川东北地区陡山沱组页岩地球化学特征与古环境分析

Geochemical characteristics and paleoceanic environment analysis of black shale in Doushantuo Formation in northeastern Sichuan Basin

下震旦统陡山沱组页岩是四川盆地震旦系成藏的主要烃源岩,是近年来海相古老地层天然气勘探的重点层位。根据川东北城口地区白包溪、柿坪两个剖面的46件样品的有机地球化学(总有机碳、岩石热解、干酪根碳同位素)和元素地球化学(主微量元素、稀土元素)特征,分析了陡二段和陡四段烃源岩的地球化学特征与沉积环境的变化情况。结果表明,该地区陡山沱组页岩有机质丰度高,总有机碳(TOC)含量在0.19%~20.38%(均值4.76%),有机质类型为Ⅰ型干酪根,成熟度已达到过成熟;黑色页岩较高的δEu异常值与异常富集V、Mn、Mo、Ba和U微量元素,揭示了该时期强烈的热液作用;稀土元素配分模式与氧化还原指标识别出川东北地区陡二段沉积时期水体在缺氧-贫氧-硫化的水体条件下反复波动,陡四段水体环境则由贫氧演变为缺氧-硫化环境,总体盆地环境水体连通性较强,沉积速率较高,上升流携带热液来源的营养元素造成了生产力的繁盛,促进了有机质页岩的形成。

The shale in Doushantuo Formation is the main source rock of the Ediacaran in Sichuan Basin and the major hydrocarbon source layer of the ancient marine strata for natural gas exploration in recent years.Forty-six samples were collected from two outcrops(Baibaoxi and Shiping)in Chengkou county in northeastern Sichuan Basin.We analyzed the change of the sedimentary environment of the MemberⅡandⅣof Doushantuo Formation in terms of their organic geochemical characteristics(total organic carbon,rock pyrolysis,kerogen carbon isotopes)and element geochemical characteristics(major elements,trace elements and rare earth elements).Results show that the shale of Doushantuo Formation in northeastern Sichuan Basin exhibit a high abundance of organic matter,with TOC content ranging from 0.19%to 20.38%(mean value is 4.76%).The dominant type of organic matter is typeⅠkerogen,and it has reached over-maturity stage.HighδEu anomalies and abnormally richness in V,Mn,Mo,Ba and U elements of the black shale revealed strong hydrothermal action during this period.The distribution pattern of rare earth elements and various redox indexes indicated that the redox fluctuates repeatedly under the conditions of anaerobic-anoxic-sulfidic environment during the depositional period of the MemberⅡ.The water environment of MemberⅣchanged from anoxic to anaerobic-sulfidic environment.In general,the water connectivity of the basin is strong,the depositional rate is relative high.The nutrients carried by upwelling from hydrothermal sources proliferate productivity,which promotes the formation of organic shale.