Characteristics Of Fill And Palaeoenvironment Evolution Since The Late Quaternary In The Qiantang River Incised Valley

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.