Role of Karstic and Basinal Fluids in Porosity Evolution in the Buda Hills, Hungary
Erőss

, Anita ¹; Poros, Zsófia ¹; Madl-Szőnyi¹, Judit ¹; Mindszenty, Andrea ¹; Molnár, Ferenc ²; Ronchi, Paola ³; Csoma, Anita .⁴
(1)Physical and Applied Geology, Eötvös Loránd University, Budapest, Hungary. (2) Mineralogy, Eötvös Loránd University, Budapest, Hungary. (3) Exporation and Production Division, ENI S.p.A., Milan, Italy. (4) Subsurface Technology Basin and Sedimentary Systems, ConocoPhillips, Houston, TX.

The Buda Thermal Karst (BTK) is located at the boundary of an uplifted Mesozoic-Tertiary carbonate unit and the adjacent Neogene sedimentary basin. Being a “marginal” area, the BTK serves as the discharge zone of regional fluid flow. This implies that it may receive fluid components from several sources, resulting in a wide range of discharge features, including springs, caves and mineral precipitates.

Recent- and palaeo-hydrogeology were studied in parallel to understand the effects of palaeo-fluid migration on the recent hydrogeological setting and the distribution and development of the extensive, still active hypogene cave-systems in the BTK. On the basis of geochemical analyses of recent waters, petrography and fluid inclusion analyses of palaeo-precipitates, three types of fluids from different sources were distinguished: (i) regional karst water, (ii) local karst water and (iii) basinal fluid. Based on the HC indications detected in both the palaeo- and the recent waters and on the changes of mineral assemblages it is proposed that the basinal contribution had been continuous from the late Middle Miocene on. In addition, an increase in dissolution at the expense of precipitation from the Miocene to present suggests an evolving groundwater system in which the proportion of karst waters increased at the expense of the basinal component. Along with mixing corrosion, also aggressive gases (H₂S, CO₂, CH₄) related to hydrocarbon accumulations in the basin have been efficient dissolving agents. The timing of the initiation of basinal fluid migration to the Buda Hills was estimated by structural geological considerations and fluid inclusion data. Compressional tectonics is believed to have been maintaining flow from the basin side up to now.

In addition to the effects of different fluids on the formation of caves and cave minerals microbial activity has had also significant influence (as proved in the case of the recent spring caves).

Microbial activity is also reflected by extremely negative δ34S values (down to -32‰) measured in gypsum samples. Nevertheless, wide range of sulphur isotopic values revealed also the multiple sources of sulphur, i.e. basinal H2S (~10 ‰), marine evaporite sulphate from the nearby basin (20-30‰) and sulphur compounds of the host rocks (~6 ‰).

Porosity evolution of the BTK has been most likely influenced by complex hydrogeological systems of various fluid components and affected also by microbial activity.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.