Abstract Co-accumulation of carbon dioxide and hydrocarbon gas is common and widely existing in oil and gas basins. Such two natural pools are located in Lishui sag (East China Sea Shelf Basin), as a favorable prospecting area for exploration. CO2 gas derived from inorganic mantle sources and CO2 accumulation times have been investigated. In this study, petrographic observation shows pure CH4, CH4-rich, H2O-rich and hybrid CH4-CO2 fluid inclusions exist in healed micro-fractures and overgrowths of quartz mineral in Palaeocene reservoirs of Lishui sag. The pressure–temperature–time–composition (PVT–x) properties of CH4-bearing inclusions were obtained by Raman quantitative analysis and thermodynamic models. These results provide important constraints on the P-T entrapment conditions and predict relative accumulation time of CH4 fluid. Combination of carbon isotope analyses, inclusion petrography, microthermometry, PT entrapment conditions of CH4-bearing inclusions, hydrocarbon generation and simulation of burial history permit to assign periods of CH4 fluid activity and its sequence with CO2 fluid. Methane charging time occurs during two stages in LS-A: low density CH4 fluid (~0.10 g/cm3 or lower) were trapped during late Oligocene (30-20 Ma) and high density (>0.115 g/cm3) during a Pliocene (8-1 Ma) CH4 accumulation event. CO2 fluid accumulation (Miocene, 21-4 Ma) displaced the methane gas trapped in the early stage. Pliocene CH4 fluid partial displaced Miocene CO2 fluid. In well B, only high density (0.144–0.153 g/cm3) recorded a late Oligocene to early Miocene (36-19 Ma) CH4 accumulation event, Pliocene (5-3 Ma) CO2 fluid displaced early CH4 fluid. Micro-thermometry, composition of fluid inclusion, features of reservoir, thermal maturity reflect hydrothermal fluid vertical migration and the existence of Supercritical CO2 extraction.

中文翻译：

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东海盆地丽水凹陷CH4成藏特征及与深层CO2流体的关系

摘要 油气盆地中二氧化碳与烃类气共聚现象普遍且广泛存在。这两个天然池位于丽水凹陷（东海陆架盆地），是勘探的有利找矿区。已经研究了源自无机地幔源的 CO2 气体和 CO2 积累时间。本研究岩相学观察表明，丽水凹陷古新统储集层石英矿物的愈合微裂缝和过度生长中存在纯CH4、富CH4、富H2O和混合CH4-CO2流体包裹体。通过拉曼定量分析和热力学模型获得含 CH4 夹杂物的压力-温度-时间-成分 (PVT-x) 特性。这些结果为 PT 截留条件提供了重要的约束，并预测了 CH4 流体的相对聚集时间。结合碳同位素分析、包裹体岩相学、显微测温、含 CH4 包裹体的 PT 包封条件、油气生成和埋藏历史模拟，可以确定 CH4 流体活动的时期及其与 CO2 流体的序列。LS-A 中甲烷充填时间发生在两个阶段：低密度 CH4 流体（~0.10 g/cm3 或更低）在渐新世晚期（30-20 Ma）和上新世期间高密度（>0.115 g/cm3）被捕获（ 8-1 Ma) CH4 积累事件。CO2 流体聚集（中新世，21-4 Ma）取代了早期捕获的甲烷气体。上新世 CH4 流体部分置换中新世 CO2 流体。在 B 井中，只有高密度（0.144-0.153 g/cm3）记录了渐新世晚期到中新世早期（36-19 Ma）CH4 聚集事件，上新世（5-3 Ma）CO2 流体取代了早期 CH4 流体。显微测温，