ABSTRACT The Nanpu Sag, located in the Bohai Bay Basin of eastern China, is a significant reservoir of natural gas resources. However, a comprehensive understanding of the overarching natural gas accumulation model in this region is lacking. This study conducted natural gas geochemical and isotopic composition analyses on 290 samples from 30 wells in the Nanpu Sag to study the origin and genetic types of natural gas, revealing that the Eocene Shahejie Formation (Es) is the main source rock, clarifying the vertical migration characteristics and establishing two accumulation models to guide future exploration strategies. The natural gas in the Nanpu Sag is dominated by methane, with contents ranging from 44.89% to 98.60%. The locally high content of non‐hydrocarbon components is due to the widespread development of fracture systems within the depression, which directly connect the deep and shallow layers. By evaluating the relationship between the natural gas vitrinite reflectance ( R o ) and the source rock R o and then comparing the gas types to the source types, we ascertain that the primary source of natural gas stems from the third member of the Shahejie Formation (Es 3 ) source rock. The methane content and carbon isotope values of methane variations highlight the primary migration traits from Es 3 and the first member of the Shahejie Formation (Es 1 ) to adjacent strata. Considering the faults and magmatic activity in the study area, combined with the characteristics of natural gas migration, we have summarised two gas accumulation models: magma‐dominated accumulation and magma/fault co‐controlled accumulation.