Geothermal carbonate reservoirs and their sustainability: what can natural hydrothermal systems tell us?

Abstract

Minerals that make up carbonate rocks (i.e. calcite) have very high chemical reactivity, allowing for various types of porosity-altering deformation processes such as dissolution and recrystallization (e.g. dolomitization, silicification), that produce secondary porosity. Even though water enriched with CO2 may quickly equilibrate by dissolving calcite, carbonate is also very dynamic, and cemented veins may still act as planes of weakness compared to the host rock allowing fluids to open up and then seal these fractures again. Therefore, natural carbonate rocks often show not only the numerous deformation stages overprinting one another, but also the same veins opening up again and again forming crack-seal structures.Natural fossil hydrothermal systems were investigated within outcrops at Nowa Wioska, Czatkowice, Pomorzan, Trzebionka, Orzel Bialy mines, Dziura cave and Hucianski Klin crest in the south of Poland to evaluate the influence of hydrothermal activities on the rock pore evolution. We present in more detail the Triassic carbonate sequence near Nowa Wioska village in the Silesian-Kraków region, where hydrothermal water had been flowing through an open fault zone, forming various hydrothermal products: secondary intercrystalline pores, vugs, breccias, metal ore and mineral precipitates. The impact of these products on the permeability of the geothermal system was analyzed in comparison with the adjacent host rock unaffected by tectonic and hydrothermal processes. We observed that at least three successive episodes of fracturing occurred during the Tertiary demonstrating that fluids were introduced episodically in the Triassic rocks. Fracture- and vug-filling calcite, saddle dolomite, pyrite, sphalerite, galena, quartz and clays were preferentially observed on the downthrown side of the fault that underwent a more intense tectonic deformation that had a decisive influence on the migration of mineralizing hydrothermal solutions. Even though mineral and metal-ore precipitates, sediments and solution / collapse breccias fill most of these karsts and fractures, the porosity and permeability of the hydrothermally altered rocks remain quite high compared to the host rock. Moreover, secondary dissolution forming vugs increase this porosity and permeability even further. The studied examples suggest that once geothermal system has been active, it stays as a preferential zone for the following deformation events, making it sustainable for tens of thousands or even millions of years.