Analysis And Research On The Mechanism Of Geothermal Formation Mechanism In Qingfeng Area Of Bohai Bay Basin

The study area is located inside the southwest of Bohai Bay Basin,with well-developed faults and abundant-medium low-temperature geothermal resources.Currently,geothermal research in the Qingfeng area is mainly confined to the shallow strata.There is only qualitative study for the deep thermal structure,but no more systematic research and analysis,which seriously restricts the development and utilization of geothermal resources in this area.Therefore,it is of great significance to study the mechanism of geothermal genesis and the characteristics of deep thermal structure in the Qingfeng area.Based on the analysis of the regional geological background,geothermal field conditions,and chemical data of geothermal water,a genetic model of geothermal water in the area has been established.The heat generation rate of rocks in the shallow layer is calculated using the relationship between the natural gamma-ray logging data and the heat generation rate of rocks in the area.By using the empirical formula of the seismic wave velocity and the heat generation rate,the heat generation rate of rocks in the deep layer and the heat flow distribution ratio of the crust and mantle in the area are obtained.Also,the conceptual model of the heat structure of the crust in the area is constructed.The deep structure reveals that the buried depth of the Moho surfaces in the Qingfeng area is about 34-35 km.The lithosphere is thin,and the buried depth of Curie's isothermal surface is shallow.So,the study area has good deep thermal and structural conditions.The geothermal field research shows that the geothermal resources in the Qingfeng area have the characteristics of a conduction-type geothermal system.The geothermal gradient value ranges from 2.1?to4.2?/100m,and the average geothermal gradient is 4.1?/100m.The terrestrial heat flow value in the Qingfeng area ranges from 65.46 to 66.06m W/m²,with an average value of 65.76m W/m²,indicating a relatively high geothermal background in this area.Hydro-chemical data reveal that the geothermal water in the Qingfeng area is weakly alkaline.The main cations are Ca²⁺,Na⁺,mg²⁺,whereby SO₄^2-and Cl^-are the anions.The main hydro-chemical types were SO₄·Cl-Ca·Natype,followed by Cl·Na type.The hydrogen and oxygen isotopes show that the geothermal water mainly comes from atmospheric precipitation,first in the uplift area,receiving atmospheric precipitation supply,then infiltrating into the underground,and then heating by deep source heat,entering the shallow strata.Based on the calculated heat generation rate,the crustal thermal structure model of the Qingfeng area is established.The crustal/mantle heat flow ratio is estimated to be 0.57,and the thermal structure type is"hot mantle cold crust".Based on the comprehensive analysis and study of the source reservoir communication cap,the geothermal genetic mechanism of the Qingfeng area has been clarified:the heat source in the area is the mantle and crust heat from the deep earth.The geothermal water mainly comes from the atmospheric precipitation in Taihang Mountain and the Southwest Shandong uplift area.The geothermal fluid migrates along the developed faults and enters the Ordovician reservoir,continuously absorbs the heat from deep conduction and surrounding rock,and finally accumulates higher heat in the reservoir.At the same time,the conceptual model and genetic model map of crustal thermal structure in the Qingfeng area is established.These research results provide a theoretical basis for developing and utilizing geothermal energy in this area and have important guiding significance.