The Upper Triassic carbonates of the Opponitz Formation and Hauptdolomit Formation represent reservoir rocks of gas deposits in the basement of the Vienna Basin Neogene fill. During study of the reservoir rock properties of dolomites in the well Kuklov-3 (K-3) significant variations in porosity as a result of late diagenesis processes in buried sediment were found. On the basis of petrographical, SEM and mineralogical-geochemical methods, we identified neomorphic calcite layers, also found in the Kuklov-4 (K-4) well. Coarse crystallized calcite crystals are idiomorphic (e.g. ditrigonal scalenohedron). They have a relatively high content of Sr, Fe and Na, and decreased isotopic ratio of O (δ¹⁸ O: -6 to -9 ‰) or also C (δ¹³ C: -0.7 up to +1.8 ‰) in comparison with values in dolomites (δ¹⁸ O: -4.8 up to ; 1.3 or also δ¹³ C: 0.1 to +4.1 ‰) or also in limestones (mostly δ¹⁸ O: -3.9 to -3.6 ‰). Microstructural analysis indicates that they substitute dolomites as a result of dedolomitization under conditions of deep burial. Diagenesis under conditions of deep burial results in forming of new minerals such as kaolinite, pyrite and illite. The observed changes (increasing) of reservoir rocks porosity of dolomites both in the well K-3 and K-4 (at the depth of 3660 to 3830 m) were caused by diagenetic processes taking place in the deep burial environment and these processes were probably limited to a layer of (originally dolomitic) breccias.