| Citation: |
Jin-qiang Liang, Wei Deng, Jing-an Lu, Zeng-gui Kuang, Yu-lin He, Wei Zhang, Yue-hua Gong, Jin Liang, Miao-miao Meng, 2020. A fast identification method based on the typical geophysical differences between submarine shallow carbonates and hydrate bearing sediments in the northern South China Sea, China Geology, 3, 16-27. doi: 10.31035/cg2020021
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A fast identification method based on the typical geophysical differences between submarine shallow carbonates and hydrate bearing sediments in the northern South China Sea
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Abstract
Bottom simulating reflector (BSR) has been recognized as one of the indicators of gas hydrates. However, BSR and hydrate are not one-to-one correspondence. In the Xisha area of South China Sea (SCS), carbonate rocks wildly develop, which continuously distribute parallel to the seafloor with high amplitude on seismic sections, exhibiting reflections similar to BSRs in the Shenhu area nearby. This phenomenon causes some interference to hydrates identification. In this paper, the authors discussed the typical geophysical differences between carbonate rocks and hydrates, indicating that the main difference exists in relationship between porosity and velocity, causing different amplitude versus offset (AVO) characters. Then the authors proposed a new model assuming that the carbonates form the matrix and the hydrate fill the pore as a part of the matrix. The key modeling parameters have been optimized constrained by P-velocities and S-velocities simultaneously, and the model works well both for carbonate rock and gas hydrate bearing sediments. For quantitative identification, the authors calculated the velocities when carbonates and hydrates form the matrix together in different proportions. Then they proposed a carbonate and hydrate identification template (CHIT), in which the possible hydrate saturation (PHS) and possible carbonate content (PCC) can be both scaled out for a group of sample composed by P-velocity and S-velocity. If PHS is far larger than PCC, it is more likely to be a hydrate sample because carbonates and hydrates do not coexist normally. The real data application shows that the template can effectively distinguish between hydrates and carbonate rocks, consequently reducing the risk of hydrate exploration.
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References
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Jin-qiang Liang, Wei Deng, Jing-an Lu, Zeng-gui Kuang, Yu-lin He, Wei Zhang, Yue-hua Gong, Jin Liang, Miao-miao Meng, 2020. A fast identification method based on the typical geophysical differences between submarine shallow carbonates and hydrate bearing sediments in the northern South China Sea, China Geology, 3, 16-27. doi: 10.31035/cg2020021
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Figure 1.
a–Location of the study areas; b–submarine landform map of SH area; c–submarine landform map of XS area.
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Figure 2.
Typical BSRs in the SH on the post-stack seismic profile. The arrows indicates the BSRs. The seafloor owns positive reflection with red color. BSRs are displayed with black. Gas blanking and low-frequency anomaly below the BSRs are obvious, indicating gas migrating pathways.
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Figure 3.
Typical BSRs of thin hydrate reservoir in the SH on the post-stack seismic profile. The arrows indicates the BSRs. W18 drilled through the BSR and thin layer of hydrates were found. Hydrate top and BSR are in the same half wavelength.
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Figure 4.
Typical seismic section of carbonate rocks in the XS. The arrows indicate the bottom of carbonate layers.
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Figure 5.
AVO forward modeling in XH1. a–synthetic traces at different incidence; b–P-wave velocity and S-wave velocity logs; c–reflection coefficient versus incidence at different depths in the carbonate layers. The curves with different colors refer to the coefficients at different depths. The red dashed line refers to the AVO curve at the top of the carbonate layer. The blue curve refers to the AVO curve in the carbonate layer. The black curve refers to the AVO curve of the carbonate bottom.
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Figure 6.
AVO forward modeling in XH2. a–synthetic traces at different incidence; b–P-wave velocity and S-wave velocity logs; c–reflection coefficient versus incidence at different depths in the carbonate layers. The curves with different colors refer to the coefficients at different depths. The red dashed line refers to the AVO curve at the top of the carbonate layer. The blue curve refers to the AVO curve in the carbonate layer. The black curve refers to the AVO curve of the carbonate bottom.
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Figure 7.
AVO forward modeling in SH1. a–synthetic traces at different incidence; b–P-wave velocity and S-wave velocity logs; c–reflection coefficient versus incidence at different depths in the hydrate layers. The curves with different colors refer to the coefficients at different depths. The red dashed line refers to the AVO curve of BSR. The blue curve refers to the AVO curve of gas top at 1540 m below the BSR. The black curve refers to the AVO curve at 1530 m.
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Figure 8.
AVO forward modeling in SH2. a–synthetic traces at different incidence; b–P-wave velocity and S-wave velocity logs; c–reflection coefficient versus incidence at different depths in the hydrate layers. The curves with different colors refer to the coefficients at different depths. The black bold curve refers to the AVO curve of the BSR.
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Figure 9.
Cross-plot of velocity and porosity in the SH when there is no hydrate and free gas.
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Figure 10.
Cross-plots of velocity and porosity in hydrate and gas reservoirs in the SH. The color axis is hydrate saturation calculated by resistivity. a–cross-plot of P-wave velocity and porosity; b–cross-plot of S-wave velocity and porosity.
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Figure 11.
Cross-plots of velocity and porosity in carbonate layers in the XS. The color axis is resistivity. a–cross-plot of P-wave velocity and porosity; b–cross-plot of S-wave velocity and porosity.
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Figure 12.
Flow chart of two-phase hybrid modeling.
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Figure 13.
P-velocity and S-velocity plots with different PHS and PCC combinations; a–P-velocity and S-velocity contours with different PHS and PCC combinations; b–planar graph of P-velocity and S-velocity in PHS-PCC coordinates.
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Figure 14.
PCC-PHS curve corresponding to a carbonate samples; a–PCC-PHS curve in the P-velocity cube; b–PCC-PHS curve in the S-velocity cube.
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Figure 15.
PCC-PHS curves and their intersections. Red dots refer to the P-velocity contour of 2585 m/s. Blue dots refer to the S-velocity contour of 1267 m/s.
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Figure 16.
XH1 fast identification results; a–PCC-PHS curve and intersection point distribution for different porosities; b–P-wave velocity and S-wave velocity; c–PCC/PHS distribution for different porosities.
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Figure 17.
SH2 fast identification results. a–PCC-PHS curve and intersection point distribution for different porosities; b–P-wave velocity and S-wave velocity; c– PCC/PHS distribution for different porosities.