Carbon Isotopic Composition and Components of the Petroleum Gases from Fields in the Western Pre-Caucasus Area (Russia)

Summary This study is about differences and similarities in the isotopic and component composition of the petroleum gases from fields in the Western Pre-Caucasus area. These gases were differentiate into three genetic types. The 1st genetic type (Akhtyrsky type) were generated by the rocks of the Middle Eocene Kuma Formation. Gases with the higher δ13C values of gas components from ethane to pentane were assigned to the 2nd (Sladkovsky) genetic type. These gases are associated with oils, the source of which was marine organic matter but the exact stratigraphic confinement of the source rock remains unclear. And the 3rd genetic type of gases (Nikolaevsky type) which the initial organic matter associated with higher terrestrial plants, which accumulated under oxidizing conditions (for oils Pr/Ph = 3.1–4.3, C27–C28–C29 steranes is 0.23–0.22–0.55). The source rock of these gases and oils remains unknown, but its age should be no younger than the Lower Cretaceous.