Félix Schubert

Integrated petrographic – rock mechanic borecorestudy from the metamorphic basement of thePannonian Basin, Hungary

Abstract The integrated evaluation of borecores from the Mezősas-Furta fractured metamorphic hydrocarbon reservoir suggests significantly distinct microstructural and rock mechanical features within the analysed fault rock samples. The statistical evaluation of the clast geometries revealed the dominantly cataclastic nature of the samples. Damage zone of the fault can be characterised by an extremely brittle nature and low uniaxial compressive strength, coupled with a predominately coarse fault breccia composition. In contrast, the microstructural manner of the increasing deformation coupled with higher uniaxial compressive strength, strain-hardening nature and low brittleness indicate a transitional interval between the weakly fragmented damage zone and strongly grinded fault core. Moreover, these attributes suggest this unit is mechanically the strongest part of the fault zone. Gougerich cataclasites mark the core zone of the fault, with their widespread plastic nature and locally pseudo-ductile microstructure. Strain localization tends to be strongly linked with the existence of fault gouge ribbons. The fault zone with ∼15 m total thickness can be defined as a significant migration pathway inside the fractured crystalline reservoir. Moreover, as a consequence of the distributed nature of the fault core, it may possibly have a key role in compartmentalisation of the local hydraulic system.

Structural controls on petroleum migration and entrapment within the faulted basement blocks of Szeghalom Dome (Pannonian Basin, SE Hungary)

The basement of the Pannonian Basin contains several fractured metamorphic hydrocarbon reservoirs that typically form structural highs between the Neogene sedimentary sub-basins. One of the largest reservoirs, the Szeghalom Dome, is located on the northern margin of the Bekes Basin and is mainly composed of Variscan gneisses and amphibolites with different metamorphic evolutions. These petrologically incompatible blocks were juxtaposed by post-metamorphic tectonic activity that was accompanied by the formation of brittle fault zones with elevated transmissibilities. The aim of this study was to define the spatial arrangement of these fault zones and their internal architecture by integrated evaluations of borecore and well-log data from a group of wells in the central part of the field. Spatial correlations between the reconstructed 1D lithologic columns revealed the main structural elements of the Szeghalom Dome. The low-angle (<15°) thrust faults most likely developed due to north-northwest vergent Cretaceous nappe tectonics, which was probably responsible for the juxtaposition of the different metamorphic blocks. A complex system of normal faults throughout the basement high provides evidence of intense Miocene extensional tectonic activity. This phase of the geodynamical evolution of the basin is believed to be responsible for the horst-graben structure of the Szeghalom Dome. The integration of the structural results with datasets of the paleo-fluid evolution, recent production and fracture network geometry indicates the importance of these fault zones in both the migration of hydrocarbons from the adjacent sub-basins to the overlying sediments and the development of significant storage capacity within the strongly fractured rock masses (mainly the amphibolite bodies). These observations of fluid flow also emphasized the impact of strong permeability anisotropy of the faults throughout the fractured reservoir.

Near vein metasomatism along propylitic veins in the Baksa Gneiss Complex, Pannonian Basin, Hungary

In many parts of the metapelitic (gneiss, mica schist) rock section of the Baksa Complex, significant wall-rock alteration is observable along the Ca-Al silicate veins, which show a di → ep ± czo →sp → ab ± kfs → chl → adu → prh → py → cal mineral sequence (FINTOR et al., 2009). These alterations appear as narrow (few cm thick) bleached margins beside thin veins, and broad alteration bands along thick veins where detailed epidotization and chloritization of the adjacent rock are recognizable. Based on petrographic and mineralogical examination of the altered wallrocks, metasomatic zones with characteristic mineral paragenesis can be distinguished: Zone 1 (ab + ttn ± ep), Zone 2 (ep + chl+ ttn + ab ± ser), Zone 3 (chl + ep + ser + rt ± ttn), Zone 4 (ser ± chl). Bulk rock chemical analyses were made from the different metasomatic zones. The results show that fluid circulated in the propylitic veins caused metasomatic alteration of the wall-rock, with transport of considerable amount of Ca 2+ toward the adjacent rocks. The hydrothermal leaching almost totally removed the K, Fe, Mg, and Mn ions from the wall rock. The main alteration processes are the epidotization and chloritization of biotite, and albitization of micas (muscovite + biotite) content of metapelites. Based on mobilization of different cations alteration was due to? to a near neutral fl uid (~pH 5–7). The pervasive hydrothermal leaching caused significant secondary porosity (cavities) in the altered domains, which were partially filled by epidote. Fluid inclusions of cavity filling epidote indicate a similar character (Th: 180–360 °C; Salinity: 0.2–1.6 mass% eq. NaCl) to that can be found in Ca-Al silicate veins. The alteration most probably occurred in the 360–480 °C temperature range as products of near vein metasomatism and the altered rock can be related to the propylite metasomatic family.

New geothermal well-completion and rework technology by laser

Our research team has developed a new well completion and rework technology involving lasers. The system is made up of a high-power laser generator and a custom-designed directional laser drilling head. The laser head is attached to a coiled tubing unit to maximize production and to carry out special downhole tasks. In this phase of the development effort, laser technology is particularly well suited to cost-efficiently drill short laterals from existing wells in a single work phase, drilling through the casing and cement as well as the formation. The technology, which is an extended perforation solution, enables a more intensive interaction with the downhole environment and supports cutting edge subsurface engineering scenarios such as barite removal. Laser-induced heat treatment appears to be a suitable alternative to effectively remove the almost immovable deposits and scales from thermal water-well pipes.