Controls on Hydrogen Sulfide Formation in a Jurassic Carbonate Play, Turkmenistan
Gary H. Isaksen, Mukhammetnur Khalylov, 2007. "Controls on Hydrogen Sulfide Formation in a Jurassic Carbonate Play, Turkmenistan", Oil and Gas of the Greater Caspian Area, Pinar O. Yilmaz, Gary H. Isaksen
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An integrated geological and geochemical evaluation of the Charjou terrace area of the Amu Darya Basin in Turkmenistan and Uzbekistan shows that the main controls on hydrogen sulfide (H2S) distribution are the lithofacies of the reservoir and seal rocks and reservoir temperature. Thermo-chemical sulfate reduction (TSR) has likely occurred where reservoir temperatures are greater than approximately 100°C, and the reservoired gas is trapped in dolomitic reservoirs in contact with anhydrite. These chemical reactions occur both in carbonate platform and reefal buildups capped by anhydrite, as well as in structural traps that have undergone thrust-induced juxtapositioning of anhydrite with the dolomite gas reservoir. The reservoirs can have an H2S concentration as much as 4 vol.%. This, together with temperatures of 100–110°C and sulfate-rich formation waters, suggests that these reactions represent the earliest stage of TSR. Carbonate buildups, including reefs, capped by marine shales have very low to trace levels of H2S because the thermochemical sulfate reaction is not initiated, presumably caused by a lack of sulfate. Furthermore, H2S concentrations can vary greatly within a single field with stacked pay zones. Reservoir compartments sealed by marine shales have no H2S gas. A covariance between increased gas wetness and lower initial temperatures for TSR is also suggested.