Geochemical characteristics and origin of natural gas and gas -filling mode of the Paleozoic in the Yanchuannan gas field, Ordos Basin, China

Abstract In order to determine the origin of Carboniferous-Permian (C-P) coal measure gas and Ordovician weathered crust gas, and their gas-filling modes, the chemical and isotopic compositions of the natural gas along with the co-produced groundwater from Paleozoic in the Yanchuannan (YCN) gas field of the Ordos Basin, China, were analyzed. It was found in this work that the gas in the upper gas group of weathered crust (O 1 m 5 1+2 ) reservoirs and the C-P gas had similar δ 13 C 1 and δ 13 C 3 compositions and they were less negative than those of the gas in the lower gas group (O 1 m 5 4 ). While all weathered crust gases had δ 13 C 2 values that are more negative than the C-P gases. The O 1 m 5 4 gas and C-P gas had a very clear boundary in δDCH 4 value, while the O 1 m 5 1+2 gas had the δDCH 4 ranges intersecting with the two others. The C-P gas is inferred derived from coal measures. The O 1 m 5 1+2 gas near erosional valley-form is a mixture of coal-derived gas and marine sapropelic gas, and the remaining Ordovician gases are inferred to originate from marine sapropelic source rock. Both gases were characterized by secondary cracking gas, and thermochemical sulfate reduction (TSR) altered the chemical and isotopic composition of weathered crust gas to a lesser extent due to short TSR reaction time. The changes in water type and total dissolved solids from the Permian to the Ordovician and the D shift phenomenon of O 1 m 5 1+2 water, reveals the fluid vertical migration between the Upper and Lower Paleozoic. Coupled with geological evolution analysis, the ancient erosional valley-form and the surrounding solution cavities and fractures are inferred the main migration channels for coal-derived gas into weathered crust reservoir. Moreover, three vertical gas-filling modes are proposed, i.e., gas seepage in free phase, diffusion phase and water-solution phase, with the importance in descending order.