Study On Formation Pressure Evolution In Sikeshu Sag,Junggar Basin

The Jurassic reservoir in the Sikeshu sag in the southern margin of the Junggar Basin has great potential for oil and gas exploration,and the oil layer is deeply buried and overpressured.The Sikeshu sag has a low degree of exploration and little drilling data.At present,there are few studies on the formation mechanism of abnormal pressure and pressure prediction methods in the study area.Combining the geological data of the study area,this paper obtains the present pressure value by means of computational simulation,and analyzes the present formation pressure characteristics;according to the logging response characteristics and the Bowers chart,the overpressure mechanism is analyzed,and the numerical model is used to quantitatively calculate the contribution of different overpressure causes.;And the evolution process of the main formation pressure in the Sikeshu sag was recovered by applying the differential stress method,the inclusion thermodynamic simulation method and the basin simulation method.The results show that the current formation pressure in the Sikeshu sag generally has a certain degree of overpressure,which can be divided into three pressure systems vertically.The Neogene Dushanzi Formation and the formation above are under normal hydrostatic pressure.The Neogene Taxihe Formation to the Paleogene Ziniquanzi Formation are weak overpressure systems with a maximum pressure coefficient of 1.6-1.7;the Cretaceous strata are strongly over-pressured systems with a pressure coefficient of 2.0-2.2.The overpressure in the Sikeshu sag is mainly caused by unbalanced compaction and tectonic compression.In the deep sag in the east,the contribution rate of unbalanced compaction to overpressure can reach up to 80%.In the piedmont in the south of the sag,the contribution rate of tectonic compression to overpressure can reach up to 60%.The overpressure in the eastern part of the sag started at the early stage of N1 t deposition,the overpressure in the central part started at the early stage of N2 d deposition,and there was basically no overpressure development in the western part.Under the joint control of structure and deposition,from the initial stage of N1 t deposition to the N2 d deposition stage,the formation overpressure intensity increased rapidly.