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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30～80m、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5～35m、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0.48MPa。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。
In recent years, a large number of Late Quaternary shallow bio-gas reservoirs have been discovered and developed in the estuary of Qiantang River in the northern part of Zhejiang Province. During the last glaciation, the decline of global sea level increased the river gradient and the undercutting effect, resulting in the formation of the undercut Qiangtang River. Sedimentary sequences in the undercut valleys can be divided into four types of sedimentary facies from bottom to top: river bed facies, floodplain - estuarine facies, estuarine - shallow marine facies and estuarine sand bar facies. All commercial shallow gas fields and gas reservoirs are located in the underplated riverbank of the Taihu Lake and the undercut river valley of the Qiantang River and its rift valley. The river floodplain - estuarine sandbodies of the Qieten River undercut river valleys are 30-80 m deep and 3-7 m thick, which are surrounded by non-permeable clays and may represent the tidal current sand ridge distributed in the valley. Rapid accumulation of estuarine-shallow marine sediments provides ample source rock and good preservation conditions for biogas formation. The floodplain - estuarine clay layer is the direct capping layer of the shallow biogenic gas reservoir in the study area, and is mainly distributed in the undercut valley. The buried depth, residual formation thickness and porosity are 30 ~ 80m, 10 ~ 30, m And 42.2% ~ 62.6%. The silt layer of the estuarine and shallow marine facies is an indirect capping layer and covers the entire undercut valley. The buried depth, remaining formation thickness and porosity range from 5 to 35 m, 10 to 20 m and 50.6 to 53.9%, respectively. The pore water pressure in the clay and silt layers is much larger than the pore water pressure in the underlying sand bodies, with a difference of up to 0.48 MPa. At the top of shallow reservoirs at the interface between the reservoir and caprock, pore water pressure reaches the maximum. The pore water pressure in the clay and silt layers can exceed the sum of the gas pressure in the sandy reservoir and the pore water pressure. The high pore water pressure of clay and mud cover may be the most important factor for the shallow biogas to be completely enclosed. Direct capping is better than indirect capping. The pore water pressure in the clay and silt layers dissipates for a long time and is sometimes difficult to reach steady state, indicating poor permeability and tightness of the clay and silt layers. As the depth increases, the degree of compaction and sealing performance increases. Compared with the clay and silt layers, the pore water pressure in the sand layer dissipates rapidly and it is easy to reach a steady state. The dissipation time has nothing to do with the burial depth, indicating good sand permeability and poor sealing. Once the gas enters the sand layer, the pore water can not be effectively released, resulting in the longer pore water pressure of the sand layer to dissipate than the clay and silt layers, which may be related to the rapid replenishment of biogas after the release of pore water pressure.