Thermal properties and thermal structure in the deep-water coalbed basin off the Shimokita Peninsula, Japan

Abstract

Heat transport properties are significant parameters that affect geothermal processes in deep sedimentary basins. We measured the thermal properties of deep sediment core samples to a depth of approximately 2500 m below sea floor (mbsf), whereas most previous studies have only presented data <800 mbsf for marine sediments, taken from the basin off Shimokita, Japan during Integrated Ocean Drilling Program (IODP) Expedition 337. Thermal conductivity and thermal diffusivity increased with depth, and heat capacity decreased with depth, although the data were highly scattered at greater depths. The increase in thermal conductivity is explained primarily by the porosity reduction of sediment resulting from consolidation during sedimentation. The high variations in thermal conductivity at the similar depth are reflected by various lithological rocks formed at the same core section. Coal exhibits the lowest thermal conductivity of 0.4 Wm−1 K−1, and calcite-cemented sandstone/siltstone exhibits the highest conductivity of approximately 3 Wm−1 K−1. The other thermal properties are also influenced by the porosity and lithological contrast. The correlation between thermal conductivity and porosity is explained by the mixed-law model of the geometric mean.The one-dimensional temperature-depth profile at site C0020 estimated from measured thermal conductivity reveals a regression of the thermal gradient with depth, and the surface heat flow of 30 mWm−2, which is consistent with the heat flow data near this site, was evaluated. The obtained results suggest that the linear geothermal gradient estimated from the average thermal conductivity value at shallow sediment materials could overestimate the temperature at greater depths and underestimate the heat flow.