Carbon isotope characteristics of the Middle-Upper Cambrian Xixiangchi Group and bottom boundary marks of Furongian Series in the Sichuan Basin and its adjacent areas

The Middle-Upper Cambrian Xixiangchi Group in the Sichuan Basin is mainly a set of dolomites of platform facies. In the past, there was no effective way to determine the bottom boundary mark of the Upper Cambrian strata, so natural gas exploration of this horizon lagged behind. In this paper, it is revealed from the C and O isotopic profile of debris that in the middle of the Xixiangchi Group, there is a positive excursion event of carbon isotopes that can be correlated all over the world, so it can be used as an important mark to identify the Furongian Series (equivalent to the Upper Cambrian strata). Then, based on the characteristics of electric curves, the Furongian Series boundary mark was determined and its classification program was worked out preliminarily. It is shown that during the deposition of the Xixiangchi Group, it is a paleo-geomorphologically gentle-slope carbonate platform which is high in the east and low in the east, and a thin-moderate dolomite layer which is thin in the west and thick in the east is deposited. This layer is vertically divided into 4 intervals. Besides, the bottom boundary mark of the Furongian Series in the Sichuan Basin and its adjacent areas can be determined by combining the SPICE (Steptoean Positive Carbon Isotope Excursion) event of carbon isotopes with the characteristics of electric curves. Finally, the carbon isotope mark of the Furongian Series bottom boundary is the SPICE event corresponding to the positive excursion which is regionally correlated, the corresponding electrical mark is the GR small peak areas in the middle of the Xixiangchi group, and the corresponding lithologic symbol is the argillaceous dolomite. The research results are conducive to the further subdivision of the Xixiangchi Group in the Sichuan Basin and its adjacent areas and promoting the study on sedimentary evolution and reservoir distribution laws so that gas exploration in this series of strata can be speeded up.