中国东南沿海平原晚第四纪超浅层生物气藏盖层研究

STUDY ON CAP BEDS OF SHALLOW BIOGENIC GAS POOLS IN LATE QUATERNARY DEPOSITS OF SOUTHEAST CHINA COASTAL PLAIN

根据晚第四纪地层的物性特征、力学性质及烃浓度值实验资料和孔隙水压力及其消散历时现场试验结果表明，覆盖下切河谷之上的近岸浅海相灰色淤泥质粘土层为区域盖层，下切河谷内的河漫滩相灰—灰黑色粘土及淤泥质粘土层为局部盖层。粉细砂、粉砂、粉砂质粘土、淤泥质粘土和粘土的粒径依次变小，比表面积依次增大，使其孔隙率、孔隙比、天然含水量及孔隙水压力值依次增大。粘土的压缩系数、塑性指数、液性指数及抗剪强度最大，其抗变形能力相对最弱，可压缩相对最大，相对封闭性最好；而淤泥质粘土、粉砂质粘土的封闭性次之。随着埋藏深度的增加，盖层的封闭性增强。盖层的封闭机理主要为物性封闭、烃浓度封闭和孔隙水压力封闭，后者可能对生物气藏保存起着最主要的封闭作用。

Based on experimental data of mechanical and physical properties, hydrocarbon concentration, as well as field test of pore water pressure and its disappearing time, a comprehensive investigation has been conducted to study the cap beds and its sealing mechanism in shallow biogenic gas pools of the Late Quaternary in the Southeast China coastal plain. The result of this study has indicated that the gas generating beds above the reservoirs could be considered as the cap beds. The gas cap beds in this area are primarily grey mud deposited in inner sublittoral environment and grey greyish black mud deposited in fluvial flood plain environment. The grey mud usually occurs above the incised valleys formed in the last glaciation (such as Qiantangjiang incised valleys) and could be considered as the regional cap beds, while the grey greyish black mud occurs only within the incised valleys and forms a local cap beds. Such parameters reflecting physical properties of sediments as the porosity, pore ratio, natural water content and pore water pressure are related to specific surface area of particles in the sediments. The smaller the grain size is, the larger the parameter values are. Therefore, the parameter values increase successively in fine silt, silt, siltic clay and clay. As for the petro mechanical properties, the clay has not only the largest compressibility coefficient, plastic and liquidity indexes, as well as the shear strength, but also the weakest deformation resistance and volumetric deformation. Consequently, the clay is the best sealing layer comparing with the silt mud and silty clay. The sealing ability of the cap beds increases with its buried depth. Although the sealing mechanisms of the cap beds include physical and hydrocarbon concentration sealings, the pore water pressure sealing probably is the most important mechanism in the shallow biogenic gas field.