Medium to low enthalpy geothermal reservoirs estimated from geothermometry and mixing models of hot springs along the Malawi Rift Zone

<p>We investigated the relationship between the geothermal fluids and the fracture networks that control the storage and fluid pathways of geothermal systems in the northern part of the Malawi Rifted Zone (MRZ). The MRZ is a magma-poor rift located in the Western Branch, where potential geothermal energy is postulated from elevated heat flow and the emergence of hot springs through fracture zones. However, there is a lack of knowledge about the fracture networks that control fluid pathways and storage of the geothermal systems in the region.</p><p>Preliminary geothermometer calculation studies of hot springs in the MRZ suggested that the highest geothermal reservoir temperatures (200 °C) are found in the northern region. Additionally, the hot springs are associated with the local meteoric water that seeps through deep fracture zones. These structurally-controlled geothermal systems are characterized by geothermal fluids stored in fracture zones with vertical fluid rise (upflows) and/or in shallow sedimentary rocks with  horizontal geothermal circulation (outflows) deposited in basins along the MRZ. </p><p>The guiding hypothesis is that interconnected regional joints, inherited reactivated structures, and Quaternary faults comprise a complex fracture network that controls the geothermal fluid transport and storage of geothermal systems in the northern part of the MRZ. Therefore, this study aims to quantify the relationship between complex fracture networks and geothermal fluids in this region. Here the term “complexity” means fracture networks that show a wider range of orientations and higher intensity than other areas. We use digital elevation models to map structures, density maps of fracture intensity, and topology characterization to identify surface level connectivity. Additionally, we use high-resolution aeromagnetic data to identify possible conductive structures at depth and the relationship between Precambrian structures and geothermal systems. </p><p>The preliminary results show that most of the hot springs in the Karonga area are located in Permian-Triassic and Quaternary basins with ~NNW-SSE fault trends. Also, the hot springs are focused on a region of higher fracture intensity with a favorable setting related to terminations of ~NNW-SSE faults and intersections with reactivated Precambrian foliations (NW-SE). The Chiweta hot spring, the highest reservoir temperature in Malawi, is located at an intersection between NE-SW, N-S, and NW-SE fault systems. Aeromagnetic data shows that most of the hot springs are aligned with the deep conductive structures ~NW-SE oriented of the Karonga Fault Zone (KFZ). The KFZ has been associated with the reactivation of the Precambrian Mughese Shear Zone.</p><p>The expectations of this research are: 1) to provide a better understanding of the fracture networks that transport the geothermal fluids, 2) to identify permeable areas to mitigate the high-risk of drilling non-productive wells, and 4) the low-cost methodology used in this study can be applied in similar areas of the Western Branch.</p>

Geochemistry of thermal fluids and the genesis of granite-hosted Huangshadong geothermal system, Southeast China

Geothermal energy, with associated low-carbon emissions compared to conventional fossil fuels, is presently one of the potential renewable energy sources. The decryption of the chemistry of thermal manifestations of geothermal systems, in terms of relative abundances of various chemical facies, is mandatory before launching systematic geothermal exploration in a given area as it provides valuable information on the basic characteristics of the geothermal reservoir. For the present study, three clusters of hot springs have been selected -1) Shyok-Nubra valleys geothermal prospect in NW Himalaya, 2) Subansiri valley geothermal areas in NE Himalaya and 3) the hot spring system occurring along Munger-Saharsa Ridge Fault Zone (MSRF) in the Eastern part of India. The study of these geothermal systems with low to moderately high temperatures of 35−75°C, has been carried out with two main objectives. The first objective is to estimate reservoir temperature and other characteristics in the three above-mentioned hot spring groups using conventional chemical geothermometry techniques for the first-hand assessment of the quality of the geothermal resource and postulating its possible application. The second objective of the study is to understand the distribution of stable isotopes (δ2H and δ18O) in the studied geothermal systems to reveal reservoir-related hydrological aspects of the geothermal system. The three groups of hot springs show large variations in total dissolved solids (TDS) from <150 to about 1800 mg/L. Enigmatic and inexplicably low TDS values of 126 to 150 mg/Lare reported from the three Bhimband hot springs along MSRF Zone. These springs discharge Ca−Mg−HCO3 type water. The dearth of dissolved ionic species in these springs indicates lack of water-rock interaction in Bhimband area. Silica is the most abundant solute species and corresponds to a reservoir temperature of about 75°C. It may be inferred that relatively inert lithology in the reservoir and very high water-to-rock ratio might have rendered the observed chemical signatures to Bhimband hot springs. Shyok-Nubra springs at Changlung and Panamik are predominantly Na-HCO3 type with TDS values of about 1700 and 550 mg/L, respectively. These hot springs give an indication of water-rock interaction in different types of lithology at temperatures of 120 to >150°C resulting in the observed chemical differences between the two. Thermal water at Taksing in NE Himalaya is also Na−HCO3 type. Chetu hot spring with TDS of >1100 mg/Lis anomalous in two ways. First, it has a very high SO4 content of 358 mg/Land second, it has the lowest silica value. There is a possibility that the Chetu hot spring has its chemistry influenced by the dissolution of sulfates of Ca and Na. Preliminary stable isotope study indicates that the geothermal fluid is derived from meteoric sources. The lack of any indication for positive δ18O-shift suggests that reservoir temperatures of the springs are generally low. Keywords: Thermal Waters, Hydrogeochemistry, Reservoir Temperature, Stable Isotopes, Himalayas, Bihar

Wuhan and its surrounding areas have obvious geothermal spring outcrops, which are unexplored potential geothermal resources. The degree of geothermal resource development in Wuhan is low, and there is a lack of systematic research on their hydrochemical characteristics and formation mechanism. The Wuhan area is bounded by the Xiang-Guang fault, the South Qinling-Dabie orogenic belt in the north, and the Yangtze landmass in the south, with Silurian and Quaternary outcrops and little bedrock outcrops. The Silurian is the main water barrier in the region, which separates the upper Triassic and Paleogene as shallow aquifers and the lower Cambrian and Ordovician as deep aquifers. Different strata are connected by a series of fault structures, which constitute Wuhan’s unique groundwater water-bearing system. Eleven geothermal water (23~52 °C) and six surface water samples (around 22 °C) were collected from the study area. The geothermal water in the study area is weakly alkaline, with a pH of 7.04~8.24. The chemical type of geothermal water is mainly deep SO42− with a higher TDS and shallow HCO3− type water with a lower TDS. Isotopic analysis indicates that atmospheric precipitation and water-rock interaction are the main ionic sources of geothermal water. The chemical composition of geothermal water is dominated by ion-exchange interactions and the dissolution of carbonates and silicates. The characteristic coefficients, correlation analysis, water chemistry type, recharge elevation, geothermal water age, reservoir temperature, and cycle depth were also analyzed. The performance was similar in the same geothermal reservoir, which could be judged as an obviously deep and shallow geothermal fluid reservoir, and the genetic conceptual model of Wuhan geothermal was preliminarily deduced. DXR-8 and DXR-9 had the best reservoir conditions, hydrodynamic conditions, rapid alternation of water bodies, and large circulation depth, which is a favorable location for geothermal resource development and will bring considerable economic and social benefits.

Hydrogeochemistry of the Indian thermal springs: Current status

Taiyuan Basin is a Cenozoic fault basin. The formation of faults block and horst in the basin is controlled by the north-south and east-west faults. The karst thermal mineral water is mainly distributed in the area between the Sanji horst and the Tianzhuang fault. The main aquifers of karst thermal water are the carbonate of the Ordovician Fengfeng group, the upper and lower Majiagou groups. We took 18 geothermal water samples in the field, and collected the hydrochemical data of 9 geothermal water and 3 geothermal water in the existing literature. According to the analysis and test results, the hydrochemical type of karst thermal water is SO4-Ca ·Mg type. According to the ion concentration relationship and the saturation index of the main mineral in the thermal mineral water, it can be inferred that the hydrochemical type is mainly affected by the gypsum layer. In addition to the dissolution of calcite and dolomite, the dissolution of gypsum plays a leading role in the process of groundwater dissolution and filtration. According to the mineral saturation index, Ca2+ produced by gypsum dissolution and the increase of geothermal mineral water temperature also lead to the saturation of calcite or dolomite, which may lead to precipitation. The effect of the dissolution of gypsum on the dissolution of calcite and dolomite has an inhibitory effect. The age of karst thermal mineral water in the Xiwenzhuang uplift is more than 20, 000 a, which is mixed with ancient water. The temperature of karst thermal reservoir is 72.6~91.1 ℃, and the depth of circulation is 2123~2663 m. Long-term water-rock interactions provide the time of conduction heating and enriched mineral components for the thermal mineral water. As the high value area of the temperature, TDS and Sr concentration in karst thermal mineral water, Xiwenzhuang uplift is the catchment area of thermal mineral water in the basin, and also the best area of regional thermal mineral water. The increasing trend of TDS and Sr concentration reflects the obvious dissolution and filtration of groundwater from recharge area to drainage area in the basin. There are occurred such a trend: cold underground water with low TDS and hydrochemical type of HCO3-Ca ·Mg to karst thermal mineral water with high TDS and hydrochemical type of SO4-Ca ·Mg.

Based on the geothermal setting of the Awang Township of Tibet, we analyzed the characteristics and genetic mechanism of geothermal water with the aid of field investigation, hydrogeochemistry, and environmental isotopes. The hydrochemical types of geothermal water belong to HCO3-Na type, resulting from leaching and cation interaction between geothermal water and surrounding rocks. The composition of hydrogen and oxygen isotope indicates an origin of meteoric water. The slight oxygen drift is produced by water-rock interaction and high temperature of geothermal reservoir. The recharge area with estimated altitude of 4600~4800 m is located in northwest mountain areas. The analyses of Na-K-Mg triangle graph and saturation index collectively show the geothermal water is mixed by shallow cold water with the proportion of 60% to 70%. The temperatures of deep geothermal reservoirs are 170~200 ℃ suggesting by quartz and Na-K geothermometers. The circulation depth of geothermal water is 4500~5300 m. The genetic mechanism of geothermal water in Awang Township can be concluded as follows: the thermal water recharged from precipitation is heated by terrestrial heat flow, arises and exposes as geothermal springs along the fault zone.

Hydrochemical and environmental isotope study of the geothermal water in Mae Chan (North) and Ranong (South) geothermal areas in Thailand

Re-visiting Geothermal Fluid Circulation, Reservoir Depth and Temperature of Geothermal Springs of India

Origin, transport and discharge of CO 2 in central Italy

Banjarnegara and Cisukarame are potential areas for geothermal development. This research was conducted to obtain geothermal fluid characteristics, reservoir temperature, geo-thermometer and heat loss. Reservoir temperature is determined based on geothermometer calculations of Na-K-Ca, K-Na-Mg and Na-K. This type of manifestation like hot springs, it can be used to calculate the amount of heat loss. There are four geothermal reservoirs in the Banjarnegara area. Based on the analysis of the geothermometer value, reservoir 1 until 4 temperature is around 81°C - 374°C, the heat loss value reaches 31.6 MW. Beside it, in Cisukarame areas, the highest value of geothermometer temperature average 185°C - 212°C, the heat loss value obtained 48.3 MW. Heat loss value is used to determine the geothermal potential for build prospect power plant. Thus, a more efficient ratio of temperature and heat loss value is Cisukarame area, but both of them are potential for geothermal power plants development.

Applicability of Na/K geothermometer to the metapelitic non-volcanic geothermal fields in the Taiwan orogenic belt

Terrestrial heat flow in the Malawi Rifted Zone, East Africa: Implications for tectono-thermal inheritance in continental rift basins

Evolution of deep parent fluids of geothermal fields in the Nimu–Nagchu geothermal belt, Tibet, China

Pakistan geothermal renewable energy potential for electric power generation: A survey