松辽盆地基底流体记录及其对深层天然气成因的指示意义

The potential of natural gas in volcanic rocks is very large in deep Songliao Basin. However, the origin of deep natural gas are being disputed, especially abiogenic hydrocarbon being or not. The key problem of which is lack of geological evidence. Deep fluid activities can be saved in basement rocks because of its unique position. The research for deep fluid records of basement is expected to become a new approach to resolve this probleum. Boreholes with basement rocks in Songliao basin are chosen to be the study object. According to petrology, mineralogy, isotopic geochemistry and chronology, fluid records as deep activities can be identified and their periods can be divided. Geochemistry criteria of alkanes in basement is established with hydrocarbons composition and carbon isotopes in veins associated with deep fluids and secondary fluid inclusions in basement rocks. Basing on homogenization temperature, elements and gas-liquid ratio of fluid inclusions, entrapped press and temperature can be simulated. The origin and mechanism of hydrocarbons in basement rocks will be discussed by synthetic analysis of petrographic, geochemistry and host minerals. It will be confirmed whether mantle-derived abiogenic gas being or not by contrasting to biogenic and abiogenic gas. It is necessary to study the fluid records in basement rocks, because it can not only provide geological proof for origin of deep natural gas, but also offer scientific basis for further exploration of deep Songliao Basin.

松辽盆地深层火山岩天然气藏勘探潜力巨大，然而深层天然气成因（尤其无机烷烃是否存在）仍存在激烈争议，其核心问题是缺乏直接地质证据。盆地基底独特的空间位置决定其具有保留深部流体活动的天然条件，详细地基底流体记录研究可望成为解决这一问题的新途径。本项目以基底钻孔岩心为研究对象，通过结晶基底及脉体岩石学、矿物学、同位素地球化学及年代学研究，识别深部流体活动记录；利用脉体以及基岩中次生流体包裹体烷烃成分、碳同位素组成，明确基底流体记录中烷烃地球化学特征；根据包裹体的均一温度、成分及气液比等参数，模拟含烃流体包裹体捕获压力及温度；综合分析包裹体岩相学、宿主矿物组合、地球化学、捕获条件等，探讨基底岩石中烷烃的成因；通过基底岩石中烷烃与典型有机及无机烷烃对比分析，阐述盆地深层是否存在无机幔源贡献。基底岩石中深部流体记录的精细研究，不仅为天然气成因提供直接证据，且将为深层气藏进一步勘探提供科学依据。

松辽盆地深部来源无机成因烃类的存在与否一直存在较大争议，本研究利用盆地基底内产出的热液石英岩脉和盆地内产出的新生代玄武岩及地幔捕虏体对深部来源挥发分的组成及成因进行探讨。对热液石英岩脉内赋存的流体包裹体进行了激光拉曼、显微测温及H-O同位素测试，以判断流体来源及活动特征。同时，利用加热爆破法对包裹体内挥发分进行提取，并对挥发分化学组成及CO2和烃类的碳同位素特征进行测定，以判断CO2和烃类的成因和来源，并进一步探讨深部流体对于天然气成藏的潜在贡献。H-O同位素及测温结果显示，石英岩脉为岩浆热液活动产物，其中挥发分代表了原始岩浆体系内挥发分特征。松辽盆地岩浆热液挥发分主要组成为H2O，CO2，N2，H2及少量烃类。碳同位素特征表明，大量岩浆来源CO2在脱气作用过程中脱出岩浆体系，为形成无机成因CO2气藏创造了条件。岩浆热液挥发分中存在少量烃类，其来源包括：费托合成反应生成的无机成因烃类，岩浆中原始无机成因烃类及再循环沉积有机质热解产生的有机成因烃类。其中后岩浆期费托反应能够生成大量无机成因烃类，使得岩浆热液挥发分中烃类含量显著提高，为盆地烃类成藏提供重要的物质输入。另外，本项目对松辽盆地内双辽火山群火山岩及地幔捕虏体内流体包裹体所含流体挥发分组成和碳同位素特征进行测试，以反映地幔流体特征及成因。研究发现双辽玄武岩橄榄石斑晶和地幔捕虏体内CO2和烃类碳同位素均具有有机成因特征，表明岩石圈地幔及玄武岩岩浆源区内均存在有机质热解产物的混入，玄武岩岩浆源区及岩石圈地幔交代流体中的有机成因挥发分主要来自于俯冲太平洋板片之上的沉积有机质。