OIL CHARGING ORIENTION AND ACCUMULATION CHARACTERISTICS OF OIL RESERVOIRS IN THE FUSHAN SAG, BEIBUWAN BASIN

By using reservoir geochemical approach,this paper mainly studies on the hydrocarbon migration and charging orientation of Eocene Liushagang Formation(E₂l) oil reservoirs in the Fushan Sag,Beibuwan Basin.Combined with the petroleum geological background,the reason that caused the difference between oil pools in upper(E₂l₃¹) and mid-lower parts(E₂l₃²—E₂l₃³)of Liushagang Formation has also been discussed.It shows that the oils in E₂l₃¹ horizon in Huachang oil and gas field are mainly sourced from the Bailian sub-Sag.The unconformity between E₂l₂ and E₂l₃,the sandstone of fan delta front and the faults are good carrier beds for these oils.Comparison with the E₂l₃¹,the oils in E₂l₃²-E₂l₃³ horizons have relatively higher density,lower maturity,high content of CO₂ and lower oil and gas yields.All these characteristics are determined by the special fault systems and specific geological background.The oils in E₂l₃²-E₂l₃³ horizons are mainly derived from the source rock of the third member of the Liushagang Formation in Huachang area,which have relatively lower thermal maturity and limited hydrocarbon generation potential.Controlled by the giant faults that cut through the E₂l₃ stratum and terminated in the basement,the volcanic-mantle derived CO₂ can migrate through these faults and charge in E₂l₃²-E₂l₃³ traps.The geochemical research provides direct evidence for hydrocarbon charging orientation and pathways.