H₂S Abundance in Natural Gas with Emphasis on Thermochemical Sulfate Reduction

Wilson L. Orr, Martin B. Goldhaber

Natural gases range from <1ppm to >90% H₂S, and the level is a major environmental concern with a significant impact on production, transportation and processing economics. General geologic and geochemical controls on H₂S abundance have been evident for more than three decades, but current understanding of the complex processes and subsurface conditions are not sufficient to allow abundance predictions with the desired accuracy. H₂S in petroleum (oil & gas) reservoirs reflects the balance between sources and sinks which are site specific and time dependent. The major source is sulfate reduction by both microbial and chemical means. The major sinks are (1) precipitation of sulfide minerals (e.g., pyrite), and (2) oxidatio by hydrodynamic waters containing oxygen or other oxidizing agents.

Below 60-80°C (possibly below 100°C) microbial sulfate reduction (MSR) is the major source. Above 250°C, thermal energy allows rapid chemical and isotopic equilibrium between SO₄^2-, H₂S and other sulfur species. The 100-200°C temperature range in oil and gas reservoirs is our major concern. This is too hot for MSR and too cool for rapid chemical equilibration. Thermochemical sulfate reduction (TSR), kinetically controlled, is considered the major source of H₂S in the 100-200°C range where most gases with >10% H₂S are found (Orr 1974, 1977).

Field studies indicate significant TSR rates on a 10⁴-10⁶ year time-scale at 100-160°C. Laboratory simulations at 175-250°C have measured rates in hours to days, but with sulfate concentrations somewhat higher and pH somewhat lower than in natural settings. Conclusions arc that major controls on TSR rate are (1) temperature, (2) sulfate availability (in solution or solid phase), (3) fluid pH (buffer systems), and (4) organic matter (type and abundance). In natural reservoirs, distributions of reactants between gas, oil, water and solid phases, as well as mixing (mass transfer) processes may have significant rate-limiting effects not evident from laboratory experiments. Inferred constraints from both field studies and laboratory experiments will be summa ized indicating remaining problem areas needing further research.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995