The studied oil and gas field are located in the northern part of Croatia. More specifically, in the Bjelovar depression of the Pannonian Basin. The geological development is closely connected with the complex geological history of the southern part of the Pannonian Basin, more precisely to the North Croatian Basin during the Neogene and Quaternary. Bjelovar depression presents a separate geotectonic unit formed by the main transcurrent fault system, together with other accompanying fault systems transverse or diagonal to the direction of the central Drava fault. It was formed in the Early Miocene syn-rift phase linked to regional extension tectonic phase which generated several rift-type depressions in North Croatian Basin The dissertation presents a methodological approach to the characterization of hydrocarbon reservoirs. It is based on the advanced methods of processing and interpretation of geophysical data. Hydrocarbon reservoir characterization refers to the quantitative analyzes of seismic, wellbore, and production data that contribute to a better understanding of the spatial distribution of reservoir characteristics such as porosity, lithology, and structural features. Different geophysical and laboratory methods are presented with the aim of integrating the available geological, laboratory and petrophysical data required for a better interpretation of seismic data. The seismic interpretation was performed on 3D seismic data in time domain and it was controlled on depth migrated 3D seismic data. The analysis of seismic attributes verified the interpreted structural setting. Attributes used in the interpretation belong to the group of physical attributes (RMS amplitude, instantaneous frequency, sweetness) and to the group of geometric attributes (variance). Shear wave velocities are fundamental for detailed seismic characterization of the reservoir because the interpretation is based on the velocity ratio Vp/Vs, impedance and Poisson's ratio. This dissertation presents the procedure for predicting Vs in wells without recorded Vs. Predicting Vs is a great challenge and requires a lot of effort to provide reliable input for seismic modeling and seismic inversion. In the Grubišno Polje gas reservoir, Vs velocities were determined using the Kuster-Toksöz and Xu-Payne models, and in the Letičani oil reservoir, Vs velocities were determined by the Greenberg-Castagna empirical relationship. VI To define the elastic properties of limestone, the laboratory measurements of ultrasonic velocities under different saturation conditions, were made for the first time for a hydrocarbon reservoir in the southwestern part of the Pannonian Basin. The empirical relationship between shear wave velocities (Vs) and compressional wave velocities (Vp) was defined based on the laboratory results. The results of the laboratory measurements of ultrasonic velocities indicate that the porosity and density have the main influence on velocities, while the influence of calcite or dolomite is less significant. In the seismic inversion process geological settings are reconstructed based on seismic data. Inversion seeks to define the physical parameters that also have geological meaning. In this dissertation, the methodology of the different seismic inversions and Poisson impedance is presented. Inversion results define the distribution of elastic parameters and the quality of reservoir rocks. The seismic inversion results obtained by the different methods show a similar range of the impedance values and Vp/Vs ratios for the same lithologies. The stochastic inversion shows results with a better vertical and lateral resolution that enable better integration with the well data, which is especially visible in the results of the Letičani field. The main scientific contribution of this research is to establish the optimal methodology for seismic interpretation of partially depleted oil and gas fields using shear wave velocity data in seismic inversion. The parameters of the reservoir quality are defined using elastic properties of the reservoir, like Poisson's ratio, the shear modulus and the volume modulus for the first time, which was not used in the previous methods. The results of the ultrasonic laboratory measurements on the limestone samples under the different effective stresses and saturation states enable the analysis of the elastic properties of limestones and the definition of the pore space, i.e. shape and pore connection. The most important scientific contributions of this thesis include establishing empirical relationships of shear wave velocities (Vs) and compression wave velocities (Vp), together with the empirical relationship of porosity and velocities. Throughout, this work determines the first empirical relationship for limestone in the southwestern part of the Pannonian Basin.