江西修武盆地早寒武世热液活动对有机质富集的影响

摘要

The Lower Cambrian is the main target intervals of marine shale gas exploration and exploitation in China.Shale gas content varies greatly in different intervals and how to determine favorable targets is an important problem to be solved urgently.Taking Well XN-1 in Xiuwu Basin as an example,according to core description,logging data analysis,mineral and elemental composition analysis and total organic carbon (TOC)content tests,the shale in Lower Cambrian Wangyinpu Formation and Guanyintang Formation was studied.Results show that water redox conditions and bioproductivity control the enrichment of sedimentary organic matter.Hydrothermal activities are helpful for the increase of reducibility of seawater and enhancing bioproductivity,and thus controls the enrichment of sedimentary organic matter and the enrichment of shale gas is further affected.The water reducibility during the deposition of SQ1 at early Cambrian varies greatly,and it is generally strong during the deposition of SQ2 and SQ3 of the Wangyinpu Formation.The reducibility of all the three sequences is stronger than that during the deposition of SQ4 and SQ5.The bioproductivity during the deposition of SQ3 is the highest and little difference in the bioproductivity has been found between SQ 1 and SQ2,both of which show higher values than that of SQ4 and SQ5.Siliceous origin of SQ1 varies greatly in the study area;most siliceous minerals of SQ2 and SQ3 are hydrothermal origin,whose content is high;content of hydrothermal origin siliceous minerals drops in SQ4 and terrigeneous siliceous content increases;siliceous minerals of SQ5 originate from normal terrigeneous clastic deposition.SQ2 and SQ3 are favorable targets of shale gas in the Lower Cambrian.%下寒武统是中国海相页岩气勘探的主要目的层,不同层段页岩含气性变化较大,如何寻找勘探目标是亟待解决的重要问题.以江西修武盆地XN1井为例,通过岩心描述、测井数据分析、矿物和元素组成分析、总有机碳含量测试等手段,对下寒武统王音铺组和观音堂组页岩进行研究.结果表明,水体下部的氧化还原环境和水体表层的生物生产力控制着有机质的富集.热液活动使水体还原性增强,生物生产力提高,从而控制有机质的富集,进一步影响页岩气的富集.早寒武世SQ1沉积时期研究区水体还原性变化大,SQ2和SQ3沉积时期水体普遍具有强还原性,均远高于SQ4和SQ5沉积时期;以SQ3沉积时期生物生产力最高,SQ2和SQ1沉积时期生物生产力相差不大,均高于SQ4和SQ5沉积时期的生物生产力.研究区SQ1沉积时期的硅质来源变化较大,SQ2和SQ3沉积时期大部分层段存在热液成因硅且含量较高,SQ4沉积时期开始出现大段的陆源碎屑沉积,也存在热液成因硅,SQ5沉积时期的硅质矿物基本来源于陆源碎屑沉积.SQ2和SQ3为研究区下寒武统页岩气有利发育层段.