Relationship Between Characteristics Of Pressure Structure And Hydrocarbon Accumulation In Qikou Sag

Formation pressure is a direct factor controlling the hydrocarbon accumulation and distribution in sedimentary basins.In this paper,the geological,logging,and seismic data are used to study the longitudinal and lateral distribution characteristics of the formation pressure structure and the formation mechanism and evolution history of overpressure in the Qikou Sag.The relationship between fluid properties and hydrocarbon distribution and pressure structure is discussed.It is clarified that formation pressure controls the accumulation and distribution of the hydrocarbon.The research results show that three overpressure belts in the longitudinal direction can be identified,and then the pressure structure should be divided into hydrostatic pressure structure,single overpressure structure,double overpressure structure,triple overpressure structure.The degree of overpressure development,quantity and depth increase from the deep concave to the buried hills and slopes.Compaction and hydrocarbon generation are the main reasons for the formation of overpressure in this sag,and the transformation of clay minerals also contributes,while hydrothermal pressure and tectonic effects are not the main cause mechanism.The overpressure in the sag begin to accumulate after the Minghuazhen Formation starts to deposit and a few can be advanced to the deposition period of the Guantao Formation.And the overpressure development time in the deep concave area is the earliest.The total mineralization degree,total salinity,chloride ion content and crude oil density,viscosity,wax content and chemical composition of crude oil in oilfields are significantly different in different pressure zones due to the presence of the sealant layer.It has been found that the hydrocarbon are mainly distributed in relatively low pressure locations such as buried hills,slopes,hydrostatic pressure zones,transition zones,and etc.,which are close to the main concave of the sag.The paper analyzes and establishes the typical accumulation model of the Qibei Slope in the source area and out of the source area,and then points out that the Banqiao-Beidagang and Chengbei fault zones are favorable zones for exploration.