A Study On The Source Rocks Thermal Evolution Characteristic In The Zifusi Sag

Zifusi sag is located in the southeast of Jiangling depression which is one of the beneficial hydrocarbon-generation sag in Jiangling depression.The research areas include the Zfusi sag of the east Jiangling depression which is west to the Tianezhou synclinal-huduhe and is east to the Qingshuihe fault and is north to Jingxi oil field-north of Shashi oil field and is south to Gongan fault.On the basic of collection the previous sedimentary structure and hydrocarbon source rocks evaluation findings of the research area, we originally develop the comprehensive research of restoration of thermal history in palaeotemperature scale model and parameter settings by the sedimentary basin thermal history simulation system which is based on palaeotemperature scale and the geochemistry data of source rock and combined with the analysis of the well drilling, seismic and logging data and combined with evolution model and restoration of structural evolution of basin. It objectively recover the geothermal temperature feature and the thermal history of the source rock in the original basin. Further more,it provides foundations of evaluation of oil and gas exploration according to evaluation of source rocks and hydrocarbon distribution.Based on research of the predecessors, the area performances the sunken alternate with bulge in northwest and unfold successively as Gongan monoclinal zone, Zifusi sag, Jingzhou anticline, Qingshuihe synclinal which is from southwest to northeast. After the formation of basement from the late Jurassic-the early Cretaceous, it includes the first fault depression period in late cretaceous-Eocene Xingouzui group, the second intensely fault depression period from Jingsha group to Oligocene Jinghezhen group and the third fault depression period from Neogene-Quaternary.Developed from the belt of middle Yangtze platform, Jianghan basin is a fault depression salt lake basin whose basement is hot basin which experienced several fault depression and constantly cooled during the Chalk. The last Xingouzui group heat flow data is55-60mW/m2.Due to the volcanic rocks invasion, the heat flow data of the Jinjiachang structure which is much higher up to70mW/m2distributes much higher when it is much closer to the center of the depression. The heat flow data began to degree from early Himalayan period. During the middle of Jinghezhen period, it is down to51-66mW/m2which distributes much higher when it is much closer to the center of the depression. Effected by the Himalayan movement which lead to formation fiercely uplift during the late tertiary period, the heat flow data descent greatly and hardly changed in the plane which is down to38-40mW/m2nowadays.As the same with the distribution of the heat flow data, maturity evolutional degree of source rock in Zifusi sag is higher and higher when it from the anticline to the center of depression. According to the research of source rock maturation history, it is in accordance with burial history of source rock in Zifusi sag,so well as the hydrocarbon generating and expulsion types. When the formation continued sinking during the Shashi-Xingouzui group, the maturity evaluated rapidly which got into hydrocarbon generation threshold. During the late Xingouzui-Jingsha group, the formation rised modestly and then accelerated sinked. The depression had reached moisture-dry gas phase, at the same time, anticline and broken slope belt had get into early maturity stages until the formation uplift ablation during the middle of Jinghezhen group. There are four stages of organic evaluation, such as immature, early mature, mature and high mature. The evaluational degree of source rock is much higher from the anticline to the center of depression. The hydrocarbon generation stage of source rock in the upper segment of Shashi group and the lower segment of Xingouzui group is single-peak whose hydrocarbon generation peak mostly are in40-30Ma(Qianjiang-Jinghezhen chron).According to the hydrocarbon generation model, the hydrocarbon generation amount of the lower segment Xingouzui in Zigusi sag is the most5.78×108t. The second oil group of lower sement Xingouzui group is3.57x10t. The hydrocarbon generation amount of Shashi group is1.37×108t. The hydrocarbon expulsion of second oil group of he lower segment Xingouzui is1.06×108t and Shashi group is0.415×108t. There are three hydrocarbon generation areas, such as the first distributes in the second oil group of lower sement Xingouzui which is in the center of Zifusi sag; the second is outside the fomer which distributes at the third oil group and the Shashi group in Jingsha anticline area; the third is at the south of the sag which distributes at Fanjiatai structure and Zifusi sag.