Diagenetic differences of the Zhenzhuchong Member of Ziliujing Formation in the Jiulongshan–Jiange area, Sichuan Basin

The rocks of the Zhenzhuchong Member of the Ziliujing Formation, Jiulongshan–Jiange area in the Sichuan Basin, were analyzed by petrography, XRD, and SEM techniques to investigate their diagenetic history and properties, such as authigenic mineral types, evolution of mixed-layer illite–smectite minerals, the clay assembly, and the fraction of mixed-layer clay minerals. The results revealed that the Zhenzhuchong Member has experienced several important episodes of diagenetic alteration since the deposition, including compaction (pressure-solution), cementation, metasomatism, dissolution, fracturing, and infilling of caves and cracks. It was also observed that diagenetic properties of the Jiulongshan area were significantly different from those of the Jiange area. The rock samples from the Jiulongshan area were characterized by the composition of siliceous and calcareous cements, varying amounts of detrital grains, clay matrix and kaolinite replaced by calcites, a certain amount of rarely dissolved early-stage kaolinite, dickite, and infillings by late-stage calcite. On the other hand, for the rock samples from the Jiange area, the dissolution is a common phenomenon with features of abundant aluminosilicates-dissolution pores or components, but the replacement phenomenon has rarely been seen. These rock samples were characterized by the presence of clay mineral cements, quartz, and dolomite infillings. It indicated that there was a great difference of diagenesis between the two areas in the types, phase, and temperature of diagenetic fluids. Revealing the difference would provide theoretical and practical implications for the exploration of high quality oil and gas reservoirs.