The study of dolomite characteristics and its formation mechanism is of great significance for dolomite theory and dolomite reservoir exploration. In this paper, the dolomite of the Middle Permian Qixia Formation in the northwestern Sichuan Basin is studied. The characteristics and genesis of the dolomite are studied by field profile and core observation and using the cast thin section and cathode luminescence and geochemical analysis methods. The results show that the main types of dolomite in Qixia Formation are massive grain dolomite, karst brecciate dolomite and porphyry dolomite. The carbon isotope is positively low and the oxygen isotope is negative which reflects the participation characteristics of early fresh water in the process of rock formation. Based on the tectonic and sedimentary background of the work area and compared with the previous classical dolomitization model, we find that the dolomitization model of the Middle Permian Qixia Formation in the northwestern Sichuan Basin has its own characteristics. By extending the "magnesium cage effect" theory from Folk, the dolomitization model of Qixia Formation can be divided into four zones：fresh-water seepage zone, mixed seepage zone, mixed underflow zone and seawater underflow zone, in which the mixed seepage zone and mixed underflow zone are the main layer of dolomite. In the mixed seepage zone, freshwater and seawater enter alternately. During the diagenesis process, the sediment is flushed by fresh water, resulting in a large number of magnesium ions flowing into the lower stratum along with fresh water, and then the sea water carries magnesium ions for replenishment, which provides favorable conditions for the formation of dolomite. The mixed fluid constantly entering the mixed underflow zone carries a large number of magnesium ions, which makes the zone have a high Mg/Ca value. The earth temperature rise caused by mantle upwelling overcomes the dynamic barrier of dolomite formation and effectively promotes the formation of dolomite.
Li Bo Wang Xingzhi Xie Shengyang Feng Mingyou . Characteristics and genesis of the Middle Permian Qixia Formation dolomite in the northwestern Sichuan Basin[J]. Chinese Journal of Geology, 2020, 55(1): 183-199.