Genesis of tight sand gas in the Ordos Basin, China

Based on molecular and stable carbon isotope composition of a large collection of gas samples and integrated with the geological context, this article discusses the genesis and migrational characteristics of natural gases from the giant tight sand gas fields of the Upper Paleozoic strata, Ordos Basin (UPOB). Our results show that the UPOB gas is mostly derived from coaly source rocks, with a minor contribution of oil-associated gas probably from the interbedded carbonate source rocks. Gas isotope data suggest that the Yulin–Zizhou gas field was likely formed from a single gas charge with no significant alteration, whereas the Sulige field contains a mixture of gases from successive charges with multiple maturity levels. The geographical variation in the observed gas composition is attributed primarily to the effect of source rock maturity, with no explicit geochemical fractionation caused by long distance lateral migration.