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IODP
Expedition 311: |
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Cascadia Margin Gas Hydrates
Expedition
311 Scientific Party
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| Introduction |
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Figure
1. A) Location
of IODP Expedition 311 on the Cascadia margin. B) Bathymetry
and location of the different sites visited (bathimetry
courtesy of D. Kelley, J. Delaney, and D. Glickson,
University of Washington, and C. Barnes, C. Katnick,
NEPTUNE Canada, University of Victoria; funded by
the University of Washington
and the W.M. Keck Foundation).
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Figure 2. Seismic
section (line 89-08) showing the position of the
Expedition 311 sites across the accretionary front.
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Expedition
311 of the Integrated Ocean Drilling Program (IODP) investigated
the occurence and the formation of gas hydrate in the
accretionary prism of the northern Cascadia margin. The
five sites visited during the expedition defined a SW-NE
transect across a ~30km wide Bottom Simulating
Reflector (BSR) that runs parallel to the coast along
most of the continental slope (see
Figure 1).
From
Site U1326, at the SW tip of the accretionary prism, to
Site U1329, the shallowest site located at the landward
limit of the BSR, the transect was designed to sample the
complexity of the evolution of a gas hydrate system (see
Figure 2).
To
constrain the formation of gas hydrates in subduction zones,
Expedition 311 had an ambitious drilling program including
extensive pressure coring to recover gas hydrate at in
situ conditions. Because of the unstability of gas
hydrate at surface conditions, and of the strong response
of some logging tools such as electrical and acoustic logs
to the presence of gas hydrate, logging was a critical
component of the operations. The logging program consisted
of two phases - the first week of the expedition was dedicated
to Logging While Drilling (LWD), in order to identify intervals
likely to contain gas hydrate where pressure coring tools
should be deployed; the second phase consisted in wireline
logging following coring operations in order to complete
the geophysical characterization of the sites. A
complete overview of the expedition results and preliminary
conclusions is available in the Expedition
311 Preliminary Report.
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Logging Operations
Logging
While Drilling/ Measurements While Drilling (LWD/MWD) |
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| Figure 3a. Logging
While Drilling/Measurement While Drilling (LWD/MWD)
bottom hole assembly used during Expedition 311. |
Following
a strategy that was successfuly appliedduring ODP
Leg 204 on Hydrate Ridge, offshore Oregon, the detailed
planning of the coring operations was determined by the
LWD results. The presence and the distribution of gas hydrate
should be indicated by high resistivity values in the the
resistivity logs and images.
A
number of the LWD/MWD tools had been used during ODP Leg
204: Resistivity at Bit (RAB, GeoVISION); the Azimuthal
Density Neutron tool (adnVISION); the Nuclear Magnetic
Resonance (proVISION); and the Measurement While Drilling
(TeleScope, an update of the MWD tool used previously).
In addition to these tools, the LWD/MWD tool string used
during Expedition 311 included the SonicVISION, which had
been used in an earlier version during ODP Leg 196 in the
Nankai Trough, and the EcoScope, which had never been used
during ODP/IODP. With all the measurements provided by
the ADN, the EcoScope also provided several additional
resistivity measurements, elemental capture spectroscopy,
and the borehole annular pressure while drilling
(APWD). The complete bottom hole assembly is shown in Figure
3a.
Wireline logging
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| Figure
3b. Results Wireline
logging tool strings used during Expedition 311.
Some of the tools had to be recombined differently
because of difficult sea states.
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The
planned wireline logging program included two logging
runs for each site: the triple
combo followed by
the FMS/Sonic tool
string. The first run was to provide data similar to
some recorded by the LWD for correlations (resistivity,
density, neutron porosity, gamma ray), but was also to
provide a caliper log indicating the quality of the hole
for the subsequent run. The second run with the FMS/Sonic
was to provide high resolution electrical images, and
most importantly for the gas hydrate characterization,
an acoustic log. Because of difficult sea conditions,
the ship heave often exceeded the operating range of
the wireline heave compensation system (> 4m), and
after damaging a caliper arm early in the expedition,
the wireline program was limited in several holes to
tool strings devoid of protruding arms. Despite this
constraint, it was possible to acquire acoustic logs
at all sites, allowing further hydrate characterization
and seismic/well integration. In addition, two VSP were
acquired as planned in Sites U1327 and U1328. The tool
strings including all the tools originally planned are
shown in Figure 3b.
The
following table summarizes the variuos tool combinations
used during Expedtion 311.
Water
depths are measured in meters below rig floor. They were
identified during each tool run by a sharp increase in
measured natural radioactivity gamma ray when the gamma
ray tool crosses the seafloor.
Hole |
Water
depth
(mbrf |
Max.
dept
(mbsf) |
Tools
run |
| 1325A |
2203 |
350 |
GeoVISION/EcoScope/SonicVISION/TeleScope/ProVISION/adnVISION |
| 1325C |
2205 |
259
185 |
DIT/HNGS
DSI/SGT/TAP |
| U1326A |
1838 |
300 |
GeoVISION/EcoScope/SonicVISION/TeleScope/ProVISION/adnVISION |
| U1326D |
1838 |
300 |
DIT/DSI/SGT |
| U1327A |
1316 |
300 |
GeoVISION/EcoScope/SonicVISION/TeleScope/ProVISION/adnVISION |
| U1327D |
1314 |
295
276 |
DIT/APS/HLDS/HNGS/TAP
WST |
| U1327E |
1313 |
290 |
DIT/DSI/SGT |
| U1328A |
1278 |
300 |
GeoVISION/EcoScope/SonicVISION/TeleScope/ProVISION/adnVISION |
| U1328C |
1279 |
292
292
285 |
DIT/APS/HLDS/HNGS
FMS/DSI/GPIT/SGT
WST |
| U1329A |
956 |
220 |
GeoVISION/EcoScope/SonicVISION/TeleScope/ProVISION |
| U1329D |
956 |
10
195 |
DIT/APD/HLDS/HNGS/TAP
FMS/DSI/GPIT/SGT |
Gas monitoring
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| Figure
4. Summary
of the LWD/MWD data used to monitor for gas. None
of the anomalies observed in the pressure and waveform
coherence records indicate any significant amount
of free gas. |
When gas
was detected or expected in previous ODP cruises, the
lack of any blowout prevention system required a strict monitoring
of the gas composition in the cores. The monitoring procedure relied
on the analysis of headspace gas samples after each core was recovered
to decide whether it was safe to proceed with coring. Because the
LWD/MWD tools
were deployed prior to any coring, the monitoring for
gas was performed by watching carefully several measurements transmitted
in real time by the MWD tool: the annular pressure (APWD) and the
sonic waveform coherence. For this purpose, the SonicVISION was
configured to identify the wave propagating through the borehole
fluid. The possible occurence of gas should be indicated by a sharp
pressure decrease, possibly preceded by a pressure increase, and
by a loss of coherence in the sonic waveforms. The safety protocol
designed for expedition 311 required preventive actions for any
pressure anomaly exceeding 100 psi. Figure
4 shows a summary of all the APWD records, after subtraction of
the best fit linear trends to enhance the pressure anomalies,
and of the sonic waveforms coherence records. Overall, it shows that
no significant anomaly was ever detected, allowing to drill and core
each site to its target.
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| Data
and Results |
Overview
We
present here a summary of the logging data and some highlights
for each site visited. Because of hole instability in the
shallower sediments, wireline data are usually not recorded
in the upper ~60 m. In addition, the very short time elapsed
after the initial formation penetration makes LWD measurements
much less affected by borehole degradation than wireline
logs. Therefore, we use the LWD measurements preferably
for data that were recorded by both sets of tools, such
as density and porosity. For still unexplained reasons,
the LWD gamma ray readings were generally higher than the
wireline data and we display both curves for completeness.
Despite
the proximity of the holes in any site, the considerable
heterogeneity in lithology and gas hydrate distribution
results in apparent discrepancies between the LWD and wireline
data. Some of these discrepancies are discussed in greater
details for individual sites. Finally, we have added
to each figure the compilation of the infrared images (IR)
recorded in each site. These images, recorded immediately
after core recovery, were the primary means onboard to
identify and isolate gas hydrate samples, which were associated
with cold anomalies due to the endothermic nature of gas
hydrate dissociation.
The
data are presented from the most seaward site (U1326),
following the transect upward the deformation front to
the most landward site (U1329) (see Figures 1 and 2 for
locations).
Water saturations
Preliminary
estimates of the amounts of gas hydrate are also given
in these figures, expressed as water saturation. Since
the primary conductor of electric current in the formation
is the pore water, its substitution by electrically insulating
gas hydrate generates resistivity anomalies that can
be used to estimate the fraction of the pore space occupied
by water, or water saturation (Sw). Archie (1942) defined
a relationship to estimate water saturation from resistivity
and porosity logs in traditional hydrocarbon reservoirs,
and Collett (1998) has shown that the same relationship
can be used in the presence of gas hydrate. The gas hydrate
saturation (Sh) can be assumed to be the complement of
Sw (i.e. Sh = 1 - Sw) within the gas hydrate stability
field. (see the Initial
reports of ODP Leg 204 for
a complete description). For each site, a value for the
cementation exponent (m) in the Archie relationship was
estimated from Archiees equation using the resistivity
and porosity logs and salinity values measured on core
samples, with the Archie coefficient (a) and the saturation
exponent (n) were assumed to be 1 and 2, respectively.
Site U1326
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| Figure 5. Summary
of the logging data recorded at Site U1326. In the
resistivity column, the Deep Induction and SFLU (Spherically
Focussed Log Unfiltered) curves were recorded with
the wireline tools, the others with LWD. The last
column on the right is a compilation of the Infra
Red (IR) images recorded on the core liner of the
recovered sections to detect gas hydrate. |
Located at the SW end of the Expedition 311 transect, Site U1326 was drilled Êon an uplifted ridge of accreted sediments. This site was the last one visited for coring and wireline logging operations. Because of time constraints and concerns about an upcoming storm, wireline operations were limited to a single run with a tool string composed of the Gamma Ray/Resistivity/Sonic tools (SGT/DIT/DSI). A summary of the data recorded in this site is shown in Figure
5. The most significant feature in the LWD data is a ~20m interval above 100 mbsf, characterized by bright RAB images and high resistivity values (Phase shift 16 in and Button deep average). The wireline resistivity curves (deep induction and SFLU) and the compressional velocity log (Vp) almost parallel each other in this interval, following a pattern identical to the LWD resistivity, but ~10 m shallower. The high resistivity and Vp values, while density or porosity do not change, suggest significant amounts of gas hydrate, but the offset between the two holes indicates significant lateral heterogeneity. Indeed, the water saturation is possibly as low as 40% between 80 and 90 mbsf in Hole U1326A, but steeply dipping features identified in the RAB images show that gas hydrate is present in beds dipping by as much as 85¡, indicating the sediment deformation in this ridge at the tip of the accretionary prism.
Below ~100 mbsf, the uniformly low resistivity values suggest that only little, if any, gas hydrate is present, except for Êa ~2m interval above 260 mbsf in Hole U1326A. This interval coincides with a loss in sonic waveform coherence in the LWD monitoring data (see Figure 4), that could be associated with free gas.
Site U1325
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Figure
6. ummary
of the logging data recorded at Site U1325. In the
resistivity column, the Deep Induction and SFLU (Spherically
Focussed Log Unfiltered) curves were recorded with
the wireline tools, the others with LWD. The last
column on the right is a compilation of the Infra
Red (IR) images recorded on the core liner of the
recovered sections to detect gas hydrate.
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Figure 7. Summary
of the logging data recorded at Site U1327. In the
resistivity column, the Deep Induction and SFLU (Spherically
Focussed Log Unfiltered) curves were recorded with
the wireline tools, the others with LWD. The comparison
of the Infra Red (IR) images from Hole U1327C and
U1327D with the resistivity logs acquired in Holes
U1327A, U1327D and U1327E illustrates the strong
lateral heterogeneity of gas hydrate distribution
at this site.
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Site
U1325 was drilled in a slope basin located seaward of of
the deformation front, behind the ridge of accreted sediments
drilled in U1326 (see Figure 1).
This site was the first one visited by Expedition 311,
and a strict application of the safety protocol guidelines
resulted in excessive pumping rate and poor data in the
first 20 m of Hole U1325A. Pumping rates were subsequently
reduced at the time of spud-in for the following sites,
while maintaining the capability to monitor the safety
of the drilling operations. Because of difficult sea state,
wireline operations were limited to two strings without
arms (HNGS/DIT and SGT/DSI). Rapidly deteriorating hole
conditions prevented the recording of acoustic data below
175 mbsf. A summary of the logging data is shown
in Figure 6.
The
highly variable resistivity log and the succession of dark
and bright layers in the RAB image show that gas hydrate
may be present in a series of thin layers alternating with
gas-hydrate-free sediments. The most distinct occurrences,
between 175 and 240 mbsf, result in water saturation values
as low as 40 %. Unfortunately, no acoustic data could be
recorded in this interval. However, because of the location
of this site in an undisturbed basin, the wireline and
LWD logs agree very well over the entire interval logged,
and tend to confirm the finely layered distribution of
gas hydrate.
Site U1327
Site U1327 was drilled near ODP Site 889, where the largest amounts
of gas hydrate had been identified during ODP Leg 146.
While LWD operations proceeded without any trouble, the
wireline logging program encountered several drawbacks
due to bad sea conditions. At the end of the first triple
combo run in Hole U1327D, during which heave was measured
at ~3.5 m, the HLDS caliper arm was damaged, and the subsequent
Vertical Seismic Profile (VSP) also resulted in damage
to the WST tool. Despite these difficulties, the data recorded
were of good quality, but it was necessary to drill a new
hole, U1327E, in order to acquire an acoustic log, crucial
for the complete characterization of the gas hydrate distribution.
The arm-free tool string used in Hole U1327E did not meet
any further obstacles. A summary of the data collected
in Site U1327 is shown in Figure
7.
As
in Hole U1326A, the most striking feature in the data recorded
in Hole U1327A is a high resistivity 20-meter thick interval,
showing as a bright lawyer between 120 and 140 mbsf in
the RAB image, which was interpreted as a significant occurence
of gas hydrate. The water saturation estimate suggest that
gas hydrate occupies up to 60% of the pore space in this
interval. However, the different pressure coring tools
deployed in the same interval in adjacent Hole U1327C failed
to recover any gas hydrate, and the IR images recorded
on the cores from Hole U1327C suggested that the main occurrence
of gas hydrate in this hole was ~20 m deeper than
in Hole U1327A. The subsequent IR images in Hole U1327D
and wireline resistivity logs in Holes U1327D and U1327E
showed more heterogeneity in gas hydrate distribution,
the only agreement being between the log data and IR images
recorded in the same hole (U1327D). Overall, the combination
of the logs and IR images suggest that some amounts of
gas hydrate is present in all the holes between 80 and
240 mbsf, but this distribution seems to be different in
every hole. VSP The
original plan for the VSP was to acquire a high resolution
survey with only 5 meters between each station over the
entire hole. Difficult sea conditions and an enlarged hole
did not allow us to record more than 16 stations in Hole
U1327D, none shallower than 182 mbsf. The results of the
VSP in Hole U1327D are shown in Figure
8.
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Figure
8. VSP
results in Hole U1327D. A) Stacked
traces with original automated travel time picks. B) Corrected
picks. The two lines indicate least square linear
fit to the data above and below the BSR. C) Comparison
of the VSP inversion results with the sonic log
recorded in Hole U1327E and the inversion performed
on the VSP data from Hole 889B.
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| Figure 9. Summary
of the logging data recorded at Site U1328. In the
resistivity column, the Deep Induction and SFLU (Spherically
Focussed Log Unfiltered) curves were recorded with
the wireline tools, the others with LWD. The last
column on the right is a compilation of the Infra
Red (IR) images recorded on the core liner of the
recovered sections to detect gas hydrate. |
The
primary purpose of the VSP was to define an accurate
time vs. depth relationship to tie precisely the well
data with the seismic line crossing the site. The arrival
times picked in the stacked traces in Figure
8a, after correction for firing delays and tool position,
define two distinct velocity trends above and below ~250
mbsf in Figure 8b. The contrast
between the higher velocity above and lower velocity
below, presumably corresponding to the occurrence of
gas hydrate and free gas, respectively, causes the BSR
observed in this site. Figure 8c shows
the results of a Bayesian
inversion of these data (see Malinverno and Briggs, 2004),
giving a more complete picture of the velocity changes
at the origin of the BSR. The
comparison with logging and VSP data in nearby Holes
U1327E and 889B shows that the velocity transition zone
responsible for the BSR occurs at different depths despite
the proximity of the holes (U1327D and U1327E are ~15
meters apart. Hole 889B is ~500m to the west).
Site U1328
Located slightly offset from
the expedition transect (see Figure
1), Site U1328 was drilled to investigate an active
cold vent system, where focused fluid flow is feeding
the formation of massive near-seafloor gas hydrate and
the growth of chemosynthetic communities. The vents are
identified by blank zones in the seismic data, whose
origin was one of the questions to answer at this site.
The sea remained calm during the operations, allowing
the full wireline program to be completed in Hole U1328C,
including the triple combo, the FMS/Sonic and the VSP.
A summary of the data collected in Site U1328 is shown
in Figure
9.
The
LWD data recorded in Hole U1328A show the highest concentrations
of gas hydrate encountered during Expedition 311. The water
saturation derived from the LWD resistivity drops below
20% in intervals between 5 and 20 mbsf, indicating that
gas hydrate could occupy more than 80% of the pore space.
The RAB images show that some of the highest concentrations
occur within steeply dipping fractures that act as conduits
feeding the near surface gas hydrate accumulation. Gas
hydrate concentrations then decrease with depth and gas
hydrate is only sparsely present below 50 mbsf, in particular
within a thin hydrate-filled steep fracture at ~95 mbsf. VSP A
clement weather and good hole conditions allowed to record
stations every 5 meters over most of the open interval
in Hole U1328C, with 35 stations recorded successfully
between 286 and 106 mbsf. Apparent interference from the
pipe and/or the wireline prevented from recording any meaningful
signal at shallower depths. The results are shown in Figure
10.
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Figure
10. VSP
results in Hole U1328C.
A) Stacked
traces with original automated picks. B) Corrected
picks. The red line indicates the least square
linear fit for the entire data set. C) Comparison
of the Bayesian inversion results with the two
passes of the sonic log recorded in the same
hole.
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Figure 11. Summary
of the logging data recorded at Site U1329. In
the resistivity column, the Deep Induction and
SFLU (Spherically Focussed Log Unfiltered) curves
were recorded with the wireline tools, the others
with LWD. The last column on the right is a compilation
of the Infra Red (IR) images recorded on the core
liner of the recovered sections to detect gas hydrate.
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The
5 meters spacing provided a high resolution image of the
velocity structure in this seismically 'blank' area, and
both the time vs. depth relationship (Figure
10b) and the results of the Bayesian inversion (Fig
10c) show low velocity values and a very low variability
that are consistent with the low seismic reflectivity.
The comparison with the Vp logs recorded during two passes
in the same hole confirm these results.
Site U1329
At the shallowest water depth encountered during our operations,
Site U1329 marks the landward end of the Expedition 311
transect, at the NE edge of the regional gas hydrate
occurrence (see Figure 1). While
it was the last site drilled with the LWD/MWD tools, it
was also the first one for coring and wireline logging
operations. All scheduled logging operations proceeded
without incidents, but the wireline logs in Hole U1329D
revealed an enlarged hole over most of the open interval,
impairing the quality of the data. A summary of the data
collected in Site U1329 is shown in Figure
11.
As
expected from its location at the limit of the regional
area of gas hydrate occurrence, the logging data recorded
in Site U1329 reveal only little, if any, gas hydrate in
the sediments penetrated at this site. The data are
characterized by a gradual increase in density and resistivity
with depth, with a marked sharpening of the trend at 165
mbsf and then at 185 mbsf. The high resistivity below this
depth, illustrated by the bright RAB images, and combined
with high density and low porosity, is the indication of increasingly
consolidated sediments, possibly composed of thick debris
flows. Unfortunately, the low core recovery in this extremely
indurated formation did not allow us to fully characterize
the nature of this interval.
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| Summary |
These
results are still preliminary, and much remains to be learned
from the data recorded during Expedition 311, but a few
conclusions can be already drawn at this stage:
(1)
As part of an adequate protocol, the LWD/MWD tools provide
a safe and reliable way to monitor the occurrence of free
gas, flows, or other drilling hazards.
(2)
The use of LWD tools to help identify targets for pressure
coring tools was again a valuable strategy to optimize
the recovery of gas hydrate. However, this strategy has
to be considered carefully in highly heterogenous settings
such as accretionary complexes.
(3)
The combined interpretation of the LWD and wireline data
along the Expedition 311 transect, particularly of the
resistivity and acoustic data, provides a rich picture
of the complex distribution of gas hydrate across the Cascadia
margin. These preliminary results suggest in particular
that, except for short 'anomalous' intervals, gas hydrate
concentrations are generally lower than previously estimated
from ODP Leg 146. One of the primary objectives of the
upcoming work will be to calibrate the different methods
to quantify accurately the amounts of gas hydrate identified
from the logs.
References: Collett,
T.S., Well log evaluation of gas hydrate saturations, Trans.
SPWLA 39th Logging Symposium, paper MM 1998.Malinverno,
A. and Briggs, V.A., 2004. Expanded uncertainty quantification
in inverse problems: Hierarchical Bayes and empirical Bayes.
Geophysics, 69: 1005-1016.
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Gilles Guerin: Logging Staff Scientist, Borehole
Research Group, Lamont-Doherty Earth Observatory of Columbia
University, PO Box 1000, 61 Route 9W, Palisades NY 10964,
USA.email: uerin@ldeo.columbia.edu
Alberto Malinverno: Logging Staff Scientist, Borehole
Research Group, Lamont-Doherty Earth Observatory of Columbia
University, PO Box 1000, 61 Route 9W, Palisades NY 10964,
USA.email: alberto@ldeo.columbia.edu
Greg Myers: LWD LoggingScientist, Borehole
Research Group, Lamont-Doherty Earth Observatory of Columbia
University, PO Box 1000, 61 Route 9W, Palisades NY 10964,
USA.email: gmyers@ldeo.columbia.edu
Peter Jackson : Logging
Scientist, Geophysics and Marine, British Geological
Survey, Kingsley Dunham Centre, Keyworth, Nottingham
NG12 5GG, UK email: pdj@bgs.ac.uk
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Additional Leg-related
publications:
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