Study on the spatial characteristics and controlling factors of subsurface temperature field in concealed Salt Lake geothermal reservoir, Yuncheng city, Shanxi Province, China

Abstract

The middle- and low-temperature sandstone-type Salt Lake geothermal field in the Yuncheng region, located in the southern part of the Shanxi Rift, is situated beneath densely populated urban areas, which poses significant challenges for the exploration and development of geothermal fields. This study first revealed the subsurface temperature characteristics from temperature logs, further employed microtremor survey to explore the underlying structures controlling this temperature distribution and finally elucidate the underlying mechanism with numerical simulation. The results obtained from ordinary kriging-based interpolation of available borehole temperature logs show that the geothermal gradient and temperature maps at different depths all have the characteristics of high values in the southwestern part and low in the northeastern part. This pattern was interpreted to be separated by an NW-trending belt. The applied 2D microtremor survey identified a vertical low-velocity anomaly which is inferred to be a highly fractured zone. The numerical simulation, incorporating coupled heat transport and fluid flow processes, indicates that this highly fractured zone may facilitate the movement of cold surface lake water into deeper regions. During this process, the water absorbs heat, which effectively accounts for the low thermal anomalies observed in the eastern sections of Salt Lake geothermal reservoir. By integrating thermal investigation, microtremor survey and numerical simulation, this study successfully delineated temperature maps at different depths, but also explained the mechanism of geothermal anomalies. These findings offer valuable insights and a solid basis for exploring deep geothermal resources within Salt Lake district of Yuncheng city.