Link Between Gas Hydrates and Conventional Petroleum System?

Double-BSRs on the Hikurangi Margin: Link
Between Gas Hydrates and Conventional
Petroleum System?
Ingo Pecher1,2, Gareth Crutchley2, Joshu Mountjoy3, Andrew
Gorman4, Douglas Fraser4, Karsten Kroeger2, Stuart Henrys2
1University
of Auckland, [email protected]
2GNS Science, Lower Hutt, New Zealand
3NIWA, Wellington, New Zealand
4University of Otago, Dunedin, New Zealand
Acknowledgments
• Co-authors
• TAN1114: Captain & Crew R/V Tangaroa; OS2020
• MBIE-commissioned PEG09 survey
• Funding: MBIE, internal GNS, NIWA funds
Methane Gas Hydrate Cage
• Solid form of water, similar
to ice, with gas in its
molecular cavities
• In nature, the most
common hydrate-forming
gas is methane
Modified from Schlumberger (SEED)
http://www.seed.slb.com/en/scictr/watch/gashydrates/index.htm (16 Oct 06)
Phase boundary in nature
gas + water
gas hydrate
seafloor
base of gas
hydrate
stability
gas bubbles
Bottom Simulating Reflections
BSR
Bottom simulating
reflections (BSRs)
are caused by free
gas at the base of
the gas hydrate
stability zone
BSR on Hikurangi
Margin, New
Zealand
Double-BSRs
Double BSR in Tuaheni Basin. Hikurangi Margin, New Zealand
Paleo-BSRs
• Paleo-BSR (here, deeper
BSR) may reflect past
environmental
conditions
• Gas migration to new
level of base of gas
hydrate stability takes
time, some still “stuck”
at paleo-level
Paleo BSR on Hydrate Ridge, Oregon (Bangs et al., 2008)
Thermogenic Gas?
0.5% propane
1% propane
100% methane
(in fresh water)
~3 °C = 120 m for
thermal gradient of
0.025 °C /m
°
• Addition of some higherorder hydrocarbon gases
(e.g., propane) leads to
formation of different
hydrate structure (Structure
II) with pronounced increase
in gas hydrate stability
• Deeper BSR with
thermogenic gas, shallower
pure methane (fractionation
of gases)?
Thermogenic Gas?
Double-BSR in South China Sea, deeper
BSR potentially linked to thermogenic gas
(Wu et al., 2005)
Gas hydrate mound in Gulf of Mexico
(GC 185), formed from thermogenic gas
(Sassen et al., 2001)
Links Between Gas Hydrate and
Conventional Petroleum Systems?
Potential links to conventional petroleum system:
• Source of gas: Do gas hydrates form from gas leaking from
conventional gas reservoirs?
• Some early studies suggested BSRs may indicate existence of
deeper petroleum system but in most places, gas hydrates found
to be formed from biogenic gas as part of an independent
“petroleum” system
• Double-BSRs from thermogenic gas hydrates would suggest link
to deeper petroleum system
Australian
Plate
Pacific
Plate
Tuaheni Basin
Gas Hydrates on
the Hikurangi
Margin
• BSRs widespread
• A number of geochemical
surveys indicate biogenic
origin of methane in gas
hydrates – similar to most
locations worldwide
• No link to conventional
petroleum system?
Pegasus Basin: Work in Progress
Polarity: Probably flat-spot from
gas-water contact
Could be artifact (bubble)
Study Area
Tuaheni Slides
After Mountjoy et al.
(in press)
Line 8
Line 7a
Double-BSRs
08
2 km
TAN1114-07A
0.25 s
Both BSRs negative polarity compared to seafloor
Double-BSRs
07A
2 km
TAN1114-08
0.25 s
Two-way traveltime (TWT) seafloor v. TWT-difference BSR-seafloor:
• Depth of BSR beneath seafloor increases with water depth
Ritchie Banks (undisturbed)
Line 7a
Predicted: Methane, 1% propane
0.5% propane
Line 8
East coast gas
mix (no propane;
Giggenbach et al., 1993;
Pecher et al., 2005)
• Shallower BSRs are “normal”
• Convergence Line 7a, not 8
• Deeper BSRs can easily be
explained by thermogenic gas
Discussion: Scenarios for double-BSRs
Key observations (in addition to presence of double-BSRs)
• Very localized
• Convergence of BSRs along Line 8 but not Line 7a
Change of environmental conditions (paleo-BSRs)
Or: Addition of thermogenic gases
Change of Environmental Conditions
Margin-wide process unlikely
(localized double-BSR)
Uplift
dissociating hydrate? gas bubbles
slow to migrate
New BSR
Paleo-BSR
uplift
hydrate
BSR
BGHS
gas
• Sediment deposition
(slump), thicker slump
mass where gap between
BSRs is larger
• Related to propagation of
thermal disturbance into
seafloor?
Thermogenic gases
hydrate from pure methane
BSR-1
BSR-2
thermogenic
gas
• BSR-2 is gas trapped
beneath hydrates from
decrease of
methane + higher-order
thermogenic fraction
HCs (probably Structure-II)
• Some methane migrates
upward to form BSR-1 at
phase boundary for pure
methane hydrate
hydrate from
thermogenic gas mix
biogenic methane
admixing of thermogenic gas
Conclusions
P-Cable 3-D seismic
• Double-BSRs present on Hikurangi
Margin
• Distinction between paleo-BSRs and
double BSRs from thermogenic gas not
straight forward
• At selected locations, studies of gas
hydrate systems may offer insights into
underlying (conventional) petroleum
system (e.g., gas chemistry)
• Work in progress
MeBo deep sampling

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