| Changling sag is located in the low plain area in the middle of Songliao basin,which is rich in geothermal water resources in the middle and deep layers.Under the background of"double carbon",promoting clean and low-carbon transformation has become the international development direction.In order to realize the efficient development and utilization of geothermal water resources,it has become an urgent requirement to find out the occurrence law and genetic mechanism of geothermal water resources in the study area in response to the national call.This study relies on the Geological Prospecting Fund of Jilin Province"Geothermal Resources Investigation Project in Central and Western Jilin Province"（No.:2018-13）,and takes the sedimentary basin geothermal water resources in Changling Sag area of western Jilin Province as the research object.Firstly,the regional geological background and hydrogeological conditions of the study area are analyzed;Secondly,the geothermal geological conditions such as thermal reservoir conditions,gravity and magnetic anomalies and geothermal field characteristics are analyzed,and the optimal thermal reservoir in the study area is determined,and the geothermal geological structure model is established Thirdly,the hydrochemical characteristics of geothermal water in thermal reservoirs in the study area are analyzed by using hydrochemical data,and the reserves and exploitable amount of geothermal resources are evaluated;Finally,the genetic mechanism of geothermal water resources in the study area is analyzed from the aspects of geothermal water source,heat source,reservoir,caprock,heat transfer mode and the influence of fault zone,and the genetic model of geothermal water resources in the corresponding area is put forward according to the different genetic factors.Based on the above research,the following main conclusions are drawn:（1）The main thermal reservoir in Changling sag is sandstone layer from the third member of Quantou Formation to Yaojia Formation,which has the characteristics of large thickness,moderate buried depth and wide distribution range.The lithology of thermal reservoir is mainly sandstone,fine sandstone and siltstone,which are distributed in layers in the study area,with buried depth ranging from 750 to 2500 m.The cumulative thickness of thermal reservoir in the whole formation is 150-450 m,porosity ranging from 5%to 30%,and permeability is mostly less than 100 m D.The thermal reservoir of the third member of Quantou Formation is the best thermal reservoir in the middle of the study area.There are thick mudstone caprock in the upper part of the thermal reservoir,in which the cumulative thickness of mudstone in the first and second members of Nenjiang Formation ranges from 200 m to 800 m and the buried depth ranges from 800m to 1400 m;The mudstone between the thermal reservoirs of each group formed the inner caprock of the thermal reservoir,the cumulative thickness ranged from 348 m to1007 m.Together,they constitute the caprock of thermal reservoir,which provides good heat preservation and insulation for thermal reservoir.（2）The geothermal gradient ranges from 3.5-4.4℃/100m,with an average of3.8℃/100m;The Earth heat flow ranges from 68.4 to 77.9 m W/m2,with an average value of 71.7 m W/m2.The average values of the two groups are greater than those of Songliao Basin,showing good thermal storage conditions.At different depths,the variation law of stratum temperature level in the study area is relatively uniform:the temperature in the center of Changling sag is low,and the geotemperature near the fault zone is high,which accords with the geothermal field characteristics of warming sedimentary basins.As a whole,the temperature of vertical strata increases with the increase of depth,which indicates that the geothermal field in the study area is mainly heat conduction,which transfers the heat of upper mantle and magma.（3）There are obvious differences in the content of various ions in geothermal water.The order of the cationic component in milligram equivalent is Na+>Ca2+>Mg2+,and the order of the anionic component in milligram equivalent is Cl->HCO3->SO42-,in which Na+is dominant.The hydrochemical type of geothermal water is mainly Cl-Na.The water in the thermal reservoir in the study area has a high degree of sealing,experienced long-term concentration,leaching and water-rock interaction,and is mainly supplied by lateral runoff,belonging to continental sedimentary-buried water and leaching-infiltration water.（4）The amount of geothermal resources and the exploitable amount of geothermal resources in the study area are evaluated by thermal reservoir method.The total amount of geothermal resources in the study area is 34496.1×1016 J,which is equivalent to117.7×108 t of standard coal.The total amount of geothermal resources stored in the third member of Quansan is the highest,accounting for 42.2%of the total geothermal resources.The total recoverable amount of geothermal resources is 2759.7×1016 J,which is equivalent to 9.4×108 t of standard coal.The recoverable coefficient of geothermal water resources is calculated by 3%,the total recoverable amount is 65.7×108 m3.The energy storage and utilization life is calculated by 100 years,and the total geothermal field productivity in Changling sag is 689.68 MW,which is rich in geothermal productivity and belongs to large geothermal field.（5）Using hydrogen and oxygen isotopes,geothermal water quality and silicon-enthalpy model,it is determined that the geothermal water sources in the study area are mainly atmospheric precipitation and deep circulating water,the average mixing ratio of cold water is 0.31;Using"back stripping method",it is determined that the heat sources of geothermal water are mainly the decay heat of radioactive elements in crust（20.0 m W/m2）and convection heat supply in deep mantle（51.7 m W/m2）;Geothermal system depends on the deep circulation of geothermal water and normal geothermal gradient to increase temperature,and convective heating in deep mantle is the main heat source of geothermal system.Influenced by the differences of reservoir structure,caprock,fault zone and other genetic factors,the study is divided into three regions according to the genetic model,and there are some differences in water source,heat source,reservoir,caprock and heat transfer mode among different regions. |
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