Subsurface temperature anomalies reveal blind geothermal systems: A case study from the Aryskum Depression, South Torgai Basin, Kazakhstan

The petroliferous Aryskum Depression, located within the South Torgai Basin, Kazakhstan, comprises several grabens up to 6 km deep, separated by horsts with more than 4 km of structural relief. Spatial analysis of reported reservoir and measured bottomhole temperatures from oil and gas wells revealed elevated geothermal gradients, ranging from 29 to 45 °C/km. An underlying active mantle plume is the main source of this anomalous heat flux. Higher gradients (>35 °C/km) are likely related to additional factors, including radiogenic heat production from granites in the basement, high thermal conductivity of the crust (including intrusive bodies), and convective heat transfer by hydrothermal fluids circulating through open faults within the highly fractured basement and sedimentary basin-fill. The overlay of basement lithotypes, faults, and reservoir-quality sands allowed for robust delineation of isotherms and prediction of geothermal reservoirs. Favorable conditions exist across the basin for both direct electricity generation with temperatures of ∼150 °C at depths of ∼4.3 km, and direct heat utilization from shallower aquifers. The study demonstrates how integration of data from various sources, in a structural and sedimentological framework, can advance exploration for blind geothermal systems in analogous basins worldwide.