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

ZHANG Qin; LIANG Feng; WANG Hongyan; LEI Zhian; QI Lin

Abstract：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.