THE ORIGIN OF JURASSIC OIL-SOAKED SANDSTONE IN THE PIEDMONT ZONE OF NORTH LONGMENSHAN, THE SICHUAN BASIN

The piedmont zone of the North Longmenshan is located at the fault front structural zone of buried faults of the nappe structural belt.Zhongba and Hewangchang Gasfeilds and more hydrocarbon show are discovered.The Jurassic oil-soaked sandstone is discovered to be wide distribution by field survey and well drilling.It believes that the crude oil of oil-soaked sandstone is the product which generates in the oil generation peak of Cambrian muddy source rock.It is the production by more time charging,movement and redistribution,which is after the varying degrees biodegradation and long distance movement,and under the polytectonic movement.The main criterions are following.First,the normal alkane and branched paraffin of oil-soaked sandstone disappear by strong biodegradation.Second,the carbon isotope of aromatic hydrocarbon is Closed.It is-34.41‰～-35.03‰.Light hydrocarbon normal alkane in the hydrolytic product is predominantly to compare with hydrocarbon branched paraffin.It belongs to the product of marine Ⅰ-Ⅱ₁ type kerogen.Third,the pattern of biomarker is similar to Cambrian dark Claystone.At last,the maturity which is converted by methylphenanthrene index is 0.82%～1.25%.