Source of the Upper Paleozoic natural gas in the Dingbei area, Ordos Basin, China

Abstract The Dingbei area is an important exploration area in the Ordos Basin for SINOPEC, and natural gas exploration in the Upper Paleozoic strata in the area achieved a great breakthrough in recent years. However, the origin and source of natural gas are weakly studied, wherein there is no consensus on whether the Upper Paleozoic gas in the basin has experienced large-scale lateral migration. Studies on the geochemical characteristics of natural gas indicate that the Upper Paleozoic natural gas in the Dingbei area is typically dry gas with the dryness coefficient (C1/C1-5) ranging from 0.977 to 0.986, and the δ13C1, δ13C2, and δD1 values range from −30.6‰ to −28.6‰, −25.9‰ to −24.1‰, and −191‰ to −177‰, respectively. The above has positive carbon and hydrogen isotopic sequence between CH4 and C2H6. Carbon and hydrogen isotope compositions of alkane gas suggest that the Upper Paleozoic gas in the Dingbei area is typically coal-derived gas. The calculated RO values according to the two-stage fractionation mode of coal-derived gas are consistent with the measured RO values of the Upper Paleozoic source rocks; this indicates that the Upper Paleozoic gas in the Dingbei area was mainly derived from in-situ source rocks in the Taiyuan and Shanxi formations. In comparison to the Dingbei area, natural gas from the Daniudi gas field in northeastern Ordos Basin displays lower δ13C1, C1/C1-5, and C1/C2+3 values and higher C2/C1 ratios, suggesting the lower maturity of source rocks rather than migration effect. The Upper Paleozoic natural gas in the Ordos Basin has not experienced large-scale lateral migration.