摘要
奥陶系赫南特阶碳同位素正漂移(HICE)事件已为人熟知,但受限于样品质量和地层对比精度,目前尚存在一些争议。本次研究对黔北习水县习科1井中的五峰组至龙马溪组底部共22个岩芯样品进行了总有机碳(TOC)和δ13Corg分析,识别出HICE正漂移事件,并将其与华南和国外其他地区的同期事件进行了对比。对华南HICE事件记录的分析表明,五峰组和龙马溪组两个页岩气有利层段与该HICE事件存在较为稳定的层位关系。五峰组页岩气有利层段的结束层位大致对应于HICE事件的起始层位,即笔石Paraorthograptus pacificus带(WF3)顶部至Metabolograptus extraordinarius带(WF4)底部;而龙马溪组页岩气有利层段的起始层位大致对应于HICE事件的结束层位,即笔石Metabolograptus persculptus带(LM1)中部至Akidograptus ascensus带(LM2)底部。在两个有利层段间夹有一段显著正漂移事件的这种模式可在化石材料较为缺乏的剖面和钻井中为有利层段的识别提供较好的参考。分析对比结果还表明,华南HICE事件的特殊性与奥陶纪末冰期期间扬子海的区域氧化还原条件波动相关。在冰盛期,扬子海处于弱还原环境,海底埋藏的有机质大量分解并返回海水中,使得海水δ13CDIC下降,这是华南扬子区HICE事件正漂幅度明显低于全球广海沉积环境的主要控制因素。
The Hirnantian isotope carbon excursion(HICE)has been well known around the world,which is largely based on samples from outcrops but rarely on fresh samples from a drillcore.The HICE event records in South China exhibit significant differences compared with those from sections in other continents,and the reason for the differences remains a debate.In this study,22 samples from the Ordovician-Silurian transition interval(from the upper Wufeng Formation to the basal Longmaxi Formation)of the Xike-1 drillcore in northern Guizhou are analyzed for total organic carbon(TOC)andδ13Corg.Theδ13Corg values show an apparently significant positive excursion,which can be well correlated with the HICE event observed in the outcrop sections in South China and other regions in the world.A comparison of the present and published data from South China indicates that theδ13Corg HICE event constantly occurs between two shale-gas-favorable layers in the Ordovician-Silurian transition,i.e.,one in the Wufeng Formation and another in the lower Longmaxi Formation.The upper boundary of the first shale-gas-favorable interval is coincident with the start of the HICE event,which lies from the topmost Paraorthograptus pacificus Biozone(WF3)to the lowermost Metabolograptus extraordinarius Biozone(WF4).The lower boundary of the second shale-gas-favorable interval coincides with the end of the HICE event,which lies from the middle Metabolograptus persculptus Biozone(LM1)to the basal Akidograptus ascensus Biozone(LM2).These patterns provide a useful reference for the recognition and stratigraphic correlation of the shale-gas-favorable layers in the Ordovician-Silurian transition in South China,especially for the outcrops or drillcores with poor biostratigraphical constraint.Comparison of the HICE records in South China and other continents has revealed a significant difference in the magnitude of the excursion,which is much less prominent in the former.We thus suggest that this is related to the end-Ordovician glaciation and the consequent regional redox fluctuation in the Yangtze Sea.During the maximum glacial stage,the Yangtze Sea was under a weak-reduction condition without dissolved sulfide,and the buried organic matter enormously degenerated,which released abundant 12C back to the seawater and led to the decrease in seawaterδ13CDIC and consequently in a lower HICE peak value.
作者
李超
武学进
樊隽轩
陈清
李关访
孙宗元
张元动
LI Chao;WU Xue-jin;FAN Jun-xuan;CHEN Qing;LI Guan-fang;SUN Zong-yuan;and ZHANG Yuan-dong(State Key Laboratory of Palaeobiology and Stratigraphy,Nanjing Institute of Geology and Palaeontology and Center for Excellence in Life and Paleoenvironment,Chinese Academy of Sciences,Nanjing 210008,China;University of Chinese Academy of Sciences,Beijing 100049,China;University of Science and Technology of China,Hefei 230026,China;School of Earth Sciences and Engineering,Nanjing University,Nanjing 210023,China;Institute of Geology and Geophysics,Chinese Academy of Sciences 100029,China;State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation,Chengdu 610059,China;Institute of Sedimentary Geology,Chengdu University of Technology,Chengdu 610059,China)
出处
《地球化学》
CAS
CSCD
北大核心
2019年第6期533-543,共11页
Geochimica
基金
国家科技重大专项(2017ZX05035002-001,2017ZX05036-001-004)
国家自然科学基金(U1562213,41725007)
中国地质调查局项目(DD20190009)
中国科学院战略性先导科技专项(B类)(XDB10010100)
关键词
赫南特阶
五峰组
龙马溪组
碳同位素正漂移
页岩气
有利层段
Hirnantian
Wufeng Formation
Longmaxi Formation
isotope carbon excursion
shale gas
favorable layer
