Origin and process of carbon isotopic reversal of tight sandstone gas in the Shixi gas field, eastern margin of Ordos Basin

Abstract

Stable carbon isotopic compositions of natural gas have long been employed to determine its origin, accumulation, and migration. A reversal of the natural gas carbon isotope sequence, i.e. δ13C1 > δ13C2 > δ13C3, was discovered in the tight sandstone gas (TSG) of the Shixi gas field at the eastern margin of the Ordos Basin. This paper presents a study to determine the causes of carbon isotopic reversal hence to better understand the gas accumulation in these tight sandstone reservoirs. The molecular composition and carbon isotopes on 81 wellhead gas samples were measured from the TSG reservoirs of the Pennsylvanian in the Shixi gas field. Results indicate that the molecular compositions of the Shixi gas field comprise methane ranging from 88.9 wt% to 98.9 wt%, ethane ranging from 0.49 wt% to 1.12 wt%, and minor propane with content <0.01 wt%. The non-hydrocarbons are mostly CO2 which is <1.2 wt% and the rare helium ranges from 0.02 wt% to 0.23 wt% with an average of 0.09 wt%. The CO2 was probably derived from the decomposition of carbonate sediments under a high temperature owing to the varied δ13CCO2. Hydrocarbon is considered predominantly from coal-derived humic source rocks and minor oil-type gas. The isotopes of CO2, helium and vitrinite reflectance of the source rocks, i.e. coal and carbonaceous mudstone, indicate that the study area is lacking conditions for the formation of biogenic gas. The carbon isotopic reversal with δ13C1>δ13C2>δ13C3 in the Shixi gas field was interpreted as a result of secondary cracking under a local high temperature >200 °C.