Origin and source of natural gas in the Upper Paleozoic in Hangjinqi area, Ordos Basin

The analyses of the chemical composition, stable carbon and hydrogen isotopes as well as noble gas isotopes of natural gas from the Upper Paleozoic in the Hangjinqi area of Ordos Basin showed that the natural gas was mainly composed of hydrocarbon gas, which appeared to be wet gas with a dryness coefficient from 0.80 to 0.94, lower than 0.95. The δ¹³ C₁ and δ¹³ C₂ values range -36. 2‰--31. 7‰ and -27. 8‰--23. 3‰, respectively, and the δD_CH4 values range -199‰--172‰. The carbon and hydrogen isotopes of alkane gas are generally characterized by positive sequence, indicating for an organic origin. Gas origin identification and gas-source correlation indicated that the natural gas from the Upper Paleozoic is coal-derived gas with a normal to high maturity and was sourced from coa-measure source rocks in the Taiyuan-Shanxi formations. There are some obvious differences of gas-source to the south and the north of the Boerjianghaizi Fault. The natural gas in the Shilijiahan play in the south was derived from the P₁t-P₁s source rocks in situ with a high maturity, whereas the natural gas in the Shiguhao play in the north was mainly sourced from the Shilijiahan area, and then suffered a certain degree of damage and escaped.