Carbon isotopes and organic matter types in the kerogens from the Wufeng Formation-Longmaxi Formation in the frontal areas of the Daba Mountains

The argument still remains current for the organic matter types in the kerogens from the Wufeng Formation-Longmaxi Formation in the frontal areas of the Daba Mountains. The present paper presents the carbon isotopic analyses of kerogens, total organic carbon contents and organic matter maturity for the samples collected from the stratigraphic sections including the Caojia section in Wanyuan, the Zhouxi section in Chengkou, the Tianba and Bailu sections in Wuxi, and the Lianghekou section in Badong, and gives the δ¹³C values from -31.1‰ to -28.2‰ for the Wufeng Formation-Longmaxi Formation. There are correlations between δ¹³C values, total organic carbon contents and maturity of organic matter. There is a negative linear correlation between the organic carbon contents and δ¹³C values while the maturity values are less than 2%, and a linear positive correlation between the maturity values and δ¹³C values while the maturity values are more than 2%. Referenced to the previous classification schemes, the organic matter types in the Wufeng Formation-Longmaxi Formation are assigned to the type I (sapropelic) and type II-1 (humosapropelic), which are graded from the type I organic matter in the Tianba section in Wuxi to the type II-1 organic matter eastwards and westwards.