Carbon isotope reversal and its relationship with natural gas origins in the Jingbian gas field, Ordos Basin, China.

The geochemical characteristics of natural gases from Lower Paleozoic Jingbian gas field are abnormal and the carbon isotope of most natural gases has reversed, which led to a dispute over the source of natural gas in the Lower Paleozoic. The Upper Paleozoic Yulin gas field is adjacent to the Jingbian gas field, but the geochemical characteristics of the Yulin gas field did not appear abnormal. Besides, the ethane carbon isotope of Upper Paleozoic Yulin gas field is much higher than that of Lower Paleozoic Jingbian gas field. In order to identify the reasons for the carbon isotope reversal of natural gas in Lower Paleozoic Jingbian gas field and its relationship with natural gas source, We conducted the components and stable carbon isotopes of natural gas from Upper Paleozoic Yulin gas field and Lower Paleozoic Jingbian gas field. On the basis of these experiments, we compared the natural gas components and carbon isotope of 100 samples from both Upper Paleozoic Yulin gas field and Lower Paleozoic Jingbian gas field. The results indicate that higher maturity will break the conventional rule, when maturity reaches a certain threshold; δ 13 C ethane value will decrease with maturity. Although δ 13 C methane value will also increase with maturity, the extent will be relatively smaller. This leads to unusual δ 13 C methane  > δ 13 C ethane in the region with higher maturity. In the process of downward migration of coal-derived gas from Carboniferous-Permian coal measures, component fractionation and carbon isotope fractionation might occur, resulting the ethane components and carbon isotope value of Lower Paleozoic less than those of Upper Paleozoic, which will lead to the carbon isotope reversal. The mixing of oil-derived gas and coal-derived gas at different ratios may also lead to the reversal of carbon isotopes. The natural gas types of Jingbian gas field are mainly coal-derived gas generated from Carboniferous-Permian coal measure strata, with a small amount of oil-derived gas generated from Carboniferous-Permian marine-terrigenous limestone. [ABSTRACT FROM AUTHOR]