Аннотация: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, hightemperature
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.
