1. 1 Guangsheng Gu 1 Advisors: George J. Hirasaki 1, Walter G. Chapman 1 Collaborators: Colin A. Zelt 2, Priyank Jaiswal 2 1 Dept. of Chemical & Biomolecular Engineering 2 Dept. of Earth Science Rice University, Houston, TX, 77005 Consortium on Processes in Porous Media, 15th, April 26, 2011 Rice University, Houston Seismic Characteristics in Marine Hydrate Systems

2. 2 Stable at high pressure and low temperature, typically in deep marine sediments or in permafrost environments What is Gas Hydrate Crystalline compounds, with gas molecules (e.g. CH 4, C 2 H 6 ) captured in water molecular cages Dissociation: 1m 3 methane hydrate = 168 m 3 CH 4 + 0.8 m 3 H 2 O

3. 3 Why Study Hydrates? World-wide distribution; huge potential amount, as energy resource Geohazards - Submarine slope failure Influence on global climate change T.S. Collett, Offshore Technol. Conf. (OTC) 2008.

4. 4 Major Seismic Characteristics Used to identify hydrates in marine sediments Bottom Simulating Reflector (BSR) Seismic Blanking in Lateral Strata Wipeout in Gas Chimeny

5. 5 Bottom Simulating Reflector (BSR)  A strong reflector below seafloor  Parallel to the seafloor  Indicating the abrupt transition from hydrate to free gas phase below  In good accordance with 3-phase equilibrium of a pure-methane system Taylor et al., 1992; M.W. Lee et al, 2001 Hydrate or Gas Saturation Abrupt Change

6. 6 Seismic Blanking in Lateral Strata Hydrate accumulation induces blanking

7. 7 Seismic Blanking MJ. Hornbach, WS. Holbrook, et al., Geophysics, v. 68, n. 1, 92–100,2003.

8. 8 Seismic Blanking Weak reflection in seismic profiling: R < R BSR /10 Typically R < 0.02

9. 9 Geologic Setting Layer 1 Layer 2 (shale/clay) InReflection Reflection Coefficient: Transmission Coefficient:

10. 10 Estimation of Acoustic Properties Revised from the Time- average Equation (Pearson et al., 1983). Average P-wave Velocity: Average Density: phase i =w,H,V

11. 11 Intrinsic Properties of Phases ComponentVp (m/s)  (kg/m3) Sea Water (w)15001030 Hydrate (H)3300900 Mineral1 (sand)200 ~ 20002500 Mineral2 (diatomite)2000 Reference Mineral (shale/clay)2000 ~ 24002600 ParameterValue Porosity1 (in sand layer)0.2 ~ 0.3 Porosity2 (in shale layer)0.2~0.7 ShSh 0~1 Table 1: Acoustic properties of components Table 2: Porosity and saturation ranges Acoustic velocities from W.J. Winters and W.F. Waite (2007); Sloan (2007), etc.. Nick Barton, Rock Quality, Seismic Velocity, Attenuation and anisotropy, Taylor & % Francis Group, 2007, p. 12. The ranges of porosity were obtained from Hirasaki (lecture note, 2006), Jenyon (2006), Magara (1980).

12. 12 (Case 1) Impossible to be blanking Blanking Range

13. 13 (Case 2) Possible to be blanking Blanking Range

14. 14 (Case 3 ) Impossible to be blanking Blanking Range

15. 15 Reflection Coeffiecient Layer porosityVp (m/s)Density (kg/m3) Layer 1 (quartz)0.310002650 Layer 2 (Clay/Shale)0.4~0.724002600 Blanking region Just possible to be blanking S h in sand layer

16. 16 Reflection Coeffiecient Layer porosityVp (m/s)Density (kg/m3) Layer 1 (quartz)0.315002650 Layer 2 (Clay/Shale)0.4~0.724002600 Very possible to be blanking Blanking region

17. 17 Reflection Coeffiecient Layer porosityVp (m/s)Density (kg/m3) Layer 1 (quartz)0.320002650 Layer 2 (Clay/Shale)0.4~0.724002600 Just Possible to be blanking Blanking region

18. 18 Different Layer (Diatomite vs. Clay) Layer porosityVp (m/s)Density (kg/m3) Layer 1 (Diatomite)0.652000 Layer 2 (Clay/Shale)0.4~0.724002600 Blanking region Very possible to be blanking

19. 19 Conclusion  Hydrate accumulation in marine sediment is helpful for blanking;  Sensitive to parameters and stratum lithology;  Hydrate accumulation doesn’t guarantee a blanking.

20. 20 S. Horozal et al., Marine Geology 258: 126–138, 2009. KIGAM data showing BSR in debris-flow deposits (DFD). BSR is weak and discontinuous. Seismic chimneys look very narrow due to vertical exaggeration (ca. 14×). Seismic chimney, marked by S, is about 820 m wide and 110 m tall above the BSR, forming a rather horizontal zone of amplitude reduction. DFD, debris-flow deposits; THS, turbidite/hemipelagic sediments. Wipeout in gas chimney Wipe out in vertical columnar regions

21. 21 gas chimney Geological Society of America Bulletin, Riedel, 2006. Northern Cascadia margin near Ocean Drilling Program (ODP) Site 889/890.

22. 22 Riedel, 2006.

23. 23 chimney S. Horozal et al., Marine Geology 258: 126–138, 2009.

24. 24 Mechanisms Due to gas bubbles in the GHSZ in the Cascadia Margin (Wood et al., 2002). These gas bubbles may be coated with hydrate that prevents the inflow of water (Riedel et al., 2006). Due to a thermal (Wood et al., 2002) or a thermo- chemical effects (Hornbach et al., 2005) Due to presence of gas hydrate, and intrinsic acoustic properties in sediments (Chand and Minshull, 2003.).

25. 25 Acknowledgement DOE Grant (No. DE-FC26-06NT42960) Rice University, Hirasaki Group, Chapman Group Colleagues in Earth Science Department