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- Author or Editor: Tivadar M. Tóth x
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Abstract
The Kiskunhalas-NE (KIHA-NE) fractured hydrocarbon reservoir is part of the structurally rather complex crystalline basement of the Great Hungarian Plain. In the course of petrologic and thermometric examinations various rock types of the investigated area have been classified and characterized. There are four basic lithological units in the area. In the lowest structural position orthogneiss is common, which according to its petrographic features is assumed to be identical to the orthogneiss body of the adjacent Jánoshalma (JH) basement high (metamorphic peak temperature T < 580 °C according to Zachar and M. Tóth 2004). The next rock unit upward is the highly mylonitized variety of the orthogneiss with textural features suggesting deformation in an extensional stress regime. In the higher section of the mylonite zone graphitic gneiss mylonite is characteristic, with a peak metamorphic T of 410±45 °C. The lithology in the shallowest position of the area is a graphitic carbonate phyllite, with a T of 375 ± 15 °C. Estimation of the deformation temperature for both mylonitic rocks results in approximately Tdef ∼ 455 °C. All data together suggest that between the top (graphitic carbonate phyllite) and the bottom (orthogneiss) of the ideal rock column there is about 200 °C peak metamorphic temperature deviation. The two extreme metamorphic blocks probably became juxtaposed along an extensional fault zone in the basement at approximately 15 km depth.
The Variscan metamorphic basement of the Great Hungarian Plain (Tisza Block, E Hungary) consists of deeply buried crystalline highs comprising several types of gneiss and metabasic rocks. A common gneiss type is an orthogneiss variety that can be followed across the basement from Jánoshalma northeastward within the Tisza Block. We believe the Jánoshalma High to be a uniform orthogneiss block of peraluminous composition; this is suggested by the presence of idiomorphic accessory phases (apatite needles, zircon), polygonal feldspar texture, occurrence of rock bodies of exotic origin, xenoliths (amphibolite, eclogite) and xenocrysts (feldspar megacrysts, amphibole, garnet) of varying composition and metamorphic evolution. The peculiar features of the orthogneiss body are indicative of processes that may have taken place in an ancient Alaskan-type orogenic belt (subduction-accretionary complex). In these situations, because of the continuous ample sediment supply, the accretionary prism retreats oceanward, forming structures of increasing steepness with depth, accreting against the continental margin. Thereafter, this causes oceanward retreat of the subduction trench as well. For this reason mantle-derived magmas do not penetrate the continental margin but the accretionary prism itself, resulting in the melting of its material. In the case of the Jánoshalma High, following the emplacement of the onetime granodioritic body at shallower crustal levels, it metamorphosed via to a two-stage overprint. An early high-temperature (HT) and a succeeding medium-temperature (MT) event formed orthogneiss from the ancient intrusive rock. This is the first detailed investigation of the Jánoshalma high that has been published.
The metamorphic basement of the Pannonian Basin consists of uplifted highs and deep sub-basins among them. One of the best-known highs is the so-called Szeghalom Dome, which is surrounded by less intensely explored ones. The eastern neighbour, the Mezõsas-Furta Dome (MFD), is studied in this paper. Based on detailed petrologic investigation, six main lithologies are distinguished for the MFD, which can be well compared to those described previously for the Szeghalom Dome. All these rock types (orthogneiss, mafic-ultramafic xenolith, granite, sillimanite-biotite gneiss, garnet-bearing amphibolite, amphibole-biotite gneiss) are classified into three main units based on different metamorphic and deformation history. Understanding their relative spatial position permitted the elaboration of geologic map and sections of the MFD
Abstract
The Algyő High (AH) is an elevated crystalline block in southeastern Hungary covered by thick Neogene sediments. Although productive hydrocarbon reservoirs are found in these Neogene sequences, numerous fractured reservoirs also occur in the pre-Neogene basement of the Pannonian Basin. Based on these analogies, the rock body of the AH might also play a key role in fluid storage and migration; however, its structure and therefore the reservoir potential is little known. Based on a comprehensive petrologic study in conjunction with analysis of the spatial position of the major lithologies, the AH is considered to have been assembled from blocks with different petrographic features and metamorphic history. The most common lithologies of garnet-kyanite gneiss and mica schist associated with garnetiferous amphibolite are dominant in the northwestern and southeastern parts of the AH. The first regional amphibolite facies metamorphism of the gneiss and mica schist was overprinted by a contact metamorphic (metasomatic) event during decompression in the stability field of kyanite. Garnet-bearing amphibolite experienced amphibolite facies peak conditions comparable with the host gneiss. Regarding the similarities in petrologic features, the northwestern and southeastern parts of the area represent disaggregated blocks of the same rock body. The central part of the AH area is characterized by an epidote gneiss-dominated block metamorphosed along with a greenschist-facies retrograde pathway as well as a chlorite schist-dominated block formed by greenschist-facies progressive metamorphism. The independent evolution of these two blocks is further confirmed by the presence of a propylitic overprint in the chlorite schists. The different metamorphic blocks of the northwestern, southeastern and central parts of the AH probably became juxtaposed along post-metamorphic normal faults developed due to extensional processes. The supposed brittle structural boundaries between the blocks could have provided hydrocarbon migration pathways from the adjacent over-pressured sub-basins, or could even represent suitable reservoirs.
Abstract
Analytical data of primary oxidized manganese ores were processed by statistical methods. Six hundred and twenty-one samples were measured (Mn, Fe, Si, and P); thus 2,426 assay data were available. The statistical pointer numbers, the distribution of the elements and the results of the correlational analysis showed the heterogeneity of the ore samples where the measured elements correlated weakly. The samples were grouped by the 4 elements to decrease the heterogeneity and the concentration of elements, and these relationships in the groups were examined. Very few and weak relationships were proved in the groups by the results of the correlational and regressional analysis. It is possible that not the heterogeneity of the samples but one or more syngenetic or postgenetic processes caused the absence of relationships. The multivariate statistical processes (principal component analysis, discriminance analysis) allow the determination of the background factors, namely which are the effects that produced the ore. Consequently — with high probability — the ore was formed by two processes. The most likely are hydrothermal and microbial ones (on the basis of geochemical results), but supergene enrichment processes can also be taken into consideration. Both hydrothermal and microbial processes played a significant role in the majority of the samples (81%), which are the ferruginous manganese ores. In the smaller group of samples (19%) the hydrothermal process predominates but the microbial one is also influential, namely for the low iron-bearing manganese ores of excellent quality.
Abstract
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.
Hazánk kedvező geotermikus adottságai miatt nagy mennyiségben termelhető ki a változatos hasznosítási módoknak eleget tevő termálvíz. Balneológiai felhasználás után a visszasajtolás tilalma miatt a felszíni csurgalékvíz-elvezetés többnyire földcsatornákon keresztül történik. A vizsgálati területül választott cserkeszőlői gyógyfürdőből a természetbe kikerülő használt hévíz szigetelés nélküli csatornán keresztül a Körösbe ömlik. Az így elszikkadó csurgalék hévíz talajtani hatásait 11 diagnosztikai talajparaméter figyelembevételével főkomponens analízis és diszkriminancia analízis segítségével értékeltük. Megállapítottuk, hogy a beszivárgó használt hévíz nagy Na+-koncentrációja miatt a talajban ioncsere folyamatok játszódnak le, melyek a Na+-koncentráció növekedése és ezzel egyidejűleg a Ca2+-tartalom csökkenése irányába hatnak. A diszkriminancia függvény alapján a legjelentősebb háttérfolyamatként a Mg2+mobilizáció azonosítható. A szikesedést jelző paraméterek (EC, szóda-, Na+-, HCO3 --tartalom) csak kis mértékben növekedtek, a szikesedési részfolyamatok megjelenése így jelenleg kezdetinek tekinthető. A csatorna környékén a humusz- és K+-tartalom növekedése is kimutatható volt. A diszkriminancia függvény alapján diszkriminancia diagramot szerkesztettünk, melyen közel 90%-os helyességgel különültek el a talajminták a csatornától való távolság szerint a priori ismereteink alapján képzett „kontroll” és „termálvízhatás alatt álló” talajmintacsoportokra. A diszkriminancia függvény alapján a továbbiakban a mintaterületről származó bármely talajminta – diagnosztikai paramétereinek meghatározása után – besorolhatólesz a termálvíz hatása által érintett vagy nem érintett csoportok valamelyikébe. Így közvetetten a hatásterület kiterjedéséről is információt nyerhetünk. Ezen az 50 éve termálvízhatás alatt álló területen igazolható volt a hévíz szikkadás talajmódosító hatása, azonban a terepi megfigyelések, laboratóriumi vizsgálatok és a statisztikai analízis tanúsága szerint ezen folyamatok nem érik el azt a mértéket, mely esetén már talajdegradációról beszélhetnénk.
The pre-Cenozoic basement of central Hungary is partly made up of different types of carbonate rocks. These carbonates are often good hydrocarbon reservoirs, and hydrocarbon production is significant in this region in Hungary. Nonetheless, the petrography of the reservoir rocks has not yet been investigated in detail. In this study, the results of the investigations of the lithology of a carbonate hydrocarbon reservoir from central Hungary (Gomba Field) are presented. Based on this work, two types of pure limestone, a dolomitic limestone and a polymictic breccia, could be distinguished in the study area. The limestone types are similar to the Kisfennsík Limestone Member and the Berva Limestone of the Bükk Mountains, but they contain significant amounts of framboidal pyrite and dead oil as vein fillings. The breccia is predominantly composed of angular carbonate clasts and minor metamorphic and sedimentary rock fragments in a chaotic pattern. The breccia has some grains that may be speleothems (e.g., stalactite or stalagmite) based on their structure and isotopic compositions. The fabric of the breccia suggests that it may have been formed by fluid-related processes. Cross-cutting relationships of the veins and petrography of the vein fillings suggest that there are four different fracture generations and two different hydrocarbon migration phases to be distinguished. The composition of the hydrocarbon-bearing fluid inclusions related to the second migration event is similar to the crude oil currently produced from the Gomba Field. During the Eocene, the Triassic basement was buried and brecciated. Subsequently, a primary hydrocarbon migration can be assumed, but the hydrocarbons became overmature, apparently due to the high temperatures of the burial environment. Finally, an uplift phase began and the youngest fracture generation formed, which serves as a primary pathway for the more recent hydrocarbon migration.
Fractured fluid reservoirs are of key importance for recovering water and hydrocarbon supplies and geothermal energy, or in predicting the subsurface flow of pollutants. There are several fractured metamorphic-basement HC reservoirs in the Pannonian Basin; one of the largest among them is the Szeghalom Dome in SE Hungary. Previous production and fluid inclusion data infer that in this case several unconnected fluid regimes must coexist in the basement, making modeling of the fracture network essential. Because the representative volume of a fractured rock mass is usually too large to measure hydraulic properties directly, stochastic calculations should be carried out, which are consistent with observed deformation history and stochastic patterns. Input statistical data (orientation, length, distribution, fractal dimension for fracture seeds) were determined for amphibolite and gneiss samples representing the Szeghalom Dome. Data were measured simultaneously using binocular microscope and computerized X-ray tomography. Comparison of the two data sets suggests that they are comparable and both can be used for modeling. A new computer program, called REPSIM has been developed recently, which follows a fractal geometry-based discrete fracture network (DFN) algorithm to simulate the fracture network. The evaluation of simulated networks suggests that amphibolite and gneiss-dominated parts of the basement behave differently; large amphibolite bodies have a connected fracture network, while gneiss domains usually are well below the percolation threshold.
