Abstract Samples for inclusion analysis were collected to investigate reasons for carbon isotope rollover in the Yan'an gas field (YAGF). Homogenization temperature of fluid inclusions, constituents and carbon isotope of gases in fluid inclusions were measured. Geochemical parameters of gas in wells and fluid inclusions in the northern gas fields (NGF) and YAGF were compared. The results show that geochemical characteristics of well and fluid inclusions gas in the NGF are almost the same, suggesting that it was originated from one set of source rocks. The direct reason for carbon isotope rollover in the YAGF is the depleted ethane carbon isotopes, while the methane carbon isotopes do not change much compared with the primary gas. There was a period of high temperature fluid filling and accumulation (140–160 °C) in the YAGF. The higher thermal maturity has disparate effects on methane and ethane. Higher thermal maturity is an important inducement of carbon isotope rollover. Mixture of oil-derived gas generated from the marine Ordovician Majiagou Formation will also result in carbon isotope rollover.

中文翻译：

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鄂尔多斯盆地南部延安气田碳同位素翻转成因——来自流体包裹体气与井地球化学对比的证据

摘要 采集包裹体分析样品，探讨延安气田（YAGF）碳同位素翻滚原因。测量了流体包裹体的均一化温度、流体包裹体中气体的成分和碳同位素。对比了北方气田(NGF)和YAGF的井气和流体包裹体地球化学参数。结果表明，NGF 中井和流体包裹体气体地球化学特征几乎相同，表明其起源于一套烃源岩。YAGF中碳同位素翻转的直接原因是乙烷碳同位素耗尽，而甲烷碳同位素与原生气相比变化不大。YAGF 中存在一段高温流体充填和积聚期（140-160°C）。较高的热成熟度对甲烷和乙烷有不同的影响。较高的热成熟度是碳同位素翻转的重要诱因。海相奥陶系马家沟组油成气混合也会导致碳同位素翻转。