Abstract: A breakthrough has been made recently in natural gas exploration of deep volcanic rocks in Nanpu sag of Bohai Bay Basin, East China, indicating a very high potential in natural gas exploration of deep volcanic rocks in Nanpu sag. Therefore, it is quite important to study the origin and hydrocarbon accumulation model of natural gases for the future exploration. Natural gases accumulate mainly in deep volcanic rock reservoirs within the Paleogene Shahejie Formation. They are dominated by hydrocarbon gases in chemical composition, of which the methane content varies from 70% to 90%, ethane and heavy hydrocarbons are relatively high, ranging between 4%~27%, the dry coefficient varies from 0.71 to 0.96, indicating mainly a wet gas. Carbon isotopes of natural gases are relatively heavy, with δ¹³C₁ mainly varying from −35‰ to −43‰ and δ¹³C₂ mainly from −24‰ to −28‰. According to plots of C₁/(C₂+C₃) versus δ¹³C₁ and δ¹³C₁ versus δD, the natural gas in deep volcanic rock reservoirs belongs to an oil-type condensate gas. The calculated thermal maturity of the gas ranges between 1.00%~1.50% by the δ¹³C−R_o method, it matches the thermal evolution of typeⅡ₁ source rocks in the Es₃ member. In addition, Pr/Ph ratios of associated condensates, light hydrocarbon parameters and hydrocarbon isotopes also show that the deep volcanic rock gas is an oil-type gas and it is dominated by a condensate gas. Carbon isotopes of individual hydrocarbons of associated condensates show that the gas in deep volcanic rocks is generated from source rocks of the Es₃ and Es₁ members, namely the natural gas below the Es₁ stratum is mainly derived from source rocks of the Es₃ member and composed of mainly a condensate gas, while the natural gas above the Es₁ stratum is mainly derived from source rocks of the Es₁ member, which generate chiefly oils. Thick mudstones of the Es₁ member serve as not only a high-quality cap rock but also a good source rock. The highmaturity oil and gas generated from deep source rocks in the sag migrate laterally through inherited abysmal faults to and accumulate in deep volcanic rock traps, hydrocarbon accumulations are controlled both by volcanic individuals and by volcanic phases.