下扬子西南部前渊带晚奥陶世—早志留世黑色页岩沉积环境与有机质富集机理——以WDD1井为例

Sedimentary Environment and Genesis of Organic Matter Enrichment of Late Ordovician-Early Silurian Black Shale in the Fore Deep Zone, the Southwestern Lower Yangtze Basin, China——A Case Study of Well WDD1

岩相古地理研究表明奥陶纪—志留纪之交下扬子地区主要发育一套前陆盆地前缘带黑色页岩,与同处扬子陆块的中上扬子地区存在明显的区域性差异。该套黑色页岩沉积环境的演变研究相对薄弱,特别是与有机质富集相关的研究缺乏。因此,搞清楚下扬子地区五峰组—高家边组页岩沉积环境和有机质富集机理,对深化下扬子地区页岩气富集规律认识具有重要意义。基于下扬子西南部最新的奥陶系—志留系页岩气地质调查井(WDD1)获取的连续地下岩芯资料,对38个岩芯样品进行了总有机碳和主量、微量元素测试,分析了主量、微量元素含量垂向上的变化特征及其与海盆古沉积环境的关系,讨论了控制下扬子西南部奥陶纪—志留纪之交页岩有机质富集的主控因素。结果表明:上奥陶统五峰组和下志留统高家边组二段为有机质富集层段,校正后的化学蚀变指数指示研究区所处的江南过渡带在奥陶纪—志留纪之交气候相对温暖湿润;生源钡、Cu、Zn含量等指标印证了五峰组—高家边组沉积期具有较高的古生产力;V/Cr、V/(V+Ni)和V/Sc值等揭示五峰组沉积期底水为厌氧—硫化状态,高家边组一段沉积期底水转化为贫氧—氧化状态,高家边组二段沉积期再次出现硫化—缺氧的底水环境;Mo-TOC图解和EF_(U)-EF_(Mo)协变图显示五峰组沉积期为中等滞留沉积环境,高家边组沉积期则转化为中等—弱滞留沉积环境;五峰组沉积期总有机碳与Al、Ti含量表现为弱的正相关关系,而高家边组沉积期则转化负相关关系。五峰组和高家边组黑色页岩有机质富集机理存在差异:五峰组黑色页岩有机质富集的主控因素为还原—硫化的半滞留环境,陆源碎屑输入带来的营养物质一定程度上促进了有机质富集;高家边组二段黑色页岩有机质富集受多种因素控制,高的古生产力和还原—硫化底水环境为有机质赋存提供了必要条件,同时低沉积速率进一步促进了有机质的富集。二者富集机理的差异揭示奥陶纪—志留纪之交下扬子地区古海洋环境发生了重大的改变。

Lithofacies paleogeography shows that a set of black shale in the foreland basin front zone is mainly developed in the Lower Yangtze region at the turn of Ordovician-Silurian,and the Lower Yangtze region is obviously different with the Middle and Upper Yangtze regions.The study on the sedimentary environment evolution of the black shale is relatively weak,especially the related study on organic matter enrichment.Therefore,it is necessary to clarify the sedimentary environment and organic matter enrichment mechanism of Wufeng Formation-Gaojiabian Formation shale in the Lower Yangtze region,which is of great significance for deepening the understanding of shale gas enrichment law.Based on the continuous underground core data obtained by the latest shale gas geological survey well(WDD1)in the southwest of the Lower Yangtze region,38 core samples were tested for organic carbon,major and trace elements.The vertical variation characteristics of major and trace elements in the profile and their relationship with the sedimentary environment of the basin were analyzed.The main controlling factors of organic matter enrichment of Ordovician-Silurian shale in the southwest of the Lower Yangtze region were discussed.The results show that Wufeng Formation and the second member of Gaojiabian Formation are enriched in organic matter,and the CIA_(corr) value indicates that the climate of Jiangnan transitional zone is relatively warm and humid at the turn of Ordovician-Silurian.Contents of Ba_(bio),Cu,Zn and other indicators confirm Wufeng-Gaojiabian deposition period has a high level of paleoproductivity;V/Cr,V/(V+Ni)and V/Sc values reveal that the bottom water of Wufeng Formation during the deposition period is an anaerobic sulfide state,and the bottom water of the first member of Gaojiabian Formation during the deposition period is converted into an oxygen-poor oxidation state.Diagram of Mo-TOC and covariation diagram of EF_(Mo)-EF_(U) show that Wufeng deposition period is medium-retention sedimentary environment,and Gaojiabian deposition period is medium-weak retention sedimentary environment.TOC shows a weak positive correlation with contents of Al and Ti in Wufeng deposition period,while it turns into a negative correlation in Gaojiabian deposition period.The enrichment mechanism of organic matter in black shale of Wufeng Formation and Gaojiabian Formation is different.The main controlling factor of organic matter enrichment in black shale of Wufeng Formation is the semi-retention environment of reduction-sulfidation,and the nutrients brought by terrigenous clastic input promote to some extent.The differences in the enrichment mechanism between Wufeng Formation and Gaojiabian Formation show that the paleo-ocean environment in the Lower Yangtze region has undergone significant changes at the turn of Ordovician-Silurian.