| Guided by the theory of accumulation geochemistry, took the fluids of Upper Triassic and Jurassic formation in West Sichuan as the main study object, and based on the fluids geochemical test data, such as the gas component, gas isotope, light hydrocarbon gas and noble gas of natural gas, and the salinity, ions and isotope of formation water. This thesis firstly explicated the basic geochemical feature of fluids, secondly analyzed of the genesis and source of natural gas,discussed the genesis and evolution of the formation water,and determined gas migration phase,the migration direction and accumulation time,evidenced the accumulation process by formation water, finally established the mechanism of natural gas migration and accumulation, and summarized the gas accumulation pattern and main control factors. The followings are the main conclusions and innovative results of the thesis.(1) In study area, the main types of natural gas is hydrocarbon gas, which is dominant by mature-high mature dry gas, and the content of non-hydrocarbon gas is low, without H2S.(2) The paper cleared out the cause of alkane carbon isotopic reverse in West Sichuan depression,and discussed the relationship between structure and carbon isotope reverse. The main gas is carbon isotope normal with small amount of carbon isotopes reverse. According to the causes of alkane carbon isotopic reverse, the samples proportions, differences of appearing layers and the differences of tectonic deformation, conclude the tectonic deformation has an impact on alkane carbon isotopic reversal, and resulted alkane carbon isotopic reversal was more common in the southern member (stronger tectonic deformation) than the northern and middle member (weaker tectonic).(3) The criterion that can distinguish the salinity of condensate water and formation water was be established in study areas:When TDS<10000mg/1, it was condensate water; when TDS≥10000mg/1, it was formation water. The salinity of formation water in the Upper Jurassic is higher than it in Jurassic.The characteristics of formation water ionic composition had also been made clear:Na+and Cl", which has a significant control effect on salinity of formation water, is the most important ions in formation water in study areas. The relations of formation water cationic content in the study areas is that Na+>Ca2+>K+>Mg2+. The anion content relations of formation water from the 2th member of Xujiahe Formation to Middle Jurassic showed:Cl->HCO3->SO42-,but that of the Upper Jurassic was that:Cl->SO42-HCO3-,which showed the formation water of it was different from others.(4) The genetic types of formation water and the significance had been explored systematically. The main formation water in the Upper Triassic is the type of CaCl2, which reflected the characteristics of deep waters. The Middle Jurassic formation water is mainly CaCl2, reflected the mixing water between the upper Triassic deeply buried water and the Middle Jurassic native water. Less other water represented the Middle Jurassic native water.The genetic types of formation water in different regions of Upper Jurassic differed obviously. Xiaoquan-Xinchang area was mainly Na2SO4, representing the native continental water in Upper Jurassic. Luodai, Majing, and Hexing area was mainly CaCl2, the formation water was mixing water about the Upper Triassic water and the Upper Jurassic native water.. Xindu area is mainly Na2SO4, also with a high proportion CaCl2 and NaHCO3. Formation water was given priority to with mixed water, with certain native water.(5) The causes of hydrocarbon gas in study areas had been made clear. The gas is coal gas in Upper Triassic and Jurassic, and there was little oil cracking gas caused the second member of the Xujiahe Reservoir. Taking further study of characteristics of oil-gas in other parts of the Sichuan basin in Upper Triassic and Jurassic, oil gas had been founded in central and south Sichuan basin. By comparing, it can be founded that the distribution characteristics, the causes and the sources of oil gas in western, central and south Sichuan basin differed greatly.(6) The causes of non-hydrocarbon gas in study areas had been made clear. Noble gas was the main cause of crust. N2 was the products of a high organic mature-mature stage. CO2 was mainly inorganic origin, which was related with the dissolution of carbonate minerals, with a little of organic origin.(7) It is unlikely that natural gas were from the underlying marine strata in Upper Triassic and Jurassic in study area, taking account of the hydrocarbon generating quantity of source rock, the formation sealing conditions, the formation pressure difference and the geochemical characteristics of natural gas. The gas in Upper Triassic was mainly from the Upper Triassic coal measures source rocks, and the gas in Jurassic was mainly from hydrocarbon source rocks in fifth member of the Xujiahe, as well as the fourth and lower Jurassic. It found that coal and mudstone made a significant contribution to the natural gas about coal measure source rocks in Upper Triassic using rare gas isotopes.(8) The conclusion is that Triassic formation water was the mixture of seawater and freshwater, Jurassic formation water was the mixture of seawater in Upper Triassic and the continental water in Jurassic, and some of the Jurassic formation maintained the original continent freshwater’s features,through a comprehensive analysis of formation water components, oxygen and hydrogen isotope, C-O isotope in carbonate cement, and Pr/Ph of organic matter.(9) Taking example of Jurassic gas in study area, the effectiveness of gas migration tracer indicators had been discussed fully. CH4 and N2 content were more effective indicators of natural gas migration tracing, but CO2 content and iC4/nC4 values were less effective.(10) Make sure of migration and accumulation mechanism of natural gas in study area. The gas in Upper Triassic and middle Jurassic in study area took water dissolving phase migration as the core, and separate phase migration to complement. In Luodai, Majing, Hexingchang, and Xiaoquan-Xinchang, gas with the Upper Triassic formation water flew into the middle Jurassic strata as dissolving water phase, but there was little free phase gas only in Middle Jurassic in Luodai. As same as the Middle Jurassic natural gas, Upper Jurassic natural gas flew into the upper Jurassic strata as dissolving water phase, but separate phase in Upper Jurassic in Xiaoquan-xinchang. Formation water chemical characteristics demonstrated gas migration process in the study area. Summarize the Upper Triassic and Jurassic gas accumulation pattern and main control factors... |
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