Subsurface thermal properties in the deepwater South Makassar Basin, Indonesia

Abstract

Sandwiched between southeastern Kalimantan and southwestern Sulawesi, the widespread presence of bottom simulating reflectors (BSRs) in the deepwater failed rift of the South Makassar Basin strongly indicates the extensive occurence of methane hydrates. Despite the region’s long petroleum exploration history, the understanding of thermal signatures in the basin remains enigmatic. This study utilized thirty-two legacy multi-channel seismic data surveys to derive heat flow, geothermal gradients, and thermal conductivity from BSRs. By assuming the depth range where methane hydrates remain stable, BSRs can act as a proxy for deducing information related to thermal properties when direct thermal measurements are scarce. Estimated thermal properties indicate a mean heat flow of 86 mW/m², a mean geothermal gradient of 73 °C/km, and a mean thermal conductivity of 1.19 W/m.°C at water depth between 400 and 2050 m. These values differ from marine-probe references in the area, with heat flow and thermal conductivity being on average 34 % and 58 % higher, respectively, geothermal gradients being 13 % lower, and geothermal gradients averaging 30 % higher than exploration wells. The variance between the drilling depth at which methane hydrates were penetrated in three exploration wells and the theoretical base of methane hydrate stability zone vary between a deviation of 6–19 m. Discrepancies in thermal properties values and depth of methane hydrates are attributed to methodological disparities, uncertainties in theoretical assumptions, and geological factors such as thermal uplift from the mantle beneath the thinned lithosphere and localized fluid advection.