Félix Schubert

The fracture network modelling and hydrogeological assessment were performed in an 845 m deep borehole of the potential high-level waste repository formation and its caprock. The geometry of the fracture network was simulated using the discrete fracture network (DFN) modelling method, which is based on the geometric characteristics of the individual fractures. The hydrogeological evaluation was based on changes in porosity and permeability along the borehole using flow zone indicator (FZI) values that denote hydraulic flow units (HFU) within the rock body. Fracture network characteristics and hydrogeological features are mainly determined by the wellbore lithology, which can be divided into three zones. The sandstone body was intersected in the upper 300 m of the borehole, which forms a single HFU. The second zone was developed along with the transition zone between the sandstone and the underlying claystone bodies. Here the predominant rock type is claystone, but the characteristics of the fracture network are distinctly different from the deeper parts of this rock body. Below 400 m is the third zone, where distinct and extensive HFU-s could not form, probably due to different water–rock interaction processes that could have changed the porosity and permeability from point to point in the claystone.

Az utóbbi években a szerkezetföldtani kutatások mind gyakrabban foglalkoznak a diagenezis tartományában lejátszódó szerkezetalakulási folyamatokkal. Ennek látványos bizonyítékai az elmúlt években, hazai lelőhelyekről származó deformációs szalagokat és azok képződési körülményeit bemutató tanulmányok. E cikkek szerzői – többek között – terepi, szerkezetföldtani adatok feldolgozásával készítettek részletes deformációtörténet- és feszültségmező-rekonstrukciót az adott képződményekre vonatkozóan. Munkánk során egy orientálatlan fúrómagban megjelenő, ebből kifolyólag a klasszikus szerkezetföldtani módszerekkel nem vizsgálható deformációs szalagot tanulmányoztunk. Olyan kőzettani, mikroszerkezeti és geokémiai alapú módszertani összefoglalást kíséreltünk meg összeállítani, amelynek segítségével természetesen a felszíni feltárások adta lehetőségeknél jóval szerényebb – információk gyűjthetők a szalagképződés körülményeiről egy pontszerű feltárás/minta esetén. A deformációs szalaghoz kapcsolódó átalakulások minél pontosabb megértése céljából részletesen vizsgálatuk a szalag és a befoglaló homokkő kőzettani jellemzőit és diagenezistörténetét.

Numerous fractured hydrocarbon reservoirs exist in the metamorphic basement of the Pannonian Basin in Hungary. Many decades of experience in production have proven that these reservoirs are highly compartmentalised, resulting in a complex mosaic of permeable and impermeable domains situated next to each other. Consequently, in most fields, only a small amount of the total hydrocarbon reserve can be extracted. This paper aims to locate the potential migration pathways inside the most productive basement reservoir of the Pannonian Basin, using a multiscale approach. To achieve this, evaluation well-log data, DFN modelling and a composition analysis of fluid trapped in a vein-filling zeolite phase are combined. Data on a single well are presented as an example. The results of the three approaches indicate the presence of two highly fractured intervals separated by a barely fractured amphibolite. The two zones are probably part of the communicating fracture system inside the single metamorphic mass. The gas analysis further specifies the migrated fluids and indicates hydrocarbons of a composition similar to that of the recently produced oil. Consequently, we conclude that the two zones do not only form an ancient migration pathway but are also members of a more recent hydrocarbon system.

Detailed mineralogical and geochemical study of red mud samples from Hungary suggests geological and geochemical processes that determine the spatial distribution of certain elements inside the red mud pitfalls. The major processes are the following: (1) Heavy mineral grains (anatase, rutile, titanomagnetite, etc.) tend to subside due to gravitational differentiation and at present accumulate in the deepest horizons of the pitfalls. (2) Kaolinite reacts to cancrinite under hyperalkaline conditions. (3) Due to diagenetic processes, goethite-cancrinite aggregates form in situ. (4) Light mineral grains (e.g., cancrinite) move upward. (5) Cancrinite transforms to calcite at the shallowest horizons, due probably to the reaction with atmospheric CO₂. All these processes have a significant role in accumulation tendencies of different groups of elements inside the pitfalls. The behaviour of chalcophile elements and the HFSE elements follow common geochemical rules and remind features of the host bauxite or even its precursor igneous or metamorphic lithologies. The REEs and Sc are possibly adsorbed on goethite and in the channels of cancrinite. Based on linear mixing model calculations, the major container of these elements is cancrinite. The proportion of the REEs and Sc in the Ti-phases, carbonates, phosphates, zircon, etc. is subordinate relative to the amount accumulated by goethite and cancrinite.

Extensive Miocene volcanic activity produced basaltic and pyroclastic successions, which were penetrated by many wells in the area of Üllés-Ruzsa-Bordány, in the western and central part of the Great Hungarian Plain. The Kecel Basalt comprises primary porosity from a high proportion of vesicles and significant secondary fracture porosity, as well. Due to the significant fracture porosity, some depth intervals show good reservoir characteristics, although the fractures crosscutting the rock body are partly or entirely cemented by various minerals. Based on the textural and mineralogical features, four distinct vein types can be distinguished, named after their volumetrically most abundant cement phases, i.e. potassium-feldspar (Kfp-), calcite (Cal-), laumontite (Lmt-) and analcime (Anl-) types. This study focuses on probably the youngest of these vein types, the Anl-type. Based on the study of veins and mineral sequences, the direction of temperature changes cannot be given unequivocally for every stage of cementation, but crystallization of the Anl-type veins might have occurred at lower temperatures than the formation of the Lmt-type veins. Fluid inclusion studies suggest that hydrocarbon migration and accumulation took place after cementation of the first three vein types (Kfp-, Cal- and Lmt-types). In the newly opened fracture system, two types of hydrocarbon (HC1 and HC2)-bearing fluid inclusion assemblages were captured during precipitation of analcime and later zeolites. This refers to two stages of hydrocarbon migration in the fracture system. Observations of the fluorescence colours and low temperature behaviours of the hydrocarbon-bearing inclusions, the earlier HC1 petroleum-inclusions captured heavier (presumably less mature oils), while the later ones (HC2) lighter (presumably more mature) oils. The HC2 petroleum seems to be very similar to the crude oil sampled in a well in the area based on their fluorescence parameters.

Volcanic successions of the Kecel Basalt Formation (KBF) occur in the southern part of the Pannonian Basin. As a result of periodic submarine eruptions, the basaltic and pyroclastic rock horizons were intercalated with layers of the Late Miocene Endrod Marl Formation, which is regarded as one of the most important hydrocarbon source rocks in the area. The KBF was discovered through almost 30 wells between 2,200 and 2,900 meters of depth. Due to the high fracture porosity, some parts of the formation show good reservoir characteristics and act as important migration pathways of hydrocarbon-bearing fluids. Since the reservoir is presumably fracture-controlled, this study concentrates on the evolution of fractures crosscutting the rock body. Based on textural and mineralogical features, four distinct vein types can be distinguished, of which the first three types are discussed in this paper. Beside calcite, quartz, feldspar, and chlorite, the veins are cemented by various zeolite minerals. The vertical dimension of the dominant zeolite zone indicates the burial-diagenetic type of zeolite zonation and suggests subsidence of the subaqueous basalt after formation