Quantitative study on vertical distribution of heat flow in Niutuozhen geothermal field, Xiong'an New Area—Evidence from heat flow determination in the Archean of D01 well

The karst geothermal reservoir in Xiong'an New Area is a representative example of an ancient buried hill geothermal system. However, published heat flow data are predominantly derived from the Cenozoic sedimentary cap. Due to the limited depth of borehole exploration, heat flow measurements and analyses of the Archean crystalline basement in the study area are rare. Further investigation of the heat flow and temperature field characteristics within the Archean crystalline basement beneath the karst geothermal reservoir is necessary to understand the vertical distribution of heat flow and improve the geothermal genetic mechanism in the area. The D01 deep geothermal scientific drilling parameter well was implemented in the Niutuozhen geothermal field of Xiong'an New Area. The well exposed the entire Gaoyuzhaung Formation karst geotheremal reservoir of the Jixian system and drilled 1,723.67 m into the Archean crystalline basement, providing the necessary conditions for determining its heat flow. This study involved borehole temperature measurements and thermophysical property testing of core samples from the D01 well to analyze the vertical distribution of heat flow. The findings revealed distinct segmentation in the geothermal gradient and rock thermophysical properties. The geothermal reservoir of Gaoyuzhuang Formation is dominated by convection, with significant temperature inversions corresponding to karst fracture developments. In contrast, the Archean crystalline basement exhibits conductive heat transfer. After 233 days of static equilibrium, the average geothermal gradients of the Gaoyuzhuang Formation and the Archean crystalline basement were determined to be 1.5°C/km and 18.3°C/km, respectively. These values adjusted to −0.8°C/km and 18.2°C/km after 551 days, with the longer static time curve approaching steady-state conditions. The average thermal conductivity of dolomite in Gaoyuzhuang Formation was measured as 4.37±0.82 W/(K·m), and that of Archean gneiss as 2.41±0.40 W/(K·m). The average radioactive heat generation rate were 0.30±0.32 μW/m3 for dolomite and 1.32±0.69 μW/m3 for gneiss. Using the temperature curve after 551 days and thermal conductivity data, the Archean heat flow at the D01 well was calculated as (43.9±7.0) mW/m2, While the heat flow for the Neogene sedimentary cap was estimated at 88.6mW/m2. The heat flow of Neogene sedimentary caprock is significantly higher than that of Archean crystalline basement at the D01 well, with an excess of 44.7 mW/m2 accounting for approximately 50% of the total heat flow in the Neogene sedimentary caprock. This is primarily attributed to lateral thermal convection within the high-porosity and high-permeability karst dolomite layer, and vertical thermal convection facilitated by the Niudong fault, which collectively contribute to the heat supply of the Neogene sedimentary caprock. Thermal convection in karst fissure and fault zone contribute approximately 50% of the heat flow in the Neogene sedimentary caprock. This study quantitatively revealed the vertical distribution of heat flow, providing empirical evidence for the genetic mechanism of the convection-conduction geothermal system in sedimentary basins.