GEODYNAMICS

Abstract

The purpose of this work is to study the Upper Albian-Lower Cenomanian psammitic complex of the platform basement of the Pokuttya-Bukovyna part of the Carpathian Foredeep, which is one of the main reservoir strata of the Lopushna oil field. Methodology. The complex of investigations includes correlation of the well-log data, lithological studies of core material, mineralogical and petrographic analysis of rocks, and paleogeographic studies of the sedimentary basin. Results. On the basis of the analysis of all existing wells, it was established that the deposits of the Late Albian-Early Cenomanian age within the entire territory of the autochthon of the Pokuttya-Bukovyna Carpathians are mostly represented by sandstones. They are composed of quartz (50-80%), feldspar (1–5%), muscovite (0.1-3%), pyrite (0.1-3%) and accessory minerals – zircon and epidote. A significant part of sandstone is compoed of the authigenic minerals – glauconite (10-15%, sometimes up to 45%) and phosphate (1–3 up to 10%). The matrix in the rocks is mainly porous and contact-porous (10-15% of the rock volume), which provides these psammites with good reservoir properties. On the basis of paleogeographic reconstructions, it was established that in the Late Albian-Early Cenomanian, terrigenous sedimentation prevailed within the entire shelf basin of the Carpathian branch of the Meso-Tethys, which caused the accumulation of the thick sandstones’ strata. The burial history and post-sedimentary evolution of rocks took place under the influence of the thrusting of the Carpathian Fold Belt. This resulted in the formation of reservoir rocks and traps in the Upper Albian-Lower Cenomanian sequence. The peculiarities of hydrocarbon migration in the studied region indicate a high probability of discovering new deposits in the autochthon of the Pokuttya-Bukovyna Carpathians. Originality. The applied complex of lithological, geological-geophysical and paleogeographical studies made it possible to investigate the features of distribution of the Albian-Cenomanian psammite complex of rocks, both laterally and in section, their mineralogical and petrographic characteristics, sedimentary environments. The post-sedimentary evolution of the study strata and features of hydrocarbon migration were caused by the dynamics of the Carpathian Flysch Belt thrusting over the margin of the East-European platform. Practical significance. The obtained results show that the Upper Albian-Lower Cenomanian sandstones, which are oil and gas-bearing in the Lopushna field, represent the potential reservoirs in terms of both thickness and mineral composition within the entire autochthon of the Pokuttya-Bukovyna Carpathians. Since the Pokuttya fault could serve as a migration path for the hydrocarbons from the Carpathian flysch sequence into the platform basement strata, there is a high probability of the discovery of new accumulations in the structures similar to the Lopushna, revealed by seismic studies in the autochthon of the Pokuttya-Bukovyna Carpathians.