Geochemical and petrophysical characterisation of igneous rocks in the southern Sydney Basin: implications for radiogenic heat production and geothermal potential

Anomalously high bottom-hole temperatures were recorded in a historical petroleum exploration well (‘Lake Nepean 1’) in the southern Sydney Basin. The deepest wireline records reached 36.7–37.8 °C at approximately 334 m but were acquired 7.5–10.4 h after circulation and form part of a non-equilibrium thermal-recovery sequence; an earlier 28 °C drill-stem test was a documented misrun. We investigated whether igneous rocks enriched in radioactive isotopes and sedimentary thermal resistance could explain this local thermal anomaly. Forty-two igneous samples from drill core and surface exposures were analysed by X-ray fluorescence and density measurements, and 74 thermal-conductivity measurement points were obtained from igneous and sedimentary drill core. Syenites and tuffs have loss-on-ignition-normalised mean radiogenic heat-production values of 2.50 and 3.90 μW/m³, respectively, whereas dolerites returned lower values (0.52 μW/m³). Measurements of coal returned the lowest thermal conductivities (0.29–1.50 W/mK), although the bulk regional conductivity will depend on the unit volume (i.e. thickness and lateral extent). At Lake Nepean 1, the upper 333.8 m has an effective conductivity of approximately 1.44 W/mK. A first-order steady-state 1D conductive calculation using values of 60–64 mW/m2 predicts temperatures of approximately 30–31 °C at a depth of 333.8 m (i.e. below the late-time historical wireline records). Regional shallow-groundwater data and sediment-thickness patterns provide complementary spatial context for this interpretation. The combined data indicate greater geological geothermal prospectivity where radiogenic rocks are concealed beneath thicker sedimentary cover north of the Southern Highlands, particularly toward the Picton–Camden region and other parts of southwestern Sydney. These results constrain the thermal architecture of the southern Sydney Basin and provide a geological basis for prioritising further geothermal investigation.