Origin and Source of Deep Natural Gas in Nanpu sag, Bohai Bay Basin, China

: Natural gas exploration in Nanpu sag, Bohai Bay Basin, has achieved breakthroughs in recent years, and a number of natural gas and condensate wells with high yield have been found in several structures in the beach area. Daily gas production of single wells is up to 170,000 m3, and high-yield wells are mainly distributed in the Nanpu No. 1 structural belt. Studies have shown that these natural gases are mainly hydrocarbon gases, with methane content about 80% to 90% and ethane 6%–9%, so they are mainly wet gas; and non-hydrocarbons are at a low level. Carbon isotopes of methane range from −42‰ to −36‰, and ethane from −28‰ to −26‰. Calculated maturity based on the relationship between δ13C and R0 of natural gas, the gases are equivalent to those generated from organic matter when R0 is 1.0%–1.7% (mainly 1.25%–1.32%). The natural gas is oil-type gas generated from the source rocks at mature to high mature stage, associated with condensate, so carbon isotopes of the gases are heavier. Natural gas in the Nanpu No.1 structural belt is mainly associated gas with condensate. The analysis of the origin and source of natural gas and condensate, combined with the monomer hydrocarbon carbon isotopes and biomarker, indicated that the main source rocks in the Nanpu No.1 structural belt were Es3 (the lower member of the Shahejie Formation), followed by Es1 (the upper member of the Shahejie Formation). The high-mature hydrocarbons from source rocks in the deep sag mainly migrated through deep inherited faults into shallow traps and accumulated to form oil and gas pools. Therefore, there is a great potential for exploring gas in deep layers.