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SCIENCE CHINA Earth Sciences, Volume 59, Issue 11: 2223-2230(2016) https://doi.org/10.1007/s11430-016-5005-3

Influence of foraminifera on formation and occurrence characteristics of natural gas hydrates in fine-grained sediments from Shenhu area, South China Sea

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  • ReceivedJan 5, 2016
  • AcceptedMay 11, 2016
  • PublishedSep 8, 2016

Abstract

Marine gas hydrates accumulate primarily in coarse-grained, high-permeability layers; however, highly saturated natural gas hydrates have been discovered in the fine-grained sediments of Shenhu area, South China Sea (SCS). This may be explained by key factors, such as the great abundance of foraminifera shells. In this paper, by analyzing the SCS foraminifera structure and performing hydrate formation experiments in the foraminifera shells, the contribution of foraminifera to hydrate accumulation in the SCS was investigated from a microscopic point of view. Simulations of hydrate formation were carried out in both pure SCS foraminifera shells and the host sediments. Pore structures in typical foraminifera were studied by use of micro-focus X-ray computed tomography (CT) and scanning electron microscopy (SEM). Hydrate growth and occurrence characteristics in the foraminifera shells were observed in-situ. The results showed that the presence of foraminifera significantly enhanced the effective porosity of the SCS sediments. Moreover, while the hydrates grew preferentially in the chambers of the coarse-grained foraminifera by adhering to the inner walls of the foraminifera shells, no apparent hydrate accumulation was observed in the fine-grained or argillaceous matrix. These findings provide a basis for further studies on the accumulation mechanism of hydrates and physical properties of hydrate reservoir in the South China Sea.


Funded by

This study was supported by the National Natural Science Foundation of China(41506082,41474119,41376078,41306065,41306062),the Scientific,Technological Innovation Project Financially Supported by Qingdao National Laboratory for Marine Science,Technology (Grant No. 2015ASKJ03)


Acknowledgment

This study was supported by the National Natural Science Foundation of China (Grant Nos. 41506082, 41474119, 41376078, 41306065 & 41306062) and the Scientific and Technological Innovation Project Financially Supported by Qingdao National Laboratory for Marine Science and Technology (Grant No. 2015ASKJ03).


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  • Figure 1

    Sampling location in Shenhu area of the South China Sea. Modified from Wang et al. (2011).

  • Figure 2

    Samples and experimental device: (a) pure foraminifera shells selected from SCS sediment (grain size: <63 μm, 63–150 μm, >150 μm); (b) sample from the intact SCS sediments; (c) low-temperature and high- pressure vessel suitable for CT scanning.

  • Figure 3

    Surface and inner images of typical foraminifera shells in the SCS sediment.

  • Figure 4

    2D section showing the inside of the SCS sediment. Black represents methane gas, bright white represents foraminifera shells, and the rest is sediment.

  • Figure 5

    X-ray CT grayscale histogram of the sample shown in Figure 4.

  • Figure 6

    Transverse sections of microscopic distribution before (a) and after (b) the methane hydrates formed in the media of pure foraminifera shells.

  • Figure 7

    Means of entry of hydrates into the inner and outer the shell walls of a single foraminifer.

  • Figure 8

    2D section of a single foraminifer with methane hydrate.

  • Figure 9

    Comparison of mineral composition (a) and grain size distribution (b) between SH2 porous sample from the Shenhu area and sample in this study.

  • Figure 10

    Microscopic distribution of the growth of methane hydrates in the SCS sediments. Black represents methane gas, blue-green represents water, yellow represents hydrates, bright white represents foraminifera shells, and the rest is sediment.

  • Figure 11

    Before and after hydrate formation: (a) and (b) show the same location, (c) and (d) show a different location. Black represents methane gas, blue-green represents water, yellow represents hydrates, bright white represents foraminifera shells, and the rest is sediment matrix.

  • Table 1   Grain size of sample and abundance of foraminifera

    Sediment grain size (μm)

    Weight (g)

    Characteristics of foraminifera abundance

    >150

    1.717

    Primarily planktonic foraminifera with a few columnar pyrite

    63–150

    0.613

    Primarily young foraminifera and broken shells with a few of coarse-grained sediments

    <63

    7.179

    No foraminifera, mostly fine-grained sediments

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