Abstract:

Through studying the ICP- MS trace elements and organic geochemistry of the Middle Permian Wujiatun Formation mudstones in eastern Inner Mongolia, it indicates that Wujiatun Formation mainly belongs to semi-salt water sedimentary environment; the range of palaeosalnity is 16.6‰ to 28.7‰. The paleosalinity and the change of water depth are showed by the trace elements and their ratios, which are impacted by the relative change of sea- level. Palaeosalinity and relative change of sea-level have a good correlation with sedimentary facies evolution. Using the quantitative calculation palaeosalinity and the values of Sr/ Ba and B/Ga which reflect the paleosalinity and the water depth, the curve of relative change of sea-level of Wujiatun Formation was established. The Wujiatun Formation had undergone two transgression-regression cycles and the transgressions took place in the bottom and middle part of Wujiatun Formation. According to the geochemical index of V/(V+Ni) values, rare earth element Ce_anom and saturated hydrocarbon gas chromatogram, combined with sedimentary facies analysis, showed that main body of Wujiatun Formation was formed in a neritic shelf environment characterized by reductive conditions, which is conducive to the preservation of organic matter and development of hydrocarbon source rock.

Key words: Wujiatun Formation, mudstone, geochemistry, palaeosalnity, sea-level change, eastern InnerMongolia