Sedimentary Facies Feature And Origin Of Dolomite Of The Maokou Formation In The Northwest Sichuan Area

At present, practice from the marine carbonate oil and gas exploration in China shows, grain bank, dolomite, karst weathering crust is an important oil and gas reservoir types in the carbonate. In addition to karst weathering crust, the formation of the other high quality reservoirs is closely related to dolomitization. Therefore, the research on dolomite has become an important part of the study of carbonate reservoir, and dolomite diagenesis become the core of the research. Dolomite Genesis problem is dolomite diagenesis. So, to clarify the origin of dolomite, for the understanding of the dolomite and carbonate reservoirs related to oil and gas exploration work is particularly important. In this paper, on the basis of previous studies, through the casting thin sections, scanning electron microscope, X- diffraction analysis, the order degree of trace elements, carbon and oxygen isotope, fluid inclusion testing analysis methods, characteristics of petrology and earth system research in Northwest Sichuan Maokou Formation dolomite chemical characteristics. Mainly achieved the following understanding:1, In combination with previous knowledge on the basis of Northwest Sichuan Permian,Using the latest data(core, logging, field etc.), combing the Northwest Sichuan Permian, a detailed analysis and compared of the lithologic assemblage of Permian Maokou group rules and characteristics, set up Northwest Sichuan Permian stratigraphic division.2, Based on sedimentology and Palaeontology, by analyzing the typical drilling core and section structure analysis, determine the Northwest Sichuan Maokou Formation of carbonate platform facies sedimentary facies open platform. Based on the analysis of sedimentary facies in the study area, combined with the ancient tectonic and paleogeographic background, established in Northwest Sichuan Maokou Formation sedimentary carbonate platform slope slow mode.3, The analysis of trace elements in showed that the Sr content of various types of fine grain grade dolomite is higher, reflecting the burial dolomitization rock environment, occurred in the evaporation of high salinity of formation water in Haiyuan fluid, and thick- caused by the heavy crystallization effect of Microtek dolomite and dolomite and deepened during burial the effect of Sr on. From the relationship between 13 C and 18 O,it can be seen in various types of dolomite values and 18 O values were relatively concentrated, the diagenetic fluid properties similar to or homologous, with stable value delta 13 C which grain dolomite, while the delta 18 O has a large negative bias, shows that dolomitization occurred in temperature high, diagenetic fluids probably derived from the deep circulation, magmatic hydrothermal in formation water, so the grain dolomite is the product of buried dolomitization.4, Analysis of characteristics of fluid inclusions, according to the analysis showed that the prescription of Maokou calcite is mainly formed in the deep buried high temperature environment, further prooved the Maokou Formation dolomite is the burial dolomitization.