Effect of thermal non-equilibrium, seafloor topography and fluid advection on BSR-derived geothermal gradient
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The seafloor and bottom simulating reflectors (BSRs) are interpreted from the 3D seismic data acquired in Krishna–Godavari (KG) offshore basin in the vicinity of sites drilled/cored during National Gas Hydrate Program (NGHP) Expedition-01. The shallow structures such as inner toe-thrust fault system, regional and local linear fault systems and mass transport deposits are inferred from attributes of seafloor time structure as well as from the seismic profiles. The geothermal gradient is estimated from the depths and temperatures of the seafloor and the BSR. The temperature at the BSR depth is estimated from the methane hydrate and seawater salinity phase boundary assuming that the BSR represents the base of the gas hydrate stability zone. The spatial variations in geothermal gradient (GTG) show a strong correlation with seafloor topography in the KG basin. The GTG decreases by ∼13–30% over the topographic mounds formed due to inner toe-thrust faults and recent mass transport deposits. The GTG decreases by only 5–10% over the mounds, likely due to defocusing of heat flux based on one-dimensional topographic modeling. Hence, the GTG perturbation due to topography alone cannot explain the observed GTG anomaly. The temperature profile beneath these mounds may not be in equilibrium with the surroundings either due to the recent upliftment of sediments along the inner toe-thrust faults or rapid deposition of sediments due to slumping/sliding. In contrast, an increase in GTG by 10–15% is observed in the vicinity of major fault systems. We presume that the likely mechanism for the increase in GTG is fluid advection from a deeper part of the basin. A detailed thermal modeling involving the effect of surface topography, high sedimentation rates, fluid advection and sediment thickening due to tectonics is required to understand the thermal profile in KG offshore basin.
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