Diagenesis and reservoir properties of the Middle Miocene sandstones in the Polish segment of the Carpathian Foredeep

Aleksandra Kozłowska, Marta Kuberska, Paweł Lis, Anna Maliszewska

Abstract


The Upper Badenian and Sarmatian sandstones from eight wells of the Carpathian Foredeep Basin have been studied. The following research methods were used: petrographic analysis with the use of polarizing microscope (staining analysis, cathodoluminescence studies – CL, porosity determinations, fluid inclusion ana- lysis), scanning electron microscope (SEM) investigations and energy dispersive spectrometer studies (EDS ISIS), X-ray diffraction analyses (XRD), isotopic analysis and petrophysical studies.

The sandstones are very-fine to medium-grained subarkosic and sublithic arenites and wackes. The main components of the sandstone grain framework are quartz, feldspars (potassium feldspar and plagioclase), lithoclasts (fragments of carbonate rocks, clastic rocks, granitoids, volcanic rocks and quartz-mica schists) and micas (mainly muscovite). Bioclasts (mainly foraminifera), glauconite, ooids, organic matter and accesory minerals are subordinate. Pore spaces between the grains are filled by matrix and by cement (mostly Fe-calcite, quartz overgrowth, dolomite, siderite and kaolinite).

The Middle Miocene sandstones show good and very good filtration abilities. Sandstones porosity very often exceeds 20% and permeability is above 100 mD. Primary intergranular porosity is considerably more frequent than secondary intragranular porosity (mainly dissolution of feldspar grains) and intercrystalline porosity (clays microporosity). Effects of the following diagenetic processes can be observed in the sandstones: compaction, cementation, dissolution, replacement and alteration. Primary porosity reduction in sandstones was predominantly caused by mechanical compaction by about 26% and cementation, mainly by calcite, by approximately 35% on the average. Some increase in porosity was caused by dissolution of detrital grains, mainly feldspars, and decay of the soft parts of organisms. Diagenetic and related reservoir properties evolution of the Middle Miocene sand- stones have been accomplished during eo- and mesodiagenesis.

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