CAJ | 학술논문

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. 近年來,在浙江省北部錢塘江河口灣地區髮現併開髮瞭大量的晚第四紀淺層生物氣藏.末次盛冰期,全毬海平麵的下降使河流梯度增加,下切作用增彊,導緻錢塘江下切河穀的形成.下切河穀內的沉積序列從下到上可劃分為4種沉積相類型,分彆為河床相、河漫灘-河口灣相、河口灣-淺海相和河口灣砂壩相. 所有的商業淺氣田和氣藏都分佈于太湖下切河穀和錢塘江下切河穀及其支穀的河漫灘-河口灣相砂體中.錢塘江下切河穀的河漫灘-河口灣砂體埋深30～80m,厚3～7m,被非滲透的黏土包圍,可能代錶瞭下切河穀內分佈的潮流沙脊.快速堆積的河口灣-淺海相沉積物為生物氣藏的形成提供瞭充足的源巖和良好的保存條件. 河漫灘-河口灣相的黏土層為研究區淺層生物氣藏的直接蓋層,主要分佈在下切河穀內,其埋深、殘留地層厚度和孔隙度範圍分彆為30～80 m、10～30m和42.2% ～ 62.6%.河口灣-淺海相的淤泥層為間接蓋層,覆蓋瞭整箇下切河穀,其埋深、殘留地層厚度和孔隙度範圍分彆為5～35 m、10～20m和50.6% ～ 53.9%.黏土層和淤泥層的孔隙水壓力遠大于下伏砂體的孔隙水壓力,其差值可達0.48 MPa.在儲集層和蓋層分界麵即淺氣藏的頂部,孔隙水壓力值達到最大.黏土層和淤泥層的孔隙水壓力可以超過砂質儲集層中氣體壓力和孔隙水壓力之和.黏土和淤泥蓋層的高孔隙水壓力可能是淺層生物氣被完全封閉住的最重要因素.直接蓋層的封閉能力比間接蓋層要好.黏土層和淤泥層的孔隙水壓力消散時間很長,有時候很難達到穩定狀態,這錶明黏土層和淤泥層的滲透性差、封閉性好.隨著埋深的增加,其壓實程度和封閉性能增加.與黏土層和淤泥層相比,砂層的孔隙水壓力消散較快,很容易達到穩定狀態,而且消散時間與埋深無關,錶明砂層滲透性好、封閉性差.氣體一旦進入砂層,孔隙水就不能有效釋放,導緻砂層的孔隙水壓力消散時間比黏土層和淤泥層的要長,這可能與生物氣在孔隙水壓力釋放後的快速補充有關.
근년래,재절강성북부전당강하구만지구발현병개발료대량적만제사기천층생물기장.말차성빙기,전구해평면적하강사하류제도증가,하절작용증강,도치전당강하절하곡적형성.하절하곡내적침적서렬종하도상가화분위4충침적상류형,분별위하상상、하만탄-하구만상、하구만-천해상화하구만사패상. 소유적상업천기전화기장도분포우태호하절하곡화전당강하절하곡급기지곡적하만탄-하구만상사체중.전당강하절하곡적하만탄-하구만사체매심30～80m,후3～7m,피비삼투적점토포위,가능대표료하절하곡내분포적조류사척.쾌속퇴적적하구만-천해상침적물위생물기장적형성제공료충족적원암화량호적보존조건. 하만탄-하구만상적점토층위연구구천층생물기장적직접개층,주요분포재하절하곡내,기매심、잔류지층후도화공극도범위분별위30～80 m、10～30m화42.2% ～ 62.6%.하구만-천해상적어니층위간접개층,복개료정개하절하곡,기매심、잔류지층후도화공극도범위분별위5～35 m、10～20m화50.6% ～ 53.9%.점토층화어니층적공극수압력원대우하복사체적공극수압력,기차치가체0.48 MPa.재저집층화개층분계면즉천기장적정부,공극수압력치체도최대.점토층화어니층적공극수압력가이초과사질저집층중기체압력화공극수압력지화.점토화어니개층적고공극수압력가능시천층생물기피완전봉폐주적최중요인소.직접개층적봉폐능력비간접개층요호.점토층화어니층적공극수압력소산시간흔장,유시후흔난체도은정상태,저표명점토층화어니층적삼투성차、봉폐성호.수착매심적증가,기압실정도화봉폐성능증가.여점토층화어니층상비,사층적공극수압력소산교쾌,흔용역체도은정상태,이차소산시간여매심무관,표명사층삼투성호、봉폐성차.기체일단진입사층,공극수취불능유효석방,도치사층적공극수압력소산시간비점토층화어니층적요장,저가능여생물기재공극수압력석방후적쾌속보충유관.
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.