钱塘江下切河谷充填物地质特征及浅层生物气的孔隙水压力封闭机理

近年来，在浙江省北部钱塘江河口湾地区发现并开发了大量的晚第四纪浅层生物气藏。末次盛冰期，全球海平面的下降使河流梯度增加，下切作用增强，导致钱塘江下切河谷的形成。下切河谷内的沉积序列从下到上可划分为4种沉积相类型，分别为河床相、河漫滩-河口湾相、河口湾-浅海相和河口湾砂坝相。 所有的商业浅气田和气藏都分布于太湖下切河谷和钱塘江下切河谷及其支谷的河漫滩-河口湾相砂体中。钱塘江下切河谷的河漫滩-河口湾砂体埋深30～80 m，厚3～7 m，被非渗透的黏土包围，可能代表了下切河谷内分布的潮流沙脊。快速堆积的河口湾-浅海相沉积物为生物气藏的形成提供了充足的源岩和良好的保存条件。 河漫滩-河口湾相的黏土层为研究区浅层生物气藏的直接盖层，主要分布在下切河谷内，其埋深、残留地层厚度和孔隙度范围分别为30～80 m、10～30 m和42.2％～62.6％。河口湾-浅海相的淤泥层为间接盖层，覆盖了整个下切河谷，其埋深、残留地层厚度和孔隙度范围分别为5～35 m、10～20 m和50.6％ ～53.9％。黏土层和淤泥层的孔隙水压力远大于下伏砂体的孔隙水压力，其差值可达0.48 MPa。在储集层和盖层分界面即浅气藏的顶部，孔隙水压力值达到最大。黏土层和淤泥层的孔隙水压力可以超过砂质储集层中气体压力和孔隙水压力之和。黏土和淤泥盖层的高孔隙水压力可能是浅层生物气被完全封闭住的最重要因素。直接盖层的封闭能力比间接盖层要好。黏土层和淤泥层的孔隙水压力消散时间很长，有时候很难达到稳定状态，这表明黏土层和淤泥层的渗透性差、封闭性好。随着埋深的增加，其压实程度和封闭性能增加。与黏土层和淤泥层相比，砂层的孔隙水压力消散较快，很容易达到稳定状态，而且消散时间与埋深无关，表明砂层渗透性好、封闭性差。气体一旦进入砂层，孔隙水就不能有效释放，导致砂层的孔隙水压力消散时间比黏土层和淤泥层的要长，这可能与生物气在孔隙水压力释放后的快速补充有关。

Geology and pore-water pressure sealing of shallow biogenic gas in the Qiantang River incised valley fills

Late Quaternary shallow biogenic gas reservoirs have recently been discovered and exploited in the Qiantang River(QR)estuary area, northern Zhejiang Province, eastern China. The fall of global sea level during the Last Glacial Maximum enhanced the fluvial gradient and river cutting, resulting in the formation of the large-scale QR incised valley. From bottom to top, the incised valley successions can be grouped into four sedimentary facies: river channel facies(Facies Ⅳ), floodplain-estuarine facies(Facies Ⅲ), estuarine-shallow marine facies(Facies Ⅱ), and estuarine sand bar facies(Facies Ⅰ).All commercial shallow biogenic gas fields or pools occur in floodplain-estuarine sandbodies of the Taihu and QR incised valleys or its branches. In the QR incised valley, the sandbodies, with burial depths of 30-80m, thicknesses of 3.0-7.0m, are surrounded by impermeable clays and may represent tidal ridges. Overlying estuarine-shallow marine sediments supplied not only abundant gas, but also good preservation conditions.The clay beds of Facies Ⅲ that serve as the direct cap beds of the shallow gas pools are mostly restricted within the QR incised valley, with burial depths ranging from 30 to 80m, remnant thicknesses ranging from 10 to 30m, and porosity of 42.2% - 62.6%. In contrast, the mud beds of Facies Ⅱ cover the whole incised valley and occur as the indirect cap beds, with burial depths varying from 5 to 35m, thicknesses from 10 to 20m, and porosity of 50.6% - 53.9%. The pore-water pressures of clay and mud beds are higher than those of sandbodies, and the difference can be as much as 0.48 MPa. The maximum pore-water pressure occurs at the top of the shallow gas reservoirs, just at the interface of gas reservoirs and cap beds. The pore-water pressures of clay and mud beds can exceed the total pore-water pressure and gas pressure of underlying sandy reservoirs. Shallow biogenic gas can be completely sealed by the clay and mud beds, whose high pore-water pressures are likely the most important factor for the preservation of the shallow biogenic gas. The direct cap beds of Facies Ⅲ have better sealing ability than the indirect cap beds of Facies Ⅱ.Generally, the pore-water pressure dissipation time of clay and mud beds is conspicuously long, and sometimes a steady state is even difficult to achieve. This indicates that the clay and mud beds have bad permeability and good sealing ability. With the increasing burial depth, compaction degree and sealing ability are enhanced. On the other hand, pore-water pressure of sand beds tends to dissipate more rapidly than the clay and mud beds to achieve a stable state, and dissipation time does not appear to be related to the burial depth. This indicates that sand beds have better permeability and worse sealing ability. However, once the gas enters the sand lenses, the pore-water pressure cant release efficiently and the pore-water pressure dissipation time is longer than those of the clay and mud beds. This condition may be caused by the prompt supply of biogenic gas after the pore-water pressure dissipation of the sandy reservoirs.