页岩元素地球化学特征及古环境意义——以渝东南地区五峰—龙马溪组为例

Elements geochemistry and paleo sedimentary significance:A case study of the Wufeng-Longmaxi shale in southeast Chongqing

为了深入研究渝东南地区奥陶纪—志留纪黑色页岩垂向上沉积环境的精细变化,选取渝东南地区黔江1井对五峰—龙马溪组进行了系统采样并进行了详细的元素地球化学研究,认为LM1-LM3时期古海平面最高、古生产力最高和水体还原性最强,页岩的有机质含量最高,是页岩气勘探重点区域;页岩的古生产力、氧化还原条件等地球化学元素决定的指标仅影响富有机质页岩的厚度及品质,对页岩气产量有一定影响,但并非主要影响因素.研究结果表明:1)黔江地区五峰—龙马溪组页岩沉积于大陆边缘环境,陆源碎屑为主要物源,沉积成岩过程中局部受到热液活动和生物作用的影响;2)五峰—龙马溪组页岩沉积时期经历了五峰晚期(WF1—WF4顶部)海退和龙马溪组早期(LM1—WF3)海侵;3)黑色页岩总体形成于水体缺氧的环境,在五峰组(WF1—WF4)由于海退处于弱氧化环境,龙马溪组早期(LM1—LM3)由于海侵处于强还原条件,而在龙马溪组晚期由于水体变浅,水体还原性逐渐减弱;4)古生产力在龙马溪组早期(LM1—LM3)达到最大值,生物钡Ba(bio)质量分数达到1 025.43×10-6,向下和向上均有减小趋势;5)相同时段位于水下古隆起附近的黔江地区的古生产力、氧化还原条件、富有机质页岩厚度及品质均不如坳陷区的巫溪2井,水下古隆起对富有机质页岩沉积产生了不利影响,在该区进行勘探应格外谨慎.

To study the delicate changes in the vertical sediment environment of the Ordovician Wufeng and Silurian Longmaxi black shale formations,and provide guidance for the exploration and development of shale gas in southeast Chongqing,Wufeng-Longmaxi shale in well QJ1 in southeast Chongqing was selected for this study.Systematic measuring,continuous sampling and detailed element geochemistry researches were conducted.It is concluded that LM1—LM3 should be considered as the favorable targets for shale gas exploration and development in southeast Chongqing because of the highest sea level,highest paleoproductivity and highly reductive conditions during that sedimentary period.Although the paleoproductivity,the redox conditions and other geochemical elements have some effect on the shale gas production,they decisively affect the organic richness and the thickness of the shale.The following results were obtained:1)Wufeng-Longmaxi shale in Qianjiang area was deposited on the passive continental margin at the southeast edge of the Yangtze platform and the clastic deposits from the continent are the major material source and some intervals are affected by hot water and biological sedimentation.2)Wufeng-Longmaxi formation underwent sea regression in the late Ordovician and sea transgression in the early Silurian.3)Generally,Wufeng-Longmaxi shale was formed in an environment with water poor in oxygen.During the formation of LM1—LM3,water bodies experienced the most intensive reduction and from both upward and downward the reduction capacity of water reduced gradually.4)Biological productivity reached the peak in LM1—LM3 with the value of Ba(bio)reaching 1 025.43×10-6.5)The paleoproductivity,redox conditions,thickness and quality of the organic-rich shale in well QJ1 are not as good as those of the well WX2,implicating that the paleouplift of Yichang has adverse effect on the shale deposition and should be paid more attention to in later research.