THE ORIGIN OF JURASSIC OIL-SOAKED SANDSTONE IN THE PIEDMONT ZONE OF NORTH LONGMENSHAN, THE SICHUAN BASIN
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摘要: 龙门山北段山前带处于推覆体构造带的断层隐伏前锋构造带,已发现中坝、河湾场气田和较多油气显示。野外调查和钻井发现侏罗系油浸砂岩广泛存在,认为油砂岩中原油是寒武系泥质烃源岩在生油高峰期生成的原油经历了不同程度的生物降解和长距离的运移后,在多期构造幕作用下,多次充注、运移、再分配的产物。主要依据为:油砂岩正构烷烃和支链烷烃均已消失,遭受强烈生物降解;芳烃碳同位素相近,为-34.41‰~-35.03‰;裂解产物轻烃正构烷烃相对支链烷烃占优势,属海相Ⅰ—Ⅱ1干酪根生成产物;生物标志化合物分布特征与寒武系黑色泥岩相似;甲基菲指数折算的成熟度为0.82%~1.25%。Abstract: 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%.
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