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盐城凹陷发现了中等规模的天然气藏，与天然气伴生还有少量凝析油，目前对天然气成因还有争议。天然气甲烷含量高、重烃含量低、干燥系数高。天然气碳同位素呈正碳分布，δ^13C1介于－37．7‰～-38.1‰。结合地质背景，认为天然气为混源气，主体为油型裂解气，海相古生界烃源岩可能是其主力气源岩。凝析油碳同位素为－28．8‰～-30.5‰，全油色谱中含有一定的长链正构烷烃，Pr/Ph为1．56，三环萜烷含量高，C29Ts、C30重排藿烷含量高，重排甾烷含量高，C29甾烷＞C27甾烷＞C28甾烷。石蜡指数、庚烷值表明凝析油为高成熟凝析油，C29甾烷20S／（20S＋20R）值表明其为成熟原油，凝析油为不同成熟度油气混合物，主体来源于泰州组烃源岩。凝析油的形成与蒸发分馏作用密不可分。烃类主要充注时间为6Ma-15Ma。天然气成藏为“古生新储”模式。

The Formation Modes of Gas Reservoirs in Yancheng Sag

??A medium scale gas reservoir was found in Yancheng sag with a small amount of condensate.However,the origin of the natural gas is still in dispute. It is discovered in the gas with high aridity that methane content is high,the quantity of weight hydrocarbon is low,and the carbon isotope is presented as normal carbon distribution with ??¹³C₁ ranging from 37.7% to 38.1%.Under the geological setting,the natural gas has multiple source origins with the oil type pyrolysis gas dominating.Its main gas source rocks may be marine Paleozoic strata.The carbon isotope of whole condensate of Well Yancan 1 is about 28.8%??30.5%.In the whole oil chromatogram,the condensate contains significant long alkanes,with Pr/Ph 1.56,high content of tricycle terpane and large qu antity of diahopane C₂₉ Ts and C₃₀ and high content of diasterane (C₂₉ > C₂₇ > C₂₈).It is suggested that it is an overmature condensate by the paraffin index and the value of heptane.The value of 20s/(20s+20R) of sterane C₂₉ indicates it is a mature crude oil.This condensate is found to be a mixture of oil and gas with different maturity,mainly originat ing from hydrocarbon generating rocks in Taizhou Formation.The formation of cond ensate is ascribed to the process of evaporative fractionation.The hydrocarbon charge experienced for a long time between 6 and 15 magaannuals.This formation mode of gas reservoir is also called "Paleo generated and newly reserved accum ulation".