Deep-seated radiogenic heat production characteristics in the northeastern Gonghe basin (northeastern Qinghai-Tibet plateau) from deep borehole samples: Implications for the formation of hot dry rock resources

With the advancement of Hot Dry Rock (HDR) geothermal resources exploration in deep buried geological formations, the high radiogenic characteristics have garnered increasing attention and are considered to be the primary heat source for HDR development. The deep wells in the northeastern Gonghe basin geothermal area revealed a geothermal gradient of up to 45.2 ℃/km within the basal granitic basement, indicating its suitability for the exploration and development of HDR geothermal resources. Nonetheless, the research on HDR formation mechanisms is still under debate so far. Understanding the distribution of radiogenic heat production (RHP) with depth is crucial for comprehending the origin of the heat source mechanisms. Previous RHP calculations were mainly focused on samples extracted from outcroppings. Due to the limited available RHP datasets from deep wells, the variations of RHP with depth remain unclear.

In the present research, 134 new samples were continuously extracted from a 4 km-deep geothermal well to decipher the in-situ RHP characteristics with depth within the Gonghe basin. These samples contain, on average, 2.40 % potassium, 12.93 ppm thorium and 2.87 ppm uranium for sedimentary cover, and on average 3.61 % potassium, 24.11 ppm thorium and 14.01 ppm uranium for granitic basement. On average, the radioactive isotopes in sediments and granitoids generate 1.65 ± 0.81 and 5.54 ± 0.61 µW/m³ of heat, respectively. Additionally, a 1D thermal simulation model was established to assess the impact of RHP on the formation of HDR. Modeling results indicate that RHP has a significant impact on the origin of the geothermal anomaly. For the proposed geothermal models, the presence of RHP within granitoids contributed 39–70 °C to the formation of HDR at the depth of 4 km. While RHP may not be the sole origin of the geothermal anomaly in the Gonghe basin, it does have a substantial impact on the thermal structure. Our findings in this study enhance the understanding of the heat source of the HDR resource within the Gonghe basin.