THE STRUCTURE AND DEPOSITIONAL SETTINGS OF THE CLASTIC OIL RESERVOIRS OF THE VEREISKY HORIZON OF THE MELEKESSKAYA DEPRESSION
The paper presents the results of the study of clastic reservoirs of the upper part of the Vereisky horizon on the eastern side of the Melekesskaya depression, southeast of Tatarstan. Clastic reservoirs are represented by fine-grained quartz sandstones and calcareous quartz siltstones, based on the lithological and mineralogical study of the core material. Rocks were formed during the regressive cycle of a shallow marine basin. The history of formation studied clastic reservoirs includes two stages of diagenetic transformations. The first included deposition and early diagenesis. At this time there was the deposition of material, compaction, squeezing out of pore solutions, and the predictions of layered texture, due to the predominant subhorizontal orientation of mineral fragments. Burial processes determined dense structural packing close to rhombohedral. Large muscovite and orthoclase grains acquired traces of mechanical deformation and oriented by cleavage planes to the normal of rock pressure. Cementation of the clastic component with fine-grained calcite occurred in early diagenesis. The second stage of diagenesis was associated with the migration of water-gas and water-oil fluids. The incoming carbonic solutions contributed to the dissolution of calcite, which formed the cement of clastic rocks, and removal of the dissolved material outside the reservoir intervals and formation of clear-grained calcite. Framboidal pyrite formed along large grains of calcite due to the vital activity of sulfate-reducing microorganisms. The increasing alkalinity of pore solutions took the process of dissolution of fine-grained quartz and regeneration of larger quartz fragments. The formed pore space was filled with oil. It suspended further processes of the transformation of the mineral substance. Oil reservoirs from the upper part of the Vereisky horizon are currently in the stages of destruction. Two mechanisms of destruction have been established. One of them is associated with the oxidation of oil due to natural water flooding by reservoir waters. The second is caused by the processes of degassing light fractions of hydrocarbons. In some cases, wells produce water, in others the production wells operate only in the accumulation regime, depending on the destruction mechanism.