Havryshkiv H. Mineralogical and petrographic features of the Paleocene deposits of the Berehova and Oriv Nappes of the Ukrainian Carpathians in the context of their oil and gas bearing

The dissertation is devoted to the study of the mineralogical and petrographic features of the Paleocene deposits of the Yamna suite of the study territory, and to the prospects of their oil and gas bearing. Based on the analysis of the petrographic composition of Paleocene rocks and the features of their occurrence, different types of sections were distinguished, the litho-facial variability of the Paleocene sequence was investigated, and the areas of sandstone, siltstone, and mudstone occurrence within the study area were identified. The paleoenvironments during the Paleocene time in the Carpathian segment of the northern margin of the Tethys Ocean were reconstructed. It has been investigated that Paleocene sediments were formed as a result of turbiditic displacement of a large amount of sediments from the first (shelf) to the second (base of the continental slope) levels of avalanche sedimentation, where huge submarine fans were formed. The lithofacial scheme of the Paleocene fan of the investigated sediment-rock basin and the paleo-oceanographic map-scheme on the palinspastic basis have been constructed. On this basis the influence of sedimentogenesis on the formation of potential hydrocarbon reservoirs has been analyzed. It has been investigated that in the Paleogene rocks the matrix is represented by clayey, mixed siliceous-clayey and carbonate composition. High reservoir properties are most often characteristic for sandstones with hydromica or siliceous-hydromica, pore and film-pore matrix. Rocks that contain a relatively higher amount of secondary carbonates within the matrix mass are characterized by low reservoir properties. In clay fraction of aleuro-psammites of Yamna suite the mixed-layer minerals of hydromica-montmorillonite and chlorite-montmorillonite composition have been established. It has been found that the Paleocene rocks have undergone postsedimentary transformations of the mesocagenesis stage of МК1–МК4. The lithogenetic transformations and the influence of the material composition of the Paleocene rocks on their reservoir properties have been investigated. The individual types of reservoir rocks have been identified and the associated areas prospective for hydrocarbons have been allocated. It is revealed that the most prospective from the point of view of potential reservoir rocks in the deposits of the Yamna suite, which is represented mainly by sandstones, is the northwestern part of the study area, where the reservoir properties of the oil- and gas-bearing Paleocene sediments are largely determined by the factors that facilitated the preservation of the high reservoir properties of the rocks at great depths. These factors are the thickness of the layers and the type of cementation of the clastic material. The complex of obtained results of investigations allowed allocation of the areas of potential reservoir rocks occurrence in the Paleocene sequence prospective for hydrocarbons accumulations.