Authigenic albites occur widely in clastic reservoirs with important implications for diagenesis and reservoir formation. The middle Permian Lucaogou Formation in the Jimusaer Sag (Junggar Basin, NW China), where major exploration breakthroughs in shale oil have been achieved, reveals a new phenomenon that authigenic albites are abundant in unique mixed carbonate–volcanic–clastic sequences. This has not been reported in the literatures. To fill the knowledge gap, the origin of these authigenic albites and their relationship with dissolution pores (i.e. diagenesis implications) were investigated. Results show that two types (I and II) of authigenic albite were identified within the shale oil reservoirs. Euhedral Type I authigenic albites with 3–10 μm only occur in dolarenite intraclasts and are symbiotic with amorphous dolomite minerals with a pure chemical composition of >99% albite-end-member content. Larger Type II authigenic albites with 10–50 μm are widely distributed in reservoirs, primarily in dissolution pores, and coexist with authigenic dolomite minerals or dolomite overgrowths. Their chemical composition is less pure with anorthite-end-member contents that range from undetectable to 9.77%, with an average of 1.34%. A symbiotic relationship, pure chemical composition, size, and euhedral morphology indicate that Type I authigenic albites precipitated during syngenetic hydrothermal action. However, the morphology of dissolution pores, residual symbiotic “orthoclase”, impure chemical composition and carbon–oxygen isotope indicate that Type II were the products of the dissolution and reprecipitation of “perthite” crystal pyroclasts influenced by acid organic fluids in latter diagenesis. The differential dissolution of “orthoclase” and “albite” components in “perthite” crystal pyroclasts formed enormous intergranular secondary pores in the presence of dolomite minerals in the shale oil reservoirs.

The first member of the Qingshankou Formation (Qing Member 1) is rich in oil and gas resources and represents the first lacustrine transgression period, during which the lake basin area reached its maximum. This study utilizes major and trace element analyses, along with pyrolysis, to investigate the sedimentary environment and mechanisms of organic matter enrichment in the hydrocarbon source rocks of the Heiyupao Depression Qing Member 1. The results indicate that the hydrocarbon source rocks in this area exhibit good to excellent organic richness, mainly comprising Type I and Type II₁ organic matter, and are at a high stage of maturity. Furthermore, the paleoclimate conditions during the Qing Member 1 period in the study area were characterized by a warm and humid climate, with an open lake basin and freshwater to brackish water conditions. The water was low in oxygen, suboxic to anoxic, and had relatively high primary productivity. Multiple marine transgressions occurred during the Qing Member 1, transporting substantial nutrients into the lake, which promoted algal blooms in the water. The correlation analysis of TOC content in the Qing Member 1 shale and various indicators shows that the enrichment of organic matter in the study area is primarily influenced by paleoproductivity and paleosalinity, while paleoclimate, paleoredox conditions, and paleowater are not the main controlling factors for organic matter enrichment in the area. Organic matter only accumulates under relatively high salinity and paleoproductivity conditions. Event-driven marine transgressions also play an essential role in enhancing paleoproductivity. Therefore, the organic matter enrichment model in the study area is more aligned with a productivity-driven model. Finally, a comprehensive organic matter enrichment model of hydrocarbon source rocks in the Qing Member 1 of the Heiyupao Depression is proposed.

Microbes are known to mediate dolomite precipitation in laboratory experiments; however, the linkage of specific microbes to ancient dolomites remains poorly constrained due to scarce diagnostic biogeochemical signatures and mineralized microbial relics in the rock record. Here, we report the occurrence of methanogen-mediated dolomite in the Lower Permian lacustrine Lucaogou Formation in northwestern China. The clumped isotope (Δ₄₇) temperature provides direct evidence of a low-temperature origin (typically <40 °C). The extremely positive δ²⁶Mg_DSM3 (up to +0.44‰) and δ¹³C_VPDB (up to +19‰) values in the dolomite indicate authigenic precipitation in methanogenic lake sediments. Micron-sized spheroidal bodies and filamentous and sheetlike structures are interpreted as mineralized coccoid methanogenic archaea and extracellular polymeric substances (EPSs), respectively. Dolomite nanoglobules (primarily 40−100 nm in diameter) are interpreted as mineralized viruses attached to the archaea and EPSs and between the cells. A combination of geochemical and microscale evidence confirms the microbial origin of the dolomite induced by methanogens and their associated bacteriophages. Furthermore, dolomite nanoglobules initially nucleated on the surfaces of methanogen cells, EPSs, and viruses and then merged into larger aggregates. The formation of microbial dolomite is characterized by a metabolic incubation, heterogeneous nucleation, and aggregative growth pathway. These findings provide valuable clues to decipher the biosignatures of these particular ancient dolomites.

Strategic breakthroughs have been made in the exploration and evaluation of Gulong shale oil in the Songliao Basin. However, the Heiyupao Depression, located near the Gulong Depression, hosts a thick section of the Qingshankou Formation shale that has not been extensively studied. This paper presents novel insights into the lithofacies characteristics, depositional environment, and reservoir features of the Qingshankou Formation shale in the Heiyupao Depression, with a specific focus on the origin and maturation of organic-rich shale. Four core wells were drilled, and 152 core samples were analyzed through a variety of techniques, including rock type classification, mineral composition, TOC content, rare earth elements, rock pyrolysis, organic matter type determination, and CT scanning. Results indicate that the Qingshankou shale is dominated by felsic compositions and Type I kerogen, with organic maturity varying across the section. Based on lithology, sedimentary structures, mineralogy, and organic matter abundance, five distinct lithofacies are identified: high-organic mud-rich felsic shale, high-organic sand-rich felsic shale, medium-organic sand-rich felsic shale, medium-organic massive shale, and low-organic sand-rich felsic shale. Notably, the Type A lithofacies (high-organic mud-rich felsic shale) is identified as a primary source rock due to its intergranular and organic matter pores, albeit with low porosity and poor connectivity. In contrast, the Type E lithofacies (low-organic sand-rich felsic shale) have high porosity, well-developed micro- and nano-scale pores, and strong connectivity, marking them as the primary reservoirs. The characteristics of this region differ significantly from those of Gulong shale oil, requiring different extraction strategies. The mineral composition of such shale is predominantly felsic rather than mixed. The findings not only provide theoretical support for the exploration of complex lacustrine shale in the Songliao Basin but also offer valuable insights for the resource development of similar non-marine shale systems worldwide.

Fluid modification critically affects the quality of clastic rock reservoirs. At present, the mechanism of multistage fluid alteration in clastic rock reservoirs under deep burial conditions is still a matter of debate. To solve this problem, this study focuses on the deep clastic reservoirs of the Zhuhai Formation in the Wenchang A Sag of the Zhujiangkou Basin as an example and conducts a series of petrological and geochemical analyses. The results indicate that the fluids that primarily affect reservoir development are hydrothermal fluids and organic acids, and the hydrothermal fluid activity occurred over multiple stages, leading to a diagenetic environment with alternating acidic and alkaline conditions. In this low-permeability reservoir, hydrothermal fluids under acidic conditions were favorable for reservoir development. However, these fluids gradually became alkaline as the reaction proceeded, and under alkaline conditions, carbonate precipitation blocked the pores. In contrast, organic acids had a very positive effect on improving the reservoir. Thus, in this relatively sealed low-permeability reservoir, regions with significant hydrothermal development typically exhibit poor reservoir quality, whereas areas with substantial influxes of organic acids exhibit better porosity and permeability. Furthermore, the order of fluid injection caused substantial variations in reservoir properties. Reservoir areas that first experienced strong hydrothermal injection tend to have very poor properties. In contrast, reservoir areas that first experienced weak hydrothermal fluid influence followed by strong organic acid activity exhibit better properties. The reservoir areas initially improved by organic acids show good connectivity, and any precipitates from later hydrothermal fluids did not accumulate in reservoir pores. As a result, these reservoirs display the best reservoir characteristics. This study defines a model of reservoir development under the combined effects of multiphase fluids and diagenetic environments, which is highly important for evaluating the exploration potential of deep clastic rock reservoirs.