Mechanism of organic-rich shale formation and shale gas enrichment in the Carboniferous Tian’eping Formation from the Xiangzhong Depression
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摘要: 【研究目的】本文旨在通过页岩成因和页岩气富集机理研究，确定湘中坳陷下石炭统天鹅坪组页岩气形成富集主控因素和富集模式； 【研究方法】以湘新地4井为重点，通过下石炭统碳酸盐岩稳定碳、氧同位素、页岩全岩氧化物和微量元素含量测定，分析页岩形成的古气候、古环境特点，确定富有机质页岩的成因。在系统查明页岩岩石矿物学、有机地化和储存物性特征以及页岩气赋存方式和构造保存条件基础上，结合中－低成熟页岩储存的热演化模拟结果，确定页岩气富集机理。【研究结果】结果表明： （1）下石炭统天鹅坪组富有机质页岩是全球早石炭世杜内期气候剧烈波动引起海水分层，海底缺氧的沉积产物。（2）涟源地区中生代广泛而强烈的岩浆事件导致区域古地温梯度升高，并引起天鹅坪组页岩发生二次生烃和储层物性的改善。天鹅坪组页岩气是印支期油气调整后，页岩中原油裂解和有机质二次生烃的共同结果。（3）发育在下石炭统测水煤系中的滑脱构造部分封堵了下伏天鹅坪组页岩气的垂直逸散通道，有利于页岩气的保存富集。【结论】总之，湘中坳陷下石炭统天鹅坪组页岩气是有利相带控制总有机碳含量、岩浆热作用控制储层物性和滑脱构造控制保存的共同结果。Abstract: [Objective] According to study on the shale formation and shale gas enrichment mechanism, the current study aims at finding out main factor controlling shale gas reservoir in the Carboniferous Tian'eping Formation in the Xiangzhong Depression as well as its enrichment patterns.[Methods] Carbonates carbon and oxygen isotopes as well as shale trace elements and major elements were analyzed at Xiangxindi 4 core in order to recover paleo-environment and investigate the origin for the shale formation. Combined with the thermal evolution simulation of medium-low maturity shale gas reservoirs, the mechanism for shale gas enrichment is identified through petromineralogy, organic geochemistry, physical properties of shale gas reservoirs, existence forms of shale gas and tectonic preservation condition.[Results] Our results show that (1) the organic-rich shale in Lower Carboniferous Tian'eping Formation were formed due to seawater stratification and seabed anoxia caused by the intensive climatic fluctuations in the Early Carboniferous, (2) the extensive and intense magmatic events in central Hunan led to the increase of locally paleogeothermal gradient and further caused secondary hydrocarbon generation in the Lower Carboniferous organic-rich shale. Shale gas in the Tian'eping Formation was formed owing to crude oil cracking and secondary hydrocarbon generation of organic matter, and (3) shale gas preservation were promoted due to decollement in the Ceshi Formation of the lower Carboniferous blocking the vertical escaping channel of the shale gas from the underlying Tian'eping Formation.[Conclusions] In conclusion, the shale gas of Lower Carboniferous Tian-ping Formation in central Hunan Depression is the common result of favorable facies zone controlling the total organic carbon content, magmatic thermogenesis controlling the reservoir physical properties and detachment structure controlling the preservation.
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