Combination of inorganic and organic geochemical characteristics to study the causes of carbon isotopic anomaly of natural gas: A case study from the central gas field of the Ordos Basin, China

Carbon isotopic anomaly occurs in the central gas field of the Ordos Basin (δ13C1 > δ13C2), and natural gas is mainly stored in the Ordovician Majiagou Formation dolomite (OMFD). Inorganic geochemical characteristics of the OMFD, including oxygen, carbon, strontium isotopes, order degree, and rare earth elements (REEs) were measured. Organic geochemical parameters, including natural gas and source rock, were also investigated. The causes of the carbon isotopic anomaly were studied based on the combination of inorganic and organic geochemical characteristics. The δ18O of the OMFD ranges from −11.9 to −5.8‰ with an average −8.4‰. Some δ18O values are more depleted than the average value of the Ordovician seawater, indicating that the dolomite is high-temperature burial dolomite, which is consistent with the fluid inclusion. The 87Sr/86Sr varies from 0.7088 to 0.7369, which is higher than the contemporaneous seawater. This suggests that the OMFD is high-temperature burial dolomite. The order degree changes from 0.72 to 0.98 with an average value of 0.89, which also indicates that the OMFD may have formed in the burial diagenesis stage at higher temperature. The REEs distribution patterns illustrate that the OMFD was influenced by the high-temperature hydrothermal alteration in the later period. The δ13C of some OMFD are more depleted than the Ordovician seawater, indicating that there was organic carbon mixing in the process of dolomitization. Inorganic and organic geochemical characteristics confirm that carbon isotopic anomaly of natural gas is the result of high temperature and mixing of the OMFD organic carbon.