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通过分析、对比鄂尔多斯盆地和渤海湾盆地67个来源于石炭系—二叠系煤系天然气的组分、稳定碳同位素组成,并结合实际地质背景分析,取得以下认识:鄂尔多斯盆地和渤海湾盆地来源于石炭系—二叠系烃源岩的天然气绝大部分是煤成气,靖边气田少量油型气来源于二叠系太原组灰岩。鄂尔多斯盆地天然气主要为干气,而渤海湾盆地天然气主要为湿气。鄂尔多斯盆地20%天然气样品发生倒转,主要倒转形式为δ13C2>δ13 C3,渤海湾盆地27%天然气样品发生倒转,主要表现为δ13C3>δ13C4倒转,倒转均是由同源不同期煤成气的混合造成。鄂尔多斯盆地天然气主要是烃源岩在成熟—高成熟阶段生成;渤海湾盆地石炭系—二叠系煤成气主要是成熟阶段的产物,少量天然气为烃源岩在高—过成熟阶段所生成,这部分天然气主要分布在东濮坳陷。成熟度和母质组成的差异使得2个盆地甲烷碳同位素组成存在差别,随着成熟度的增加,2个盆地重烃气与甲烷碳同位素差值逐渐减小。
By analyzing and comparing the compositions and stable carbon isotopic composition of 67 natural gas components derived from the Carboniferous Permian coal series in the Ordos Basin and Bohai Bay Basin and combining with the actual geological background analysis, the following conclusions can be drawn: The origin of Ordos Basin and Bohai Bay Basin Most of the natural gas in the Carboniferous-Permian source rocks is coal-derived gas, and a small amount of oil-gas in the Jingbian gas field comes from the Permian Taiyuan limestone. Natural gas in Ordos Basin is mainly dry gas while natural gas in Bohai Bay Basin is mainly moisture. In the Ordos Basin, 20% of natural gas samples were inverted, mainly in the form of δ13C2> δ13C3, and 27% of the natural gas samples in the Bohai Bay Basin were inverted, mainly showing the δ13C3> δ13C4 inversion, which was caused by the mixture of coal gas of different stages . The natural gas in the Ordos Basin is mainly derived from the source rock at the mature-high maturity stage. The Carboniferous-Permian coal-to-gas in the Bohai Bay Basin is mainly the product of the mature stage. A small amount of natural gas is generated during the high-over maturity stage. This part of the natural gas is mainly distributed in the Dongpu depression. The differences of maturity and parent composition make the difference of carbon isotopic composition of methane in the two basins. With the increase of maturity, the difference of carbon isotope between heavy hydrocarbon and methane in two basins decreases.