Spatio-temporal variations of shallow seismic velocity changes in Salton Sea Geothermal Field,California in response to large regional earthquakes and long-term geothermal activities

We measure spatio-temporal variations of seismic velocity changes in Salton Sea Geothermal Field,California based on cross correlations of daily seismic traces recorded by a borehole seismic network from December 2007 to January 2014.We find clear co-seismic velocity reductions during the 2010 M 7.2 El Mayor–Cucapah,Mexico earthquake at~100 km further south,followed by long-term recoveries.The co-seismic reductions are larger with longer post-seismic recoveries in higher frequency bands,indicating that material damage and healing process mostly occurred in the shallow depth.In addition,the co-seismic velocity reductions are larger for ray paths outside the active fluid injection/extraction regions.The ray paths inside injection/extraction regions are associated with smaller co-seismic reductions,but subtle long-term velocity increases.We also build 3D transient water flow models based on monthly injection/extraction rates,and find correlations between several water flow parameters and co-seismic velocity reductions.We interpret the relative lack of co-seismic velocity changes within the geothermal region as unclogging of fracture network due to persistent fluid flows of geothermal production.The long-term velocity increase is likely associated with the ground water depletion and subsidence due to net production.

Integrated application of gravity,aeromagnetic,and electromagnetic methods in exploring the Ganzi geothermal field,Sichuan Province,China

The Ganzi geothermal field is located in the Songpan-Ganzi orogenic belt in Sichuan Province.Many hot springs are exposed along the Yalahe valley in Ganzi geothermal field,which is a favorable area for hightemperature geothermal resource exploration.However,the geological model of heat exchange,the regional structure controlling hydrothermal convection and the development model of geothermal reservoirs are still unclear.Therefore,further studies are necessary to meet the geothermal exploration requirements in the middle and deep strata of this geothermal field.In this study,a geological model of the geothermal system of Ganzi geothermal field is proposed.We are convinced that there exists a hydrothermal convection system in the Ganzi geothermal field,the heat transfer of which is accomplished through deep-rooted major faults.Therefore,the identification of deep-rooted major faults and the description of geothermal reservoirs are the research objects of the integrated geophysical methods.The main factors controlling the geothermal reservoirs in the deep-rooted Xianshuihe major fault and Yalahe fault zones are analyzed by using gravity,aeromagnetic,and electromagnetic methods and techniques.The analysis results of regional gravity and aeromagnetic anomalies show that the Xianshuihe major fault has produced obvious gravity and aeromagnetic anomalies on the surface,and thus the position and strike of this fault can be accurately predicted by inversion of the aeromagnetic anomalies.Geothermal reservoirs show low-resistivity anomalies in the electromagnetic profile.The inversion results of the controlled source audio-frequency magnetotelluric(CSAMT)data show that geothermal reservoirs are mainly developed along the Yalahe valley,and the west side of the valley is more favorable for geothermal exploration.This study is of guiding significance to the efficient exploitation and utilization of the Ganzi geothermal field.

Factors controlling the distribution of granite reservoirs of hydrothermal system type in South China:A case study of Huangshadong geothermal field in Yuezhong Depression,China

The granitoids widely distributed in South China are characterized by multi-stage evolution via episodic intrusions,in a complex geodynamic setting.Since granites have high radioactive heat generation and excellent thermal conductivity,a deep moderate-to high-temperature geothermal system can be formed in the presence of high-quality,fissured granite geothermal reservoirs and thermal insulation with appropriate cap rocks.The key to exploring deep geothermal resources is to identify high-quality fissured granite geothermal reservoirs of a certain scale in a thermal anomaly zone with high background heatflow values.To determine the controlling effects of the distribution and development characteristics of granite geothermal reservoirs on the generation and enrichment of deep geothermal resources,this study analyzed the characteristics of the geothermal reservoirs in the Huangshadong geothermal field in the Yuezhong Depression,Guangdong Province,and their controlling effects on the formation of geothermal resources.The results are as follows.The hydrothermal system in the Huangshadong geothermal field mainly distributed in the contact zones between magmatic plutons and surrounding rocks,is significantly controlled by faults,followed by neoid volcanic apparatus and magmatic activities.That is,the geothermal system therein is under joint control of structures and magmas.Moreover,fractured zones of neoid transtensional faults conduct the geothermal water in the hydrothermal system and control its shallow discharge.Therefore,the hydrothermal system in the study area is characterized by the control of transpressional tectonic zone and volcanic apparatus,and geothermal water conduction through fractured zones of transtensional faults.

Geology and development of geothermal field in Neogene Guantao Formation in northern Bohai Bay Basin: A case of the Caofeidian geothermal heating project in Tangshan, China

Taking the Gaoshangpu-Liuzan geothermal field in the Nanpu sag of the Bohai Bay Basin as the research object, this paper discusses the geological conditions and potential of the geothermal resources of the Guantao Formation in the study area, and introduces the development practice of geothermal energy heating in Caofeidian. The average buried depth of the Guantao Formation is 1500–2500 m, the lithology is dominated by sandy conglomerate, and the average thickness of thermal reservoir is 120–300 m. The average porosity of thermal reservoir is 28%–35%, the permeability is(600–2000)×10^(-3) μm^(2), and the temperature of thermal reservoir is 70–110 ℃. The formation has total geothermal resources of 13.79×10^(18) J, equivalent to 4.70×10^(8) t of standard coal. Based on a large amount of seismic and drilling data from oil and gas exploration, this study carried out high quality target area selection, simulation of sandstone thermal reservoir, and production and injection in the same layer. The geothermal heating project with distributed production and injection well pattern covering an area of 230×10^(4) m^(2) was completed in the new district of Caofeidian in 2018. The project has been running steadily for two heating seasons, with an average annual saving of 6.06×10^(4) t of standard coal and a reduction of 15.87×10^(4) t of carbon dioxide, achieving good economic and social benefits. This project has proved that the Neogene sandstone geothermal reservoir in eastern China can achieve sustainable large-scale development by using the technology of "balanced production and injection in the same layer". It provides effective reference for the exploration and development of geothermal resource in oil and gas-bearing basins in eastern China.

Geochemical characteristics of geothermal water in Weiyuan geothermal field, Huzhu County, Qinghai Province

According to the chemical composition of thermal water from Geothermal Well DR2010 located in the Weiyuan Geothermal Field of Huzhu County in Qinghai Province, the groundwater recharge, age and geothermal resource potential of the thermal water are discussed by using the methods of Langelier-Ludwig Diagram, isotopic hydrology and geochemical thermometric scale. The analysis results indicate that the Weiyuan Geothermal Field is located in the northern fringe of Xining Basin, where the geothermal water, compared with that located in the central area of Xining Basin, is characterized by greater water yield, shallower buried depth of thermal reservoir and easier exploitation. Due to its active exchange with the modern cold water, the thermal water here shows relatively younger age. These findings provide a hydro-geochemical evidence for the exploitation of Weiyuan Geothermal Field.

Structural Controls on the Geochemistry and Output of the Wells in the Olkaria Geothermal Field of the Kenyan Rift Valley

The Olkaria geothermal field is located in the Kenya Rift valley, about 120 km from Nairobi. Geothermal activity is widespread in this rift with 14 major geothermal prospects being identified. Structures in the Greater Olkaria volcanic complex include: the ring structure, the Ol’Njorowa gorge, the ENE-WSW Olkaria fault and N-S, NNE-SSW, NW-SE and WNW-ESE trending faults. The faults are more prominent in the East, Northeast and West Olkaria fields but are scarce in the Olkaria Domes area, possibly due to the thick pyroclastics cover. The NW-SE and WNW- ESE faults are thought to be the oldest and are associated with the development of the rift. The most prominent of these faults is the Gorge Farm fault, which bounds the geothermal fields in the northeastern part and extends to the Olkaria Domes area. The most recent structures are the N-S and the NNE-SSW faults. The geochemistry and output of the wells cut by these faults have a distinct characteristic that is the N-S, NW-SE and WNW-ESE faults are characterized by wells that have high Cl contents, temperatures and are good producers whereas the NE-SW faults, the Ring Structure and the Ol’Njorowa gorge appear to carry cool dilute waters with less chloride concentration and thus low performing wells. Though the impacts of these faults are apparent, there exists a gap in knowledge on how wide is the impact of these faults on the chemistry and performance of the wells. This paper therefore seeks to bridge this gap by analysis of the chemical trends of both old wells and newly drilled ones to evaluate the impacts of individual faults and then using buffering technique of ArcGis estimate how far and wide the influence of the faults is. The data was obtained after the sampling and analysis of discharge fluids of wells located on six profiles along the structures cutting through the field. Steam samples were collected with a stainless steel Webre separator connected between the wellhead and an atmospheric silencer on the discharging wells whereas the analysis was done in house in the KenGen geochemistry laboratory. The results indicates that Olkaria field has three categories of faults that control fluid flow that is the NW-SE trending faults that bring in high temperature and Cl rich waters, and the NE-SW trending Olkaria fracture tend to carry cool temperature waters that have led to decline in enthalpies of the wells it cuts through. The faults within the Ol Njorowa gorge act to carry cool, less mineralized water. Though initially, these effects were thought to be in shallow depths, an indication in OW-901 which is a deeper at 2200 m compared to 1600 m of OW-23 well that proves otherwise. This is, however, to be proved later as much deeper wells have been sited.

Crustal Thermo-Structure and Geothermal Implication of the Huangshadong Geothermal Field in Guangdong Province

Dense distribution of granites and surrounding hot springs, the high anomalous heating rates of geothermal fluids and the high geothermal gradients in shallow crust in Southeast China are revealed by previous geothermal explorations. However, there have always been debates on the genesis of geothermal anomalies of Southeast China. It is imperative to look into the genesis mechanism of geothermal anomalies through selecting a typical geothermal field, and constructing fine crustal thermostructure. In this study, in-depth excavation is implemented for the previous data of geophysical exploration and deep drilling exploration in the Huangshadong area. We synthetically analyze the results of radioactive heat productions(RHPs), thermophysical properties of rocks and audio-frequency magnetotellurics(AMT) sounding. This study concludes that the coefficient of radioactive heat generation(RHG) of crustal rocks and conduction heat of concealed granites is the main formation mechanism of geothermal anomalies of South China, where occurs a Great Granite Province. There is a regional indicating implication for the genesis of geothermal anomalies, taking the Huangshadong geothermal field as a typical example. It is also an important reference to guide the exploration, evaluation, development and utilization of geothermal resources in this region.

Three-dimensional electrical structure model of the Yangbajain geothermal field in Tibet:Evidence obtained from magnetotelluric data

The Yangbajain Geothermal Field in Tibet is located in the fault subsidence basin of the central Yadong-Gulu Rift Valley.The spatial distribution of the field is controlled by mountain-front fault zones on the northwestern and southeastern sides of the basin.Geothermal power has been generated in Yangbajain for more than 40 years.However,owing to the lack of threedimensional(3D) geophysical exploration data,key geological issues related to the partial melt body of the Yangbajain Geothermal Field,such as its location,burial depth,and geometric form,as well as the ascending channel of the geothermal fluid,have for a long time been controversial.In this study,3D inversion was performed using measured geo-electromagnetic total impedance tensor data from 47 survey points.The extracted horizontal sections at different depths and profiles,and at different lines,reflect the 3D electrical structure model of the geothermal field in the study area.Subsequently,three findings were obtained.First,the partial melt body,located below the China-Nepal Highway extending along the northeast direction,is the heat source of the Yangbajain Geothermal Field.The burial depth range of the molten body was determined to range between approximately 6.2 and 14 km.Moreover,the geothermal fluid ascended a horn-shaped circulation channel with an up-facing opening,located in the northern section of the sulfur ditch area.The study results revealed that deep rock fissures(>2 km) were not well developed and had poor permeability.In addition,no layered heat reservoirs with high water richness were observed in the northern part of the study area.However,the application of enhanced geothermal system(EGS) technology in the northern region would be essential to improving the power generation capacity of the Yangbajain Geothermal Field.In addition,the study found no deep high-temperature heat storage areas in the southern region of the study area.

Controlling effects of cap rocks on the formation of deep geothermal resources

Cap rocks with high thermal insulation are important for deep geothermal systems at a depth of 3000‒6000 m.Based on the deep geothermal geological conditions in the Fujian-Guangdong-Hainan area of South China,this study established an ideal geological model of reservoir-cap rock assemblages and simulated the geothermal field distribution of cap rocks of different thicknesses and thermal conductivity.The simulation results show that the vertical geothermal temperature distribution in an uplifted area of a depression was present as inverted mirror reflections relative to the elevated area of the basement.Specifically,the isotherms above the elevated area are convex in shape,while those below the elevated area are concave.There is a temperature equilibrium line between the convex and concave isotherms.The heat flow moves from the depressed area to the uplifted area below the temperature equilibrium line and migrates in an opposite direction above the line.On this base,this study conducted the inversion of geothermal temperature fields in typical areas with thin,moderately thick,and thick cap rocks.The results indicate that,at the depth of 3000e6000 m,areas with thin cap rocks(igneous rock zone in the coastal area of Fujian)mainly host moderate-to low-temperature hydrothermal resources;areas with moderately thick cap rocks(Yuezhong Depression)have the geothermal temperature ranging between 100℃and 200℃and may develop moderate-to high-temperature hydrothermal resources and hot dry rocks(HDRs),with the former superimposing on the latter;and areas with thick cap rocks(onshore Beibuwan Basin)have a geothermal temperature of 120‒220℃,and contains mainly moderate-to high-temperature hydrothermal resources and HDRs.Therefore,it is recommended that the evaluation,exploitation,and utilization of deep geothermal resources be carried out according to the burial depth of the temperature equilibrium line and the specific demand for geothermal resources.

Surface geochemical anomaly pattern of medium-to-high temperature geothermal systems in South China and its application

The existence of thermal storage will correspondingly increase the temperature of surrounding strata and promote the continuous expansion,volatilization,upward migration,and loss of gas in the strata.As a result,a low-concentration gas field will be formed in the strata above geothermal reservoirs.Geothermal reservoirs could in turn heat formation water and increase the solubility of soluble inorganic salts in the surrounding rocks and the total dissolved solids(TDS)content in the formation water.Since water can strongly wet and permeate strata,the dissolved inorganic salts migrate into upper strata along with water,giving rise to the formation of a high-concentration inorganic salt field in the strata above geothermal reservoirs.A higher geothermal reservoir temperature corresponds to more significant characteristics mentioned above.Therefore,a medium-to-high temperature geothermal system has a surface geochemical anomaly pattern of high inorganic salt concentrations and low gas concentrations(also referred to as the high-salt and low-gas pattern).This pattern is applied to the surface geochemical exploration of the two geothermal fields in Guangdong Province,i.e.,the Huangshadong geothermal field in Huizhou City and the Xinzhou geothermal field in Yangjiang City,revealing low-concentration gas fields above both.The application results also show that the exposed thermal spring water in both geological fields has higher concentration of dissolved inorganic salt than the surface water and nearby seawater,forming high-amplitude anomalies on the surface above geothermal reservoirs.These characteristics,as well as the measured temperature at known geothermal wells,verify the validity of the high-salt and low-gas pattern of medium-to-high temperature geothermal systems proposed in this study.Moreover,the high-salt and low-gas pattern proposed predicts three favorable medium-to-high temperature geothermal zones in the surface geochemical exploration of the Shiba Basin near the Huangshadong geothermal field.

Fluid Dynamic Field in BozhongDepression, Bohai Bay Basin

The data from regional geology, boreholes, geophysics and tests are integrated to analyze the fluid dynamic field in the Bozhong depression, Bohai Bay basin. The current geothermal gradient is determined to be about 2.95 /100 m by integrating 266 drill-stem test (DST) measurements and comparing with the global average value. The paleogeothermal gradients are calculated from the homogenization temperatures of saline inclusions, which vary both laterally and vertically. The data from sonic logs, well tests and seismic velocities are used to investigate the pressure variations in the study area. The mudstone compaction is classified as three major types: normal compaction and normal pressure, under-compaction and overpressure, and past-compaction and under-overpressure. The current pressure profile is characterized by normal pressure, sight pressure and intense overpressure from top to bottom The faults, unconformity surfaces and interconnecting pores constitute a complex network of vertical and horizontal fluid flows within the depression. The fluid potential energy profiles present a "double-deck" structure. The depocenters are the area of fluids supply, whereas the slopes and uplifts are the main areas of fluids charge.

The accumulation characteristics and exploration potential of oil and gas in the back-arc basin of Japan under the background of high heat flow

The Sea of Japan is located in the southeast margin of Eurasia, in the triangle area of the western Pacific Ocean. Due to the interaction of the Pacific plate, Eurasian plate and Philippine plate, its tectonic environment is complex, forming a typical trench-arc-basin system. At present, 148 oil and gas fields have been discovered in Japan, with an oil and gas resource of 255.78×10^(6) t, showing a good prospect for oil and gas exploration. Based on the previous research and the recently collected geological and geophysical data, the characteristics of tectonic-sedimentary evolution and geothermal field in the basins around the Sea of Japan are analyzed. The results show that the tectonic evolution of the basin is mainly controlled by plate subduction and back-arc oceanic crust expansion, and it mainly undergone four tectonic-sedimentary evolution stages: Subduction period, basin development period, subsidence period and compression deformation period. The overall heat flow value of Japan Sea is high, and it is distributed annularly along Yamato Ridge. The geothermal heat flow value is about 50–130 MW/m^(2), and the average heat flow is75.9±19.8 MW/m^(2), which has a typical “hot basin ”. The high heat flow background provides unique thermal evolution conditions for hydrocarbon generation, which leads to the high temperature and rapid evolution. The authors summarized as “early hydrocarbon generation, rapid maturity and shallow and narrow hydrocarbon generation window”. The type of oil and gas is mainly natural gas, and it mainly distributed in Neogene oil and gas reservoirs. The trap types are mainly structural traps, lithologic traps and composite traps. In addition, the pre-Neogene bedrock oil and gas reservoirs also show a good exploration prospect. The resource prospecting indicates that Niigata Basin, Ulleung Basin and kitakami Basin are the main target areas for future exploration and development.

Thermal Lithospheric Thickness of the Sichuan Basin and its Geological Implications

Thermal lithospheric thickness is an important parameter in studying the tectonic-thermal evolution of basins and plate dynamics.Based on the measured geothermal data and thermophysical properties of the rocks,the thermal lithospheric thickness of the Sichuan Basin was calculated according to the principles of heat conduction in the crust and lithospheric mantle.The calculation results revealed that the thickness of the thermal lithosphere in the Sichuan Basin is 140-190 km and is unevenly distributed.The thickness of the thermal lithosphere in central Sichuan and southwestern Sichuan is less than 160 km,while that in the western Sichuan depression and eastern Sichuan is larger(~180 km).The distribution of the thermal lithospheric thickness in the basin has a good correlation with the geological units and the thickness of the sedimentary layers.The thickness of the thermal lithosphere in the depression area,which has thick sedimentary layers and the fault-fold zone with shallow crustal deformation and thickening,are larger than that in the basement uplifted area,which has thin sedimentary layers.The calculated thermal lithospheric thickness is in good agreement with the geophysical data and reflects the stable conduction temperature field in the Sichuan Basin.The present thermal regime and thermal lithospheric thickness of the Sichuan Basin indicate that flexural thickening of the lithosphere occurred in the eastern Sichuan fault-fold belt and the Longmen Mountain-Western Sichuan depression foreland basin system,while asthenospheric uplift occurred in the central Sichuan region,which were the result of the expansion of the Xuefeng orogeny from the east and the compression of the Tibetan Plateau from the west.