Porosity Development Controlled by Deep-Burial Diagenetic Process in Lacustrine Sandstones Deposited in a Back-Arc Basin (Makó Trough, Pannonian Basin, Hungary)
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Deeply buried Pannonian (Upper Miocene) siliciclastic deposits show evidence of secondary porosity development via dissolution processes at a late stage of diagenesis. This is demonstrated by detailed petrographic (optical, cathodoluminescence, fluorescence, and scanning electron microscopy) as well as elemental and stable isotope geochemical investigations of lacustrine deposits from the Makó Trough, the deepest depression within the extensional Pannonian back-arc basin. The analyses were carried out on core samples from six wells located in various positions from centre to margins of the trough. The paragenetic sequence of three formations was reconstructed with special emphasis on sandstone beds in a depth interval between ca 2700 and 5500 m. The three formations consist, from bottom to top, of (1) open-water marls of the Endrőd Formation, which is a hydrocarbon source rock with locally derived coarse clastics and (2) a confined and (3) an unconfined turbidite system (respectively, the Szolnok and the Algyő Formation). In the sandstones, detrital grains consist of quartz, feldspar, and mica, as well as sedimentary and metamorphic rock fragments. The quartz content is high in the upper, unconfined turbidite formation (Algyő), whereas feldspars and rock fragments are more widespread in the lower formations (Szolnok and Endrőd). Eogenetic minerals are framboidal pyrite, calcite, and clay minerals. Mesogenetic minerals are ankerite, ferroan calcite, albite, quartz, illite, chlorite, and solid bituminous organic matter. Eogenetic finely crystalline calcite yielded δ13 values from 1.4 to 0.7‰ and δ18 values from –6.0 to –7.4‰, respectively. Mesogenetic ferroan calcite yielded δ13 values from 2.6 to –1.2‰ and δ18 values from –8.3 to –14.0‰, respectively. In the upper part of the turbidite systems, remnants of the migrated organic matter are preserved along pressure dissolution surfaces. All these features indicate that compaction and mineral precipitations resulted in tightly cemented sandstones prior to hydrocarbon migration. Interconnected, secondary, open porosity is associated with pyrite, kaolinite/dickite, and postdates of the late-stage calcite cement. This indicates that dissolution processes took place in the deep burial realm in an extraformational fluid-dominated diagenetic system. The findings of this study add a unique insight to the previously proposed hydrological model of the Pannonian Basin and describe the complex interactions between the basinal deposits and the basement blocks.Keywords:
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Abstract The use of field emission electron microscopy for the study of clay mineral petrography in mudrocks and sandstones is discussed. The methodology including sample preparation is outlined and three examples of the application of the technique are described: the formation of authigenic illite in mudrocks, the multiple generation of authigenic illites in sandstones and the effects of KCl drilling muds on shale fabrics. In the study of authigenic illite formation in Palaeocene mudrocks from the North Sea, the FESEM analyses have demonstrated the formation of illites with increasing burial depth that conventional SEM and XRD analyses had failed to show. The FESEM analyses of the authigenic illites in Carboniferous sandstones from the southern North Sea revealed at least three different habits representing different generation episodes rather than one illite formation event.This has important repercussions with regard to the interpretation of stable isotope and dating data for the illites. Significant petrographic changes in shales after treatment with KCl drilling muds have been observed from FESEM analyses, suggesting reactivity between the shales and the KCl muds.
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Abstract The Late Devonian-Early Carboniferous Drummond Basin of central Queensland, Australia is one of the oldest units of the New England Fold Belt. Samples of authigenic clay minerals have been collected from across the Drummond Basin and analysed using mineralogic, isotopic and geochronologic techniques. X-Ray diffraction analysis on 36 samples selected from six exploration wells was carried out on volcaniclastic and argillaceous sandstones and mudstones of the Drummond Basin sequence in order to ascertain authigenic illite content. Sixteen samples were found to be suitable for K-Ar dating purposes, containing abundant illite and illite/smectite mixed layers (I/S). K-Ar age dating was carried out on these selected samples yielding three age groups at 300, 250 and 200 Ma. Temperature of formation was estimated for the authigenic illite and illite/smectite minerals, based on illite crystallinity and illite content of illite/smectite mixed layers. Two groups of temperatures have been delineated, with a high temperature assemblage of between 275¨¬C-320¨¬C and a low temperature assemblage of approximately 100¨¬C. When compared with the age groups, it is seen that the older event was typically hotter. Stable isotope compositions for the authigenic clay minerals were analysed with ¥aD and ¥a18O values of between -68¢¶ to -117¢¶ and +7.5¢¶ to +14.4¢¶ (SMOW) respectively. Isotopic compositions for the fluid in equilibrium with the clay minerals were calculated using palaeotemperatures estimated from the clay mineralogy and range between -90¢¶ and -41¢¶ and -0.6¢¶ and +8.7¢¶ (SMOW) for hydrogen and oxygen respectively. These compositions are more depleted in deuterium and more enriched in 18O than those previously reported for basins in the region. The data also produced three groups of fluid compositions, which correlate with the age distributions delineated by the K-Ar geochronology. The first fluid composition, which correlated with the Permo-Carboniferous age (~300 Ma) is interpreted to result from a mixing of Permo-Carboniferous meteoric water with highly evolved formation water trapped within the Drummond Basin since deposition of the basin. The regional extensional event at the Permo-Carboniferous boundary, which initiated the opening of the Bowen Basin to the east of the Drummond Basin allowed meteoric water to percolate deep into the basin, enabling the growth of illite rich clay minerals at this time. The second fluid group is considered to represent the evolution of Late Triassic meteoric water migrating through the Drummond Basin during another extensional period in the region at approximately 200 Ma. This event precipitated a new period of clay mineral growth in the basin (reflected in the K-Ar dates), raised the geothermal gradient in the region and extended the crust, forming new sedimentary basins within the New England Fold Belt. The third assemblage of fluid compositions were found to be very similar to the Late Triassic group, with ¥aD and ¥a18O values plotting very close to the Late Triassic meteoric water evolution trend. This is interpreted as representing the same fluid as that which produced the Late Triassic compositions and therefore the 250 Ma ages may represent partial resetting of older ages by the Late Triassic thermal episode. However, if it is assumed that these K-Ar ages are not old ages partially reset by a thermal event, the stable isotope data could represent a mixing of Mid-Late Permian meteoric water with formation water trapped in the basin. This event may represent the regional compressive Hunter-Bowen event and may be responsible for dewatering the basin before the Late Triassic extensional episode.
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Based on the previously available data of clay minerals in the surface sediments of the Southern Yellow Sea,this paper mainly deals with the distribution of 4 types of the clay minerals(illite,kaolinite,smectite and chlorite).The research results show that the content of the illite in this area is the highest,the smectite or kaolinite are the second and chlorite is the lowest;the illite-smectite-kaolinete-chlorite-type rock is the main type of the clay minerals assemblage in the area,and the second one is illite-kaolinete-smectite-chlorite-type;the material source of clay mineral is mainly from the Yellow River and theYangtze River.
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