• 主管:

      天津市地质矿产勘查开发局

    • 主办:

      天津地热勘查开发设计院

    • ISSN

      1001-1412  CN 12-1131/P

    普光地区中二叠统茅口组天然气地球化学特征及差异成藏模式

    Geochemical Characteristics and Differential Accumulation Model of Natural Gas in the Middle Permian Maokou Formation, Puguang area

    • 摘要: 近年来,普光地区中二叠统茅口组天然气勘探取得重要突破,显示出该层系气藏具备较好的开发潜力。但该区天然气的成因类型、多套烃源岩的贡献比例及其成藏演化与差异富集机理尚不明确,特别是不同构造区带气源构成的差异性及区域构造运动控制下的成藏模式认识不清,制约了下一步勘探部署。基于普光地区最新天然气样品测试分析数据,结合烃源岩发育特征,综合利用天然气成因判别与气-源对比分析,探讨了其成藏过程与富集规律。研究结果表明,普光地区茅口组天然气中CH4含量最高,均值>90 %,重烃C2+含量低,干燥系数均>0.99,属于典型高演化干气;茅口组天然气甲烷碳同位素值为−30.4 ‰~−28.8 ‰,乙烷碳同位素值为−33.7 ‰~−30.32 ‰,且碳同位素系列发生倒转,判别结果显示普光地区茅口组天然气属于高成熟-过成熟阶段形成的原油裂解气;气-源对比分析表明,该套天然气主体为中二叠统烃源岩与志留系龙马溪组烃源岩共同贡献的混源气,且不同井区的气源贡献占比存在明显差异,明1井以龙马溪组来源为主,而杨柳1井和川岳84井中二叠统来源贡献显著增加。结合区域构造演化及断裂发育程度,建立了研究区两种成藏模式。在通源断裂发育带,表现为志留系供烃为主的“断裂沟通-外源主导”模式;在断裂不发育区,表现为中二叠统自源供烃为主的“层间微裂缝-近源主导”模式。

       

      Abstract: In recent years, exploration breakthroughs in the Middle Permian Maokou Formation of the Puguang area have demonstrated significant development potential for natural gas reservoirs in this series. However, the genetic types of the gas, the contribution proportions of multiple source rocks, and the mechanisms governing accumulation evolution and differential enrichment remain unclear. In particular, the heterogeneity of gas-source composition across different structural zones and the unclear understanding of accumulation models driven by regional tectonic movements have constrained the deployment of further differentiated exploration. Based on analytical data from recent natural gas samples, combined with source rock characteristics and the fault tectonic background, this study integrates natural gas genetic identification with gas-source correlation. It clarifies the origin and source of natural gas and further discusses the the processes of accumulation and enrichment laws. The results indicate that:Natural gas in the Maokou Formation of the Puguang area is dominated by methane (CH4) with a mean content of >90 %, while the content of C2+ heavy hydrocarbons is low. The dryness coefficient is >0.99, classifying it as a typical highly evolved dry gas. The carbon isotopic composition of methane (δ13C1) ranges from −30.4 ‰ to −28.8 ‰, and that of ethane (δ13C2) ranges from −33.7 ‰ to −30.32 ‰, showing a reversal in the carbon isotope series. Comprehensive identification results suggest that the gas is oil-cracking gas formed at the high-to-over mature stage. Gas-source correlation analysis reveals that the gas is primarily a mixed origin derived jointly from Middle Permian source rocks and the Silurian Longmaxi Formation source rocks. Furthermore, there are significant differences in source contribution proportions across well areas: Well Ming-1 is dominated by the Silurian Longmaxi Formation, whereas the contribution from the Middle Permian source increases significantly in Well Yangliu-1 and Well Chuanyue-84. Combining regional tectonic evolution with the degree of fault development, two differential enrichment models were established for the study area: In zones with developed source-connected faults, it exhibits a “fault conduit-external source dominated” model, primarily charged by Silurian source rocks. In areas with underdeveloped faults, it presents an “intraformational microfracture-proximal source dominated” model, mainly supplied by self-sourced Middle Permian source rocks.

       

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