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SOME FEATURES OF THE OIL-SATURATED DOLOMITE RESERVOIRS FROM THE LOWER MISSISSIPPIAN OF THE VOLGA-URAL BASIN, RUSSIA

A. N. Kolchugin

Abstract

The dolomite problem is very popular among geologists, who are especially working with oil-saturated carbonate rocks. Probably, this is the most common type of diagenetic transformation of limestone. However, the problem of secondary dolomitization is still insufficiently discussed, as a process associated with oil migration and oil-charge. The main purpose of this study is the geological reconstruction of diagenetic changes for the Tournaisian and Visean dolomites based on the application of data from cathodoluminescence microscopy, isotope studies, and analysis of data from microthermometry of fluid inclusions. Administratively studied oil-fields are located in the eastern part of the Russian platform. The Dolomites in Tournaisian and Visean strata are present not everywhere. Only small part of reservoirs contains dolomite in cross-section. According to data from of optical microscopy Tournaisian and Visean dolomites are represented by small-medium-grained dolomites, with an idiomorphic and hypidiomorphic-grained structure. According to the isotope geochemistry, the Tournaisian and Visean dolomites are quite similar in ?13? and ?18O ratios. They are characterized by variations in ?13? values from -0.28 to 1.76, with an average value of 1.18; ?18O from - 2.37 to - 11.02, with an average value of -6.26. It was done detailed microthermal studies for dolomites, in addition to geochemical studies. Fluid inclusions were recognized in all studied dolomites. The homogenization temperatures of inclusions vary from 160 oC to 215 oC. Salinity of inclusions varies in the range of 22.8 - 27.7 wt.% (NaCl + CaCl2). It can be said that all studied dolomites are quite similar both in geochemical characteristics and according to the analysis of the composition and temperature of the homogenization of fluid inclusions, despite external differences according to cathodoluminescence microscopy. Such a high similarity of the composition of inclusions in Tournaisian and Visean dolomites from various oil fields suggests that fluids caused secondary dolomitization and have the same source. In this case, the fluids were rather hot, as indicated by the homogenization temperatures on average 190 °C. The authors suggest that the studied dolomites are associated with the processes of oil migration and oil-charge since they always contain an abundance of hydrocarbon inclusions in the grains.

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