Abstract: Analyzing the characteristics of geological factors in a geothermal system is the basis to establish its genetic model and to evaluate the geothermal resource. Combined with previous research and regional geothermal well data, we analyze the main geological factors including “source, reservoir, migration, and cap” of the geothermal system in the Shulu Sag, Jizhong Depression with an aim to establish the genetic model of the geothermal system and to evaluate geothermal resources. The Shulu Sag is a typical Cenozoic half graben, where heat may be supplied by a low-resistance body about 20 km in the deep crust. The Guantao Formation (Fm) sandstone thermal reservoir and Ordovician karst thermal reservoir make up two sets of geothermal systems, respectively. The sandstone thermal reservoir in the Guantao Fm is stably distributed in the whole area with itsbottom boundary at depth between 1100-2000 m. The reservoir is about 200-320 m thick with 15%-35% porosity. The permeability can reach up to 1200 mD. The temperatures at its bottom boundary are mostly 57-78℃; The distribution of the Ordovician carbonate reservoir is controlled by the half-graben and the reservoir shows a monoclinal dip with its top boundary at depth of 1800 to 6000 m. The reservoir is about 100 to 550 m thick with the porosity of mostly 2 to 18% and permeability of mostly 0.5 to 50 mD. The wellhead temperatures of the thermal reservoir are about 75-92℃. The two geothermal systems are replenished by atmospheric precipitation from the Taihang Mountains to the west of the study area. Water migrated along the unconformity of the layers and the faults, then warmed by deep heat conduction and local heat convection, and finally enriched in the reservoir. The overlying loose Quaternary sediments and Minghuazhen formations fluvial clastic sediments are 300-1400 m thick with thermal conductivity of 0.9-1.8 W/(m·K), and thus can be a good cap layer. Evaluation of the geothermal resources in the Shulu Sag shows that the sandstone geothermal system of the Guantao Fm contains 244.430×10⁸ GJ and the Ordovician karst geothermal system contains 203.752×10⁸ GJ. The total amounts to 448.182×10⁸ GJ, which is equivalent to 15.296×10⁸ t standard coal. The annual geothermal resources can meet the heating area of 1.106×10⁸ m2. Therefore, the development potential is huge.

Key words: geothermal system, genetic model, resource evaluation, Shulu Sag