Limited thermochemical sulfate reduction in hot, anhydritic, sour gas carbonate reservoirs : The Upper Jurassic Arab Formation, United Arab Emirates

Limited thermochemical sulfate reduction (TSR)in hot (130–160°C)and anhydrite-rich sour gas reservoir carbonates of the Arab Formation (Upper Jurassic)is manifested by rare calcitization of anhydrite with slightly lower δ13CVPDB values (−3.2 to −0.1‰)than calcite precipitated in equilibrium with Late Jurassic seawater. Fluid inclusion microthermometry of calcite that has replaced anhydrite indicates that TSR occurred between 130°C and 160°C. The lack of evidence for extensive TSR, despite the suitable current temperatures and abundant sulfates in the gas reservoir, coupled with the relatively more common TSR-related calcite in the flanks (water zone)than crest (gas zone), indicate that: (1)gas emplacement while the reservoir was buried at shallower depth slowed down or inhibited TSR in the crest even when it subsequently reached depths where extensive TSR would occur, and (2)H2S (up to 38 vol%)has migrated from the underlying Permo-Triassic and/or Jurassic sulfate-carbonate deposits. This study demonstrates that constraining the timing of hydrocarbon emplacement within the context of burial-thermal history is crucial for a better understanding of the origin and distribution of H2S in hot, anhydrite-rich, sour gas reservoirs.