Origin and Source of the Cenozoic Gas in the Beach Area of the Nanpu Sag, Bohai Bay Basin, China

Abstract

Natural gas and condensate oil and gas have been discovered in structures in the beach area of the Nanpu sag, Bohai Bay Basin, eastern China. The gas is dominantly hydrocarbon gas with less non–hydrocarbons, it is wet gas and has a dry coefficient in the range of 0.7–0.9. The gas has a methane carbon isotope in the range of −44‰–36‰ and a ethane carbon isotope in the range of −30‰– −24‰. Analysis of the relationship charts between 13C1–C1/C2+3 and δ13CCH4–δDCH4, δ13C– Ro equation, Pr/Ph of associated oil or condensate, suggests that the Cenozoic gas in the beach area of the Nanpu sag is mature – high mature oil–type gas. According to analysis of origin and source of the gas and condensate oil, in combination of monomer hydrocarbon carbon isotope and biomarker, the Cenozoic gas in the beach area of the Nanpu sag is sourced from the Tertiary Shahejie Formation Sha 3 Member (Es3) source rocks and Sha 1 Member (Es1) source rocks. Among these gases, the gas from Es3 source rocks is equivalent to condensate gas formed when source rock vitrinite reflectance ( Ro) is at or above 1.3%. This type of gas has heavier carbon isotope, ethane carbon isotope generally more than −27‰, associated oil C24–tetracyclic terpane /C26–tricyclic terpane ratio less than 0.7, the distribution of monomer hydrocarbon carbon isotopes being flat. The gas from Es1 source rocks is equivalent to oil–associated gas formed when source rock vitrinite reflectance ( Ro) is at or below 1.0%. This type of gas has lighter carbon isotope, ethane carbon isotope generally less than −28‰, associated oil C24–tetracyclic terpane /C26–tricyclic terpane ratio more than 1.0, the distribution of monomer hydrocarbon carbon isotopes having clear inflection points. This study provides reference to study of gas origin in a complicated region, and makes a further step to show exploration potential and fields for gas in the Nanpu sag.