The Study of H2S Origin in Natural Gas Reservoirs in South Iran

Rabbani, Ahmad Reza, AmirKabir University of Technology, Tehran-Iran, Iran

Hydrogen sulfide (H2S) is generally an undesirable component of natural gas. Where present, H2S not only can critically affect the economic proportion of hydrocarbon gas in the reservoir, but it is highly toxic and corrosive for production equipment. Therefore the ability to predict the occurrence of H2S in undrilled prospects would be a very useful idea in decrease of gas exploration and production risk. A variety of discrete sources for H2S in petroleum have been identified including: •Bacterial reduction of sulfate to H2S (BSR). The sulfate can be from connate waters, anhydrite dissolution, injected seawater, or pyrite oxida­tion by injected water. Bacterial sulfate reduction typically does NOT result in gases contain­ing >5% H2S. •Thermal decomposition of sulfides in kerogen and/or oil. This process typi­cally does NOT result in gases containing >5% H2S. •Thermochemical reduction of sulfate to H2S (TSR). TSR is the reaction of sulfate minerals (primarily anhydrite) and hydrocarbons (beginning at temperatures of 120-140 deg. C) to form H2S and calcium carbonate. Because anhydrite is often associated with carbonate sequences, TSR is commonly associated with deep, hot, carbonate reservoirs and/or source rocks. TSR is the most important process for formation of high-H2S gases (>10% H2S). The highest concentrations of H2S are found in deep, post-mature gases from carbonate sources where TSR is active. In natural gas reser­voirs in the south of Iran have been seen the sign of TSR in particular in lower Dalan gas reservoir .Significant enrichment in nitrogen, occurrence of isotopically light CO2 and deple­tion in the light carbon isotope of methane, ethane and propane in the gas reservoir that buried in more than 4km . These peculiarities suggest that the composition of gases in this zone was modified by the process of the Thermochemical reduction of sulfate.