Carbonate reservoir rocks characterization of the Kazanian Stage of the Gorsky ultraviscous oil field by electron paramagnetic resonance method

Abstract

The use of the electron paramagnetic resonance (EPR) method for studying carbonate rocks is widespread. EPR makes it possible to reconstruct lithological and geochemical environments of ancient sedimentation, including redox conditions and paleo-radioactivity of sedimentation basins. The paper considers the heterogeneous structure of carbonate reservoir rocks of the Kazanian Stage of the Gorsky ultraviscous oil field by paramagnetic marks: manganese ions Mn2+, radical ions SO3–, SO2–, РO22-(РO20) in carbonate minerals and radical С600 in the remains of organic matter in the rock. The carbonate reservoirs of the Gorsky structure, tectonically confined to the eastern side of the Melekess Depression, are represented by two main lithotypes: dolomitic boundstones and dolomitic oolitic packstones, which have various post-sedimentary alterations. EPR data were obtained for 28 samples with a step of 0.5-1.5 m along the section. The EPR spectra are characterized by narrow lines that indicate the marine origin of carbonates. Based on the EPR spectra of the ions Mn2+ and its content in carbonate minerals, the lagoonal-marine genesis of the Gorsky structure rocks was revealed. The dominant dolomite mineralization of the section was established. The absence of dolomite radicals and low values of the degree of population of the magnesium and calcium positions of dolomites are due to the secondary nature of carbonates. The distribution of manganese ions, organic and inorganic radicals in the rock along the section is shown simultaneously with its reservoir properties and the conditions for the formation of two dolomite lithotypes.