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Deep Geothermal Processes Acting on Faults and Solid Tides in Coastal Physics of the Earth and Planetary Interiors 264 (2017) 76–88 Contents lists available at ScienceDirect Physics of the Earth and Planetary Interiors journal homepage: www.elsevier.com/locate/pepi Deep geothermal processes acting on faults and solid tides in coastal Xinzhou geothermal field, Guangdong, China ⇑ Guoping Lu a,b, , Xiao Wang b, Fusi Li b, Fangyiming Xu b, Yanxin Wang b, Shihua Qi b, David Yuen c a State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Wuhan 430074, China b School of Environmental Studies, China University of Geosciences, Wuhan 430074, China c Institute of Supercomputing, The University of Minnesota, Twins, United States article info abstract Article history: This paper investigated the deep fault thermal flow processes in the Xinzhou geothermal field in the Received 13 April 2016 Yangjiang region of Guangdong Province. Deep faults channel geothermal energy to the shallow ground, Received in revised form 27 December 2016 which makes it difficult to study due to the hidden nature. We conducted numerical experiments in order Accepted 27 December 2016 to investigate the physical states of the geothermal water inside the fault zone. We view the deep fault as Available online 29 December 2016 a fast flow path for the thermal water from the deep crust driven up by the buoyancy. Temperature mea- surements at the springs or wells constrain the upper boundary, and the temperature inferred from the Keywords: Currie temperature interface bounds the bottom. The deepened boundary allows the thermal reservoir to Solid tide revolve rather than to be at a fixed temperature. The results detail the concept of a thermal reservoir in Geothermal Reservoir terms of its formation and heat distribution. The concept also reconciles the discrepancy in reservoir tem- Deep fault peratures predicted from both quartz and Na-K-Mg. The downward displacement of the crust increases Circulation depth the pressure at the deep ground and leads to an elevated temperature and a lighter water density. Ultimately, our results are a first step in implementing numerical studies of deep faults through geother- mal water flows; future works need to extend to cases of supercritical states. This approach is applicable to general deep-fault thermal flows and dissipation paths for the seismic energy from the deep crust. Ó 2017 Elsevier B.V. All rights reserved. 1. Introduction the source for the hot waters (Liang, 1993). However, the deep flow processes remain relatively unknown. With elevated temperatures (one of the thermal water vent’s The thermal springs that occur in association with a fault zone temperature is as high as 98°C), the Xinzhou geothermal field is have the potential to provide data on geochemical characteristics, known for its thermal water outflows associated with a deep fault- physical states, and thermal outflows, all of which are useful in ing. Located in the coastal region of western Guangdong, it has a understanding deep flow processes and cyclic variations. In this backdrop of the southern and south-eastern China geothermal belt, study, daily and annual variations in the geothermal waters of characterized by regional faults along the longitudinal, latitudinal, Xinzhou geothermal field were recorded and modeled to under- and northeastern-southwestern directions (Fig. 1a) (Lu and Liu, stand the water flow processes of the system. The modeling study 2015). is considered as a vital tool used in understanding the underground The deep geological structure of the Xinzhou geothermal area geo-environments for the site. has not been identified until recently. The Xinzhou geothermal area Geothermal springs are commonly associated with fast flow is classified as a medium-scale geothermal field (Tian, 2012). The paths that connect the deep crust to the surface (Curewitz and hot spring waters are slightly alkaline and salty, and of Cl-Na type Karson, 1997; Zheng and Bennett, 2002; Sun and Lu, 2009) due (Table 1; Wang, 2013), mixed with the cold water and the seawater. to the horizontal impedance and vertical enhancement of the The local area’s ground waters are generally of HCO3-Ca, HCO3-Cl- groundwater flow by a fault zone (Bredehoeft et al., 1992; Caine Ca-Na, and SO4-Cl-Ca-Na types. The atmospheric precipitation is et al., 1996; Lu and Liu, 2015). Deep faults and fractured bedrocks, as infiltration and runoff channels, are critical control factors in the ⇑ flow of hot waters to the surface (Wang, 2005; Gao et al., 2009). Corresponding author at: School of Environmental Studies, China University of Faults and fractures are potential channels for seawater to intrude Geosciences, Wuhan 430074, China. E-mail address: [email protected] (G. Lu). inland concurrently (Chen et al., 2001). http://dx.doi.org/10.1016/j.pepi.2016.12.004 0031-9201/Ó 2017 Elsevier B.V. All rights reserved. G. Lu et al. / Physics of the Earth and Planetary Interiors 264 (2017) 76–88 77 Fig. 1. Geological background map for the Xinzhou geothermal field in Yangjiang, Guangdong province: a. Regional tectonic map (Guangdong Province Geological Bureau Regional Geological Survey Brigade 1988; Chinese Academy of Sciences,1959); b. Regional geological map for Xinzhou geothermal field (Geological information drawn from 1: 250,000 outline configuration diagram of Yangjiang city, 2004); and c. Water sampling sites. New Hole was a 1000 m deep scientific borehole (Wang et al., 2015). Cross section I-I’ shown in Fig.6. 78 G. Lu et al. / Physics of the Earth and Planetary Interiors 264 (2017) 76–88 Table 1 Thermal reservoir temperatures (T) and reservoir depthsa estimated using geothermometers for Na/K, K/Mg and quartz. No. Name of Spring Temperature (°C) Reservoir Depthb (m) Na-Kc K-Mg Quartz Maximum Minimum XZ01d New hole 219 140 153 3369.0 2921.5 XZ02 Dun well 209 158 154 3393.7 2942.9 XZ03 Ta well 222 141 157 3450.7 2992.4 XZ04 Jia well 213 158 155 3409.5 2956.6 XZ05 East wei well 214 153 156 3433.3 2977.2 XZ06 East mao 213 142 147 3202.8 2777.5 XZ07 Old hole 202 130 138 2985.4 2589.0 XZ08 West ting 211 156 151 3311.7 2871.9 XZ09 East tang 217 144 154 3379.5 2930.6 Note: a. Aqueous chemistry data from Xu (2016); b. Reservoir depth calculated by (Ta À T0)/g + h. Ta means temperature of quartz thermometer, while To is the average annual temperature of Guangdong (22.5°C). Constant h represents depth of annual constant temperature (13 m), and g is geothermal gradient (3.9°C/100 m–4.5°C/100 m); c. Formulation based on Giggenbach (1988); d. All samples are of Cl-Na type. As a realistic and emerging source, geothermal energy has surrounded by low mountains on the eastern and southeastern attracted increased attention (Baioumy et al., 2015). Furthermore, edges. The altitude of the Xinzhou geothermal field is about geothermal energy is clean, renewable, and does not produce car- 10–13 m (Fig. 1). bon dioxides (Smith, 2007; Younger and Gluyas, 2012; Craiget al., Quaternary sediments cover the Xinzhou geothermal field. The 2013). Studies on the potential of geothermal energy have implica- basement granites are buried on a shallow level and their outcrops tions in environmental geology. Deep and large faults are often can be seen in ditches and ponds. To the north of the site is the linked to potential geothermal energy, as well as potential earth- boundary of Precambrian-Cambrian light metamorphic clastic quakes (Skarbek and Rempel, 2016; Bai et al., 2013). rocks (Fig. 1c). We studied the geothermal system in terms of the high temper- Hot springs in Xinzhou are naturally exposed along the stream ature and high pressure states in the deep fault. We also investi- bed. Hot water flows from the boreholes and oozes from the pond gated the up-swelling of the geothermal water and the driving bottoms. The Jia well is located in the middle of the field and the force along the deep fault in the Xinzhou geothermal field in order thermal water outflow can reach up to 98.4 °C, with this being to find out the interplay of controlling factors. We did this in order the highest temperature among the springs. to further understand how the deep system responds to changes of The field site has a deep fault at a high angle (almost vertical at states. The results could potentially provide further insight into 97°), tipping to the south. The fault was initially revealed in dril- deep geothermal behavior in response to earthquakes. lings at an earlier time (Liang, 1993). Recently, the fault was fur- The objective of this paper is to study the physical and chemical ther characterized by the geophysical method of Audio processes in the geothermal site via field site monitoring and Magnetotelluric Sounding (AMT) (Wu, 2013) and was later con- numerical modeling. The site’s description, conceptual model, and firmed in a 1000-m scientific drilling in 2014 (Wang et al., 2015). research methods are described (Sections 2 and 3), as well as the The advanced geophysical method’s apparent resistivity and impe- daily and annual variations in the outflows (Section 4). We also esti- dance phase have revealed the faulting to extend to a depth about mated the thermal reservoir temperature and calculated the reser- 10 km into the crust (Wang et al., 2015). voir depth (Section 5). The modeling study is presented (Section 6), and the findings are discussed and summarized (Sections 7 and 8). 2.3. Conceptual model 2. Site description and conceptual model The flow system is conceptualized as being well connected to the deep heat source through the deep fault F1 (Figs. 1 and 2). 2.1.
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