Page 1
Modeling Shallow Pore Water Chemistry above Marine Gas Hydrate Systems
Sayantan Chatterjee, Gerald Dickens, Gaurav Bhatnagar, Walter Chapman, Brandon Dugan, Glen Snyder, George Hirasaki
Rice University, Houston, Texas, USA
April 23, 2012
Rice UniversityConsortium on Processes
in Porous MediaDE-FC26-06NT 42960
Page 2
Torres et al., Earth Planet. Sci. Lett., (2004)
2
Gas hydratesCage structure “Ice that burns” Core sample
Source: USGS
Source: USGS
- Clathrates- Ice-like solids- Guest molecules (e.g., CH4) encapsulated in H2O cages
Stability- High pressure- Low temperature- Low salinity
Occurrence- Marine sediments along continental margins - Permafrost regions
Page 3
Motivation
Potential energy resource
Subsea geohazard Global climate change
McIver, AAPG (1982) Westbrook et al., Geo. Res. Lett., (2009)
A fundamental understanding of the dynamics of gas hydrate systems
Soil1400
Gas hydrates (on shore and
offshore), 10,000
Other, 67
Peat, 500
Land Biota, 830
Dissolved organic matter in water, 980
Unit 1015 g carbonRecoverable and
Non-recoverable fossil fuels (coal, oil, natural gas), 5,000
Page 4
Hydrate dissociation due to burial below the GHSZ
Free gas recycling
Geologic time (Myr)
Concentration (mM)
Subsidence Subsidence
Hydrate layer extending downwards
Solubility
Hydrate
Free gas
Dissolved gas
Organic carbon
CH4
SO42- reduction zone
Base of GHSZ
GHSZ
Temperature (oC)
Dep
th
Seafloor
Geotherm
CH
4 3-ph
ase
equ
ilibriu
m
0 10 20 30
Sedimentation fluid flux
External fluid flux
0 100 200 300
T0
Phase relationships
Steady state Transient state Components
TOCao
Sediment flux
SO42-H
ydro
ther
m
Tran
sien
t st
ate
Ste
ady
stat
e
Schematic of hydrate formation and burial
Bhatnagar et al., Am. J. Sci., (2007); Chatterjee et al., (2012) to be submitted
Page 5
5
Methods to quantify gas hydrate amount and distribution
Dickens., Org. Geochem., (2001)
Free gas
994 995 997
Paull et al., ODP Init. Repts., 164, (1996)
Boswell et al., Mar. Pet. Geo., (2011)
Page 6
Gas hydrate systems and the SMT
6Bhatnagar et al., Geo. Res. Lett., (2008)
SMT: Sulfate Methane Transition
Page 7
SMT depth inversely proportional to upward methane flux
7
Borowski et al., Geology, (1996) Paull et al., Geo-Mar. Lett., (2005)
Gas hydrate bearing sediment
Gas hydrate free sediment
Fault line SMT Chemosyntheticcommunity
De
pth
be
low
se
afl
oo
r
SMT
SMT
Page 8
CH4 solubility and phase equilibrium curves
Sedimentation and compaction Mass balance equations:
• Sediment• Water• Organic carbon
• Methane (CH4)
• Sulfate (SO42-)
• Bicarbonate (HCO3-)
• Calcium (Ca2+)
• Carbon isotopes of CH4 and HCO3-
CH4 sources:
• In situ methanogenesis (biogenic)• Deep external sources (thermogenic)
Advection, diffusion and reaction
Key model features
8
PDEs solved using finite-difference method
(Explicit and implicit schemes)
Page 9
Geologic sites known/inferred for gas hydrates
Chatterjee et al., J. Geophys. Res., (2011)
Page 10
Modeling pore water chemistry at 3 sites
Chatterjee et al., J. Geophys. Res., (2011)
Page 11
Flux balance across the SMT using steady-state simulations
Chatterjee et al., J. Geophys. Res., (2011); Chatterjee et al., (2012) to be submitted
iiD z
c
At the SMT:
Diffusive flux
Advective flux = 0
Page 12
A 1:1 flux balance across SMT implies dominant AOM at the SMT
Chatterjee et al., J. Geophys. Res., (2011); Chatterjee et al., (2012) to be submitted
24 4 3 2( ) CH aq SO HCO HS H O
flux
flux
Anaerobic Oxidation of Methane (AOM)
Page 13
Paull et al., Geo-Mar. Lett., (2005)
SMT depth: A useful proxy
13
Gas hydrate bearing sediment
Gas hydrate free sediment
Fault line SMT Chemosyntheticcommunity
SMT dept
h
Net fluid flux
Top of
gas hydrate
Hydrate
saturation <Sh>
Bhatnagar et al., Geochem. Geophys. Geosyst., (2011)
Pe 1
<S
h>
Top
of
hy
dra
te /
SM
T
Normalized Scaled SMT depth
Scaled SMT depth
De
pth
be
low
se
afl
oo
r
SMT depth Hydrate saturation
SMT depth Top of gas hydrate
“Rule of ten”
Page 14
Top
of
hy
dra
te /
SM
T14
Seafloor and geologic
parameters
Base of hydrate stability
Local SMT depth
Top of gas hydrate
Local fluid flux
Local hydrate
saturation
Modeling to quantify hydrate amount and distribution
De
pth
Base of GHSZ
Dep
th Geotherm
T3 P
Hyd
roth
erm
Scaled SMT depth
“Rule of ten”
Pe1<Sh>
Loca
l flui
d flu
x
Page 15
Conclusions
15
A 1:1 flux balance across the SMT implies dominant AOM at the SMT
SMT depth is a direct proxy to relate upward methane flux and hydrate saturation
An empirical “rule of ten” established to relate SMT depth and top of hydrate
Developed a model to evaluate hydrate amount and distribution
Page 16
Back up slides
16
Page 17
Pore water chemistry and reaction zones
17
Sulfate reduction zone
Methanogenesiszone
Snyder et al., J. Geochem. Explor., (2007)
Page 18
Pore water chemistry data: Sites 1244 and KC151
Chatterjee et al., J. Geophys. Res., (2011)18
Page 19
Pore water chemistry data: Site 1230
Chatterjee et al., (2012) to be submitted19
Page 20
Concentration crossplot of DIC and SO42-
20Chatterjee et al., J. Geophys. Res., (2011)
Page 21
Steady state profiles: Site 1244
21Chatterjee et al., J. Geophys. Res., (2011)
Page 22
Steady state profiles: Site KC151
22Chatterjee et al., J. Geophys. Res., (2011)
Page 23
Physical property data: Site 1230
Chatterjee et al., (2012) to be submitted23
Pre
ssur
e (M
Pa)
Page 24
Evidence of a 4.3 Myr hiatus implies Site 1230 is in transience
24
Hiatus
Page 25
Seafloor 2.4 Myr ago
Pre-hiatus steady state profiles: Site 1230
25Chatterjee et al., (2012) to be submitted
Page 26
Transient state profiles: Site 1230
26Chatterjee et al., (2012) to be submitted
Page 27
pH and activity correction
27Chatterjee et al., (2012) to be submitted
Page 28
Geotherm correction
28Chatterjee et al., (2012) to be submitted
Page 29
Effect of DaAOM
29Chatterjee et al., J. Geophys. Res., (2011)
Page 30
Effect of DaPOC
30Chatterjee et al., J. Geophys. Res., (2011)
Page 31
2:1 concentration crossplot
31Chatterjee et al., J. Geophys. Res., (2011)
Page 32
Concentration crossplot: Site 1244
Da = 0.22; Cb,ext = 27
32Chatterjee et al., J. Geophys. Res., (2011)
Da = 1; Cb,ext = 50
Page 33
Concentration crossplot: Site KC151
33Chatterjee et al., J. Geophys. Res., (2011)
Page 34
Concentration crossplot: Site 1230
34Chatterjee et al., (2012) to be submitted
Page 35
Concentration crossplots CANNOT determine stoichiometry
Chatterjee et al., J. Geophys. Res., (2011); Chatterjee et al., (2012) to be submitted
Site 685/1230
Page 36
Flux crossplot: Site 1244
Chatterjee et al., J. Geophys. Res., (2011)36
Page 37
Flux crossplot: Site 1230
Chatterjee et al., (2012) to be submitted37
Page 38
1. Anaerobic Oxidation of Methane (AOM)
(1:1)
2. Organoclastic Sulfate Reduction (OSR)
(2:1)
Two potential causes for SMT
38
= Dissolved Inorganic Carbon (DIC) ~ Alkalinity
Page 39
∆ = change from seawater
AOM
SO42-, Alkalinity (mM)
Dep
th (
mbs
f)
∆ (
Alk
+C
a+M
g)
∆SO4
39
OSR
Site 1244
22 4 32 ( ) 2CH O s SO HCO HS H 2
4 4 3 2( )CH aq SO HCO HS H O
Kastner et al., Fire in the ice, (2008)
Arguments for OSR: Stoichiometry and d13C of DIC
Dep
th (
mbs
f)
d13CDIC (‰)
OSR (2:1); δ13CDIC ≈ -25‰ AOM (1:1); δ13CDIC ≈ -60‰
Page 40
Dep
th (
mbs
f)
∆ (
Alk
+C
a+M
g)
40
Site 1244
22 4 32 ( ) 2CH O s SO HCO HS H 2
4 4 3 2( )CH aq SO HCO HS H O
Dickens and Snyder., Fire in the ice, (2009)
Dep
th (
mbs
f)
Counterarguments for AOM and methanogenesis
Methanogenesis; δ13CDIC ≈ 10‰
Flux units mol/m2kyr
2 2 4 32 CH O H O CH HCO H
SO42-, Alkalinity (mM) ∆SO4
+10‰ (methanogenesis)
-60‰ (AOM)
Deep DIC flux is enriched in 13C
OSR (2:1); δ13CDIC ≈ -25‰ AOM (1:1); δ13CDIC ≈ -60‰
d13CDIC (‰)
Page 41
13C enriched DIC flux from depth impacts alkalinity and d13C of DIC at SMT
Chatterjee et al., J. Geophys. Res., (2011)
24 4 3 2( ) CH aq SO HCO HS H O
2 2 3 42 ( ) CH O s H O HCO CH H
AOM (δ13CDIC ≈ -60‰)
Methanogenesis (δ13CDIC ≈ 10‰)
OSR (δ13CDIC ≈ -25‰)2
2 4 32 ( ) 2CH O s SO HCO HS H
Page 42
42
OSR dominated systems
CH4 and SO42- DIC (or HCO3
-) Ca2+ δ13C in DIC
Page 43
BHSZ
Distinct zones of local fluid flux
43
Hig
h lo
cal
flu
id f
lux
Low local fluid flux
Seafloor
Low local fluid flux
Page 44
Local SMT depends on local fluid flux
SMT depth
Top of gas hydrate
Low flux in sediment High flux in fracture
Low flux in sediment High flux in fracture
BHSZ
PeLocal = - 29
PeLocal = - 0.85
No
rmal
ized
dep
th
Normalized concentration
BHSZ
<Sh>Local = 22%<Sh>Local = 6%
No
rmal
ized
dep
th
Hydrate and free gas
No
rmal
ized
dep
thN
orm
aliz
ed d
epth
Hydrate and free gas
Normalized concentration
44
Page 45
Generalized model to quantify amount and distribution of gas hydrates
45
Pe1<Sh>
Bhatnagar et al., Am. J. Sci., (2007)
Biogenic sources only Biogenic sources and external flux
Net fluid flux (Pe1) and org C input at seafloor
Hydrate saturation
Net fluid flux (Pe1 + Pe2)Hydrate
saturation
Page 46
Kinetic and equilibrium reaction model
Methanogenesis reaction:
AOM reaction at the SMT:
POC driven sulfate consumption above the SMT:
Calcite precipitation reaction:
46
Page 47
δ13C definition
The isotopic carbon composition (δ13C ) in any sample is defined
The isotope ratios usually reported in per mille, relative to an standard Pee-Dee-Belemnite (PDB) marine carbonate
47
Page 48
Methanogenesis reaction:
AOM reaction of biogenic methane at the SMT:
Organoclastic sulfate consumption:
Calcite precipitation reaction:
Reactions with corresponding δ13C values
48
Page 49
1-D organic carbon mass balance
Dimensionless mass balance
49
4
1
,
,
1(1 )
1 1(1 )
1
sed
POC s o lsw POC s
m SO m eqb
Pe Ut z
M cDDa S Da
D Mc
c
4
1 1
1(1 ) ( )( )
sed sed s
lsed w seP d
Sw
OsOC
vt z
S cM
Page 50
1-D methane mass balance
Dimensionless mass balance
50
1 (1 ) l
h g m h gg
h m g mhS S c S c S c
t
1 1
1 2
1 1
1
1
1
s sh h gh w h h mm h g m g
lPeU PeU
Pe Pe S c S c S ccz
4 4
4
,
,
1 1 1 2
lCH CH s o l lm
w sed w AOM m sPOC SO m eqb
M M ccS Da S Da
z z M Mc c
c
Page 51
1-D sulfate mass balance
Dimensionless mass balance
51
1
1 2
1
1
1 1 1
s hh w h s
ll l s s
w s wm
PeU cc Pe Pe S c
t z zS c
D z
DS
1 11
1 2l l ls
w AOM m s w sed POC sm
cD
S Da c Da cSD
Page 52
1-D DIC mass balance
Dimensionless mass balance
52
1
1 2
1 1
1
1
s hh w
l lb h bw
PeUc Pe Pe S cS c
t z
3 3
4
, ,
, ,
1 1 1 2
lHCO m eqb HCO s o l lb b
w sed w AOM m sm POC b o SO b o
M c M cDS Da S Da c c
D z z M c M c
c
3 3
4
, ,
, ,
1 1 1
(1 )HCO s o CaCOCa ols
w sed POC s wSO b o m b o
M c ccDS Da c S
c tM c D
Page 53
1-D calcium mass balance
Dimensionless mass balance
53
1
1 2
1 1
1
1sl lw C
hh w h Ca a
PeUPe Pe S cS c c
t z
31 1
l
CaCOCa Caw w
m
cD cS S
D z z t
Page 54
1-D δ13Cmethane mass balance
Dimensionless mass balance
54
4
131 (1 ) l
h g m h h m g g mh g
CHS S CS c c S ct
4
1 1 131 2
1 1
1 1
1 ls sh h g
h w h h m h g m g CHm
PeU PeUPe P c
ze S c S c S c C
4
4
4
1313
,
(1 1 2
)CHC
lm CH
w sedPOC
H meth
c MDa
z MCS
z
C
4
4
4
1,
,
31C
CH s o l lw AOM m s
SO mH
eqb
M cS Da
M cc c C
.
Page 55
1-D δ13CDIC mass balanceDimensionless mass balance
55
3 3
113 131 2
1 1
1
1
s hHCO h w h b HC
lb O
lwS c
PeUc C Pe Pe S c C
zt
3
3
3
1313
,,
,
(1 1 2
)HCOHCO me
lb HCO m eqbb
w sedm P
thOC b o
M cDS Da
D z z M c
c CC
4
3
3
3
4
4
13
13,
,
,
,
,
1
1 1
(1 )
HCO s o l lw AOM m s
SO b o
HCO s o lsw sed POC s
SO b o m
CH
HCO POC
M cS Da c c
M c
M c DS Da c
M c
C
CD
3 3,
,
13( 1 ) CaCOCa o
wb o
HCOccS
C
tc