羌塘盆地石炭—二叠系烃源岩地球化学特征讨论

摘要

The Jurassic source rocks have long been one of the research hotspots, but the Carboniferous-Permian source rocks' capability of hydrocarbon generation has not aroused much attention due to the scattered distribution in Qiangtang Basin. Organic geochemical characteristics of carbonate rock and mudstone samples from eight sections in the Carboniferous-Permian strata of Qiangtang Basin were analyzed for the organic matter abundance, types and thermal evolution. The authors found that source rocks, including carbonate rock and mudstone,were influenced by the evolution of sedimentary environment. The carbonate source rocks might have been the micrite limestone of the carbonate platform, and the mudstone source rocks were mainly the dark mudstone of delta and slope facies and tuffaceous mudstone. The organic carbon content of Carboniferous-Permian mudstone is higher, suggesting hydrocarbon-generative potential; most sections have moderate to high content of organic carbon except Rejuechaka section which has no or poor source rock. Kemo section and Gongri section have mainly reached the standard of mid-good hydrocarbon source rock. However, the Carboniferous-Permian carbonate rock shows a lower content of organic carbon, with no or poor source rock. The type of organic matter of Carboniferous-Permian carbonate rock is mainlyⅡ1, but the type of organic matter of Carboniferous-Permian mudstone is mainlyⅡ2 andⅢ. The degree of thermal evolution of Carboniferous Permian source rock is generally high, and all the source rocks, except for the Kemo areas where the source rock is at the mature stage, are mostly in the high mature and over-mature stage. Such a situation is very favorable for the generation of natural gas and has a good prospect of natural gas exploration.%长期以来,羌塘盆地烃源岩的研究一直限于中生代地层,而对其古生代地层生烃能力一直缺乏系统研究.针对这一问题,本文选择羌塘盆地石炭—二叠系8条剖面的暗色泥岩及碳酸盐岩样品,对其从有机质丰度、有机质类型和热演化程度等方面进行了有机地球化学特征的分析.研究发现,石炭—二叠系可能烃源岩类型包括泥岩和碳酸盐岩两种,其分布总体上受沉积相的控制,碳酸盐岩烃源岩可能为局限台地相发育的泥晶灰岩,而泥质烃源岩主要为三角洲及斜坡相发育的暗色泥岩及凝灰质泥岩.石炭—二叠纪泥岩有机碳含量较高,具有较好生烃能力,大多达到烃源岩标准,尤其是刻莫石炭系剖面及贡日二叠系剖面,大多为中等—好烃源岩.碳酸盐岩有机碳含量总体比较低,为非烃源岩.石炭—二叠系碳酸盐岩烃源岩有机质类型为Ⅱ1型,泥质烃源岩有机质类型主要为Ⅱ2-Ⅲ型.石炭—二叠系烃源岩热演化程度总体较高,除盆地东部刻莫石炭系剖面处在成熟阶段外,大都处在高成熟—过成熟阶段,非常有利于天然气的生成,具备良好的天然气勘探前景.