Characteristics and genetic mechanisms of geothermal reservoirs in the Neogene Guantao Formation, Qikou Sag

The Qikou Sag in the Huanghua Depression, Bohai Bay Basin boasts extremely abundant geothermal resources.Among them, the geothermal reservoirs of the Neogene Guantao Formation have the greatest potential for exploitation. To ascertain the properties of geothermal resources in this formation and rationally exploit these resources, it is significant to investigate the geothermal field, porosity, permeability, geothermal water circulation patterns, and genetic mode of geothermal reservoirs in the formation. Based on data from drilling, geophysical exploration, thermometry, and production of the study area, this study determined that the geothermal reservoirs of the Guantao Formation in the Qikou Sag constitute a conductive geothermal system of a sedimentary basin, with the heat flow from the deep mantle and the radioactive heat from the crust being transferred through heat conduction. The distribution of geothermal anomalies is subjected to wavy bedrock. Hightemperature geothermal anomalies with geothermal gradients up to above 5.0°C/hm are found near the Cangdong and Beidagang faults. The geothermal reservoirs of the Guantao Formation manifest high porosity and permeability in the north and south but low porosity and permeability in the central portion, with porosity greater than 26%, permeability above 400×10^-3 μm², and average single-well water yield of 60 m³/h overall. These suggest high-quality geothermal reservoirs. The geothermal water in the study area primarily originates from paleometeoric water. As the recharge source, the paleometeoric water from the Yanshan area in the north circulates deeply along strata while absorbing heat from the rock matrix, ultimately forming geothermal water. The geothermal reservoirs of the Guantao Formation in the Qikou Sag host geothermal resources of 41.81×10¹⁸ J, 95% of which are distributed in the Tianjin Binhai New Area and Huanghua City, Hebei Province. Their high match with demands makes geothermal resources in the study area have great potential for production and utilization.