HYDROCARBON POTENTIAL OF SANDSTONE RESERVOIRS IN THE NEOGENE EAST SLOVAKIAN BASIN PART I: A PETROGRAPHIC EXAMINATION OF LITHOLOGY, POROSITY, AND DIAGENESIS

Abstract

Core description and petrographic analyses were utilized to study East Slovakian Basin sandstone reservoirs. Reservoir development is largely dependent on the original sandstone composition, which is influenced by deposition in a deltaic setting, local sourcing and volcanic activity. Sandstones are texturally and mineralogically immature lithic and feldspathic arenites. The presence of unstable lithic grains and feldspars contributes to low, irregular reservoir porosity, due to deformation by compaction, and susceptibility to chemical alteration. Reservoir quality is also influenced by the subsequent diagenesis of the sandstones, which is driven by high heat flow. Lithic fragments and feldspars alter readily to form authigenic/diagenetic mineral suites, which tend to occlude porosity; however, dissolution of some of these grains also enhances secondary porosity development. Most observed porosity in the basin sandstones is secondary, developing from dissolution of both carbonate cement and unstable framework grains. Porosities suggest a weak decreasing trend with depth of burial. However, detailed examination of several localities reveals that porosity development is strongly influenced by local factors (e.g. structural evolution, sandstone lithology, and the distribution of volcanics). Sandstones of the East Slovakian Basin are generally not good hydrocarbon reservoirs (particularly for liquid hydrocarbons), due to the presence of unstable framework grains, early carbonate cementation and authigenic/diagenetic mineral suites. Exceptions are found when dissolution of cement and framework grains results in significant secondary porosity.