ALBERT

Description: The Seferihisar–Balçova Geothermal system (SBG) is characterized by complex temperature and hydrochemical anomalies. Previous geophysical and hydrochemical investigations suggest that hydrothermal convection in the faulted areas of the SBG and recharge flow from the Horst may be responsible for the observed patterns. A numerical model of coupled fluid flow and heat transport processes has been built in order to study the possible fluid dynamics of deep geothermal groundwater flow in the SBG. The results support the hypothesis derived from interpreted data. The simulated scenarios provide a better understanding of the geophysical conditions under which the different fluid dynamics develop. When recharge processes are weak, the convective patterns in the faults can expand to surrounding reservoir units or below the seafloor. These fault-induced drag forces can cause natural seawater intrusion. In the Melange of the Seferihisar Horst, the regional flow is modified by buoyant-driven flow focused in the series of vertical faults. As a result, the main groundwater divide can shift. Sealing caprocks prevent fault-induced cells from being overwhelmed by vigorous regional flow. In this case, over-pressured, blind geothermal reservoirs form below the caprocks. Transient results showed that the front of rising hot waters in faults is unstable: the tip of the hydrothermal plumes can split and lead to periodical temperature oscillations. This phenomenon known as Taylor–Saffman fingering has been described in mid-ocean ridge hydrothermal systems. Our findings suggest that this type of thermal pulsing can also develop in active, faulted geothermal systems. To some extent, the role of an impervious fault core on the flow patterns has been investigated. Although it is not possible to reproduce basin-scale transport processes, this first attempt to model deep groundwater geothermal flow in the SBG qualitatively supported the interpreted data and described the different fluid dynamics of the basin.

Description: The Seferihisar-Balçova Geothermal system (SBG), Western Anatolia, Turkey is characterized by complex temperature and hydrochemical anomalies manifested by hot springs along the major fault systems and contrasting salinity distributions in different areas of the basin. Thermal waters in the Northern Balçova are heated meteoric freshwater, whereas the hot springs of the southern Seferihisar region have a strong seawater contribution. Previous numerical simulations of coupled fluid flow and heat transport processes along a North-South transect indicated that the interaction between forced convection from the Seferihisar Highs and free convection in the faults (i.e. mixed convection) is likely the major flow mechanism. It turned out that focussed upsurge of hot water in the faults induces a convective-like flow motion in the surrounding reservoir-units, even at sub-critical Rayleigh conditions. Below the sea, these fault-induced cells stretch from the seafloor toward the inner part of the basin. The question arises whether the described fault-induced cells could be responsible for sweater encroachment in the SBG. In this contribution, this hypothesis is investigated by fully coupling salt transport to the thermally-driven flow models (i.e. thermohaline flow). New isotopes data are presented to support the numerical findings. The results show that, within the expected ranges of hydraulic permeability, fault-induced cells generate brine plumes which protrude from the seafloor toward the faults along the basement interface. At the fault intersections, seawater mixes with rising hot thermal waters. Driven by buoyant forces, the captured brines ascend along the fault flanks reaching near-surface aquifers. Shallow alluvial sediments play a major role in shaping brine plumes and controlling discharge areas. In Balçova, the thick alluvium deposits and the regional flow prevent ascending salty water from spreading at the surface, whereas the weak recharge flow in the thin alluvium unit of the southern SBG is not sufficient to flush the ascending hot brines toward the sea. Further controlling factors as permeable interfaces and minor faults cutting the seafloors are studied. Although permeability variation and local heterogeneities modify flow and chemical patterns, fault-induced cells persist. This process seems to be a plausible mechanism allowing inland seawater circulation. Because of parameters uncertainty, it is not possible to reproduce thermohaline flow at basin-scale. Nevertheless, this attempt to model seawater circulation in the SBG qualitatively supported the interpreted data and described the different fluid-dynamics of the basin. The suggested mechanisms and flow patterns could likely develop in several coastal hydrothermal systems of the world with similar tectonic features.

Description: The Seferihisar-Balçova Geothermal system (SBG), Turkey, is characterized by temperature and hydrochemical anomalies along the faults: thermal waters in northern Balçova are heated meteoric freshwater, whereas the hot springs of the southern Seferihisar region have a strong seawater contribution. Previous numerical simulations of fluid flow and heat transport indicated that focused upsurge of hot water in faults induces a convective-like flow motion in surrounding units. Salt transport is fully coupled to thermally driven flow to study whether fault-induced convection cells could be responsible for seawater encroachment in the SBG. Isotope data are presented to support the numerical findings. The results show that fault-induced convection cells generate seawater plumes that extend from the seafloor toward the faults. At fault intersections, seawater mixes with rising hot thermal waters. The resulting saline fluids ascend to the surface along the fault, driven by buoyant forces. In Balçova, thick alluvium, minor faults and regional flow prevent ascending salty water from spreading at the surface, whereas the weak recharge flow in the thin alluvium of the southern SBG is not sufficient to flush the ascending hot salty waters. These mechanisms could develop in any faulted geothermal system, with implications for minerals and energy migration in sedimentary basins.

Description: The worldwide concern about the consequences of global warming is increasing interest in developing geothermal resources, both for power generation and direct use. This is providing new field data numerical investigations of extensional geothermal systems. The numerical models should explore the role of faults and fractures on the different forces driving hot fluid flows. To date, however, there has been limited effort made to systematically determine the basic relationships between system configuration (e.g., hydraulic permeability, inherited geological structures) and the resulting thermally induced flow behavior of geothermal systems. An exceptional example to investigate the mentioned issues is the hot Seferihisar-Balçova Geothermal system (SBG) which is part of the Cesme Peninsula, Western Anatolia, Turkey. In the SBG, geothermal processes are extremely vigorous and hot waters close to boiling temperature can be observed at the surface. Here, the first numerical models of coupled fluid flow and heat transport based on the SBG are presented. The final objective is to understand the basic processes driving the thermal waters within the SBG, the role of different driving forces as well as their interactions with faults in controlling geothermal processes. The results will shed new light on the links between the migration of subsurface energy, active transport processes and tectonic structures.

Description: The Seferihisar-Balçova Geothermal area (SBG) is located in western Turkey, along a N-S trending fault system and is defined by two major geothermal systems topographically separated by the Seferihisar Horst: the Seferihisar (SG) in the south and Balçova (BG) in the north. The temperatures of cold, hot springs and shallow wells vary between 16 and 70°C and reach 138°C in drilled wells. In these areas, the geothermal waters are used for balneological purposes and district heating. Previous hydrochemical and istopes analyses indicated that the geothermal waters have two main origins: (1) meteoric waters (heated groundwater) and (2) seawater. Heated groundwater types with low total dissolved solids (TDS) content are found in the BG geothermal field whereas the thermal waters in SG originated from a mixture of seawater and local meteoric groundwater. In this presentation, stable environmental isotopes are used to investigate the distribution of recharge areas, to determine the origin of the waters, the evaporation ratios and the seawater contribution. New samplings are carried out in both SG and BG systems and in the recharge areas of the Horst. A deflection trend in the cold waters of Balçova reflect altitude effects of the BG and the contribution of both geothermal and sea waters. According to the elevation vs. D diagram, the SG waters recharge from groundwater flow at about 200-300m altitude whereas the BG deep geothermal waters recharge from 800-1000. d18O shifting in these diagrams indicates that meteoric water has been heated at great depths before rising to the surface, likely driven by buoyant forces. Therefore deepreaching regional flow from the horsts toward the coastal aquifers also determines the geothermal behaviour of the SBG. Circulation of ascending thermal waters at different depths could also explain the variation in salinity of BG and SG water samples. In summary, all isotopic analyses strongly suggest that density-driven convective flow within permeable fractured areas, forced convection imposed by the horst and seawater intrusions are likely to be the major transport processes in the SBG. These results are well supported by numerical modeling of density-driven flow presented in the HS7.6 session.