上扬子地台上奥陶统五峰组页岩有机质聚集主控因素

选择上扬子地台东北部SY井上奥陶统五峰组为研究对象,对25个页岩岩芯样品开展了有机碳含量、矿物组分、主量和微量元素测试,重点分析了五峰组主、微量元素的变化特征及其所反映的沉积环境,讨论了五峰组页岩有机质聚集的主控因素。结果表明:根据有机碳含量的分布特征,晚凯迪阶—赫南特阶五峰组页岩可以划分为3个层段(WF1,WF2,WF3),WF2具有最高的有机质丰度,WF1和WF3相当。氧化还原敏感微量元素Mo和U的富集特征指示WF1和WF3沉积于氧化-贫氧环境,WF2沉积于贫氧-缺氧的底水环境,沉积水体的还原性先增强而后有所减弱。五峰组3个层段均具有较高的Ba和生物成因硅含量,指示五峰组沉积时水体表面始终维持较高的古生产力,较高的Cd/Mo比值指示五峰组沉积时具有的高古生产力可能受控于上升洋流的存在。Al和Ti含量总体变化不大,反映五峰组沉积时海平面总体波动不大。WF1有机质含量与Mo和U的富集系数存在相似的变化趋势且与Al和Ti具有正相关性,认为有利的氧化还原条件是WF1有机质聚集的主要控制因素,而WF2有机质含量与生物成因硅存在明显的正相关性,认为较高的古生产力导致WF2有机质富集,WF3有机质含量与Mo和U的富集和生物成因硅均存在一定的相关关系,反映古生产力与氧化还原环境共同控制着WF3有机质的聚集。

Factors controlling organic matter accumulation in the Upper Ordovician Wufeng shale from upper Yangtze Platform

A series of experiments,including total organic carbon(TOC),mineral compositions,major and trace elements test,were performed on twenty-five shale samples in the Upper Ordovician Wufeng Formation from upper Yangtze Platform.This study analyzed the characterizations of major and trace elements distribution and their implications for sedimentary environment,and the factors controlling organic matter accumulation were further discussed.The results showed that late Katian through Hirnantian Wufeng shale could be divided into three intervals(named WF1,WF2,and WF3)according to TOC content.WF1 and WF3 contained similar TOC content,whereas WF2 contained the most abundant organic matter(OM).Geochemical redox proxies of Mo and U indicated that the redox conditions were dominated by oxic to suboxic when WF1 and WF3 deposited and the depositional environment for WF2 were mainly suboxic to anoxic.The reducibility of bottom water gradually increased from WF1 to WF2 and then decreased from WF2 to WF3.All the three intervals were characterized by high content of Ba and biogenic Si,indicating a high paleoproductivity,which might be controlled by the upwelling evidenced by the higher ratio of Cd/Mo.The concentration of Al and Ti were relatively stable through the Wufeng Formation,indicating a period characterized by stable sea-level fluctuations.The TOC content of WF1 had positive relationships with EFMo and EFU,whereas the TOC content had negative relationships with Al and Ti,indicating that the favorable redox conditions made WF1 OM preserved.The TOC content of WF2 had positive relationships with biogenic Si,indicating that the high paleoproductivity made WF1 OM accumulate.Both the paleoproductivity and redox conditions had effect on WF3 OM accumulation,because both EFMo and EFU and biogenic Si content had relationships with the TOC content in WF3.