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

Abstract

Abstract—: The carbon isotope composition of oil in the various regions of Tatarstan was found out to range from δ13C = –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 (δ13C < –29‰) than oil in the southeastern STA slope (δ13C > –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 (D2 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 (D3 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.