Diagenetic facies classification and characterization of a high-temperature and high-pressure tight gas sandstone reservoir: A case study in the Ledong area, Yinggehai Basin

A high-temperature and high-pressure tight gas sandstone reservoir, located in the Ledong area of the Yinggehai Basin, shows considerable resource potential. However, the diagenetic characteristics of this reservoir remain unclear. In this study, reservoir petrology and diagenetic characteristics were systematically investigated through different experimental methods. Further, the porosity and diagenetic evolution models were constructed. The methods employed included petrophysical analysis, observation of core and thin sections, and grain-size distribution analysis. Furthermore, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy, cathodoluminescence, carbon and oxygen isotope analysis, digital image analysis, mercury injection capillary pressure tests, and micro-CT imaging were also employed. In the study area, gravity-flow channel sediments composed of feldspathic litharenite primarily developed in a restricted submarine canyon. And tight sandstones in target formation have an average porosity of 9.34% and permeability of 1.79 mD. Mechanical compaction and cementation (mainly illite and ferrocalcite) were regarded as crucial roles in the densification process. Further, the diagenetic evolution model indicated that the reservoir underwent a high degree of thermal diagenetic evolution. Five diagenetic facies types were classified and characterized at multiple scales. Moreover, a vertical distribution model of different diagenetic facies was also developed considering the overpressure, depth of formation, thickness and frequency of mudstones, and injection concentration of the CO2. This study helps to improve the understanding of the diagenetic characteristics and diagenetic facies types in high-temperature and high-pressure clastic reservoirs.