Relationship Between Characteristics Of Abnormal Pressure And Hydrocarbon Distribution In Paleogene Formation Of The Baxian Depression

Based on geophysical,geochemical and geological data,we have analyzed the pressure distribution,origin of overpressure,pressure evolution and characteristics of hydrocarbon reservoirs under different pressure conditions in Paleogene of Baxianin depression,as a result of which we have confirmed the relationship between pressure and hydrocarbon distribution of hydrocarbon-enriched depression.The study shows that the pressure of formations are mainly overpressure.Group three and group four of the Shahejie formation are dominated by overpressure.The pressure center of each formation is mainly distributed at the deep zone of depression and corresponds to the sedimentary center.There are three types of pressure structures in the depression,such as single overpressured streture,double overpressured structure,and multi-overpressured streture,among which the double overpressured structure is the most widely distributed.It turns out that the seal bed is located between group two of the Shahejie formation and top of group three of the Shahejie formation.Thick gypsum or mudstone sealing together with nearby reservoir densification prevent the pressure in bottom overpressured system from dissipating.There are two kinds of origin for overpressure generation in our survey,the one is undercompaction which is the main origin for shallow formations,and the another one is hydrocarbon generation which is the main origin for deep formations.The high content of mudstone and the rapid burial rate will cause the formation to enter the fluid constrain depth at the early stage.Under the condition of constant lithology in section,the burial rate can not influence the final overpressure of undercompaction.Through paleo pressure recovery of fluid inclusions and numerical simulation based on Petromod,we have studied the pressure evolution.It turns out that pressure in the survey has experienced four stages: the primitive accumulation of overpressure,the release of overpressure,the slow accumulation of overpressure,the rapid accumulation of overpressure,which is the result of comprehensive effect between pressure origin evolution and tectonism.The characteristics of hydrocarbon distribution is different under different pressure conditions,which shows that the deep formations are mainly overpressured with lower density and lower sulfur content.In these areas,hydrocarbon distribution is constrained by seal bed and influenced by overpressured source rocks.Hydrocarbon generated from these areas tend to accumulate in lower buried hill reservoir or self-storage reservoir or areas below sealed beds.Under this condition,overpressure developed in formation tends to be the driven force of hydrocarbon migration as well as effective cover bed.The shallow formations are dominated by weak overpressure and normal pressure with higher density and higher content of sulfur.In these areas,hydrocarbon distribution is controlled by source rock and transporting conditions.Near the source rocks and being equipped with good transporting conditions can both lead to the enrichment of hydrocarbon.Because huge amount of hydrocarbon can potentially migrate along the faults into shallow anticline which forms the multi-layer distributed complex petroleum accumulation zones.And in the shallow areas,overpressure is the main resistance or sealing for vertical hydrocarbon migration.