Karst Geothermal System in the Beijing-Tianjin-Hebei Plain of North China

The Beijing-Tianjin-Hebei Plain covers an area of about 87,000 km. It is the main area for the development and utilization of carbonate Karst geothermal reservoirs in the central part of the Bohai Bay Basin, providing an accumulated heating area of more than 40 million m. This paper presents a comprehensive study that covers aspects of reservoir-caprock assemblages, reservoir characteristics, water sources, migration pathways, and heat transfer modes of the Karst geothermal system in the Beijing-Tianjin-Hebei Plain. A conceptual model of medium-low temperature conductive aquifer system is established to representing the Karst geothermal system in the Beijing-TianjinHebei Plain. It takes high geothermal heat flow of a large rift basin as background, atmospheric precipitations from Taihang and Yanshan mountains as recharge water sources, deep faults at basin margin and Karst unconformities as migration pathways. Geothermal water is heated up through deep circulations and accumulates within Karst reservoirs in the basement uplift zone. This geothermal system displays the following characteristics: (1) The water recharge area is close, but there are many migration pathways within the basin, and the migration distance is relatively long. Due to the large coverage of the rift basin, recharge water from both Taihang and Yanshan mountains migrates along four main groundwater runoff directions (i.e. low pressure zones in a basin, such as basement uplift zones and tectonic transformation zones), with the migration distance of more than 4km towards east into the sea, and the migration time of longer than 25 ka. (2) There are six types of reservoir-caprock assemblages. Influenced by the difference of denudation degree between Indosinian and Yanshanian, Jixian Wumishan Formation and Ordovician Karst geothermal reservoirs are mainly developed in the study area. And due to the control of differential uplift and fall of bedrock during extensional faulting period, six types of reservoir-caprock assemblages are mainly developed. (3) High-quality geothermal resources are mainly distributed along the basement uplift zone. The central uplift zone of Jizhong depression and the Cangxian uplift zone have not only favorable Karst facies, but also relatively shallow-buried Karst geothermal reservoirs. They are characterized by many Karstification stages, large accumulative effective thickness, good reservoir physical properties and high thermal storage temperature, which all together make them to be high-quality Karst geothermal zones. According to volumetric method, the total geothermal resource of the Karst geothermal system in the Beijing-Tianjin-Hebei Plain is estimated to be 53.821 billion GJ, which is equivalent to 184.155 billion tons of standard coal. According to current economic recoverable depth of geothermal drilling, annual recoverable geothermal resources of shallow Karst geothermal reservoirs within 4 km is equivalent to 222 million tons of standard coal and that can provide 7.65 billion square meters of heating space, which shows a great potential for geothermal development.