation in the eastern Yunnan region exhibit extensive diverse morphological features within siderite deposits. These variations in siderite deposits suggest potential differences in their formation processes. In this study, fieldwork and comprehensive indoor studies revealed four distinct forms of siderite deposits: stratiform-laminated, lens-like nodule, sandstone cementation, and fracture filling. The stratiform-laminated siderite, varying in color from bluish-grey to dark grey, is composed of uniformly sized microcrystalline to fine-grained siderite along with detrital matter, displaying precise layering and banding structures that suggest direct deposition from cyclic iron-rich seawater under reducing conditions. Lens-like-nodule siderite, which appears grey-yellow, is composed of mud microcrystalline siderite, medium to coarse-grained pseudo-ooids, and glauconite. It shows conformable distribution characteristics resulting from the diagenetic differentiation of iron-rich sediments under reducing conditions during the diagenetic and early diagenetic periods. Siderite as sandstone cementation exhibits a yellow-brown color and consists of dispersed colloidal siderite and cemented siderite clumps that fill intergranular pores of detrital particles. It precipitated under reducing conditions within those intergranular pores. Siderite filling fractures typically appear as vein-like or network-like structures intersecting bedding at large angles. They exhibit grain structures with significant variations in size. These siderite deposits exhibit exceptional purity and result from siderite dissolution during sedimentary periods, followed by reprecipitation within regional extensional fractures during the diagenetic phase. The primary occurrence of siderite deposits in the study area is within coal-bearing strata, as revealed by the integration of sedimentary profiles and sedimentary facies analysis. The coal-bearing strata, influenced by the Emeishan large igneous province, underwent iron enrichment during and after volcanic eruptions while developing a reducing environment, which was facilitated by abundant vegetation. Consequently, geological processes led to siderite layers, lens-like siderite nodules, and siderite cementation. The Yanshan orogeny induced extensive high-angle fracture development in epigenetic coal-bearing strata, facilitating fluid circulation and the redistribution of soluble siderite. This geological activity resulted in the formation of vein-like structures composed of siderite.