Porosity evolution during the diagenesis of the reef shoal dolostones from the Changxing Formation in the Daxian-Xuanhan region, Sichuan

The reef shoal dolostones from the Changxing Formation in the Daxian-Xuanhan region, Sichuan have good potentials for oil and gas reservoirs. On the basis of the observations of routine thin sections and cast sections, the reservoir spaces in the reef shoal dolostones consist mostly of intercrystal pores, intercrystal solution openings, intergranular and intragranular solution openings, visceral solution openings and organic framework solution openings. The formation and evolution of the pores are generally constrained not only by deposition, tectonism and thermal evolution of organic matter but also by diagenesis. The compaction and cementation are responsible for the disappearance of primary pores in the submarine-exposure-shallow burial environments. The penecontemporaneous mixing water dolomitization and early exposure and dissolution result in the modification in the internal structures and textures of the sediments, and formation of extensive micropore networks and solution openings. The intense recrystallization and dissolution permit the generation of abundant secondary pores and improvement of the reservoir capacity of the reef shoal dolostones in the intermediate to deep burial environments.