Ufimian bituminous sandstones of the Republic of Tatarstan: lithological and petrophysical characteristics and formation history

Research subject . Bituminous sandstones of the Sheshmian horizon of the Ufimian stage in the Republic of Tatarstan. Aim . To identify relationships between lithological and petrophysical parameters for paleoreconstructions of the conditions of formation and redeposition of natural bitumen deposits. Materials and methods . The composition and internal structure of bituminous sandstones were studied by optical microscopy methods (petrographic description of thin sections) and thermogravimetry (quantitative parameters of oil saturation and carbonation), statistical processing of the results of granulometric and petrophysical analyses in Statistica 10 software. Results . The studied sandstones belong to the graywacke group. Statistical analysis of granulometric and filtration-capacity parameters of bituminous sandstones revealed relationships between certain fractions and the degree of bituminosity. Cluster and factor analysis of lithologic and petrophysical data made it possible to identify integral indicators that can be used to reconstruct the paleofacies of the Sheshmian sandstones. The important role of the Sheshma River valley in the reformation of bitumen deposits in the Neogene-Quaternary time under the influence of Coriolis force has been established. Presence of soda water with high (more than 1 g/l) content of hydrocarbonate-ion is a hydrochemical criterion for searching the deposits of natural bitumen. The present study complements the existing hypotheses of the formation of the Sheshmian bituminous sandstones of the South Tatar arch. Conclusion. Based on a combined analysis of geological data, lithological and petrophysical parameters, it is suggested that the most bitumen-saturated upper layer of sandstones was formed by eolian processes.