Tuff-rich mixed shales, representing an important class of unconventional hydrocarbon resources, exhibit diverse components, rapid lithofacies variations, complex diagenetic evolution, and strong heterogeneity. For these shales, there remains a lack of a systematic understanding of the diagenetic evolutionary processes and reservoir formation mechanisms of different lithofacies, restricting target area selection and evaluation for shale oil exploration. In this study, we investigate the diagenesis, reservoir formation, and favorable exploration targets of mixed shales with unique components in the 2nd Member of the Lucaogou Formation (also referred to as the Lu 2 Member) in the Santanghu Basin. To this end, a range of test methods are employed, including core characterization, thin section observation, scanning electron microscopy (SEM), whole-rock X-ray diffraction (XRD) analysis, electron probe microanalysis, high-pressure mercury injection (HPMI), and nuclear magnetic resonance (NMR), along with measurements of porosity, permeability, and oil saturation. The results indicate that the mixed shales of the Lu 2 Member are composed primarily of tuffaceous materials, carbonates, and organic matter and can be classified into 10 lithofacies, which are frequently interbedded. The mixed shales mainly contain nano-to micro-scale intercrystalline pores in dolomites, devitrification-induced pores in volcanic ash, and dissolution pores, suggesting complex pore structures. Distinct lithofacies exhibit significant differences in physical and oil-bearing properties. Among these, massive lithofacies featuring low organic matter abundance and composed primarily of a single component display the most favorable properties, followed by lamellar transitional lithofacies dominated by dolomites, while lamellar transitional lithofacies composed primarily of tuffaceous materials show the poorest physical and oil-bearing properties. The shale component types and their differential diagenetic evolution govern reservoir quality. Rapid deep burial and compaction in the early stage represent primary factors responsible for the deterioration of reservoir physical properties. However, they occurred earlier than other diagenetic processes. Furthermore, dolomitization and devitrification occurred before organic acid-induced dissolution. This diagenetic evolutionary sequence provides effective spaces for organic acid migration while also offering a material basis for dissolution, serving as the key mechanism behind the formation of high-quality reservoirs. A comprehensive analysis reveals that favorable exploration targets in the Lu 2 Member include the basin margin zone and the slope zone near the basin margin.

Volcanic ash input holds significant impacts on the reconstruction of sedimentary environments in saline lacustrine basins. Specifically, when abundant trace elements from volcanic ash enter lakes and are preserved in tandem with sediment, the element contents in lacustrine strata cannot accurately reflect the original sedimentary setting. Focusing on the 2nd member of the Permian Lucaogou Formation (P₂l²) in the Tiaohu-Malang Sag within the Santanghu Basin, we investigate the mechanisms by which volcanic ash alteration affects element anomalies in fine-grained mixed deposits using major, trace, and rare earth element analyses, gas chromatography-mass spectrometry (GC-MS) of saturated hydrocarbons, whole-rock X-ray diffraction (XRD), total organic carbon (TOC) content determination, and thin section observation. Accordingly, the saline lacustrine sedimentary environment under the action of volcanic ash is reconstructed. The results indicate that the P₂l² consists of fine-grained tuffaceous materials and carbonates, with the total content of felsic and carbonate minerals reaching up to 95 % on average, suggesting gradational mixed sedimentation at facies margins in a broad sense. Under the influence of alteration such as early dissolution in water and mid-late devitrification and organic acid corrosion, substantial volcanic ash thermodynamically unstable released various high-abundance nutrient ions (including metal ions) into pore fluids, which significantly interfered with the elements including Ni, Co, S, P, and Ga in the fine-grained mixed rocks of the P₂l². As a result, these elements cannot accurately reflect the original sedimentary environment. Biomarker compounds, together with the analysis of major and trace element data after screening and correction, reveal a hot and arid climate during the deposition of the P₂l², featuring limited water supply to the lacustrine basin and extremely high salinity and low oxygen concentration of the lake. Volcanic activity played a significant role in regulating the sedimentary environment of the P₂l². The transition from intense to intermittent volcanic eruptions corresponded to an increasingly hot and arid climate, along with elevated water evaporation, water salinity, and oxygen concentration, leading to the deposition of various types of rock assemblages.