Gas Hydrate Saturations and Their Uncertainties
From Chlorinity and Well Log Data at IODP Site
U1325 (Exp. 311, Cascadia Margin)
Alberto Malinverno, Miriam Kastner,
Marta E. Torres, Ulrich J. Wortmann
and the IODP Exp. 311 Scientific Party
Timothy S. Collett, Michael Riedel, Mitchell J. Malone, Gilles
Guèrin, Fumio Akiba, Marie-Madeleine Blanc-Valleron,
Michelle Ellis, Yoshitaka Hashimoto, Verena Heuer, Yosuke
Higashi, Melanie Holland, Peter D. Jackson, Masanori Kaneko,
Ji-Hoon Kim, Hiroko Kitajima, Philip E. Long, Greg Myers,
Leena D. Palekar, John Pohlman, Peter Schultheiss, Barbara
Teichert, Anne M. Trèhu, Jiashen Wang, Hideyoshi Yoshioka
Fire and ice
clathrate
(Chemistry) a compound in which molecules
of one component are physically trapped
within the crystal structure of another.
ORIGIN 1940s: from L. clathratus, from clathri
‘lattice-bars’.
2
GHSZ on continental margins
No GHSZ above 300-500 m
3
Cascadia Gas Hydrate Model
after Hyndman and Davis, 1992
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ODP Leg 146
after Hyndman et al., 1999
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ODP Leg 204
6
7
Cascadia Plate Tectonics
8
9
Site U1328
(Cold Vent)
Exp. 311 Sites
10
Quantifying Gas Hydrate
• Visual inspection
• Infrared imaging
• Pressure coring
• Well logging
• Interstitial water chemistry
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Archie’s equation
Rw
R0 = m
φ
R0
Rw
Rt
R0
Rt =
(S w )n
Resistivity of watersaturated sample
Resistivity of water
“True” resistivity of
sample
Φ
Sw
m
n
a
!
a Rw
Sw = m
φ Rt
" 1n
Porosity
Water saturation
Cementation exponent ≈ 2
Saturation exponent ≈ 2
Constant ≈ 1
Pore water salinity
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1
0.9
0.8
0.7
w(Cb − C)
Sh =
C + w(Cb − C)
0.6
0.5
0.4
0.3
0.2
0.1
ρw NAvo Vcell
w=
Mw nw
Sh
Cb
C
ρw
Gas hydrate saturation
Baseline chlorinity
Measured chlorinity
Water density
0
0
0.2
0.4
C
Cb
0.6
0.8
1
NAvoAvogadro’s number
Vcell Volume of unit cell (Type I)
Mw Molar mass of water
nw Water molec. in unit cell (Type I)
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Uncertainty quantification
!
a Rw
Sh = 1 − m
φ Rt
" 1n
w(Cb − C)
Sh =
C + w(Cb − C)
Parameter
Measured porosity Φ
Standard deviation
3%
Measured resistivity Rt
10 %
Pore water resistivity Rw
2%
Constant a
Cementation exponent m
Fit R0-Rt in watersaturated interval
Fit between Archie
and chlorinity Sh
Saturation exponent n
Baseline chlorinity Cb
5 mM
Measured chlorinity C
5 mM
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Uncertainty propagation
y = f(x)
!! !!
∂y !
!
σy = σx !! !!
∂x
σx
x
Example: Sh = f(Φ, Rt, Rw, a, m, n)
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Measured salinity
0
20
30
Measured chlorinity (mM) Gas hydrate saturation
40300
400
500
600
0
0.2
0.4
0.6
0
Baseline
50
100
100
150
150
200
200
250
250
300
300
Hole U1325BC
Thin sands
(1-3 cm)
Depth (mbsf)
Depth (mbsf)
50
Site
U1325
Depth (mbsf)
0
20
30
Measured chlorinity (mM) Gas hydrate saturation
40300
400
500
600
0
0.2
0.4
0.6
0
50
50
100
100
150
150
200
200
250
250
300
300
Depth (mbsf)
Measured salinity
9 cm-sand
20
Hole U1325BCnts
0
Depth (mbsf)
180
10
0
10
GVR (! m)
0
10
DIT (! m)
0
10
180
185
185
190
190
195
195
200
200
205
205
210
210
215
215
220
220
225
225
230
230
Hole U1325A
Hole U1325A
Hole U1325A
Hole U1325C
Depth (mbsf)
Ph. 400 kHz (! m)Ph. 2 MHz (! m)
9 cm-sand
R0 and Rt (! m)
Porosity
0.2
0.4
0.6
0.8
0
10
Gas hydrate saturation
0
0.2
50
0.4
0.6
!
a Rw
Sw = m
φ Rt
0.8
50
" 1n
100
100
150
150
200
200
250
250
Site U1325 (a=2.11 m=1.1 n=2.4 +/! 0.52)
Depth (mbsf)
Depth (mbsf)
Sh = 1 − Sw
Rw
a, m
n
From measured
T, salinity
From fitting R0 to
Rt (0-190 mbsf)
From fitting
chlorinity results
in 9 cm-sand
Measurement uncertainties:
Φ 3%
Rt 10%
Gas hydrate saturation
0
0.2
0.4
0.6
50
GHSZ
Average Sh = 3 ± 0.7%
Depth (mbsf)
100
150
200
Average Sh = 7.8 ± 1.5%
Sh = 55 ± 5%
250
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Logs in U1325A, Cl in U1325BCnts
Greetings
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