古地理学报2009，Vol.11Issue(3)：314-329,16.
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
Geology and pore-water pressure sealing of shallow biogenic gas in the Qiantang River incised valley fills
摘要
Abstract
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 30-80m, thicknesses of 3.0-7.0m, 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 80m, remnant thicknesses ranging from 10 to 30m, 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 35m, thicknesses from 10 to 20m, 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 cant 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.关键词
生物气/封闭机理/下切河谷/钱塘江Key words
biogenic gas, sealing mechanism, incised valley, Qiantang River分类
天文与地球科学引用本文复制引用
林春明,李艳丽,卓弘春,张志萍,George W.Shurr,Jennie L.Ridgley..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[J].古地理学报,2009,11(3):314-329,16.基金项目
国家自然科学基金项目(编号:40872075和40272063)资助 （编号:40872075和40272063）
