Carbon isotopic composition of individual light hydrocarbons evolved from pyrolysis of source rocks from Ying–Qiong Basins, China

Abstract

Light hydrocarbons are an important part of natural gas, and are commonly used in gas/source rock correlation. This study determines carbon isotopic compositions of individual light hydrocarbons generated from three potential source rocks pyrolyzed at different temperatures by using GC-IRMS technique with combined glass tube pyrolysis. The results indicate that, for the natural gases from the Ying–Qiong basins, it is difficult to further identify the origin of gas in terms of carbon isotope data alone. Light hydrocarbons from pyrolysates can be divided into two main generation stages: at the lower temperature stage, light hydrocarbons are formed directly from primary cracking of kerogen, and their carbon isotopic distributions accord with isotope kinetic fractionation; at the higher temperature stage, isotopic distributions of light hydrocarbons become more complex due to the mixing of light hydrocarbons with different generation mechanisms, e.g. secondary cracking of heavy hydrocarbons. In addition, experimental results also prove that an “accumulative” effect may be one of major factors that cause the part reversal in carbon isotope ratios among light hydrocarbon components from the Ying–Qiong basins.