Tracking the origin of Ordovician natural gas in the Ordos basin using volatile sulfur compounds

Molecular and compound specific isotope analysis of volatile organic sulfur compounds (VOSC), alkanes and CO₂ in natural gases from the Ordovician Majiagou Formation of the Daniudi field (Ordos basin, China) was performed. The analysis of VOSC demonstrated abundant thiols, sulfides and thiophenes in several samples while others had low concentrations of sulfides only. Molecular analysis of main alkanes and non-hydrocarbons showed the samples vary by the nitrogen and H₂S concentrations. Thiols in the gas were at isotopic equilibrium with H₂S bearing isotopic signature similar to that of Ordovician anhydrites within the reservoirs, hence the source of H₂S was likely thermochemical sulfate reduction (TSR) of these anhydrites. In contrast, gas samples that lacked thiols also lacked H₂S. Since all gases came from the same formation and similar thermal regime, the lack of thiols and H₂S indicates that the occurrence of TSR in the field was limited, likely by contact between hydrocarbons to the anhydrites. The isotopic signature (δ³⁴S) of thiophenes in the gas samples suggests the source of the gas is unlikely to be Carboniferous or Permian coals which were previously suggested as the source rocks for the gas. The δ¹³C and δ²H of alkanes further supports this suggestion as it demonstrates Carboniferous and Permian gases are unlikely to undergo TSR and generate the gas present in the Ordovician reservoirs. Instead, the δ¹³C and δ²H of alkanes hint that the gas source is shale rather than coal. This study demonstrates the applicability of compound specific sulfur isotope analysis of VOSC in identification of H₂S sources, performing gas-source rock correlation and acting as a complementary proxy in case where routine analysis of hydrocarbons (δ¹³C and δ²H) is insufficient for identification of the gas source.