Genesis Of Dolomite Dolomite And Its Relationship With Reservoirs In The Aurora Dan Shadows In Maxi - Gaositan Area, Central

The study of the genesis, occurrence and distribution of dolomite is geological basis for oil and gas exploration in a sedimentary basin. The Sinian Dengying Formation is widely distributed in the Sichuan Basin, and eastward to Hubei (the Three Gorges area), it gradually changes to limestone. The large area of massive dolomite is the carrier of the Dengying Formation reservoirs, and the pore evolution of Dengying Formation reservoir has a close relationship to its diagenesis process. The genesis of Sinian Dengying Formation has been a hot issue to the geologists. In view of this situation, it is necessary to study the genesis of the dolomite of Sinian Dengying Formation, in Moxi-Gao shiti area, according to sedimentology, petrology and geochemistry etc.This thesis is based on the domestic and foreign related research and techniques, combining with the geological background of the study area and using the mineral identification technology and inorganic geochemistry technology(including the carbon and oxygen isotopes, strontium isotope, REE, trace element and inclusion analysis, etc.) to research the genesis of Sinian Dengying Formation of the Moxi-Gao shiti area deeply, and expound the relationship between the dolomitization and reservoir briefly.According to the research of core observation, thin slice identification etc, the dolomite of Dengying Formation have been classified with composition-genesis, including the four models of dolomite:stromatolite, grain dolomite, crystalline dolostone and breccia dolostone. Crystalline dolomite which was the product of recrystallization, and brecciated dolomite (including weathered remnants breccia, karst breccia) was the product of the tectonization and epidiagenesis. Stromatolite and grain dolostone formed during the deposition period of Dengying Formation retained the characteristics of the near surface environment, and these characteristics could provide favorable geological basis for the analysis of the genesis of the Dengying Formation.When the rock was subjected to isotope fractionation, geothermal and diagenesis transformation relatively weak, the rock’s carbon and oxygen isotope are close to the seawater isotope value of Dengying Formation sedimentary period relatively. On the contrary, oxygen isotope and carbon isotope are negative seriously. Strontium isotope was mainly controlled by mantle source and land sources, the strontium isotope is significantly higher when the diagenesis modification was strongly, which indicates that radioactive strontium had infiltrated into in the fluid. REE shows that the Dengying Formation dolomite lack of seawater feature (Ce and Eu negative anomaly), and shows the obvious characteristics of the positive anomalies of Ce and Eu. In addition, the rock which preserved primary fabric, sedimentary structure, biological feature, and affected by diagenesis weakly, its Mn and Fe content are relatively low. A series of geochemical data above could prove Dengying Formation dolomite formed in the near surface environment, but the late diagenesis resulted in change of carbon, oxygen isotope, strontium isotope, oxygen isotope, trace elements, REE.According to crystallography and mineralogy, the early seawater fibrous dolomite cements was the replacement product of aragonite and magnesium calcite. And in the near surface environment, due to the kinetic barriers, dolomite could not precipitate from seawater directly, even the fibrous dolomite cements. In the progradation process, the carbonate sediment layer, which rich in cyanobacteria, was under the action of sulfate reducing bacteria, the biodegradation of cyanobacteria mats removed SO42-from the marine pore fluid, increased the activity and ion concentration of CO32-. In addition, this process also enabled the Mg2+release from the hydration water pore water. The changes of physical and chemical properties of pore fluid overcame the kinetic barrier of dolomite formation, and promoted the occurrence of dolomitization. Dolomitization provided a prerequisite for Dengying Formation dolomite reservoir, but deposition, diagenesis and tectonization were the key factors to the formation of dolomite reservoir.