Lánczos T., Milička J. & Pereszlényi M., 2011: Geochemical indicators of hydrogeologically isolated structures in the Danube basin; implications for potential gas storage objects. Acta Geologica Slovaca, 3, 1, 43–56. (in Slovak with English summary)

Geochemické indikátory uzavretosti geologických štruktúr v Dunajskej panve; implikácie pre budovanie podzemných zásobníkov plynu

Geochemical indicators of hydrogeologically isolated structures in the Danube basin; implications for potential gas storage objects

Tomáš Lánczos¹, Ján Milička¹ & Miroslav Pereszlényi²

¹Katedra geochémie, Prírodovedecká fakulta, Univerzita Komenského v Bratislave, Mlynská dolina G, 842 15 Bratislava; lanczos@ fns.uniba.sk
²EURO - GEOLOGIC, a.s. Tomášikova 223/26, 82101 Bratislava II – Ružinov; mperesz@egeo.sk

Abstract

One of the key conditions for underground gas storage (UGS) objects establishment is the presence of hydrogeologically isolated structures. These are mainly exhausted hydrocarbon deposits and aquifers in the Cenozoic fill and the basement of the Danube Basin. The geochemical indicators used to determine the aquifer isolation level of potential underground gas storage objects are based on the ratio of stagnant and dynamic water components. The most widely used indicator is the rHCO₃/rCl molar ratio, assuming that the HCO₃^- concentration represents the dynamic origin of dissolved rock material by meteoric water and that the Cl^- concentration is the stagnant marine component. To distinguish these two components of the water in the aquifers the rHCO₃/rCl indicator was combined with other parameters, such as rCl/rSO₄ and rCl/rBr molar ratios. As for more aquifers of the Danube Basin contain higher amounts of carbon-dioxide we attemptet to quantify the influence od the carbon dioxide on the water chemistry. The  numerical model simulating rock-marine water interactions under different P_CO2 temperature and ion-exchange conditions proved the rHCO₃/rCl molar ratio increase caused higher carbon dioxide partial pressure for this purpose. Consequently under the conditions of higher partial carbon dioxide pressure the rHCO₃/rCl ratio is indicating a lower isolation level of the structure. Taking in account signs of the higher carbon dioxide pressure based on the numerical model combined with the abovementioned geochemical indicators the most convenient aquifers for UGS establishment were identified within the Slovak part of the Danube Basin. These are located within Badenian sediments in the Trakovice and Sereď areas, due to the presence of hydrogeologically isolated structures buried at depth intervals of 800 to 1150 m below ground level. Other potential convertible aquifers are present in the Lower and Middle Badenian sediments in the Komjatice and Želiezovce Depressions, especially in the following localities: Modrany at 1145 to 1670 m below the surface, Vráble approximately up to 2110 m and Zlaté Moravce at 1364 to 1374 m below surface.

Key words: Danube Basin, isolation of hydrogeological structures, underground gas storage objects, geochemical indicators, aquifers, carbon dioxide

Manuscript received: 2010-11-23

Revised version accepted: 2011-03-21