The Origin of H2S in the Arab Reservoirs in Abu Dhabi

Abstract

Abstract Two different sources of H2S have been identified in the ultra-sour gases in the Arab Formation in Abu Dhabi, based on sulfur isotope measurements of both anhydrites and H2S. In the Upper Arab reservoirs, H2S appears to be generated in-situ from intra-Arab anhydrites, leading to variable, but relatively low H2S concentrations. Sulphur isotopes suggest that the Lower Arab reservoirs receive H2S from anhydrites in the Hanifa or possibly even deeper Formations. The different sources of H2S clarify the highly variable fluid compositions in the Upper Arab reservoirs, while active migration of H2S from deep sources would explain the observed compositional gradient in the Lower Arab reservoirs. The in- situ generated H2S in the Upper Arab reservoirs is unlikely to increase during the production lifetime of the field; the externally generated H2S could increase when depleting reservoir pressures (re)-activate the faults that form the migration pathway of the H2S. The extent of the TSR reaction, and thus the H2S concentrations in the gas, can be estimated by measuring molecular proxies that can be thermally extracted from core and cuttings samples. This study is the first to document the successful use of molecular proxies to accurately estimate H2S concentrations in the gas contained in individual, isolated reservoirs.