重庆南川地区五峰组—龙马溪组黑色页岩沉积环境与有机质富集关系

Relationship between sedimentary environment and organic matter accumulation in the black shale of Wufeng-Longmaxi Formations in Nanchuan area,Chongqing

利用有机碳、硫以及主量元素和微量元素等地球化学方法,分析了重庆东南部南川县三泉剖面的上奥陶统五峰组黑色页岩、观音桥段泥灰岩、下志留统龙马溪组黑色页岩的沉积环境与有机质的富集关系。结果表明,五峰组中上段和龙马溪组下段为有机质富集层段,Ba、P显示晚奥陶世—早志留世四川盆地内具有较高的生产力水平;V/Cr、Ni/Co和S/C等指示五峰组沉积时为缺氧还原条件,冰期观音桥段为富氧—贫氧环境,龙马溪组为厌氧硫化环境;Mo/TOC表明五峰组页岩受局限盆地控制,观音桥段和龙马溪组为中等—弱滞留环境;Ce异常揭示了冰期前后古海平面的变化对海底氧化还原条件造成的影响;TOC与Ba、Ti/Al的弱相关性表明初级生产力水平与陆源碎屑对五峰组—龙马溪组黑色页岩有机质的富集影响较小;而与V/Cr、Ni/Co的正相关性则表明缺氧还原的沉积环境是影响五峰组—龙马溪组黑色页岩有机质富集的主要因素。

The enrichment of organic matter in black shale is a controversial issue among the researchers and critical problem in the exploration of shale gas.In order to discuss the relationship between sedimentary environment and organic matter accumulation in black shale of Wufeng Formation,Longmaxi Formation and limestone of Guanyinqiao Bed,multiple geochemical method,such as organic carbon,sulfur contents,major and trace elements,are tested from the Sanquan section in the southeast of Chongqing.Research results show that the organic carbon is enriched in the middle to upper part of Wufeng Formation and the bottom of Longmaxi Formation.Ba and P contents exhibit a high primary productivity during Late Ordovician-Early Silurian in Sichuan Basin.Redox proxies(V/Cr,Ni/Co and S/C)indicate that Wufeng Formation deposited in an anoxic environment and Longmaxi Formation deposited in euxinic condition,but Guanyinqiao Bed was predominated by oxic-dysoxic environment during the glaciation.Mo/TOC shows a strong restricted water mass controlled the accumulation of Wufeng Formation and a moderate-weak restriction of Longmaxi Formation and Guanyinqiao Bed,respectively.Ce anomaly reveals that the fluctuation of sealevel affected the redox condition of bottom water.The poor relation between TOCand Ba,Ti/Al suggests primary productivity and terrigenous clastics are not the major factors controlling the enrichment of organic matter,but an anoxic environment is the predominant condition causing the accumulation and preservation of organic matter suggested by apositive relation among TOCand V/Cr,Ni/Co.