| Ordos Basin is one of the most prospecting petroliferous basins for oil and gas exploration and development in China.The exploration and development for tight sandstone gas in south-eastern Ordos Basin has become the research hotspot in current.Previous researchers have systematically investigated the sedimentary and reservoir characteristics of Shanxi formation in this area,while the research of abnormal pressure and caprock are not enough.Based on regional geological data and drilling,logging and core analysis,with the help of vitrinite reflectance,fluid inclusions and other analysis methods,this paper systematically studied present formation pressure,paleopressure and fluid potential characteristics of Shanxi formation in south-eastern Ordos Basin.Finally,gas migration,comprehensive quantitative evaluation of mustone caprock capillary seal and overpressured sealing ability in Shanxi formation has been carried out,and several main conclusions of this paper are shown as follows:1.Present formation pressure: We calculate the formation pressure of Shanxi formation in south-eastern Ordos Basin with well logging method,it turns out that normal pressure,abnormal low pressure and abnormal high pressure all exist in the study area,normal pressure and abnormal low pressure are the most.On the whole,formation pressure of Shanxi formation is relatively higher in west and lower in east;while the pressure coefficient is high er in east and lower in west.We preliminary think this is due to the erosion that east area has suffered is much more serious,so its formation depth is shallower than that of the west.2.Erosion thickness and burial history: According to the characteristics of different areas in the study area,this paper respectively adopts acoustic time extrapolation,vitrinite reflectance and stratigraphic correlation method to restore formation erosion thickness since late early Cretaceous.Total erosion thickness has increased gradually from the west to the east and is in the range of 400-2000 m.3.Paleopressure restoration: Through the analysis of the fluid inclusions,we restored the paleopressure of Shanxi formation in the study area.Paleopressure mainly distributed in 10 ?88 MPa,and the paleopressure coefficient is about 1.1 ? 1.7.Besides,restricted by present pressure and temperature,we simulated the pressure evolution of several wells in the study area,it shows that at least twice overpressure had existed in Shanxi formation.4.Main accumulation periods and fluid potential: Fluid inclusion homogenization temperature and burial history analysis showed that there are at least three phases fluid flow of Shanxi formation in the study area,and Jurassic to early Cretaceous is a large-scale gas filling period.Fluid potential had different distribution characteristics in different periods of geological history : gas potential is high in east and low in west from Triassic to middle Jurassic.Affected by the uplift of late early Cretaceous,high gas potential places gradually moved to the west.Present gas potential is high in west and low in east,which inherited the characteristics of the late Cretaceous.5.Caprock quantitative evaluation: We make comprehensive quantitative evaluation of mud caprock's capillary sealing ability in Shanxi formation by grey correlation theory and analytic hierarchy process,and conbined the overpressure sealing ability to make the distribution of different kinds of caprock in Shanxi formation clear.The Shanxi formation mud caprock is divided into four categories,high yield gas provinces are mainly distributed in the second and third type caprock. |
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