Features of the structure and formation of oil deposits in the sediments of the Bashkirian stage of small oil fields on the western slope of the South Tatar arch

Abstract

Oil-bearing carbonate complexes of the Bashkirian stage of low-amplitude brachianticlinal structures on the western slope of the South Tatar arch are considered. It has been established that the prospects for oil production are associated with the upper parts of the sections, where layers of algal-foraminiferal limestones - packstone occur, overlain by dense clayed mudstone-wackestone limestones. Oil deposits of the dome type, hydrodynamically isolated, are under the pressure of regional or bottom waters. Cap rocks, reservoir rock, water-oil contacts are clearly distinguished in the structure of deposits. The formation of the main elements of oil fields occurred at the stage of sedimentation due to eustatic fluctuations in the level of the epicontinental sea basin. At the stage of transgression, the formation of future reservoir rocks took place, at the stages of regression - cap rocks. The reservoir rocks acquired high capacitive-filtration properties during the period of migration of water-oil fluids. At this time, the dissolution of micritic calcite cement took place, the removal of the dissolved substance outside the reservoir bed, the filling of the void space with oil, and the displacement of residual water to the wings of uplifts. The next stage of transformation of reservoir rocks is associated with the processes of natural waterflooding of oil deposits. The penetration of regional pressure water into the oil reservoir contributed to the formation of local zones of water-oil contacts. Due to the oxidation of hydrocarbons inside the oil deposits in the OWC zone, bituminous and water-bearing subzones were formed. In the first one, intensive leaching of calcite occurred with the simultaneous formation of patchy-banded segregations of calcitized rocks, healing of cavern cavities with diagenetic dolomite and anhydrite. In the second subzone, the process of dissolution of bituminous films on the walls of caverns was going on, accompanied by secondary cementation of pre-existing cavities with diagenetic calcite and chalcedony, which determined the light color of reservoir rocks.