Variations of natural gas carbon isotope-type curves and their interpretation - A case study

Natural gas is dominated by low-molecular weight gaseous hydrocarbons (C1-C5) whose genetic and diagenetic information is mainly obtained from stable carbon isotope compositions. Ordos Basin is one of the largest natural gas provinces in China. By means of examining the carbon isotope compositions of the Ordos basin gases, altered patterns of the isotope-type curves due to secondary cracking, thermochemical sulphate reduction (TSR) and mixing of gases generated from different sources are recognized and discussed. A typical carbon isotope-type curve is nearly linear on the natural gas plot [Chung, H.M., Gormly, J.R., Squires, R.M., 1988. Origin of gaseous hydrocarbons in subsurface environments: theoretical considerations of carbon isotope distribution. Chemical Geology 71, 97-103]. Our results show that the isotope-type curve pattern of TSR and gas secondary cracking in coal is convex due to catalysis, while the isotope-type curve of gas secondary cracking in reservoirs is concave. The natural gas of Yulin, Suligemiao and Wushenqi gas fields is coal-derived gas; both coal-derived gas and mixed gas from oil- and gas-prone sources exist in the Ordovician reservoirs of the Jingbian gas field, depending on the borehole locations. In the Ordovician carbonate reservoirs TSR is recorded but uncommon, whereas secondary cracking in reservoirs is often observed.