The Genesis Of Middle Permian Dolomite In Sichuan Basin

The genesis of dolomite is the most complex geological problem in carbonate rocks for hundreds of years.Different genetic models have been established for the genesis of dolomite,among which the diagenetic fluid and diagenetic system of dolomite have been the focus of research.These problems are of great significance for the genesis of dolomite.The middle Permian in sichuan basin has been an important natural gas reservoir since the 1960 s.An in-depth understanding of the distribution and properties of carbonate reservoirs can provide more resources and basis for the exploration of oil and gas resources in this area.Based on previous studies,this paper systematically studied the genesis of the middle Permian dolomite in sichuan basin through field investigation,microscopic observation,carbon and oxygen isotopes,strontium isotopes,rare earth element analysis and other methods,and obtained the following understanding:The Permian dolomites in the study area can be divided into three types: medium-coarse-grained dolomite,fine-grained dolomite and saddle-shaped dolomite;medium-coarse-grained dolomite has coarse grains,often occurs in layers,with developed micro-fissures filled with saddle-shaped dolomite and asphalt;mediumfine-grained dolomite often occurs in layers,with dissolved holes and structural fissures developed,and saddle-shaped dolomite and silica filled in fissures and caves.Saddle dolomite occurs in the cracks and holes of the rocks and has a distinctive boundary between medium-coarse grained dolomite and fine-grained dolomite,so it is considered that the formation of saddle dolomite is later than that of the other two dolomites.The homogenization temperature of the inclusion of the three types of dolomites is higher than the normal burial depth temperature in this region,and their oxygen isotopes are obviously negative,which are consistent with the characteristics of hydrothermal dolomites.Breccia and zebra-like structures related to hydrothermal activity can be seen in outcrop and core in the field,so it is believed that the formation of dolomite is related to hydrothermal activity.The geochemical characteristics of the three types of dolomites indicate that they have carbon isotopes,strontium isotopes,rare earth element distribution patterns and negative anomalies of cerium and europium,which are similar to the limestone.Therefore,it is believed that the chengyan fluid of the dolomites is similar to the diagenetic fluid of the limestone,and they are all seawater or Marine source flows in the strata.