Objective  The Da’an area in Jilin Province, located in the southwestern Central Depression of the Songliao Basin, exhibits significant geothermal anomalies, which suggest substantial potential for geothermal exploitation. However, there remains a lack of detailed geothermal resource assessment of this area. Among conventional assessment methodologies, the volumetric method fails to accurately characterize irregular stratigraphic geometries and the spatial heterogeneity of parameters, while the Monte Carlo method suffers from limitations such as heavy dependence on the probability distribution models of geothermal reservoir parameters.

Methods  To overcome the limitations of the conventional methods, this study established a 3D geothermal geological model by integrating regional geology, drilling data, and the thermophysical properties of reservoir rocks. Through 3D simulations of steady-state geothermal fields, this study characterized the temperature field distributions at depths of less than 2 600 m in the eastern Da’an area. Then, using the finite volume method, it assessed geothermal resources in three major Upper Cretaceous geothermal reservoirs: the fourth member of the Nenjiang Formation (K₂n⁴), the second-third members of the Yaojia Formation (K₂y²⁺³), and the second-third members of the Qingshankou Formation (K₂qn²⁺³).

Results and Conclusions  The results indicate that the formation temperatures at depths of 500 m, 1 000 m, 1 500 m, 2 000 m, and 2 500 m in the study area measure 25.8–36.4 ℃, 48.6–61.3 ℃, 63.1–84.6 ℃, 79.3–107.1 ℃, and 94.4–125.6 ℃, respectively. The geothermal contour lines are dense in the southwest and sparse in the northeast, with temperature distribution patterns roughly consistent across varying depths. In other words, the formation temperature is high in the northeast and low in the southwest. The geothermal reservoirs in K₂n⁴, K₂y²⁺³, and K₂qn²⁺³ exhibit temperatures of 27.0–67.3 ℃, 38.2–96.7 ℃, and 43.2–103.58 ℃, respectively. Using the finite volume method, the geothermal resources in K₂n⁴, K₂y²⁺³, and K₂qn²⁺³ were calculated via integration by units, yielding 19.2×10¹⁸ J, 30.4×10¹⁸ J, and 47.3×10¹⁸ J, respectively, which are equivalent to 6.55×10⁸ tonnes of coal equivalent (tce), 10.37×10⁸ tce, and 16.14×10⁸ tce, respectively. Using 25% as the recovery factor, recoverable resources of K₂n⁴, K₂y²⁺³, and K₂qn²⁺³ are estimated at 1.64×10⁸ tce, 2.59×10⁸ tce, and 4.04×10⁸ tce, respectively. Compared to the traditional volume method and Monte Carlo method, the combination of the finite volume method and numerical simulation technique enjoys unique advantages in geothermal resource assessment.