Abstract

Geothermal gradient within deep sediments down to ~7 km below the seafloor has been determined at 7 localities in the section of continental shelf off Alabama, Mississippi, and eastern Louisiana. In obtaining the geothermal gradient at each locality, we compiled bottom-hole temperature data reported from wells in the vicinity and corrected the data for disturbance due to drill fluid circulation. The so-called Horner plot method was used for the bottom-hole temperature correction. This method requires that bottom-hole temperature be measured twice or more at a fixed depth while the well is shut in for wire-line logging operations. Thermal gradient values in the northeast (i.e., landward) of the Lower Cretaceous shelf margin off Alabama are quite uniform at about 0.028° C per meter (°C/m) or 0.015° F/ft. Areas along the outer perimeter of the shelf margin also show similar thermal gradients. Further seaward of the shelf margin off Mississippi and eastern Louisiana, in areas of deep marine deposition and salt diapirs, thermal gradient values are generally lower. Especially within sediments flanking salt domes, gradient values are 0.018 to 0.025° C/m, or 0.010 to 0.014° F/ft. The difference in temperature at 5 km below the seafloor is ~50° C (~90° F) between the diapiric province and the interior of the Lower Cretaceous carbonate platform. This temperature difference may be large enough to significantly influence the hydrocarbon maturation history and the bacterial and thermochemical sulfate reduction processes that yield hydrogen sulfide gas within deep sediments.