柴达木盆地东部石炭系页岩气储层流体地球化学特征

Geochemical characteristics of fluid in Carboniferous shale gas reservoir of the eastern Qaidam Basin

柴达木盆地东部石炭系页岩气储层中多发育薄层粉细砂岩夹层,部分页岩气在形成后便近距离聚集在砂岩夹层中,因而赋存于砂岩夹层不同期次成岩矿物中的流体包裹体便成为研究页岩气储层流体形成与演化过程的"活化石"。从页岩有机地球化学特征、储层显微偏光/荧光特征、流体包裹体的温度及成分特征等方面入手,系统分析了研究区页岩气的形成和保存条件。结果表明,研究区石炭系泥页岩有机质丰度高,有机质类型为Ⅱ2—Ⅲ型,普遍处于成熟—高成熟阶段。页岩气储层存在2期次的油气充注过程:第1期发生在早二叠世末期,油气的成分成熟度较低,为砂岩夹层中环石英颗粒加大边内侧分布的重质油包裹体及现今储层中残留的褐色、灰褐色稀油沥青;第2期发生在新近纪早期—现今,为砂岩夹层中沿石英颗粒成岩期后微裂隙成带分布的凝析油及天然气包裹体,是现今页岩气储层油气的主体。盆地东缘及东北缘部分砂岩夹层中气体包裹体成分多为CH4+CO2、CO2+CH4+N2,属于岩浆热液作用下的变质流体。研究区中部石炭系页岩有机碳含量高、成熟度适中(Ro均值约为1.0%)、油气保存条件较好,现今仍存可动轻质油气,具有良好的页岩气勘探前景。

The thin interlayer of silty-fine sandstone is mostly developed in Carboniferous shale gas reservoirs of the eastern Qaidam Basin.After generation,shale gas will gather in the adjacent sandstone interlayer.Thus,the fluid inclusions in the diagenetic minerals with different stages of sandstone interlayer will become the＂living fossil＂for studying the fluid formation and evolution process of shale gas reservoir.In this paper,the generation and preservation conditions of shale gas in the study area were systematically investigated in terms of the geochemical characteristics of shale,the microscopic polarization/fluorescence characteristics of reservoir,as well as the temperature and composition characteristics of fluid inclusion.The research results indicate that Carboniferous mud shale in the study area has high organic abundance,dominated by the organic matter of Type II2-III,and is generally in the maturehighly mature development stage.There exist two stages of hydrocarbon charging processes in shale gas reservoir.Stage 1 occurred at the end of Early Permian period,in which the hydrocarbon compositions had low maturity,consisting of heavy oil inclusions distributed inside the annular quartz-grain overgrowth edge in sandstone interlayer and residual brown-beige thin-oil asphalt in current reservoirs.Stage 2 occurs from Early Neogene till now,in which the condensate and natural gas inclusions are distributed zonally along the micro-cracks of sandstone interlayer posterior to the quartz grain digenesis period,considered as the hydrocarbon main body of current shale gas reservoirs.In partial sandstone interlayer at the east and northeast edges of the basin,gas inclusions have primary compositions such as CH_4＋CO_2 and CO_2＋CH_4＋N_2,belonging to metamorphic fluid under magma hydrothermalism.The Carboniferous shale in the central study area has high organic carbon content with moderate maturity（Ro mean of 1.0%±）and good hydrocarbon preservation conditions,where movable light oil and gas exist at present,indicating favorable exploration prospects for shale gas.