Reasons of the inhomogeneity of the composition and the internal structure of bitumen-saturated sands in permian deposits of Tatarstan (Russian federation)

Abstract

© SGEM2019. In the Permian sediments of the Volga-Ural oil and gas province (east of the East European Platform) occurs layers of tar sands and sandstones. They are confined to the Sheshmian horizon of the Ufimian stage. Currently, bituminous deposits are being developed as high-viscosity oils. The internal structure of high-viscosity oil fields is very complex due to the combination of a large number of sedimentation and post-sedimentation processes, caused, among other things, by the influence of the oil substance. An important parameter characterizing the permeability of tar sands and sandstones is the nature and composition of the detrital component and the cementing substance. Carbonate cement is formed at the stage of diagenesis and fills the pore space of rocks. This negatively affects their reservoir and permeable properties. To determine the factors affecting the heterogeneity of the sands and sandstones of the Sheshmian horizon, core samples were taken from wells drilled within the natural bitumen deposit. They were used for particle size distribution analysis of the detrital material of the sands. To interpret the data of particle size distribution analysis, three main fractions of sands (0.5–0.315 mm, 0.315–0.16 mm and 0.16–0.08 mm) were used, totaling more than 90% of the mass. Analysis of these fractions showed variability in the section of the reservoir of the quantitative content of each fraction. Obviously, this is due to a change in the primary conditions of accumulation of sand deposits (hydrodynamic conditions, the nature of the introduced detritus material, etc.). The degree of carbonate content of sands was also investigated, which is determined by the content of carbonate minerals. Analyzes showed that the carbonate content of the reservoir varies widely from 4 to 28%, with an average value of 7-9%. The highest carbonate values are associated with the lower part of the reservoir. In this case, a direct correlation was found between the change in the carbonate content of the sands and the percentage of the medium-grained fraction of the sands. This may be due to favorable geochemical and thermodynamic environmental conditions for the formation of carbonate cement in this particular dimension range. The identified patterns can be useful in the industrial development of high-viscosity oils for planning methods for the development and selection of operating equipment.