Diagenesis of the Malmian Mikulov Formation source rock, Vienna Basin: Focus on matrix and pores

Abstract

Diagenetic processes and pore development in the matrix of the 1000 m thick main source rock for oil and gas in the Vienna Basin, the autochthonous Malmian mudstones of the Mikulov Formation have been studied. Core samples from wells over a true vertical depth range of 1400 m–8551 m were available. The bulk samples contain quartz, minor amounts of plagioclase, pyrite and a large but variable proportion of calcite; the clay mineral content ranges from 14 to 47%. The clay fraction contains a prominent illite-smectite (I–S) mixed-layer phase, illite, chlorite and kaolinite. The quantities of I–S and kaolinite decrease with depth, whereas illite and chlorite increase with depth. Diagenesis has involved a gradual transformation of smectite to illite through mixed-layer I–S intermediates. The ordering of the mixed layer I–S changes with increasing depth from R0 to R1 and R3. The R1 transformation of the mixed-layer I–S occurs at approximately 3000 m and vitrinite reflectance values of 0.4%–0.6%. Based on petrographic evidence, the cations resulting from the illitization of smectite were the source of a variety of late diagenetic mineral cements, such as Fe and Mg for chlorite formation and for ferroan dolomite precipitation. Illitization also provided Si for local quartz cementation. During diagenesis nanometer to micrometer size pores developed because of specific mineral frameworks and dissolution processes. Organic matter pores developed in deeper, thermally mature samples. Phyllosilicate framework pores between brittle grains are commonly observed. Pores caused by partial dissolution of carbonate grains also occur. In places diagenetic cements, such as quartz overgrowths or carbonate cements keep pores propped open. The connectivity of the pores cannot be established unequivocally from SEM photomicrographs, but they likely contribute to the creation and preservation of effective porosity and gas storage capacity of these rocks.