Application of fluorescent light microscopy to enhanced lithofacies analysis of Jurassic carbonate rocks, SE Bohemian Massif

Abstract

Upper Jurassic limestones and dolomites of the Vranovice Formation occur on the southeastern slopes of the Bohemian Massif below the Western Carpathian thrust belt. They represent a special type of reservoir rocks in the potential geothermal systems and exhibit highly variable porosity, permeability, and flow mechanisms due to different type of diagenesis and pervasive dolomitization. Fluorescence and translucent light microscopy was used for more detailed petrographic analysis of Vranovice limestones and dolomites. Isopachous, syntaxial and dolomitic cements were distinguished along with fossils recrystallization. Two generations of dolomite were identified – replacement dolomite and zonal cementation dolomite, what suggests a number of changes in the formation water chemistry during the dolomitization process. Extent of the observed dolomitization is inversely related to the porosity of the Jurassic rocks. Postdiagenetic generation of fissures cutting the strata due to tectonic deformation is documented by bright fluorescent color. In the studied samples of Vranovice limestones and dolomites the fissures represent the most important migration pathways, more than pore throats themselves. Further genetic information on petoleum migration is interpreted from intercrystalline porosity filled with precipitated solid bitumen. Present results provide basis for further investigations in pore-water-rock interactions and changes in chemical conditions during diagenesis.