Characteristics and genetic types of the Lower Paleozoic natural gas, Ordos Basin

The source of the Lower Paleozoic natural gas in the Ordos Basin has been a highly controversial issue. Using data obtained from newly drilled wells, we present the geochemical characteristics of Lower Paleozoic natural gas in various regions of the Ordos Basin. The Lower Paleozoic natural gas is dominated by hydrocarbons, of which methane accounts for 40.17%-97.24%, and the heavier gaseous hydrocarbon (C2+) ranges from 0.01% to 9.72%. Non-hydrocarbon gases in the Lower Paleozoic gas reservoir are mainly CO2 and N2, 2%-10% for CO2 and 0.04%-48.56% for N2, higher than those in the Upper Paleozoic gas reservoir. This observation showed that the high CO2 and N2 content was related to the Ordovician marine carbonate rock deposits. The carbon and hydrogen isotopes of the methane, ethane, and propane in the Lower Paleozoic natural gas of the Ordos Basin showed various degrees of reversal, indicating the mixing origin of natural gas. In the central-eastern Ordos Basin, coal-derived gas from the Carboniferous-Permian (C-P) coal measure and oil-associated from the Lower Ordovician Majiagou Formation marine source rocks gas coexist, as well as mixtures of these two gases. For the Lower Paleozoic natural gas from the western margin of the basin, coal-derived gas coexist with oil-associated gas, but no mixing gas, in which coal-derived gas originated mainly from the C-P source rocks and oil-associated gas from the Lower Ordovician Kelimoli Formation and Middle Ordovician Wulalike Formation source rocks in the western margin of the basin. The Lower Paleozoic natural gas in the southern part of the Ordos Basin was mixing gas, and the oil-associated gas is the dominant one. The natural gas was originated from the Lower Ordovician Majiagou Formation source rocks in the central-eastern Ordos Basin and the Pingliang Formation mudstone and argillaceous limestone at the southern margin of the basin.