Paleoenvironmental characteristics of the Jiusi Formation and potential for shale gas exploration in Zhaotong shale gas demonstration zone

In order to realize the sustainable development of Zhaotong shale gas demonstration area and search for new potential target strata of shale gas,Well A targeted at the carboniferous Jiusi Formation shale gas exploration was de- ployed and implemented to explore the gas content of Jiusi Formation shale gas. Based on the systematic test of organic carbon,major content,trace amount and rare earth elements in Well A,combined with petrology,biological relic fossil characteristics and chromatography-mass spectrometry,the geochemical characteristics of elements in Jiusi Formation in Well A were analyzed,and the REDOX conditions in Jiusi sedimentary stage were identified,revealing the sedimen- tary environment for the development of black shale. The research results show that the petrological characteristics,ma- jor,trace and rare earth elements of Jiusi formation in Well A of Zhaotong shale gas demonstration area are quite different from those of Wufeng formation and Longmaxi formation. The organic carbon content of Jiusi group in Well A drill- ing is relatively high,with an average value of 1. 13% ,and the TOC content is not strongly correlated with depth. The content of Al2 O3 in the main elements of Jiusi formation is high,mainly calcareous shale and clayey shale. The REDOX indexes such as trace elements and rare earth elements indicate that the depositional stage of the Jiusi formation was a relatively oxygen-rich environment,which was mutually verified with the content of TOC. Chondrites combination fossil and shale chromatogram mass spectrum characteristics in the gray mudstone of the Jiusi formation also reveal that the Jiusi formation was developed in the paleosedimentary environment of the transition facies of sea and land,and its pal- eoproductivity was lower than that of the Wufeng formation and Longmaxi formation,and the abundance of organic mat- ter in the Jiusi formation was relatively low.