中扬子宜昌地区五峰组和龙马溪组页岩发育主控因素

The study of main factors controlling the development of Wufeng Formation and Longmaxi Formation organic-rich shales in the Yichang area,Middle Yangtze Region

中扬子地区页岩气勘探潜力巨大,但页岩发育机制研究相对滞后。选取宜昌地区临湘组、五峰组和龙马溪组25个样品开展了总有机碳(TOC)和主、微量元素测试,重点调查古生产力、沉积环境和陆源输入等指标与富有机质页岩发育的关系。研究发现:五峰组和龙马溪组下段为富有机质层段,过量硅为生物来源;微量元素中P/Ti、Ba/Al和过量钡等古生产力指标与TOC无关,这可能与P、Ba沉积后期的损耗有关;过量铜、过量镍和过量锌构建的古生产力替代系数与TOC呈正相关性,表明高生产力与富有机质页岩的发育密不可分,这与前人的认识有很大不同;氧化还原敏感元素U、V和Mo在五峰组和龙马溪组下段相对富集,且V、Mo与TOC有良好正相关性,表明还原沉积环境是富有机质页岩发育的必要条件;陆源输入指标Al_2O_3在龙马溪组上段最高,且TOC与Al、Ti和Th呈负相关关系,表明陆源输入增加对有机质具有稀释效应。综上所述,高古生产力、还原沉积环境和较低的陆源输入有利于海相富有机质页岩的发育。

Although the huge potential of shale gas in the Middle Yangtze Region,mechanism of the development of organic-rich shales are not clear.In this study,total organic carbon,major and trace elements analysis were conducted on 25 samples,in order to investigate the relationships between paleoproductivity,sedimentary environment,terrigenous flux and the development of shale.The data show that the shales from Wufeng Formation and Lower Longmaxi Formation have high TOC values and high excess-Si content,which indicates that excess-Si is in biogenic origin.There is no positive relationship between conventional paleoproductivity proxies（P/Ti,Ba/Al,Baxs）and TOC,which is probably attributed to the depletion of those elements in the late diagenesis phase.However,paleoproductivity coefficient composed by excessCu,excess-Ni and excess-Zn correlates with TOC well,which illustrates that the development of organicrich shale has a close relationship with high paleoproductivity.Moreover,redox sensitive elements（U,V,Mo）are more enriched in Wufeng Formation and Lower Longmaxi Formation than Upper Longmaxi Formation,while positive correlations between V,Mo and TOC occurs.These phenomena state that reductive environment is necessary to preserve organic matter.Terrigenous flux proxies（Al2O3）represented that the strongest terrigenous flux in Upper Longmaxi Formation,and TOC shows negative relationship with Al,Ti and Th,which illustrates that the strengthened terrigenous flux diluted organic matter.In conclusion,high paleoproductivity and reductive sedimentary environment and low terrigenous flux are beneficial to the development of marine organic-rich shale.