QUANTITATIVE PREDICATION AND SIGNIFICANCE OF MIXED OILS FOR LITHOLOGIC OIL POOL IN THE DONGYING DEPRESSION OF THE BOHAI BAY BASIN

Normal geochemical approaches were utilized in quantitative predication of the proportion of the mixed oils derived from Third(E_s3)and Fourth(E_s4) Member of Paleogene Shahejie Formation and accumulated in middle interval of the E_s3(E_s3^M) in the Dongying Depression of Bohai Bay Basin,where subtle traps are primarily of lenticular basin-floor turbidite sands encompassed in mudstones mostly developed in the depression.The result shows that about 28%~85% of the mixed oils were derived from the E_s4 interval with averaged level up to 58.5%~67.7% in the Niuzhuang Sag,and about 9%~81% were E_s4 derived oils in the Central Uplift of the depression(the average is up to 32.4%~53.6%).It was observed that there are much more E_s4 derived oils located nearby where deep faults developed.The fact that high concentration of the E_s4 derived oils with long migration distances discovered in the E_s3^M subtle lithological traps in the Niuzhuang Sag suggests that surrounding source rocks might be not necessary having a full controlling on hydrocarbons supply of a subtle trap,and subtle migration pathway should play an important role in oil migration.The traditional opinion that hydrocarbons in situ accumulation in lenticular traps with short migration distance from surrounding source rocks should be revised.It is suggested that there is great petroleum potential in the depo-center of rifted basins in Eastern China,and the Niuzhuang Sag is capable to supply normal E_s4 derived oils for the pools in the South Slope,which has been proved by the high proportion of the E_s4 derived oils mixed in the oils of the Wangjiagan Oilfield.The petroleum potential of the E_s4 member in the Dongying Depression should be reevaluated.