Open Geosci. 2015; 7:53–64 Research Article Open Access László Molnár*, Balázs Vásárhelyi, Tivadar M. Tóth, and Félix Schubert Integrated petrographic – rock mechanic borecore study from the metamorphic basement of the Pannonian Basin, Hungary Abstract: The integrated evaluation of borecores from the 1 Introduction Mezősas-Furta fractured metamorphic hydrocarbon reser- voir suggests significantly distinct microstructural and Brittle fault zones of crystalline rock masses can serve as rock mechanical features within the analysed fault rock migration pathways or also as sealing surfaces for fluid samples. The statistical evaluation of the clast geometries flow in the Earth’s crust, so the understanding of their in- revealed the dominantly cataclastic nature of the sam- ternal structure is crucial for interpreting hydraulic sys- ples. Damage zone of the fault can be characterised by tems. Earlier studies [1, 2] on the architecture of fault an extremely brittle nature and low uniaxial compressive zones defined two main structural elements: first, a weakly strength, coupled with a predominately coarse fault brec- disaggregated, densely fractured “damage zone” and a cia composition. In contrast, the microstructural manner strongly deformed and fragmented “fault core”, where of the increasing deformation coupled with higher uni- the pre-existing rock fabrics were erased by fault devel- axial compressive strength, strain-hardening nature and opment. These elements can be characterised by the for- low brittleness indicate a transitional interval between mation of diverse tectonite types (fault breccias, catacla- the weakly fragmented damage zone and strongly grinded sites, fault gouges), which often also possess quite hetero- fault core. Moreover, these attributes suggest this unit is geneous rheological features. Consequently, the petrolog- mechanically the strongest part of the fault zone. Gouge- ical, petrophysical and rock mechanical properties may rich cataclasites mark the core zone of the fault, with their have an extremely heterogeneous spatial distribution in widespread plastic nature and locally pseudo-ductile mi- fault zones. crostructure. Strain localization tends to be strongly linked In addition, evaluation of tectonites from core sam- with the existence of fault gouge ribbons. The fault zone ples always provide a serious challenge to proper struc- with ∼15 m total thickness can be defined as a significant tural interpretation, particularly due to the small number migration pathway inside the fractured crystalline reser- of borecores and their quite doubtful representativeness of voir. Moreover, as a consequence of the distributed nature the whole rock mass [3]. These constraints raise the neces- of the fault core, it may possibly have a key role in com- sity of integrated analytical methods that aim to gain as partmentalisation of the local hydraulic system. much information from these samples as possible. Keywords: fractured metamorphic reservoir; rock me- The main aim of this paper was to present the results of chanical parameters; fault rocks; brittle deformation one such petrographic, microstructure and rock mechani- cal analysis: we examined the lithological and rheological attributes of a basement fault zone with a quite high rate DOI 10.1515/geo-2015-0004 Received March 25, 2014; accepted October 13, 2014 of core recovery and abundant amount of available sam- ples, via the usage of these integrated methods. The com- bined petrographic (microstructural and clast geometric) and rock mechanical evaluations were conducted partly to *Corresponding Author: László Molnár: Department of Miner- clarify the role of main structural elements in the hydraulic alogy, Geochemistry and Petrology, University of Szeged, Szeged, system of both the fault zone and the whole fractured rock Hungary, Egyetem u. 2. H-6722, E-mail: [email protected] mass. Balázs Vásárhelyi: Deptartment of Geotechnics and Engineering Geology, Budapest University of Technology and Economics, Bu- dapest, Hungary Tivadar M. Tóth, Félix Schubert: Department of Mineralogy, Geo- chemistry and Petrology, University of Szeged, Hungary © 2015 L. Molnár et al., licensee De Gruyter Open. This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 License. 54 Ë L. Molnár et al. 2 Geological setting OG group. The rather different metamorphic evolutions of these groups and the presence of wide shear zones along the group boundaries indicate these blocks were juxta- The locality of this study is the pre-Neogene basement posed following their metamorphic pressure-temperature of the Pannonian Basin (PB), which is part of Alpine- peaks. Carpathian-Dinaric orogenic belt, and which consists of According to [8] in the central range of the dome the deep sub-basins separated by subsurface topographic SG and AG units dominate on top of the basement, while highs of metamorphic basement (e.g. [4, 5]). One of the the northern slope of the MFD is mainly built up by the best-known elevated basement highs is the Mezősas-Furta OG unit. By the interpretation of these spatial data, the Dome (MFD), which is part of the series of metamorphic structure of the area can be explained as overthrusted highs bordering the Békés Basin to the north (Fig. 1). NW-SE oriented stripes of AG/SG/OG triplexes. The sim- plified framework of the MFD can be described bypa- rautochthonous OG unit with allochthonous AG/SG du- plexes on the top. Details of microstructural investigations indicate that AG/SG boundary formed during an earlier compressional event and the two groups jointly overthrust the OG unit. These statements are in good agreement with the results of numerous authors [4, 9, 10], as the struc- ture of the basement was strongly affected by Upper Creta- ceous nappe formation, as well as sinistral transpression in the Middle Miocene period. This scenario was compli- cated on the southeastern portion of the MFD, where high- angle normal faulting related to the Neogene (Badenian) extension of the area formed horst-graben structures. The multi-stage tectonic evolution of the area resulted in the presence of wide brittle fault zones throughout the crystalline highs, which also act as fractured hydrocar- bon reservoirs in the basement of the PB, with the MFD Figure 1: Topographic map of the Pre-Neogene basement of the reservoir being one example. By the inferences of [5], eastern portion of the Pannonian Basin and the distribution of the these fractured horizons, with their often-enhanced per- basement highs (shaded). Isolines denote depth below the present meability, have a great impact on the hydraulic system surface in meters. Inset: Location of the studied well in the Alpine– of the metamorphic highs, and in fact these shear zones Carpathian–Pannonian System. A – Alps, BM – Bohemian Massif, C – Carpathians, D – Dinarides, PB – Pannonian Basin. probably serve as the main migration pathways between the deep sedimentary basins and the fractured hard rock realm [5]. A wide fault zone was penetrated by a well Following the earliest studies on the area (e.g. [6] (A-2) in the northern, OG-dominated part of MFD with or [7]), detailed petrological investigations [8] defined the a quite high core recovery (Fig. 2). The well is situated metamorphic evolution of the most typical lithologies of north of the hydrocarbon producing wells of the field in MFD and clarified their spatial arrangement through the the most elevated parts of the metamorphic high (unpub- basement high. The constructed idealized rock column lished industrial data). Although, well A-2 itself is non- suggests the presence three lithological blocks: at the productive due to its structural position, the sampled fault uppermost structural level amphibolite and amphibole- zone may play a crucial role in the local petroleum sys- biotite gneiss (AG group) with medium grade metamor- tem by linking the source rocks of the adjacent sedimen- phism is common. This group structurally overlays high tary sub-basins and the metamorphic high. In the absence metamorphic grade sillimanite and garnet bearing biotite of oriented borecores, high-resolution seismic sections or gneiss that is often coupled with garnet-bearing amphi- nearby wells, the kinematic sense and the spatial position bolite, and is jointly named the SG group. At the low- of the approximately 10 meters wide fault zone are unclear. ermost structural position orthogneiss is dominant, de- Nevertheless, the examined section that consists of diverse rived from medium-grade metamorphism of an igneous fault rock samples give an outstanding opportunity for an intrusion. The orthogneiss body is often cross-cut by mi- integrated evaluation of the shear zone. crogranite dykes. These lithologies are jointly named the Integrated petrographic – rock mechanic borecore study Ë 55 sections were made perpendicular to the foliation and par- allel to lineation. 3.3 Description of Petrographic Image Analysis (PIA) The PIA of samples was conducted with the aim of defin- ing the geometric features of clasts within the fault rocks. First, polished surfaces were cut parallel to the longitu- dinal axis of the borecores, then 2D images were taken (Fig. 3). Unfortunately, as some of the available thin sec- tions did not provide a statistically sufficient amount of observable particles (>50 clasts), the micro-scale samples were omitted from the image analysis. The lower limit of the image analysis was 100 pixel2 (0.16 mm2), according to the suggestions of [11, 12] on the methods of particle size/shape analysis, while the size of the largest measured clast was 193960 pixel2 (293 mm2). However, in this case the clasts could not be segmented by the use of a colour threshold application as the matrix or cement was too het- erogeneous, so the selection of edges was accomplished Figure 2: a: Detailed sub-surface topography of Mezősas-Furta manually, using the “Magnetic Lasso” tool of Adobe Photo- Dome. Dots mark the productive wells of the field, while the dashed shop. The clast geometric measurements were completed line indicates the assumed location of the main hydrocarbon ac- using the selected parameters in ImageJ. The applied pa- cumulation.