Study On The Development Of Ordovician Karst Reservoir And The Distribution Of Favorable Geothermal Areas In Jizhong Depression

In recent years,the inner buried hills of Ordovician karst reservoir in Jizhong Depression have made major exploration breakthroughs,which fully show that the Ordovician in Jizhong Depression still has great exploration potential and research value.The high-value areas of terrestrial heat flow in Bohaiwan Basin mainly gather in Jizhong Depression,Cangxian uplift,Chengning uplift and Neihuang uplift,which also reveals the great research potential of Ordovician Karst geothermal reservoirs in Jizhong Depression.Based on previous studies and making full use of field outcrops,core slices,drilling and logging data,seismic profiles,geochemical analysis and other data,this paper mainly analyzes the Ordovician lithology,reservoir space types,paleogeomorphic distribution,karst development and geothermal contributing factors in Jizhong Depression.In accordance to the structural evolution characteristics of the study area,which is divided into three sections of the North,the Middle and the South,the genetic models of Ordovician Karst thermal reservoir in the study area have been established from the four perspectives of “source,communication,reservoir and cover”.Based on the analysis of favorable factors for the development of karst thermal reservoirs and combined with their genetic models,the plane distribution of Ordovician favorable geothermal areas in the study area has thus been defined.The research shows that the lithology of Ordovician in Jizhong Depression mainly includes limestone and dolomite,and the types of reservoir space mainly include solution pore,solution cave,solution fracture and structural fracture.The Ordovician paleogeomorphology in Jizhong Depression shows the characteristics of decreasing from northwest to southeast.Karst highlands are mainly distributed in the northwest and south of the depression,karst slopes are distributed in the middle of the depression,and karst basins are mainly distributed in the southeast of the depression.The karst zoning is stratified,and the karst zones in different stages are superimposed and developed under the influence of tectonic movement.The degree of karstification is controlled by lithology,paleogeomorphology,fault development and exposure time.In the karst highland-slope areas covered by Cenozoic caprock and developed faults,the karstification of transitional rocks such as calcite dolomite or dolomitic limestone is the most developed.The genesis of the Ordovician Karst thermal reservoir in Jizhong Depression can be summarized as the precipitation from the Taihang Mountain and Yanshan Mountain and its infiltration into deep strata along Piedmont faults such as East Taihang Fault.After being heated by the decay heat of radioactive elements in the shallow crust and the heat conduction of the mantle,the geothermal water migrates laterally or upwelling along the unconformity surface and water conducting faults,and enters the karst pores in the basement uplift area.The favorable geothermal areas in the Ordovician of the study area have been classified as Class-I and Class-II areas.Class-I favorable areas are distributed in the east of Niutuozhen uplift,the east of Renqiu,the west of Shenze low uplift and the west slope of Shulu,which are considered as the most favorable areas with ample heat source,best karst development and most prospecting potential in the study area.Class-II favorable areas are mainly distributed in the buried hills in the northeast and south of the depression.