Accumulation Mechanism Of Overpressured Large Gas Field In Kelasu Tectonic Zone Of Kuqa Depression

On the basis of a lot of techniques and methods of oil and gas geology, oil and gas geochemistry, the composition, structure of organic matter and the characteristics of pyrolytic gas of the Triassic-Jurassic coal measure source rocks in the Kuqa Depression of Tarim basin was systematically studied in this dissertation. The geochemical characteristics and origins of natural gas, features of fluid inclusions in the reservoir and accumulation periods of overpressured large gas fields were detailedly analyzed. The causes of the overpressure of the reservoir and their influences of natural gas accumulation were also discussed. Combined with the anatomy? of the Kela-2, Dabei-1and Keshen-2three typical large gas fields, the characteristics and process of natural gas accumulation in the overpressured large gas fields were also comprehensively studied. There are rich aromatic structures and have a certain quantity of short fat side chains in the Triassic-Jurassic coal measure source rocks in Kuqa Depression. The maceral compositions in these coal measure source rocks are predominated by vitrinites and have strong gas generation ability. The natural gases of Kelasu tectonic zone in kuqa Depression obviously belong to dry gases. Their δ13C1values were ranged from-33.1‰to-25.1‰, and theδ13C2values were from-28.1‰to-16.8‰. These gases belong to high-over mature coal-formed gases, mainly generated from the middle-lower Jurassic and upper-middle Triassic source rocks. Based on the analyses of the homogenization temperature and salinity of the fluid inclusions, Quantitative Grain Fluorescence (QGF) and Quantitative Grain Fluorescence Extract (QGF-E), it is concluded that the overpressured large gas fields of Kela-2, Dabei-1and Keshen-2, are late migrated and accumulated.Natural gas accumulation time of deep zones is later than that of kelasu anticline zones, and the late gas charging has a strong gas washing on the early oil filling. Finally, the main controlling factors and the accumulation model of the overpressured large gas fields in Kelasu tectonic zone were put forward.