Figure 1 - uploaded by Julia Vladimirovna Frolova
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reveals the cellular microstructure of smectite and felty microstructure of clinoptilolite-the main minerals of argillization zone in Pauzhetskoe geothermal field. The size of smectite "cells" is about 5-7 micron. Smectite swells when saturated by fluid and fills intergranular pores and fractures, forming impermeable horizon. Several boreholes were used as examples in order to show how to define the zone of hydrothermal argillites and probably the uppermost water confining horizon of geothermal field by the change of hygroscopic moisture without hydrogeological tests, geophysical log or x-ray analysis. Figure 2 illustrates a change of hygroscopic moisture along borehole section in Okeanskoe geothermal field. The zone of argillization with smectite and corrensite extends from 70-80 m down to the depth 420-430 m. It is marked by high W g values which change from 1 to 5.5 percent. Below the argillization zone W g does not exceed 1 percent.
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... It is only recently that hydrothermal alterations are considered qualitatively in parallel with the above-cited parameters for the study of the petrophysical property evolution of volcanic rocks in hydrothermal contexts. Notably, and through several papers, Frolova et al. (2001Frolova et al. ( , 2006Frolova et al. ( , 2010Frolova et al. ( and 2015 studied the influence of hydrothermal alterations on the petrophysical properties of rocks from the Kuril-Kamchatka island arc by dividing the hydrothermal alterations encountered into low-temperature alterations and high-temperature alterations. Mordensky et al. (2019) on their side focused their study on high temperature hydrothermal alterations developed in Rotokawa geothermal field. ...
The evolution of the petrophysical properties of rocks induced by hydrothermal alteration is often considered qualitatively by sorting the hydrothermal alterations encountered from low to high intensity, depending on the identified mineral paragenesis. In this paper, we studied the evolution of three different petrophysical properties (connected porosity, permeability and electrical conductivity) as a function of the argillization degree of andesites affected by argillic alteration identified in the caprock formation of the Petite Anse-Diamant hydrothermal system in Martinique. These petrophysical measurements were supplemented by quantitative mineral evaluation via scanning electron microscopy (i.e., the Quantitative Evaluation of Minerals by Scanning electron microscopy or the QEMSCAN® method), connected porosity mapping using ¹⁴C-PMMA method and measurements of methylene blue (MB) values and structural water proportions (H2O⁺). In the series of rocks investigated, an increasing trend of clay mineral abundances (montmorillonite and kaolinite) from 5.68 to 37.56% was revealed by the QEMSCAN® method. Based on the quantitative results, the structural water proportions were used as a proxy of argillic alteration progression. Comparison between the mineral maps provided by the QEMSCAN® system and connected porosity mapping observed in autoradiographs using ¹⁴C-PMMA method revealed a good correlation with clay mineral phases, dissolution vugs and fractures. The connected porosities evaluated with the triple weight method range from 3.74 to 35.35% and show a good correlation (R² = 0.8835) with the H2O⁺ values, revealing that porosity development is mainly due to the replacement of primary phases by clay minerals. In contrast, the crystallization of silica, carbonates and, to a lesser extent, iron oxides tends to clog connected pores, inducing a local decrease in connected porosity, as revealed in autoradiographs. The Darcian permeability ranges from 7.66∙10⁻²⁰ to 5.36∙10⁻¹⁷ m² and shows a moderate correlation with the connected porosity. The bulk electrical conductivity measured as a function of the conductivity of the saturating solution reveals the significant contribution of surface conduction arising from montmorillonite. At a pore fluid salinity of 2 wt% (the salinity of the Eaux Ferrées thermal spring), the bulk electrical conductivity shows a relatively good correlation with the H2O⁺ values (R² = 0.8591) and an even better correlation with the MB values (R² = 0.9579). The contribution of montmorillonite to the bulk electrical conductivity was estimated with the use of the isoconductivity point, showing an increase in the BM values and a strong correlation (R² = 0.9806).
... Smectite swells when saturated with fluid and fills intergranular pores and fractures, forming impermeable horizons. These argillized horizons are the impermeable caprock of the hydrothermal system (Frolova et al., 2006). An increase of fluid pH to subalkaline conditions causes the formation of high-silica zeolites in association with smectites, as shown in Figure 7. High-silica zeolites form filmy cement with empty intergranular pores, as shown in Figure 8. ...
This report is based on complex petrophysical data obtained on a number of hydrothermal systems in the Kuril-Kamchatka island arc (Pauzhetskaya, Mutnovskaya, Koshelevskaya, Essovskaya, Northen-Paramushir, and Baranskogo). The mineral composition and porous structure of primary rocks are intensively altered by hydrothermal processes, resulting in changes of petrophysical properties. Petrophysical alteration of rocks gradually causes a transformation of hydrothermal system structure, which leads to changes in its hydrodynamic and temperature regimes. The tendency of petrophysical alteration can be different. In some cases, rock "improvement" is observed, i.e. consolidation, hardening, a decrease of porosity and permeability, and removal of hygroscopy. In other cases, rock "deterioration" occurs, i.e. formation of secondary porosity and permeability; a decrease of density, strength and elastic modulus; and occurrence of hygroscopic moisture. The character of petrophysical alteration depends on a number of factors including peculiarities of primary rocks; temperature, pressure and composition of thermal fluids; duration of fluid-rock interaction; and fluid phase. The properties of primary rocks control the speed, intensity and character of petrophysical alterations. Effusive and intrusive rocks are dense and low permeable with high strength and elastic modulus. They form water-confining horizons in the structure of geothermal fields, and rarely fracture-type reservoir. The speed of their alteration is low. Tuffs and tuffites differ significantly by lower physical and mechanical properties, higher magnitude of alteration and permeability character. They are the most common type of the host rocks of geothermal reservoirs. The temperature and pressure in hydrothermal systems cardinally influence on rocks properties. Deep, high-temperature fluids cause consolidation, hardening, a decrease of porosity and permeability, and removal of hygroscopic moisture. The influence of low-temperature subsurface fluids is more complicated and diverse. It can lead to either an increase or a decrease in petrophysical properties depending on what process prevails – rocks leaching, sedimentation of secondary minerals in pores and cracks, or replacement of primary minerals by secondary minerals. The chemical composition and pH of thermal fluids have a large influence on the alteration of rock properties for low-temperature fluids and are not important for high-temperature fluids. Petrophysical alterations depend on the duration of hydrothermal processes. As a whole, hydrothermal systems are characterized by heterogeneity and selectivity of processing, resulting in the petrophysical heterogeneity of all structures. However, petrophysically homogeneous zones are gradually formed by contact with thermal waters over long periods, causing the systems to become more stable.
Geothermal exploration and development of geothermal sites requires the investigation of their engineering geological setting and an assessment of the hydrothermal activity impact on the environment. Hydrothermal alteration of rocks and changing in their physical-mechanical properties promotes a broad range of geological phenomena that is indicated by many researchers on various geothermal sites all over the world. There are slope instability with the initialization of landslides or rock avalanches, changes in relief, hydrothermal explosions, migration of thermal manifestations, changes in temperature and the hydrodynamic regime of a hydrothermal system, reduction of well production, surface deformation and subsidence, discharge of wastewater from production wells to the creeks with formation of silica deposits etc. These processes have an impact on the environment and influence on exploitation of the geothermal field, and in some cases, they could be hazardous for the economic or tourist development of the geothermal site. In the paper we discuss the environmental impact problems induced by hydrothermal activity, which may occurred during natural evolution of hydrothermal systems or due to the geothermal development and utilization, and describe the geological phenomena associated with hydrothermal activity on the most well-known hydrothermal systems of Kamchatka Peninsula (Far East, Russia).
The development of geothermal sites for electricity production, direct utilization, or tourist destinations has increased in recent decades worldwide. Such development requires investigation of the engineering geological setting, including an assessment of the physical-mechanical properties of rocks. Geothermal regions in the Kuril-Kamchatka arc are composed of NeogeneQuaternary volcanic formations with relatively high strength and deformation properties. Thermal fluids circulating within rocks change their compositions, porosities, and physical-mechanical properties. The most intense alterations occurring in the thermal fields are due to argillization. This study investigates the effects of hydrothermal alteration on the physical-mechanical properties of tuffites in the Upper Pauzhetsky thermal field located in South Kamchatka. The aims are to consider the entire range of hydrothermal argillization and to analyze the relationships between physical-mechanical properties and degrees of alteration. The variations in mineralogy, microstructure, particle size distribution, consistency, and rock properties along the section of the thermal field are studied. The results show that argillic alteration gradually decomposes tuffites to weak clay-rich soils, forming a cover several meters thick. The alteration is accompanied by decreases in strength and rock weakening; minor variations are observed between fresh and altered tuffites, whereas a dramatic reduction in strength occurs when tuffites disintegrate and are converted into clays. The heterogeneity of the clayey layer and the presence of secondary siliceous minerals and residual components of parent rocks are the factors that distinguish hydrothermal clays from sedimentary deposits.
В статье рассматриваются гидротермальные преобразования туфогенных пород Долины Гейзеров (п-ов Камчатка). Выделены две фации гидротермально-метасоматических пород - аргиллизиты и низкотемпературные пропилиты. Проанализирована динамика изменений физико-механические свойств пород, соответствующих гидротермальным преобразованиям. Получены зависимости прочностных и деформационных свойств от содержания вторичных минералов - цеолитов, глинистых минералов, кварца.
