川西南地区五峰组—龙马溪组沉积环境及有机质富集模式

Sedimentary Environment and Accumulation Pattern of Organic Matter in Wufeng-Longmaxi Formations in the Southwestern Sichuan,China

为了明确川西南地区五峰组—龙马溪组富有机质页岩的沉积环境演化特征及有机质主控因素,充分利用元素地球化学和有机地球化学方法,获得了YD1井五峰组—龙马溪组总有机碳(TOC)、主量和微量元素含量纵向变化特征,探讨了五峰组—龙马溪组古环境演化特征及有机质富集机制。结果表明:龙马溪组总有机碳(平均值为4.50%)整体高于五峰组(平均值为1.26%);地球化学指标V/Cr值、C_(org)/P值、Mo含量及Mo/TOC值表明,五峰组—龙马溪组沉积期,沉积环境处于中等—强的局限程度,同时海水经历了氧化—次氧化(五峰组V/Cr平均值为3.39,C_(org)/P平均值为138.16,Mo平均含量为4.28×10^(-6))向缺氧硫化(龙马溪组V/Cr平均值为6.71,C_(org)/P平均值为249.38,Mo平均含量为61.49×10^(-6))的变化过程;古生产力指标Ni含量、(Ni+Cu)/Al值、Ba_(bio)含量指示五峰组—龙马溪组均具有相对较高的生产力水平,这主要归因于同期频繁火山活动引起的丰富营养物质的输入以及上升流的发育。总有机碳与陆源输入指标、氧化-还原指标、古生产力指标相关性分析发现:五峰组和龙马溪组有机质富集主要受控于沉积环境的氧化-还原条件;氧化—次氧化的水体环境抑制了五峰组有机质的保存;强烈的缺氧条件及较高的生产力水平共同促成龙马溪组较高的有机质含量,使得龙马溪组更加有利于页岩气开发。

In order to find the evolution characteristic of sedimentary environment and main controlling factors of organic matter in Wufeng-Longmaxi Formations organic-rich shale in the southwestern Sichuan,the vertical variations of total organic carbon(TOC),and contents of major and trace elements in Wufeng-Longmaxi Formations in well YD1 were obtained by element geochemistry and organic geochemistry;and the paleo-environment evolution characteristic and organic matter accumulation mechanism of Wufeng-Longmaxi Formations were discussed.The results show that TOC of Longmaxi Formation with the average of 4.50% is higher than that of Wufeng Formation with the average of 1.26%.The geochemical indices(V/Cr,C_(org)/P,Mo content and Mo/TOC) indicate that the sedimentary environment of Wufeng-Longmaxi Formation is in a moderate to strong limitation,and the seawater undergoes from oxic-suboxic(the averages of V/Cr,C_(org)/P and Mo content of Wufeng Formation are 3.39,138.16 and 4.28×10^(-6)) to anoxic sulfide(the averages of V/Cr,C_(org)/P and Mo content of Longmaxi Formation are 6.71,249.38 and 61.49×10^(-6)).The paleoproductivity indices((Ni+Cu)/Al and contents of Ni and Ba_(bio)) indicate that Wufeng-Longmaxi Formations have relatively high productivity levels,which is mainly attributed to the input of rich nutrients caused by frequent volcanic activities and the development of upwelling in the same period.According to the correlation analysis of TOC with continental input indices,redox indices and paleoproductivity indices,it is found that the organic matter accumulation of Wufeng and Longmaxi Formations are controlled by the redox conditions;organic preservation of Wufeng Formation is suppressed by oxic-suboxic condition;intense anoxic condition and high productivity contribute to the high organic matter content,making Longmaxi Formation conducive to shale gas development.