Element Geochemical Characteristics and Their Geological Significance of Wufeng-Longmaxi Formation Shales in North Margin of the Central Guizhou Uplift

In order to discuss depositional setting and its tectonic background of the Wufeng-Longmaxi Formation organic-rich shales in the northern margin of the central Guizhou uplift, the TOC content, trace elements and rare earth elements of the samples from two wells in the area were tested. The results show that the Wufeng-Longmaxi Formation shales in the study area was mainly deposited in poor oxygen-anaerobic environment, and the oxygen-rich environment also exists occasionally, and the reduction degree of sedimentary water in Longmaxi Formation is high than that in Wufeng Formation. The sedimentation rate reflected by the changes of La N /Yb Nvalue is as follows: the lower part of the Longmaxi Formation < the Wufeng Formation < the Guanyinqiao section, which reflects the process that the sedimentation rate of the Wufeng-Longmaxi Formation shale becomes faster at first and then slows down. The Mo/TOC values show that the water mass restriction degree of the basin is strong during the sedimentation of the Wufeng Formation shale, and relatively weakened in the Silurian, the restriction degree has little positive effect on the enrichment of organic matter, while the changes of redox conditions and productivity factors caused by sea level rise and fall control the enrichment of organic matter. The characteristics of the trace elements and rare earth elements reflect that the provenance of the Wufeng-Longmaxi Formation shales comes from the upper crust,there is mixed provenance in Wufeng Formation, and the provenance of Longmaxi Formation is relatively simple, generally speaking, the source rocks are mainly feldspathic-quartz (granite). The tectonic setting was mainly passive continental margin, due to the effect of certain hydrothermal input , it also exhibits certain characteristics of continental island arc tectonic background.