Geochemical characteristics of He and CO2 from the Ordos (cratonic) and Bohaibay (rift) basins in China

According to the compositional and isotopic analyses of He, CO2 and CH4 of 95 gas samples from the Ordos Basin and 76 gas samples from the Bohaibay Basin in China, this study carried out a detailed investigation on the geochemical characteristics of helium and CO2 from the cratonic and rift basins, especially the cratonic Ordos Basin and rift Bohaibai Basin in China. Compared to the rift Bohaibay Basin, the cratonic Ordos Basin has lower level of helium and CO2, lower R/Ra ratio and lower δ13CCO2 maximum value. When the content of CO2 is <10%, both cratonic and rift basins have organic CO2 associated with the hydrocarbon generation process and inorganic CO2 from the thermal decomposition of carbonates or dissolution of organic acid. Rift basin can also have CO2>20% and δ13CCO2 of −6±2‰ or −4‰–−9‰, which is of volcanic-magmatic or mantle-derived origin, while cratonic basin has no such type of CO2. The cratonic basins (Ordos, Sichuan) have CO2<5% and relatively smaller variation range of δ13CCO2, i.e., 25.6‰ (−0.8‰–−26.4‰). The rift basins have higher level of CO2, R/Ra ratios and δ13CCO2 values. The content of CO2 can reach 100%, the R/Ra ratio can be up to 6.45 and the variation range of δ13CCO2 is 34‰ (7‰–−27‰). Bigger variation range of δ13CCO2 implies stronger tectonic activities, which is not favorable for the gas accumulation. While smaller variation range means more stable structure, which is favorable for the discovery of large gas fields. Sample in the cratonic basin in China generally has CH4/3He of 109–1012 and R/Ra<0.1 and in the rift basin it has CH4/3He of 105–1013 and R/Ra of 0.1–6.45. It is concluded that methane with CH4/3He≤108 is of inorganic origin and methane with CH4/3He≥1011 is of organic origin. The regression equation for the CH4/3He-R/Ra in the cratonic basin is: y=3E+11e−46.28x and it is: y=4E+10e−1.791x in the rift basin. Both Ordos and Bohaibay basins found gas wells with He>0.05%, which has significant commercial values and provides a clue for the exploration of helium-rich gas fields. Traps with R/Ra>2 is a favorable clue for the discovery of CO2 gas fields. Seven CO2 gas fields have been found in the Bohaibay Basin, but no systematic development has been carried out and only some liquid CO2 has been injected for tight oil from the Huagou CO2 gas field.