Diagenetic processes in reservoir rocks of the North Alpine Foreland Basin (Austria)
Research output: Thesis › Doctoral Thesis
The main goal of the present study is to gain insights into rock-fluid interactions in hydrocarbon reservoir rocks. The effects of gas generation on the diagenesis are studied using the example of reservoir rocks from different stratigraphic horizons in the northern foreland basin of the Alps. The Austrian part of the foreland basin comprises Jurassic to Miocene sediments and two petroleum systems: Lower Oligocene source rocks generated oil and thermogenic gas, which is trapped in Upper Cretaceous and Eocene reservoir rocks. Microbial gas is present within Oligocene-Miocene strata. New geochemical data suggests a mixture of microbial and thermogenic hydrocarbons in all horizons. For a better understanding of the diagenetic history, 180 core samples from 600 to 2300 m depth were investigated by optical light-, cathodoluminescence-, microprobe-, and scanning electron microscopy. Further, the mineralogical composition was analysed qualitatively and semi-quantitatively by X-ray diffractometry. Stable isotope ratios of carbon and oxygen were measured on calcitic cements. Petrophysical data were provided by RAG. Porous water- or hydrocarbon-bearing as well as low permeable sandstones within the reservoir horizons were investigated. The reservoir rocks of the thermogenic petroleum system are Upper Cretaceous shallow marine subarkoses and Eocene fluviatile to shallow marine lithic arkoses. They contain cements, which were formed during eo-, meso- and telogenesis. Eogenesis – The eogenetic processes were controlled by the depositional environment and primary mineralogical input. Clay mineral cement is the dominant authigenic phase and formed by disintegration of feldspar and phyllosilicate minerals. Abundant glauconite pellets developed within Cretaceous sandstones. Microbial gas (CH4 and CO2) was generated by metabolization of organic matter in intercalating shales. The microbial CO2 was incorporated into carbonate minerals, which formed with columnar (Cc I), microcrystalline (Cc II), blocky/homogenous (Cc III) and later poikilitic (Cc IV) morphology. Carbonate precipitation was accompanied by partial dissolution of siliciclastic components. The d13C values of carbonate cements (Cc I - Cc III) range from -6.7 to +3.3‰ [VPDB] and the d18O values from -10.2 to -4.3‰ [VPDB], comparable to the isotopic composition of marine carbonates. Lighter d18O values of Eocene fluviatile to tidal sandstones reflect the non-marine setting. Mesogenesis – Some sandstone layers were strongly cemented with carbonate (Cc III, (Cc IV)) during the mesogenetic stage. Based on the isotopic composition, two types can be distinguished: (i) calcite cement with light carbon isotopy (d13C: -32.7‰) and slight Mg-enrichment formed during “advanced” sulfate reduction, (ii) isotopically heavy cement (d13C: +8.7‰) with slightly elevated Fe contents formed during carbonate reduction. Telogenesis – An influx of meteoric water caused partial mineral corrosion (feldspar, carbonate) and late kaolinite growth in porous, laterally continuous marine sandstones. Additionally, isotope ratios of eogenetic carbonate cements were overprinted (d18O: -23.9 to -9.4‰). The Oligocene-Miocene sediments of the microbial petroleum system were deposited in a deep-marine setting. The pore filling of these lithic arkoses to litharenites consists of eogenetic cements, (Cc I - Cc III, dolomite), which show corrosion marks. Their isotopic composition is similar to marine carbonates (d13C: +1.1, d18O -5.3‰). Sediments near the gas-water contact in Oligocene-Miocene sediments are often strongly cemented by calcite (Cc III), which precipitated during the telogenetic stage. Cements of these horizons are slightly depleted in 18O (d18O: -8.0‰).
|Translated title of the contribution||Diagenetische Prozesse in Speichergesteinen des Nordalpinen Vorlandbeckens (Österreich)|
|Publication status||Published - 2017|