--> Genetic Types and Origin of Natural Gases in the Nanpu Oil field, Bohai Bay Basin, China

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Genetic Types and Origin of Natural Gases in the Nanpu Oil field, Bohai Bay Basin, China

Abstract

Natural gas exploration in Nanpu sag, Bohai Bay Basin has achieved great breakthroughs in recent years, while the genesis of the discovered gas is still unclear which impede the exploration of natural gas. Based on molecular component of large collection of gas samples, combined with stable carbon isotope composition, the geochemical characteristics and genetic mechanisms of the natural gas within the whole gas producing layers of the beach area are discussed. The results showed that the natural gas in the beach area can be divided into oil-related gas, coal-derived gas, mixed gas and secondary biodegraded gas and is dominated by the middle two categories. Oil-related gas, composed of gas samples with wetness mainly from 0.75 to 0.85 and the δ13C1, δ13C2 values lower than −40‰ and −28‰, respectively, is located in the Eogene Ed and Es1 formations of No.3, 4, 5 structural belts. Coal-derived gas is characterized by relatively low level of C2+ gas with wetness about 0.85–0.95 and is isotopic heavy with δ13C1 and δ13C2 values heavier than −40‰ and −28‰, respectively. Coal-derived gas was found in the buried hills (O, ϵ) of the No.1, 2, 3 structural belts and Eogene Es2+3 formation of No.5 structural belt. It was observed that natural gases in the Eogene Ed1 formation of No.1, 2 structural belts have intermediate molecular constituent and isotope characteristics between the oil-related gas and coal-derived gas, indicating mixtures of the two categories, which are consistent with the mixture origin of associated oils. In the shallow layers, including the Neocene Nm and Ng formations of No.1, 4 structural belts, both the gas components and carbon isotopes display evidence for microbial alteration which is in accordance with the biodegraded oils found in these layers. Detailed correlation of gas-source showed that oil-related gas and secondary biodegraded gas were derived from the Eogene Es1 source rock. Coal-derived gas was generated by Eogene Es3 source rocks and the mixed gas is derived from both the Es1 and Es3 source rocks. The genetic type and distribution of the natural gases are mainly controlled by the types of organic matter and migration channels, including faults and unconformities. This study achieves new understandings toward the origin of natural gas in the Nanpu sag which could be instructive to the natural gas exploration in the area and helpful for unraveling the genetic mechanisms of natural gas in continental rift-subsidence basins.