Geothermal gradient and temperature of hydrogen sulfide-bearing reservoirs, Alabama continental shelf

Present-day formation temperatures of the hydrogen sulfide (H₂S)-bearing reservoirs in the James Limestone and the Norphlet Sandstone in the continental shelf off Alabama have been determined to be 138-149 and 191-217°C, respectively. Hydrogen sulfide gas in those reservoirs is generated by thermochemical sulfate reduction, a process sensitive to the ambient temperature. Bottom-hole temperature data from 135 wells in the offshore lease areas of Mobile, Main Pass East Addition, and the northern section of Viosca Knoll were examined in the estimation of formation temperatures. The bottom-hole temperatures were corrected for the thermal effect of drill-fluid circulation. Estimation of formation temperatures permitted the determination of the geothermal gradient representative for the study area, leading to a temperature range estimation for the H₂S-bearing James and Norphlet reservoirs. Temperatures of offshore Norphlet reservoirs are higher than those reported previously for Norphlet reservoirs onshore. Temperatures of the James Limestone are close to the low-temperature limit for thermochemical sulfate reduction previously suggested.