Geochemical Tracking on Hydrocarbon Accumulation Periods of Ordovician in Hanilcatam Sag,Tarim Basin

The normal alkanes of crude oils of the Ordovician in Hanilcatam sag in Tarim basin are complete in distribution,with varied rising of the chromatographic baselines,widespread 25-Hopane,partial heavier and gently carbon isotopes of nC15-nC26,and gradually lighter carbon isotopes of nC27-nC30.These features indicate that the reservoirs in this area underwent at least 2-period reservoir-forming processes,the oil source of present reservoirs are dominated by late charging oil.The oil inclusions infrared spectrum analysis shows that the early charging oil is low mature oil,with m(CH2)/m(CH3) value of 4.5~7.3,straight chain carbon atom number Xstd of 18~25;the late charging oil belongs to normal mature oil,with m(CH2)/m(CH3) value of 1.9~2.8,Xstd of about 7~11.Therefore,it is concluded that the oil in Hanilcatam sag nowadays is mixed crude oil charged by normal mature oil after experiencing biodegradation of early low mature oil.