Geochemical characteristics and origin of natural gas in southern Jingbian gas field, Ordos Basin, China

Jingbian gas field is yet the only Lower Paleozoic-hosted large gas field discovered in Ordos Basin. However, much debate still exists regarding the type and main source of natural gas in the Lower Paleozoic units. Previous studies on Jingbian gas field have mainly focused on the central and northern regions, whereas few studies have addressed the southern part. Basing on natural gas components and carbon isotope compositions of 33 gas samples from 28 wells in southern Jingbian gas field, we investigated the main gas source and phenomenon of carbon isotope reversal. The results indicate that natural gas from the Upper Paleozoic and Lower Paleozoic units are all at the high-mature and over-mature stages, with average dryness coefficients (C1/C1-4) of 0.982 and 0.996, respectively. The Upper Paleozoic natural gas primarily comprises coal-derived gas from Carboniferous-Permian units, whereas the composition of the Lower Paleozoic natural gas is mostly a mixture of Upper Paleozoic coal-derived gas with additional oil-type gas produced by Upper Paleozoic marine limestone. The phenomenon of carbon isotope reversal mainly results from downward diffusion and migration of the mixed coal-derived gases; moreover, in the Upper Paleozoic unit, it is also related to secondary cracking.