Geochemical Characteristic and Significance of Elements in Lower Member of Lower Ganchaigou Formation in the Platform Area on North Margin of Qaidam Basin
In order to find out the evolution of palaeosedimentary environment in the lower member of lower Ganchaigou Formation in the platform area on the north margin of Qaidam Basin and its indicative significance for oil and gas exploration,the main trace element and carbon-oxygen isotopic data sensitive to the sedimentary environment were used to study the paleoenvironment parameters like paleoclimate,paleosalinity and redox environment of the study area.The climate index showed that the arid and warm humid climates coexisted in the sedimentary period of the study area,but the climate was warm and humid on the whole.The analysis of strontium and barium contents as well as delta~(13)C_(VPDB) and delta~(18)O_(VPDB) data showed that the sedimentary water was freshwater deposit with high salinity.The analysis of redox indexes such as vanadium,nickel,chromium and cobalt showed that there might be oxygenpoor subreduction environment and oxygen-rich environment in the study area,but only hematite reflecting oxidation environment was found in X-ray diffraction data,moreover,the depth of fossil water depth estimated by cobalt element was less than 26 m;combined with the analysis of core data,it was judged that the shallow water sedimentary system of braided river dominated by oxidation environment was mainly developed in the study area.The paleoenvironmental evolution results obtained from element geochemistry analysis showed that the dry and wet climates occurred alternately,resulting in frequent fluctuation of lake level,and the coarse-grained channel bar facies and fine-grained alluvial flat facies exhibited interbedded distribution,which created conditions for the development of favorable reservoir-caprock assemblage.The study results are of great significance for guiding oil and gas exploration in the study area.