【作者】 李艳丽；

【导师】 林春明；

【作者基本信息】 南京大学 ， 矿物学岩石学矿床学， 2010， 博士

【摘要】 下切河谷可容空间大,可以保存较完整的地层记录并蕴藏着大量的油气资源,是第四纪地质学、层序地层学、古环境变化和油气资源研究的热点和重点内容。生物气资源潜力巨大,很多能源问题都可以通过开发浅层生物气得到解决,但其封闭机理的研究仍很薄弱,勘探程度也较低。钱塘江下切河谷保留了较完整的晚第四纪沉积物,并含有大量的生物气资源,成为研究钱塘江河口区环境演变的理想的天然实验室,其充填物特征的研究还有助于浅层生物气封闭机理探讨和勘探方法的研究。本学位论文立足于研究区的大量岩心资料,以钱塘江南岸SE2钻孔为重点研究对象,通过矿物组分、有孔虫属种、粒度、磁化率、元素地球化学、渗透率和AMS’4C测年等测试分析手段,对晚第四纪以来钱塘江下切河谷充填物特征进行分析,确定了研究区详细的沉积相类型、识别出关键层序界面、建立了沉积物地层格架,总结了钱塘江下切河谷的演化过程,并与世界其它地区下切河谷的充填模式进行对比。通过对钱塘江下切河谷区浅层生物气藏盖层和储层沉积物属性的对比,探讨了浅层生物气藏盖层的封闭机理,确定了主要的封闭机理及其成因,并对直接盖层和间接盖层的封闭性能进行对比。对钱塘江下切河谷区已实施的多种生物气勘探方法进行分析,确定各种勘探方法的优劣,总结出有效的生物气勘探步骤。钱塘江下切河谷沉积物可识别出河床、河漫滩、潮坪、近岸浅海和河口湾砂坝五种沉积相。不同沉积相沉积物的矿物组分、有孔虫含量、粒度、磁化率和稀土元素特征差异明显。矿物组成特征表现为：河床相沉积物轻重矿物种类均较少,轻矿物仅见石英和长石,重矿物以绿泥石-绿帘石-角闪石-金属矿物为主；河漫滩相沉积物中开始出现云母和木屑,重矿物以透闪石的出现为特征,重矿物组合为绿泥石-金属矿物-绿帘石；潮坪相沉积物中云母和木屑含量升高,并含较多石墨,重矿物组合为绿泥石-金属矿物-绿帘石-副矿物；近岸浅海相沉积物中石墨和木屑减少,生物贝壳零星出现,重矿物以绿泥石-金属矿物-辉石-绿帘石-角闪石为主；河口湾砂坝相沉积物中石墨和贝壳含量较高,木屑缺失,重矿物组合为绿泥石-辉石-绿帘石-金属矿物。河床和河漫滩相缺失有孔虫,潮坪相偶见胶结壳有孔虫,近岸浅海相和河口湾砂坝相底栖有孔虫较丰富,且在近岸浅海相中见浮游有孔虫。相对而言,河床相沉积物粒度粗,分选差；河漫滩相和潮坪相沉积物整体粒度较细,但各粒级组分含量变化较大,分选较差；近岸浅海相沉积物粒度细且均匀,分选最好;河口湾砂坝相沉积物粒度较粗,分选较好。沉积物磁化率表现为河床相>潮坪相>河口湾砂坝相>河漫滩相>近岸浅海相。稀土元素含量则以河漫滩相最高,近岸浅海相、河口湾砂坝相、潮坪相和河床相依次递减。概率累积曲线、频率分布曲线、粒度参数之间的相关性、磁化率与平均粒径相关性、稀土元素参数间的相关性可作为沉积相的划分参考依据。概率累积曲线特征反映出河床相沉积物中跳跃组分含量高,见滚动组分；河漫滩相沉积物跳跃组分含量明显减小,缺失滚动载荷；潮坪相沉积物有两种明显不同的类型,一种以跳跃组分为主,一种以悬浮组分为主；近岸浅海相沉积物均为悬浮组分；河口湾砂坝相沉积物以跳跃组分为主。河床相、潮坪相和河口湾砂坝相沉积物的频率分布曲线为不对称的双峰式,河漫滩和近岸浅海相的频率分布曲线则呈单峰式。河床相和河漫滩相的区分可参考(La/Sm)N与HREE或(La/Yb)N的相关性、平均粒径与分选系数相关性的改变。(La/Sm)N与LREE/HREE的相关性,偏态与分选系数、峰态的相关性变化可用于区分河漫滩相和潮坪相。潮坪相与上覆近岸浅海相沉积的识别可参考LREE/HREE、(La/Yb)N、(Gd/Yb)N与∑REE、LREE、HREE相关性和平均粒径与偏态、分选系数与峰态相关性的变化。(La/Sm)N与∑REE、LREE、HRE、LREE/HREE、(La/Yb)N、(Gd/Yb)N相关性,平均粒径与偏态、分选系数与峰态、峰态与偏态的相关性改变可为近岸浅海相和河口湾砂坝相的划分提供依据。在下切河谷不同位置可识别出三种不同类型的沉积相序。根据沉积相演变和海平面变化历史,钱塘江下切河谷经历了深切、充填和埋藏三个阶段,形成一个由低水位体系域到高水位体系域的地层旋回。与其它地区下切河谷相比,钱塘江下切河谷具有广泛分布的近岸浅海相沉积,Zaitlin模式中的第二段在这里不存在,且缺乏河口砂坝、中央盆地、湾顶三角洲沉积。研究区浅层生物气藏盖层的封闭机理包括毛细管封闭、孔隙水压力封闭和烃浓度封闭,其中孔隙水压力封闭是盖层封闭性形成的最主要原因。不均衡压实、黏土矿物的膨胀和生物气生成是高孔隙水压力形成的重要原因。盖层沉积物较强的压缩性、较低的渗透率、气毛细管封闭、分散的有机质和黏土矿物膨胀为高孔隙水压力的保存提供了有利条件。直接盖层比间接盖层具有更大的毛细管封闭、孔隙水压力封闭和烃浓度封闭能力,形成更强的封闭性。静力触探、浅层横波地震、土壤气氡测量、微生物方法和电磁法是浅层生物气勘探的有效手段。静力触探曲线可用于地层划分对比和储层识别。横波地震剖面上可清晰地识别出含气砂体。氡异常高值可以确定气藏范围。土壤中甲烷消耗菌、黄杆菌属、芽孢杆菌属、不动细菌属、黄单孢菌属和假单孢菌属的含量可以反映地下生物气的分布。电磁勘探方法获得的电阻率曲线可以很好地反映出气体的平面分布和垂向厚度。上述方法的合理利用可以有效地提高浅层生物气勘探成功率。静力触探和大间距的微生物勘探可以用于有利勘探区的确定,浅层横波地震、小间距微生物勘探和土壤气氡测量可以确定含气砂体的详细分布。电磁勘探则可以确定生物气层的具体勘探深度。更多还原

【Abstract】 The accommodation space of incised valley provides a unique opportunity to study a relatively uninterrupted stratigraphic history of an area. The incised valley is also associated with oil and gas resources. Therefore, research on the incised valley has become a focus of quaternary geology, sequence stratigraphy, paleoenvironment evolution, and resource exploration. Some of the world’s gas-reserve problems can be solved by exploiting shallow biogenic gas reservoirs due to its big resource potential, but the sealing mechanism for cap beds and exploration methods of shallow biogenic gas pools is relatively poorly studied. The great thick sediments in the Qiantang River incised valley provide good opportunity for the investigation of palaeoenvironment since late Quaternary. A number of shallow biogenic gas pools were found in the Qiantang River incised valley. The research on the characteristics of fill is helpful to decipher the sealing mechanisms for cap beds of shallow biogenic gas pools, and to discuss the effective exploration methods for the shallow biogenic gas and its implementing steps.In this thesis, abundant cores with the borehole SE2as the key are analyzed. A systematic study of mineral composition, foraminifera, grain size, magnetic susceptibility, element geochemistry, permeability, and AMS14C dating of Late Quaternary sediments in the borehole SE1and SE2was carried out. Based on the results, we studied the characteristics of fill, recognized sedimentary facies and main sequence boundary, set up sedimentary architecture, and summarized the infill model of the Qiantang River incised valley. Compared the properties of sediments from cap beds and reservoirs, sealing mechanisms for cap beds of shallow biogenic gas pools is discussed, and the main sealing mechanisms and its causes is ascertained. The exploration methods for the late Quaternary shallow biogenic gas in the Qiantang River incised valley are described and the implementing steps of effective exploration methods are summarized.Five sedimentary facies, including fluvial channel, floodplain, tidal flat, shallow marine, and estuarine sand bar, can be distinguished. Mineral composition, foraminifera, grain size, magnetic susceptibility, and rare earth element of sediments from different sedimentary facies are obvious different. Sediments in the fluvial channel facies have little kind of mineral species, in which light minerals are only quartz and feldspar, and heavy minerals are dominated by chlorite-epidote-amphibole-metallic minerals. Mica, charcoal began to occur in sediments of the floodplain facies, and heavy mineral assemblage in this facies is chlorite-metallic minerals-epidote. In the tidal flat facies, the contents of mica and charcoal markedly increase with more graphite, and the heavy mineral are mainly chlorite-metallic minerals-epidote-accessory minerals. The light minerals in sediments of shallow marine sediments are characterized by the reduction of mica and charcoal, and the occurrence of shells. The heavy mineral assemblage in the shallow marine sediments is chlorite-metallic minerals-pyroxene-epidote-amphibole. Sediments from the estuarine sand bar facies are rich in graphite and shell, but charcoal are absent, and the major heavy minerals are chlorite-pyroxene-epidote--metallic minerals. Benthic foraminifera are mainly found in the shallow marine and estuarine sand bar facies, and are occasionally found in the tidal flat facies. Planktonic foraminifera are only present in the shallow marine facies. As to the grain size, the sediments of fluvial channnel facies are coarse and poorly sorted. The grain size of sediments from floodplain and tidal flat facies are relatively fine, but the contents of different grain fractions change greatly, indicating the poorly sorted. Shallow marine sediments are finest and best sorted of all. Sediments of estuarine sand bar facies are relatively coarse and well-sorted compared to other facies. Magnetic susceptibility of sediments has the relationship of fluvial channel>tidal falt>estuarine sand bar>floodplain>shallow marine. The total content of rare earth elements are highest in the floodplain sediments, and gradually decrease in an order of shallow marine, estuarine sand bar, tidal flat and fluvial channel facies.Probability cumulative frequency curves, frequency distribution curves, correlation among grain size parameters, relationship between magnetic susceptibility and mean grain size, and correlation among rare earth elements parameter sediments all can be employed to define sedimentary facies. Saltation population are abundant and rolling population occur in the fluvial channel facies. In floodplain facies, content of saltation population reduce and rolling population vanish. There are two kinds of sediments in the tidal flat facies, in detail, one is dominated by the saltation population, and the other by suspension population. Suspension populations represent100%of the distribution in the shallow marine facies. Estuarine sand bar sediments are rich in saltation population. Frequency distribution curves of sediments from fluvial channel, tidal flat, and estuarine sand bar are all asymmetric bimodal distribution; whereas, unimodal distribution in floodplain and shallow marine facies. Correlations between (La/Sm)N and HREE or (La/Yb)N, and between mean grain size and sorting coefficient are helpful to distuigish fluvial channel facies and floodplain facies. Relationships between (La/Sm)N and LREE/HREE, between skewness and sorting coefficient or kurtosis can be used to differentiate the floodplain and tial flat facies. To divide the tidal flat and overlying shallow marine facies, the change of relativity between LREE/HREE,(La/Yb)N,(Gd/Yb)N and ΣREE or LREE, HREE, between mean and skewness, and between sorting coefficient and kurtosis may be useful. Correlation between (La/Sm)N and ΣREE or LREE, HREE, LREE/HREE,(La/Yb)N,(Gd/Yb)N, between mean grain size and skewness, and between kurtosis and sorting coefficient or skewness can be referred to distinguish the shallow marine and estuarine sand bar facies.According to facies and stratigraphic architecture, combined with the sea level history, the evolution of the Qiantang River incised valley is discussed. The development of Qiantang River incised valley underwent three stages:deep-cutting, filling, and burial stage, forming a stratigraphic cycles from lowstand system tract to stable highstand system tract. Three kinds of sedimentary sequences are formed at different locations of the incised valley. Compared with other infill model of typical incised valley in the world, the Qiantang River incised valley has widespread shallow marine deposition with the absence of channel mouth bar, central basin, bayhead delta.The differences in capillary pressure, pore-water pressure, and gas concentration between cap beds and reservoirs make the cap beds can prevent gas in the reservoirs from escaping upwards. Pore, pore throat, and permeability of cap beds are smaller than those of reservoirs. Capillary pressure for gas to pass through pores in the cap beds is greater than that in the sand lenses, forming the capillary sealing. Pore-water pressure of clay and mud beds can exceed the total of pore-water pressure and original gas pressure of the underlying sand reservoirs, so the pore-water pressure sealing ability is formed. Disequilibrium compaction, swelling of clay minerals, and gas generation are the main reasons for the generation of high pore-water pressure. Large compressibility, low permeability, gas capillary seal, abundant organic matter, and volume expansion of clay minerals offer favorable conditions for the preservation of high pore-water pressure. Hydrocarbon concentration in the cap beds is obviously higher than that in the reservoirs. The downward diffusion of gas in the cap beds can restrain the vertical flow of gas in the underlying reservoirs, thus a seal is formed by the gradient of hydrocarbon concentration. Capillary sealing, pore-water pressure sealing, and hydrocarbon concentration sealing all contribute to the conservation of shallow biogenic gas pools in the Qiantang River incised valley area. Pore-water pressure sealing mechanism may be the most important factor for the formation of the sealing ability of cap beds. Sealing ability of the direct cap beds is better than that of the indirect cap beds, because of the great burial depth, low permeability, obvious volume expansion of clay minerals, and great potential to generate biogenic gas.Commercial accumulations of shallow biogenic gas have been widely found in the world. The successful development and exploitation of shallow gas are dependent on utilizing suitable technology to effectively prospect for this kind of resource. In this study, we describe the methods used for the exploration of shallow biogenic gas in the Qiantang River incised valley, including cone penetration test, shallow shear wave seismic, soil-gas radon analysis, microbiological prospecting, and electromagnetic surveying. The cone penetration test is effective in helping to establish stratigraphic divisions and correlations, especially reservoir identification. Shallow shear wave seismic profiles identify the top surface of a gas-bearing sand bed, which shows a strong reflecting boundary. The reflection will sharply decline where the gas-bearing sand body pinches-out. Thus, the delineation of a gas-bearing sand body can be visualized on a seismic profile. The content of radon is higher over the boundary of gas pools than over gas pools and outside of field limits. A high concentration of radon can indicate the boundary of a gas pool. The concentration of methane-consuming bacteria, flavobacterium, bacillus, acinetobacter, xanthomonas, and pseudomonas in the soil can act as an indicator for the presence of biogenic gas in the subsurface. Resistivity curves obtained by electromagnetic methods can aid in determining whether there is gas in the sand bodies and in determining the thickness of gas layer. Shallow gas exploration can be improved by combining the above methods. Cone penetration tests and large spacing microbiological surveys can be used to confirm the favorable exploration area. To define the detailed distribution of gas-bearing sand bodies, shallow shear wave seismic, small spacing microbiological, and radon anomaly analysis should be applied. Electromagnetic exploration methods can be used to establish the exploration depth.更多还原