The Isotope Geochemical Response Characteristics And Significance Of Carbonate Rocks In Changxing-Feixianguan Formation Of Northeastern Sichuan Basin

The isotope geochemical response characteristics of carbon and oxygen are one of the important geochemical indicators of carbonate rocks.They have been widely used in reflecting global climate change,sea level rise and fall,diagenesis,CO₂ content changes and stratigraphic correlation.It has become an important means for people to study carbonate rocks.At the same time,major breakthroughs have been made in the exploration of marine carbonate rocks in the northeastern Sichuan Basin,especially the discovery of large gas reservoirs such as Puguang and Yuanba,which show good exploration potential.Therefore,it is of great significance to study the isotope geochemical response characteristics of carbonate rocks in this area.As far as the study area is concerned,due to the extinction event at the Permian/Triassic boundary and the ecological resurgence of the early Triassic depression,therefore,the predecessors paid close attention to the carbon isotope composition of the Early Triassic marine carbonate rocks near the Permian/Triassic boundary.However,there are few studies on the carbon and oxygen isotope composition characteristics and significance of the carbonate rocks in the Changxing-Feixianguan Formation,and the degree of discussion is weak.Aiming at a discussion on the relatively weak reality of the isotope geochemistry response characteristics and significance of carbonate rocks in Changxing-Feixianguan Formation of northeastern Sichuan Basin,based on field measurements sections,microscopic slices and other rock materials,guided by isotope stratigraphy and carbonate sedimentary,combined with the carbon and oxygen isotopic composition of the carbonate rocks of the Changxing-Feixianguan Formation,focus on the regional and global comparative analysis of the carbon and oxygen isotopic and element geochemical responses,and further explore its geological significance.In the early period of Changxing,due to large-scale transgression in the study area,the carbon isotope value showed an increasing trend.After a short-term transgression,the carbon isotope value decreased accordingly.When entering the middle and late Changxing period,the marine regression turned into transgression.The carbon isotope values increased,and the hydrodynamic conditions continued to turbulent after the transgression.At the same time,the carbon isotope values decreased continuously in the context of large-scale marine regression.During this period,the oxygen isotope composition showed a relatively stable trend.The carbon isotope undergoes strong negative drift near the boundary of the Permian/Triassic,and the oxygen isotope value decreases rapidly near the boundary of the Permian/Triassic,reaching the lowest value.After entering the Feixianguan period,the carbon isotope value entered the recovery phase of low-amplitude fluctuations,and small fluctuations occurred during the period,which decreased after reaching the maximum value in the fourth part of the Feixianguan Formation.The oxygen isotope composition was also stable during the Feixianguan period.From the perspective of the Changxing-Feixianguan period,the carbon isotope composition showed a downward trend,reflecting the continuous decrease of sea level,indicating that the carbon isotope evolution curve of the Changxing-Feixianguan Formation in the study area was significantly positively correlated with sea level change.A strong negative drift in the carbon dioxide and oxygen isotopes at the Permian/Triassic boundary may be related to the methane release and volcanic eruptions.