Sedimentation mechanism and petroleum significance of calcareous cements in continental clastic rocks: Comparison between the Kongdian Formation in the Jiyang Depression and the Xujiahe Formation in the western Sichuan Basin

The mechanism of calcareous cementation in clastic rocks in the Kongdian Formation in the Jiyang Depression and the Xujiahe Formation in the western Sichuan Basin were studied by means of cathode luminescence, scanning electron microscopy, inclusion homogenization temperature measurement, isotope content of carbon and oxygen, and chemical ion. The calcareous cements mainly consist of carbonates, hard gypsum and fluorite. Their microscopic characteristics are influenced by cementation time and environment. Calcium sources and precipitation controlling factors affect the mechanism of calcareous cementation. The calcium sources for calcareous cements mainly include meteoric water eluviation, feldspar dissolution, compaction and dissolution, thermochemical action of organic salt rocks, and clay mineral transformation. The precipitation mechanism of calcareous cements is not influenced by a single factor, but by many factors such as temperature and pressure, pH value and the concentration of chemical ions. The time of calcareous cementation is divided into early, middle and late. The physical properties of reservoirs are influenced by the time of calcareous cementation. Calcareous cements during the early diagenetic stage are constructive to the reservoir, while those during the late diagenetic stage are destructive.