Genetic Mechanism Of Geothermal Resources In Zhongmu Depression And Its Characteristics

The study area is in the heart of Kaifeng depression,the eastern Henan plain,southern North China Plain.In this area,geothermal sources are abundant.The faults are developed,the effluent temperature of geothermal wells is high,and the amount of water gushing from a single well is considerable.However due primary to some un-artificial factors,there were few surveys focusing on geothermal geology,and the previous researches of the genesis mechanism are not in-depth.Moreover,the long-term of unreasonable exploitation of geothermal resource have resulted in the waste of geothermal resources and a series of environmental geological problems.Therefore,this study concentrating on the formation mechanism and characteristics of geothermal resources in Zhongmu depression is essential to the sustainable development of geothermal resources.In this study,we took the Zhongmu depression as the case study to analyze.On the basis of previous research,collecting and arranging geothermal fluid data in the area,geothermal geological setting,isotopes characteristics,and hydrochemical characteristics were studied.The genesis mechanism was characterized by calculating the recharge source and elevation,temperature of thermal storage,the depth of circulation,and the mixing ratio.The conclusion was summarized as follows:?1?Zhongmu depression was surrounded by fault and the mantle source heat is the main heat source.The caprock was composed of Quaternary with good thermal insulation property.The main thermal reservoirs were Neogene formation,Minghuazhen formation,Guantao formation as well as Paleogene formation.The caprock and thermal reservoir models established by GOCAD shown the development and distribution of various strata very well.The distribution of the geothermal field was correlated closely with the geological structure and fault activity.Generally,the closer to the sedimentary center and the fault development zone the higher the geothermal gradient would be,it was generally distributed between 2.4?/100m and 3.8?/100m.?2?The analysis based on hydrochemical characteristics shown that the geothermal fluids were weakly alkaline and most of they were fresh water.The main ion concentration was affected by regional fault and runoff conditions.To be specific the concentration of those major ions and TDS were gradually increased along the flow direction,also ions'content were distinct in different zones.Na⁺and HCO₃^-were the major for cation and anion respectively.Hence the chemistry type was mainly characterized as Na-HCO₃ and then Na-HCO₃-SO₄.The ions in geothermal fluids are mainly derived from weathering of rocks,they were partly affected by evaporation and concentration but not influenced by atmospheric precipitation,and mainly comes from water rock interaction in the process of circulation.?3?The analysis on the basis of isotopic characteristics shown that geothermal water had the phenomena of"oxygen drift"in different extent,especially in the southeastern of the study area.This proven that the enrichment of oxygen-18 was influent by thermal fluid.,and that main source of geothermal water was atmospheric precipitation nearby mountains.The recharge altitude in the northern was 705m?841m,and was 292m?863m in the southwest and southeast.Combining topography and hydrogeological setting,the recharge area of geothermal fluid of the northern was Taihang Mountain in the northwestern of the study area,while the recharge area for geothermal fluid of the southwestern and southeastern was located in Songshan mountain block in the western of Zhengzhou city.?4?The geothermal water-rock balance status was determined by using Na-K-Mg balance diagram.To the water samples in the partial equilibrium zone,cation and Si O₂temperature indicator methods were combined to estimate the geothermal reservoir temperature.The results shown that the temperature of the geothermal system varied in the range of 67??101°C and the depth of thermal cycle was 1998m?3933m.Moreover,by employing silicon enthalpy diagram,the mix ratio of thermal water to cold water was estimated varied from 0.47 to 0.73,and the geothermal reservoir temperature was quantitative characterized as ranged from 90?to 120?.?5?Combined with regional geothermal geology,hydrogeological conditions,hydrochemistry and isotope characteristics and evolution paths,we concluded that the mantle source heat of the geothermal system is the main heat source,the recharge water sources were come from the Taihang Mountains and Songshan Mountains,the basement heat channel was the deep and large faults that located outside and inside of the Zhongmu depression and the intersection zone of fault,the thermal energy of the sedimentary layer comes from the heat conduction from the basement.Conceptual model of the geothermal system in the north and south of Zhongmu depression was established.The geothermal system of Zhongmu depression was a fault-depression basin-type geothermal system,which belongs to the middle-low temperature conduction type geothermal system.The geothermal system in the basement fault zone was the convection type.This study can provide a scientific basis for the sustainable management and utilization of geothermal resources in the Zhongmu depression.