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, CO 2 and CH 4 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 CO 2 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 CO 2 , lower R/Ra ratio and lower δ 13 C CO2 maximum value. When the content of CO 2 is < 10%, both cratonic and rift basins have organic CO 2 associated with the hydrocarbon generation process and inorganic CO 2 from the thermal decomposition of carbonates or dissolution of organic acid. Rift basin can also have CO 2 > 20% and δ 13 C CO2 of − 6 ± 2‰ or − 4‰ –− 9‰, which is of volcanic-magmatic or mantle-derived origin, while cratonic basin has no such type of CO 2 . The cratonic basins (Ordos, Sichuan) have CO 2 < 5% and relatively smaller variation range of δ 13 C CO2 , i.e., 25.6‰ (− 0.8‰ –− 26.4‰). The rift basins have higher level of CO 2 , R/Ra ratios and δ 13 C CO2 values. The content of CO 2 can reach 100%, the R/Ra ratio can be up to 6.45 and the variation range of δ 13 C CO2 is 34‰ (7‰ –− 27‰). Bigger variation range of δ 13 C CO2 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 CH 4 / 3 He of 10 9 – 10 12 and R/Ra < 0.1 and in the rift basin it has CH 4 / 3 He of 10 5 – 10 13 and R/Ra of 0.1– 6.45. It is concluded that methane with CH 4 / 3 He ≤ 10 8 is of inorganic origin and methane with CH 4 / 3 He ≥ 10 11 is of organic origin. The regression equation for the CH 4 / 3 He-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 CO 2 gas fields. Seven CO 2 gas fields have been found in the Bohaibay Basin, but no systematic development has been carried out and only some liquid CO 2 has been injected for tight oil from the Huagou CO 2 gas field. [ABSTRACT FROM AUTHOR]