Study of the Sources and Mechanisms of Produced H2S in Nosoud Tunnel (West of Kermanshah)

While Tunneling in Nosoud area, west of Kermanshah, plenty of water and hydrocarbon influx were encountered. The tunnel was drilled in Pabdeh formation and drilling continued, through Gurpi into the Ilam formation, where water flow containing high amount of H₂S, reached maximum 300 lit/sec. The exceeding of flow and H₂S contamination caused considerable drilling problems and damages. H₂S seepage as a non-hydrocarbon gas is fairly common during drilling oil wells and tunneling. This occurs by several mechanisms, two of which are more significant. These are: Bacterial Sulphate Reduction (BSR) and, Thermochemical Sulphate Reduction (TSR). In order to clarify the sources of H₂S and the mechanisms involved its  generation in Nosoud tunnel, 22 samples of cuttings from Pabdeh, Gurpi and Ilam formations, 9 pieces of cores cut from Garu formation in the Pataq and Imam Hasan oil wells (nearest oil fields to the study area), as well as 3 samples from groundwater flowing in the tunnel were geochemically analyzed. Chemical analyses and Rock Eval pyrolysis were performed on the water and rock samples respectively. Subsequently, 7 cutting samples and 4 core samples were selected for complementary analyses such as: organic petrography studies, chemical analyses and Biomarker studies. After chemical analyses 4 asphalten samples, which were extracted from 4 studied formations, were selected for carbon and sulfur isotopic analyses. During the field visiting and sampling for the present study, the amount of seeped water into the tunnel and emitted H₂S was reduced severely. As H₂S sampling of water and air from the tunnel was not possible, determination of H₂S sources was performed using indirect and parallel methods. Studies and analyses on the water samples show regional ground waters containing various microorganisms and bacteria, with: low salinity, neutral pH, high ion concentrations of sulphate (SO₄^2-) and, relative low H₂S contents; have forwarded favorable environmental conditions for growing and activity of sulphate reducing bacteria. Carbon and sulphur isotopes, biomarker studies (GC, GC-MS) and chemical analyses proved that slight biodegradation and BSR have been active particularly in Gurpi and Garu formations, in the vicinity of the Nosoud tunnel. Whereas; limited evaporate minerals such as gypsum or anhydrite, the value of δ³⁴S and δ¹³C, in the same lithostratigraphic units indicate that the TSR process was not significantly active in the studied area.