摘要
晚奥陶—早志留世转折期是一个重要的地质历史时期,在这一相对短暂时间内(440~453 Ma),全球板块构造形态、古气候条件、古海洋环境、生物群类型等都发生了巨大的改变。扬子地区在这一全球大背景之下沉积了五峰组和龙马溪组两套富有机质泥页岩,是页岩气勘探的重点层位。有机质丰度决定着页岩生烃能力的强弱,同时还影响着储层吸附能力及储集空间的大小,是储层含气性的决定因素。有机质富集所涉及的因素较多,过程复杂,古海洋初级生产力、古氧相及沉积速率、成岩降解等对其都有影响,其中古生产力决定有机质数量,古氧相是有机质保存的决定性条件,这两者是富有机质泥页岩主要影响因素。然而五峰组—龙马溪组纵向上有机质丰度差异较大,具有很强的非均质性。因此,为厘清五峰组—龙马溪组垂向沉积环境演化及有机质丰度差异原因,对四川盆地南缘叙永地区田林剖面上奥陶统临湘组—下志留统龙马溪组地层剖面进行实测及连续采样,并开展了岩石矿物学及详细沉积地球化学(主量、微量及稀土元素)研究。结果表明:田林剖面五峰组—龙马溪组Al/(Al+Fe+Mn)值、Ce异常、Eu异常指示非陆源硅主要为生物成因,SiO2、Si xs与CaO、TiO2含量的强负相关性表明硅质含量受陆源输入量及古水深控制;Mo/TOC值与Mo/U值指示五峰组沉积初期具有强滞留环境,而有机质富集层段主要在半滞留环境下沉积形成;斑脱岩密集层段较高古生产力表明火山活动对古生产力有提升作用;总有机碳与古氧相代替系数的高相关性表明有机质富集整体主要受古氧相控制;而古生产力代替系数整体与总有机碳相关性不高是由于生境类型的差异及陆源输入量与古生产力、古氧相的利弊关系导致对古生产力有利的沉积环境往往对应较差的还原条件。综合分析认为,五峰组—龙马溪组不同沉积阶段有机质富集模式不同。非灭绝期为低陆源输入量的深水环境产生的良好缺氧条件;第一幕灭绝期(五峰组沉积末期),成烃母质生物勃发、强“海洋雪”作用以及高成烃母质生物沉降速率致使有机质十分富集;第二幕灭绝期(龙马溪组沉积初期),高成烃母质生物沉降速率加之大量有机质在水底分解消耗氧化剂的同时,分层水体阻碍O 2补给造就的水底长期极度缺氧环境,致使该时期有机质极为富集。
Late Ordovician-Early Silurian transition period is an important geological history period.Global plate tectonics,paleoclimate conditions,paleo-ocean environment and biota types occur during this relatively short period(440-453 Ma).With these great changes,there are two sets of organic-rich shales in Wufeng Formation and Longmaxi Formation in Yangtze area.It is a key exploration layer for shale gas exploration and has received extensive attention.A lot of influencing factors on organic matter enrichment are paleo-ocean primary productivity,palaeoredox facies and sedimentation rate,diagenetic degradation,etc.However,the two sets of organic-rich shales in Wufeng Formation and Longmaxi Formation show strong heterogeneity vertically.Therefore,systematic measuring,continuous sampling and detailed petromineralogy and sedimentary geochemistry(major,trace,and rare earth elements)of Tianlin profile in Xuyong area were conducted in order to make better sense of the evolution of the vertical sedimentary environment and the causes of differences in organic matter enrichment of Wufeng-Longmaxi Formations.The results show that the ratio of Al/(Al+Fe+Mn),Ce anomaly and Eu anomaly indicate that non-terrestrial Si is in biogenic origin;the contents of SiO2 and Si xs are significantly negative correlated with the contents of Ca and TiO2,indicating that silica contents are mainly affected by terrigenous flux and paleo-water depth;the ratios of Mo/TOC and Mo/U indicate that the early period of Wufeng Formation has a strong restricted environment,while the organic-rich intervals form in moderately restricted environment;the intervals with dense bentonite layers have higher paleoproductivity;stronger positive correlation between paleoredox proxy coefficient and TOC indicates that paleoredox is the main controlling factor of organic matter enrichment;due to the different biohabitat types and the different influences of terrigenous flux on paleoproductivity and paleoredox,the correlation between paleoproductivity proxy coefficient and TOC is poor in the mass.A comprehensive analysis suggests that organic matter enrichment patterns in different sedimentary stages of Wufeng-Longmaxi Formations are different.Organic matter enrichment of Wufeng-Longmaxi Formations is mainly affected by anoxic environment,which is caused by deep water environment with low terrigenous flux during non-extinction period.It is mainly affected by high sedimentation rate of bio-precursors,which is caused by explosion of bio-precursors and“marine snow”during the first episode extinction period(corresponding to the terminal period of Wufeng Formation),and is affected by high sedimentation rate of bio-precursors and extremely anoxic environment caused by rapid transgression and organic decomposition during the second extinction period(corresponding to the early period of Longmaxi Formation).
作者
郑宇龙
牟传龙
王秀平
ZHENG Yu-long;MOU Chuan-long;WANG Xiu-ping(College of Earth Science and Engineering,Shandong University of Science and Technology,Qingdao 266590,Shandong,China;Key Laboratory of Sedimentary Basin and Oil and Gas Resources of Ministry of Land and Resources,Chengdu Center,China Geological Survey,Chengdu 610081,Sichuan,China)
出处
《地球科学与环境学报》
CAS
北大核心
2019年第5期541-560,共20页
Journal of Earth Sciences and Environment
基金
中国地质调查局地质调查项目(DD20160019-25)
鄂西地区重点层系页岩气评价及有利区优选项目(HQYYQ-GC1075)
关键词
沉积地球化学
五峰组
龙马溪组
古生产力
古氧相
灭绝事件
有机质富集
四川盆地
sedimentary geochemistry
Wufeng Formation
Longmaxi Formation
paleoproductivity
paleoredox
extinction event
organic matter enrichment
Sichuan Basin
