Mechanism of heat transfer and accumulation in sedimentary basin geothermal anomaly belts: a case study of Bohai Bay Basin, China

The mechanism of heat transfer and accumulation in sedimentary basin geothermal anomaly belts is very complicated, resulting in a lack of in-depth understanding. The Bohai Bay Basin is the largest Mesozoic–Cenozoic rifted basin in China and taken as the research object in the present work, the relationship between geothermal anomaly characteristics and influencing factors, including the structural geometry of the bedrock bulge, the bedrock thermal conductivity, and the caprock thickness was quantitatively discussed. Based on a typical measured geothermal geologic profile in the central Bohai Bay Basin, the formation mechanism, controlling factors, and the guiding significance for geothermal resources exploration of geothermal anomaly belts were analyzed. The formation of a geothermal anomaly in the sedimentary basin was driven by the rapid transfer of uniform deep heat flow in bedrock with the high thermal conductivity and accumulation of that in caprock with the low thermal conductivity of shallow bedrock bulge belts. As a result, heat flow depletion in the deep bedrock and heat flow enrichment in the shallow caprock were formed, with the characteristics of a nearly symmetrical coupling variation pattern. The bedrock bulge amplitude and the bedrock thermal conductivity were the main factors controlling geothermal anomaly characteristics, and the sufficient caprock thickness was vital to the formation of the geothermal anomaly. Simulation results of theoretical models showed that the top surface temperature of bedrock geothermal reservoirs with a depth of 2 km can increase by up to 31.3 ℃, and positive anomaly amplitudes were up to 41.8%. Based on the difference of tectonic deformation patterns in the rifted basin and their determining role in a geothermal gradient of caprock, geothermal fields developing bedrock geothermal reservoirs in the Bohai Bay Basin were divided into seven types, including the high bulge type, the bulge type, the low bulge type, the sag type, the gentle slope type, the basin margin fault-uplift type, and the shallow burial type, with a decrease in turn of the caprock geothermal gradient from > 5.0 ℃/100 m to < 2.0 ℃/100 m. The high bulge-type and the bulge-type geothermal fields were favorable targets for the shallow-burial bedrock geothermal reservoir exploration. The low bulge-type and the sag-type geothermal fields were the main targets for exploring deep-burial bedrock geothermal reservoirs with high temperatures above 150℃.