The Bearing Features And Genetic Model For Geothermal Resources In Guanzhong Basin

Guanzhong Basin which located in the transition part of Ordos Basin and Qinling orogenic belt possesses favorable geothermal resource and geothermal conditions influenced by deep heat source and geologic structure. The geothermal resource utilization in the Basin has a long history with mostly spontaneous extensive exploration which resulting in a waste of geothermal resources, groundwater levels decreased year by year, series of geological environmental problems and even the sustainable development and utilization of new challenges. How to develop and use the geothermal resources reasonably, effectively and scientifically plays an important part in the geothermal field life and environmental influences mitigations caused by the geothermal exploration.The geothermal water stored in Guangzhong Basin was analyzed in this thesis based on the geothermal geological background, geothermal fluid chemistry, isotopic geochemistry, regional geological structural characteristics, deep features and results of previous research. The results shows low-middle geothermal occurrence feature of Guangzhong Basin improve the formation mechanism of geothermal resources which provide theoretical support for regional geothermal resources exploration and management. Ra, Rn, radioactivity distribution of total α and total β in geothermal water were firstly analyzed to verify the heat source characterisitics of radioactivity. Meanwhile, the thesis did the analysis of hot water associated He characteristics, mantle invasion ratio and typical fault features, improve the conceptual model of geothermal system formation.Affected by the lithology, geological structure and some other factors, from the marginal basin to the central regions, the hot water temperature, TDS, 14 C ages, δ34S, δ18O increase and D, HCO3/Cl ratio decrease, the water chemistry type changes from Ca, HCO3, SO4 to Cl-Na, 18 O drift and Sr concentration increases, all of which indicate a relatively closed geological environment with strong reducibility, longer replacement cycles exchange with the outside, intensive water- rock interaction and strong degree of contact with ground water aquifers and other aquifers. For the underground water retention area which maybe controlled by the regional geological structure, Water soluble gas He mainly from crust can reach up to 3.95% with intrusive mantle-derived He amount from 0.15% to 6.92%, the highest value area correspond with the highest heat flow area(96.0mw/m2) of the region. Geothermal wells lying on the intersection area of Changan-Lintong fault and north Weihe fault reveal the connectivity from the tope to the mantle area based on the deep fault indicating the weak material and energy transference. Radioactivity of the rock is another heat source showed from the radioactive activity of hot water and basement lithology distribution.Groundwater of Guangzhong Basain can be categorized as Qinling piedmont Basin fractured structure type, central Cenozoic porosity fractured type and northern Cenozoic basins Paleozoic rocks fractured type. The research of water chemistry, isotope and gas chemistry show that the ground hot water of the basin could be divided into four categories, those were open-cycle type, half-cycle semi-closed, closed, sealed type deposit. From the open loop model to the deposition sealed type, the thermal storage environment transformed from oxidizing to reductive, the water recharged from modern meteoric water on the edge of the basin to the Last Glacial ancient precipitation in the center, the cycle replacement capacity of the underground hot water becoming more deteriorative, the challenges of sustainable development and utilization of geothermal resources becoming greater and the stricter management and technical for development and utilization should be carried out.The heat of the basin mainly from heat conduction of the asthenosphere and heat radiation of radioactive elements’ decay in the earth’s crust; hot water was mainly recharged from atmospheric precipitation, modern meteoric water recharged the groundwater under the edge of the basin along the fault zone, while central region recharged by the ancient meteoric water; the pore and fracture fault and stratum developed on the front of Qinling Mountains, central and Northern of the basin provided storage space for underground hot water; the developed deep fracture zone provided a channel for geothermal fluid moving and mixing; overlying thick sedimentary strata was the good sealing cover structure for the heat resources. Because of the unique geological structure and geothermal geological condition, the heat conducted from upper mantle heated crustal rocks and basement of the basin from which the water in storage layer got heat, then transferred heat in the form of convection to the upper or the surrounding aquifer or fault zone. So the formation of the geothermal resources was mainly from upper mantle heat conductive with thermal convection cycling system in aquifers.