Diagenetic evolution and reservoir formation mechanisms of tuff-rich mixed shales: A case study of the Lucaogou Formation in the Santanghu Basin

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.