A series of samples including natural gas, formation water, and rocks were collected from volcanic rock reservoirs in the Niudong area of the Santanghu Oilfields and analyzed for their mineral and/or chemical compositions and sulfur and carbon isotopes in order to investigate the occurrence and origin of hydrogen sulfide (H2S). H2S was mostly dissolved in the formation water along with petroleum production in the study area. The δ34S values of on-well H2S samples varied in a range of 9.2‰ to 20.5‰, probably indicating thermochemical sulfate reduction (TSR) and/or thermal decomposition of organic sulfur-bearing compounds (TDS) as the genetic process for H2S. However, the chemical composition of formation waters from the Kalagang Formation (C2k) and their coefficient of desulfurization also revealed that TSR could be the main principle for H2S formation. Considering the regional geological background, especially the tectonic structures and thermal evolution features of the basin, it was concluded that H2S in the study area was dominantly produced by thermal genesis with TSR as a domain through interactions between hydrocarbons and aqueous sulfate dissolved from sulfate minerals.

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