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

Light hydrocarbons are an important part of natural gas, and are commonly u sed in gas/source rock correlation. This study determines carbon isotopic c ompositions of individual light hydrocarbons generated from three potential source rocks pyrolyzed at different temperatures by using GC-IRMS techniqu e with combined glass tube pyrolysis. The results indicate that, for the na tural gases from the Ying-Qiong basins, it is difficult to further identify the origin of gas in terms of carbon isotope data alone. Light hydrocarbon s from pyrolysates can be divided into two main generation stages: at the l ower temperature stage, light hydrocarbons are formed directly from primary cracking of kerogen, and their carbon isotopic distributions accord with i sotope kinetic fractionation; at the higher temperature stage, isotopic dis tributions of light hydrocarbons become more complex due to the mixing of l ight hydrocarbons with different generation mechanisms, e.g. secondary crac king of heavy hydrocarbons. In addition, experimental results also prove th at 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. (C) 2000 Elsevier Science Ltd. All rights reserved.