下扬子黄桥地区龙潭组致密砂岩胶结物流体特征与演化

Fluid characteristics and evolution of Longtan Formation tight sandstone cements in Huangqiao area,Lower Yangtze,China

探讨下扬子黄桥地区上二叠统龙潭组含碳质层系致密砂岩中石英次生加大和菱铁矿胶结物共存的流体环境及演化过程。通过铸体薄片、流体包裹体温度、盐度和荧光、扫描电镜、X射线衍射等测试分析,揭示石英次生加大明显早于菱铁矿胶结,两者形成的温度和盐度相近,但却表现出明显的从酸性流体到碱性流体的转变,石英次生加大与早期富含碳质的酸性流体环境相关,其碱性流体环境的转化可能与含碳层系浅埋条件下的厌氧细菌活动存在相关性。两种胶结物形成过程普遍经历了热流体活动影响,反映东吴运动热事件对该套地层的影响,并具有高盐度的记录;但形成过程中并未伴随油气的充注和影响。菱铁矿形成后的有机质成熟和油气充注使得致密砂岩中流体介质又从碱性过渡为酸性。

The environment and evolution process of fluid in quartz overgrowth coexists with siderite cement in tight sandstone of the Upper Permain Longtan Formation in Huangqiao area, Lower Yangtze region, are studied on the basis of analyses of homogenization temperature, salinity and fluorescence of fluid inclusions, microscopy of scanning electron and X ray diffraction. It reveals that the quartz overgrowth is formed obviously earlier than siderite cements, the transition from acidic fluid to alkaline fluid between quartz overgrowth and siderite cement occurs under similar temperature and salinity and the quartz overgrowth is related to early carbon rich acid fluid environment. The conversion process from acid fluid environment to alkaline fluid environment may be related to theactivity of anaerobic bacteria under the condition of shallow buried anaerobic environment in carbonaceous laminates. The formation of two types of cements with the record of high salinity in the same samples generally experienced the influence of hot fluid activity related to the thermal event of Dongwu Movement, with the record of high salinity. However, the formation process of two types of cements is not involved in the activity of hydrocarbons. Fluid medium transits from alkaline to acidic environment in tight sandstone by organic maturity and hydrocarbon filling after the formation of siderite.