Imprints of hydrocarbon-bearing basinal fluids on a karst system: mineralogical and fluid inclusion studies from the Buda Hills, Hungary

Abstract

Calcite veins and related sulphate-sulphide mineralisation are common in the Buda Hills. Also, abundant hypogenic caves are found along fractures filled with these minerals pointing to the fact that young caveforming fluids migrated along the same fractures as the older mineralising fluids did. The studied vein-filling paragenesis consists of calcite, barite, fluorite and sulphides. The strike of fractures is consistent-NNW-SSEconcluding a latest Early Miocene maximum age for the formation of fracture-filling minerals. Calcite crystals contain coeval primary, hydrocarbon-bearing-and aqueous inclusions indicating that also hydrocarbons have migrated together with the mineralising fluids. Hydrocarbon inclusions are described here for the first time from the Buda Hills. Mixed inclusions, i.e., petroleum with 'water-tail', were also detected, indicating that transcrystalline water migration took place. The coexistence of aqueous and petroleum inclusions permitted to establish the entrapment temperature (80°C) and pressure (85 bar) of the fluid and thus also the thickness of sediments, having been eroded since latest Early Miocene times, was calculated (800 m). Low salinity of the fluids (\1.7 NaCl eq. wt%) implies that hydrocarbon-bearing fluids were diluted by regional karst water. FT-IR investigations revealed that CO 2 and CH 4 are associated with hydrocarbons. Groundwater also contains small amounts of HC and related gases on the basin side even today. Based on the location of the paleo-and recent hydrocarbon indications, identical migration pathways were reconstructed for both systems. Hydrocarbon-bearing fluids are supposed to have migrated north-westward from the basin east to the Buda Hills from the Miocene on.

Karst porosity estimations from archive cave surveys - Studies in the Buda Thermal Karst System (Hungary)

“…Our method evolved during the first modeling in the PVC, where, the enclosing rock body was primarily modeled as a square prism in orthogonal the expense of the basinal component as well (Erőss et al, 2011;Poros et al, 2012). All of these effects imply that during the formation of the investigated caves the different geological and hydrogeological settings might have led to different cave size.…”

“…In the PVC and the MJC the records of this polygonal surveying was available from the archives. In the SHC the original station-target records were not preserved, but most of the original stations were physically marked in the cave and were later used (e.g., Poros et al, 2012). On the eroded surface of the Triassic (i.e., Dachstein Limestone, cherty limestone and dolomite and "Hauptdolomit" -Haas 1988; Haas et al, 2000), a Paleogene transgressional sequence (bauxitic clays, limestones, marls, clays and sandstones) has deposited (e.g., Wein, 1977;Kázmér, 1985;Báldi, 1986;Nagymarosy & Báldi-Beke, 1988;Fodor et al, 1994;Nagymarosy, 2001;Báldi-Beke, 2003).…”

“…During the Miocene, mineral veins (calcite, barite and sulfides) were formed along the faults, and later (mainly in the Pleistocene) they gave place to the cave forming as a different phase in the evolution of one single hydrothermal system (Poros et al, 2012). The BTKS has a hypogenic origin, and mixing corrosion has been the dominant cave forming process (e.g., Leél-Őssy & Surányi, 2003;Erőss et al, 2012).…”

Magmatic–hydrothermal evolution of highly fractionated granites: Evidence from the Kuiqi miarolite in Fujian province, SE China

“…Fluid evolution in shallow intrusions and its genetic link with the commonly associated mineralization were documented in previous fluid inclusion studies (Audétat, 2015;Cathelineau et al, 1988;Hurtig et al, 2014;Roedder, 1984;Weisbrod, 1981). Compositions of the fluid phases and their evolution in many mineralization systems were well investigated through fluid inclusion studies (Fan et al, 2002(Fan et al, , 2003Jiang et al, 2014;Lawrence et al, 2013;Li et al, 2012;Lu et al, 2003;Poros et al, 2012;Xie et al, 2009). Fluid inclusions entrapped in crystals are by far the most representative of the fluids existed during and after crystal growth.…”

Relationship between micro-fracture network and karstification in recent (Mecsek) and paleokarst (Gomba) reservoirs

“…The closest outcrops in the TRU to the investigated reservoir are located in the Buda Mts., where (Wein 1977;Nádor and Sásdi 1991) and young, still active hypogene processes (Kriván 1959;Kraus 1982;Nádor 1991). Paleokarst features of Mesozoic-Tertiary carbonates of the Buda Hills were studied by Korpás and Juhász 1990, Esteban et al 2009, Győri et al 2011, Poros et al 2012.…”