Features of the Isotope–Geochemical Carbon Composition of Oil in Fields at the South Tatar Arch

— The carbon isotope composition of oil in the various regions of Tatarstan was found out to range from δ 13 C = –32.5 to –28.6‰. Oil in sedimentary rocks in the central part of the South Tatar Arch (STA) is characterized by a lighter carbon isotope composition (δ 13 C < –29‰) than oil in the southeastern STA slope (δ 13 C > –29‰), whose rocks are broken by deep faults. The oil of relatively heavy carbon isotopic composition is shown to be characterized by a high sulfur content, contains relatively much polar fractions, and is less catagenically mature that the oil with a relatively light carbon isotope composition. The source organic matter of oils, regardless of their carbon isotopic composition, in the South Tatar Arch was sapropelic material with an admixture of a bacterial component, which was transformed under reducing conditions when hosted in clay–carbonate sediments. The Devonian (D 2 gv) oil of the Bavlinskoye field (Well 475) in the southeast slope of the South Tatar Arch differs in composition from the other oils. Along with high contents of saturated (53%) and aromatic (36%) hydrocarbons, this oil is characterized by a relatively heavy carbon isotope composition and a sublinear curve of the isotope-fractional distribution of components. A similar distribution of components with a heavy isotope composition of the asphaltene carbon is typical the Devonian (D 3 psv) oil of the Abdrakhmanovskoye field (Wells 719 and 312) in the crest STA part, which generally has a relatively light carbon isotope composition and a contains low concentrations of polar fractions.