Seismic Characteristics
in Marine Hydrate Systems
Guangsheng Gu1
Advisors: George J. Hirasaki1, Walter G. Chapman1
Collaborators: Colin A. Zelt2, Priyank Jaiswal2
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
1
Rice University, Houston
What is Gas Hydrate
• Crystalline compounds, with gas
molecules (e.g. CH4, C2H6)
captured in water molecular cages
• Dissociation:
1m3 methane hydrate =
168 m3 CH4 + 0.8 m3 H2O
• Stable at high pressure and low
temperature, typically in deep
marine sediments or in permafrost
environments
2
Why Study Hydrates?
Geohazards Submarine
slope failure
World-wide distribution;
huge potential amount,
as energy resource
Influence on
global climate
change
T.S. Collett, Offshore Technol. Conf. (OTC) 2008. 3
Major Seismic Characteristics
• Used to identify hydrates in marine
sediments
• Bottom Simulating Reflector (BSR)
• Seismic Blanking in Lateral Strata
• Wipeout in Gas Chimeny
4
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
Hydrate or Gas Saturation
Abrupt Change
Taylor et al., 1992; M.W. Lee et al, 2001
5
Seismic Blanking in Lateral Strata
• Hydrate accumulation induces blanking
6
Seismic Blanking
MJ. Hornbach, WS. Holbrook, et al., Geophysics, v. 68, n. 1, 92–100,2003.
7
Seismic Blanking
• Weak reflection in seismic profiling:
R < RBSR/10
Typically R < 0.02
8
Geologic Setting
In
Reflection
Layer 1
1, SH ,1, SW ,1, SV ,1, 1, Vp,1, Z1
Layer 2
(shale/clay)
2 , SH ,2 , SW ,2 , SV ,2 , 2 , Vp,2 , Z2
Reflection Coefficient:
Z 2  Z1
R
Z 2  Z1
Transmission Coefficient:
2Z 2
T
Z 2  Z1
9
Estimation of Acoustic Properties
Average P-wave Velocity:
Revised from the Timeaverage Equation
(Pearson et al., 1983).
Average Density:
phase i =w,H,V
10
10
Intrinsic Properties of Phases
Table 1: Acoustic properties of components
Component
Vp (m/s)
 (kg/m3)
Sea Water (w)
1500
1030
Hydrate (H)
3300
900
Mineral1 (sand)
200 ~ 2000
2500
Mineral2 (diatomite)
2000
2000
Reference Mineral (shale/clay)
2000 ~ 2400
2600
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.
Table 2: Porosity and saturation ranges
Parameter
Value
Porosity1 (in sand layer)
0.2 ~ 0.3
Porosity2 (in shale layer)
0.2~0.7
Sh
0~1
The ranges of porosity were obtained from Hirasaki (lecture note, 2006), Jenyon (2006), Magara (1980).11
11
(Case 1) Impossible to be blanking
3.4
x 10
3.3
6
Average Impedance in Sand Layer
Blanking
Range
Shale Layer
Vp
2
Average Impedance, kg/(m *s)
3.2
sand
= 1000
 sand = 0.3
3.1
Vp

shale
shale
= 2400
= 0.5
3
2.9
2.8
2.7
Sand Layer
2.6
2.5
2.4
0
0.2
0.4
0.6
0.8
1
Sh, %
12
(Case 2) Possible to be blanking
3.9
x 10
6
Average Impedance in Sand Layer
Vpsand = 1500
2
Average Impedance, kg/(m *s)
3.8
 sand = 0.3
Vp
shale
= 2400
 shale = 0.5
Sand Layer
3.7
3.6
3.5
Shale Layer
Blanking
Range
3.4
3.3
3.2
0
0.2
0.4
0.6
0.8
1
Sh, %
13
(Case 3 ) Impossible to be blanking
5
x 10
Average Impedance in Sand Layer
 sand = 0.3
4.8
Vpsand = 2000
4.6

shale
= 0.5
Vpshale = 2400
2
Average Impedance, kg/(m *s)
6
Sand Layer
4.4
4.2
4
3.8
Blanking
Range
3.6
3.4
Shale Layer
3.2
0
0.2
0.4
0.6
Sh, %
0.8
1
14
Reflection Coeffiecient
Reflection Coefficient from A certain layer to Clay Layer
0.02
-0. 02
0
-0. 04
-0. 02
0
0
.
-0. 06
04
-0. 02
-0. 04
-0. 06
-0. 08
-0. 06
-0. 08
-0. 1
0.65
0.6
 of clay
0.55
0.5
0.45
0.4
-0. 12
-0. 1
-0. 08
-0. 14
-0. 12
-0. 1
-0. 16
-0. 14
-0. 12
-0. 18
-0. 16
-0. 14
-0. 2
-0. 18
-0. 16
-0. 22
-0. 2
-0. 18
-0. 22
-0. 2
-0. 24
0.35
0.3
0.2
0.4
-0.1
-0.15
-0.2
-0.25
-0. 24
-0. 26
0
Blanking
region
-0.05
-0. 22
-0. 24
-0. 26
0
Just possible
to be
blanking
0.6
0.8
1
Sh sand layer
Sh in
Layer
Layer 1 (quartz)
Layer 2 (Clay/Shale)
porosity
0.3
0.4~0.7
Vp (m/s)
1000
2400
Density (kg/m3)
2650
2600
15
Reflection Coeffiecient
0.65
0.6
 of clay
0.55
0
0.5
Reflection Coefficient from A certain
0.14
0.16
0.12
0.14
0.1
0.12
0.08
0.1
0.06
0.08
0.04
0.06
0.02
0.04
0.02
-0. 02
0.45
0.35
0.3
0.16
0.15
0.14
0.12
0.1
0.1
0.08
0.06
0.05
0.02
-0. 04
-0. 02
-0. 06
-0. 04
-0. 08
-0. 12
-0. 1
0.2
0
0
-0. 02
-0. 06
-0. 1
0
layer to Clay Layer
0.18
0.04
0
0.4 -0. 0
8
Very
possible to
be blanking
-0.05
0
Blanking
region
-0. 04
-0.1
-0. 06
-0. 08
0.4
0.6
0.8
1
Sh
Layer
Layer 1 (quartz)
Layer 2 (Clay/Shale)
porosity
0.3
0.4~0.7
Vp (m/s)
1500
2400
Density (kg/m3)
2650
2600
16
Reflection Coeffiecient
Reflection Coefficient from A certain layer to Clay Layer
0.24
0.3
0.26
0.22
0.28
0.2
0.24
0.65
0.22
0.26
0.18
0.2
0.6
0.18
0.16
 of clay
0.55
0.14
0.1
0.16
0.12
0.5
0.4
0.35
0.3
0
0.2
0.4
0.6
0.15
0.1
0.12
0.05
0.1
0
0.08
0.04
0
0.2
0.14
0.06
0.02
-0. 0
2
0.24
0.16
0.08
0.04
0
0.25
0.2
0.1
0.06
0.02
0.18
0.12
0.08
0.04
0.22
0.14
0.1
0.45
Blanking
region
0.2
Just
Possible to
be blanking
0.06
0.8
1
Sh
Layer
Layer 1 (quartz)
Layer 2 (Clay/Shale)
porosity
0.3
0.4~0.7
Vp (m/s)
2000
2400
Density (kg/m3)
2650
2600
17
Different Layer (Diatomite vs. Clay)
Reflection Coefficient from Diatomite to Clay Layers
0.0
-0.
-0.
0. 0.08
0
0
2
06
04
0.
.0 06
02
4
-0.
08
14
0.
12
0.
1
0.
0.15
0.65
-0
.
0.6 -0.1
2
-0
. 14
0.55
-0
.1
-0
.
-0
.
2
-0.15
1
12
14
Blanking
region
-0.2
04
18
-0.1
.
-0
16
04
08
0.2
-0
.
0
-0.05
.
-0
0
08
-0
.
06
-0
.
0.4
14
-0
.
2
-0
.
06
.
-0
0.45 -0.2
-0
.
12
-0
.
02
02
0.
-0
.
1
0.05
0.
02
0
0
-0
.
8
-0
.
02
0.5
04
08
-0
.
6
-0
.
.
-0
-0
.1
-0
.
06
0.1
08
0.
06
0.
04
0.
 of clay
-0.
1
Very
possible to
be blanking
-0.25
0.4
0.6
0.8
1
Sh
Layer
Layer 1 (Diatomite)
Layer 2 (Clay/Shale)
porosity
0.65
0.4~0.7
Vp (m/s)
2000
2400
Density (kg/m3)
2000
2600
18
Conclusion
Hydrate accumulation in marine sediment is
helpful for blanking;
Sensitive to parameters and stratum lithology;
Hydrate accumulation doesn’t guarantee a
blanking.
19
Wipeout in gas chimney
Wipe out
in vertical
columnar
regions
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.
S. Horozal et al., Marine Geology 258: 126–138, 2009.
20
gas chimney
Northern Cascadia margin
near Ocean Drilling Program
(ODP) Site 889/890.
Geological Society of America Bulletin, Riedel, 2006.21
Riedel, 2006.
22
chimney
S. Horozal et al., Marine Geology 258: 126–138, 2009. 23
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 thermochemical effects (Hornbach et al., 2005)
• Due to presence of gas hydrate, and intrinsic
acoustic properties in sediments (Chand and
Minshull, 2003.).
24
Acknowledgement
• DOE Grant (No. DE-FC26-06NT42960)
• Rice University, Hirasaki Group, Chapman Group
• Colleagues in Earth Science Department
25