Lithological and mineralogical signs of the formation and destruction of the Bashkirian stage oil deposits within the Sokskaya saddle

Abstract

Oil deposits in sections of the Bashkirian stage of the Sokskaya saddle are considered. It has been established those oil-bearing limestones are represented by leached peloidal-clumpy grainstones. The rocks were formed within the shallow shelf of sedimentation basin of normal salinity. The active hydrodynamics of the aquatic environment predetermined the dense structural packing of organic residues. The migration of aggressive oil-water fluids contributed to leaching of calcite cement from grainstones. Subsequently, the pore-cavernous space was filled with oil. The introduction of edge formation waters into oil-bearing reservoirs contributed to the oxidation of oil and the manifestation of secondary diagenetic mineralization. The initial stage of waterflooding was indicated by dolomitization of reservoir rocks. Due to an increase in the partial pressure of carbon dioxide in the pore space of rocks, calcite is metasomatically replaced by diagenetic dolomite. At this stage of reservoir rock alteration, a relatively small amount of oil is recovered from oil-bearing formations. The introduction of formation waters enriched with sulfate ions into reservoir layers leads to the precipitation of gypsum-anhydrite aggregates in the pore space of oil-bearing limestones. Calcium sulfate minerals clog the pore-capacitive space of reservoir rocks, reducing their productivity. At the stage of precipitation of gypsum-anhydrite aggregates, mainly mineralized brines with an admixture of oil are extracted from reservoir rocks.