Genetic Mechanism of Geothermal System in Shulu Sag, Jizhong Depression, Bohai Bay Basin

Abstract

The analysis of the characteristics of geological factors in the geothermal system is the basis to establish its genetic model, and is also the basis for later exploration and development. Combined with previous research and regional geothermal well data, we analysis the main geological factors include source, reservoir, migration, cap of the geothermal system in the Shulu Sag, Jizhong Depression, establish the genetic model of the geothermal system. The Shulu Sag is a typical Cenozoic half graben, which may be supplied heat by a low-resistance body about 20 km in the deep crust structure. The Guantao Formation sandstone thermal reservoir and Ordovician karst thermal reservoir make up two sets of geothermal systems respectively. The sandstone thermal reservoir in Guantao Formation is stably distributed in the whole study area, which buried depth of the bottom boundary is between 1100~2000 m. The thickness of the reservoir is about 200~320 m, with 15-35% porosity, and the permeability can reach up to 1200mD. The temperature of the thermal reservoir bottom boundary is mostly 57~78ºC; The Ordovician carbonate reservoir distribution is controlled by the half-graben and shows a monoclinal dip, which buried depth of the top boundary is from 1800 to 6000 m. The thickness of the carbonate reservoir is about 100 to 550 m, the porosity is mostly 2 to 18%, the permeability is mostly 0.5 to 50 mD. The wellhead temperature of the thermal reservoir is about 75~92ºC. These two geothermal systems are replenished by atmospheric precipitation from the Taihang Mountains in the west of study area. The water migrated along the unconformity of the layers and the faults, and warmed by deep heat conduction and local heat convection, finally enriched in the reservoirs. The overlying loose Quaternary sediments and Minghuazhen Formations fluvial sediments are 300~1400 m thick, and the thermal conductivity is 0.9~1.8 W/(m·K), can be a good cap layer.