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O R I G I N A L R E S E A R C H P A P E R
Evidence of gas hydrate accumulation and its resource estimationin Andaman deep water basin from seismic and well log data
Anand Prakash B. G. Samanta N. P. Singh
Received: 9 February 2012/ Accepted: 31 October 2012/ Published online: 10 November 2012
Springer Science+Business Media Dordrecht 2012
Abstract 2D and 3D seismic reflection and well log data
from Andaman deep water basin are analyzed to investi-gate geophysical evidence related to gas hydrate accumu-
lation and saturation. Analysis of seismic data reveals the
presence of a bottom simulating reflector (BSR) in the area
showing all the characteristics of a classical BSR associ-
ated with gas hydrate accumulation. Double BSRs are also
observed on some seismic sections of area (Area B) that
suggest substantial changes in pressuretemperature (PT)
conditions in the past. The manifestation of changes in PT
conditions can also be marked by the varying gas hydrate
stability zone thickness (200650 m) in the area. The 3D
seismic data of Area B located in the ponded fill, west of
Alcock Rise has been pre-stack depth migrated. A signifi-
cant velocity inversion across the BSR (1,9501,650 m/s)
has been observed on the velocity model obtained from
pre-stack depth migration. The areas with low velocity of
the order of 1,450 m/s below the BSR and high amplitudes
indicate presence of dissociated or free gas beneath the
hydrate layer. The amplitude variation with offset analysis
of BSR depicts increase in amplitude with offset, a similar
trend as observed for the BSR associated with the gas
hydrate accumulations. The presence of gas hydrate shown
by logging results from a drilled well for hydrocarbon
exploration in Area B, where gas hydrate deposit was
predicted from seismic evidence, validate our findings. Thebase of the hydrate layer derived from the resistivity and
acoustic transit-time logs is in agreement with the depth of
hydrate layer interpreted from the pre-stack depth migrated
seismic section. The resistivity and acoustic transit-time
logs indicate 30-m-thick hydrate layer at the depth interval
of 1,8651,895 m with 30 % hydrate saturation. The total
hydrate bound gas in Area B is estimated to be
1.8 9 1010 m3, which is comparable (by volume) to the
reserves in major conventional gas fields.
Keywords Gas hydrate Bottom simulating reflector
Hydrate saturation estimation Resource assessmentAndaman deepwater basin
Introduction
Natural gas hydrates may have the potential of becoming
an alternate energy resource as a result of the huge deposits
estimated worldwide (Kvenvolden 1993a; Collett 2002;
Makogon et al. 2007). The potential reserves of hydrated
gas are estimated to be of the order of 1.5 9 1016 m3, and a
commercial production of just 15 % of this gas reserves
could provide the world with energy for over 200 years
from now at the current level of energy consumption. In
recent years, gas hydrates have received much attention
because of their widespread occurrence and associated
potential importance as an energy resource, sea floor sta-
bility related geo-hazard, and possible impact on global
climate (Paul et al.1991; Kvenvolden1993b). A number of
publications describing the origin, significance, occurrence
and formation/genesis of these deposits both in permafrost
regions and in marine sediments on continental slopes are
A. Prakash B. G. SamantaRegional Computer Centre, Oil and Natural Gas Corporation,
Kolkata 700088, India
e-mail: [email protected]
B. G. Samanta
e-mail: [email protected]
N. P. Singh (&)
Department of Geophysics, Faculty of Sciences,
Banaras Hindu University, Varanasi 221005, UP, India
e-mail: [email protected]
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Mar Geophys Res (2013) 34:116
DOI 10.1007/s11001-012-9163-3
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available in the literature (e.g., Kvenvolden 1993a; Hol-
brook et al. 1996; Sloan 1997; Ginsburg and Soloviev
1998; Collett et al. 1999; Collett and Ladd 2000; Milkov
and Sassen 2001; Collett 2002; Milkov 2004; Makogon
et al.2007; Economides and Wood2009; Makogon2010).
Presently, many countries have national programs for
the research and production of natural gas from gas hydrate
deposits, and as a result of which over 220 gas hydratedeposits have been discovered, more than a hundred wells
drilled, and kilometers of hydrated cores have been studied
(Makogon et al. 2007). In Japan, the methane hydrate
programme is in an advanced stage to carry out initial
production testing in the deep water Nankai Trough (the
program is called MH21; Tsuji et al.2009). Canada with its
consortium of partners has completed several drilling
programs to produce gas from the Mackenzie Delta (Dal-
limore et al.1999). Korea (Park et al. 2008), China (Zhang
et al.2007; Wang et al.2010) and Malaysia (Hadley et al.
2008) have also launched major deep-water hydrate dril-
ling expeditions successfully. Beginning in 1996, theNational Gas Hydrate Programme (NGHP) initiated and
funded by Ministry of Petroleum and Natural Gas, Gov-
ernment of India, has been documenting gas hydrate
reserves in offshore India by collecting geophysical, geo-
logical, geochemical and microbiological data. Further, the
drilling by JOIDES Resolution drill ship under NGHP
Expedition- 01 in the Krishna Godavari (KG), Andaman
and Mahanadi Basins has confirmed the presence of gas
hydrate accumulation (Collett et al. 2008a,b).
Seismic surveys (conventional 2D/3D survey, ocean
bottom seismic, vertical seismic profiling, and cross-well
seismic and multi-component), well logging, and con-
trolled source seismic surveys are the commonly used
geophysical techniques for identification and evaluation of
gas hydrate deposits. Most of the gas hydrates, worldwide
have been inferred from the detection of a bottom simu-
lating reflector (BSR) and associated Gas Hydrate Stability
Zone (GHSZ) thickness map (Shipley et al. 1979; Hynd-
man and Spence1992; Yuan et al. 1996; Sain et al. 2000;
Dewangan and Ramprasad2007; Riedel et al. 2010). The
BSR is recognized based on its characteristic features such
as; (a) mimicking the shape of sea floor because the BSR
follows isotherms, which are nearly parallel to the mor-
phology of sea floor, (b) cutting across the underlying/
overlying dipping strata and (c) exhibiting large amplitude
but opposite polarity to that of the seafloor reflections
(Brooks et al.1986). The BSR is the interface between gas
hydrate-bearing sediments above and free-gas saturated
sediments below the interface and is often associated with
the base of the gas hydrate stability field (Kvenvolden
1993b). The BSR may not be continuous, indicating an
upward gradation between a hydrate layer above and a free
gas layer below the BSR. The Gulf of Mexico, Blake
Ridge, Cascadia Margin, Mackenzie Delta and Nankai
Trough are some of the best known examples (Holbrook
et al.1996; Dallimore et al.1999; Wood and Ruppel2000;
Hyndman et al. 2001; Ashi et al. 2002; Holbrook et al.
2002; Hornbach et al.2008). In addition to the BSR, other
geophysical anomalies, such as pockmarks, gas up-thrust
zone, vents, and blanking zones are also prominent indi-
cators of gas hydrate accumulation. Gas hydrate stabilityzones thickness maps have been prepared on the basis of
available bathymetry, heat flow, seabed temperature and
geothermal gradient data within the Exclusive Economic
Zone (EEZ) of India (Chandra et al. 1998: Rastogi et al.
1999; Sethi et al. 2004; Ramana et al. 2007; Rajesh et al.
2010; Shankar et al. 2010).
Gas hydrates show relatively high acoustic velocity and
electrical resistivity values compared to unconsolidated,
water-saturated sediments. Thus, gas hydrate-bearing sed-
iments are usually characterized by increased values of
resistivity and velocity from well logs. Normally, Archies
equation (1942) is used for estimating the saturation ofhydrocarbon using resistivity data. The equation has been
applied to calculate gas hydrate saturation (Hyndman et al.
1999; Collett and Ladd 2000; Mrozewski et al. 2009). In
addition, gas hydrate saturations have also been estimated
from the modified Biot-Gassman theory by Lee (2000) and
three phase Biot-type equation using acoustic log data (Lee
2000; Lee and Collett 2006). Recently, dielectric logging
has also been used to estimate high-resolution in situ
hydrate saturation (Sun and Goldberg 2005). Lee and
Collett (2009) have suggested that gas hydrate saturations
calculated from fractured reservoir could be overestimated
as a result of the anisotropic nature of the reservoir caused
by the presence of fractures.
Geophysical, geochemical and microbiological proxies
observed in the east coast of India have indicated the pres-
ence of gas hydrate deposits in Krishna Godavari, Cauvery,
Mahanadi and Andaman Basins (Ramana et al.2006,2007,
2009; Riedel et al. 2008; Satyavani et al. 2008; Prakash et al.
2010a,b; Shankar et al.2010; Nandi and Chaudhury2011;
Sain2011). Satyavani et al. (2008) conducted seismic attri-
bute studies in order to search for gas hydrates in the And-
aman offshore and have indicated the presence of free gas
accumulation below the BSR on the basis of seismic attri-
butes, namely reflection strength and instantaneous fre-
quencies. In the present study, 2D and 3D seismic reflection
profiles, as well as bore-hole data from the Andaman deep-
water basin (Fig.1) are analyzed to investigate the evidence
for gas hydrate accumulation and its saturation and reserve
estimation. AVO analysis, pre-stack depth migration, and
velocity inversion of seismic data have been carried out.
Model-based and grid-based tomography of the seismic data
provided detailed velocity information, which indicates free
gas below the hydrate layer. The logging results from a
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drilled well in Area B have been used to estimate hydrate
saturation and reserve estimation in the area. Some seismic
sections also show double BSRs, which indicate changes in
thePT conditionsof thearea in past. DoubleBSRs have also
been reported in Nankai Trough (Golmshtok et al. 2006;
Foucher et al.2002), and in western Norway (Posewang and
Mienert1996).
Geologic setting
The Andaman Basin is the southeastern part of the Bay ofBengal around the Andaman-Nicobar chain of islands
between 6 and 14N latitude and 9194E longitude and
covers an area of more than 47,000 km2. The basin has
more than 7.5 km thick succession of marine sediments
from Cretaceous to recent (source: www.dghindia.org/
SedimentaryBasins.aspx). The basin extends about
1,200 km in the northsouth direction from Myanmar in
the north to Sumatra in the south, and 650 km in the east
west direction from Malay Peninsula in the east and Java
trench in the west. The morphology and structure of the
Andaman Islands suggest that they are an island arc
developed by subduction of Indian plate beneath the
southeast Asian plate since the Late Cretaceous (Curray
et al. 1979; Curray2005).
The regional geotectonic units in the basin area from
east to west are back arc basin, volcanic arc, fore-arc, and
trench associated with converging plate boundaries. Majortectonic eventssubduction/oblique subduction, magmatic
intrusion and back-arc spreading led to the development of
various sub-basins; fore-deep/trench, fore-arc, ponded fill,
inter-arc, and back-arc basins. During the Late Cretaceous
under-thrusting of the oceanic plate caused accretion of
overlying Bengal Fan turbidites and shallow marine sedi-
ments and part of Burmese crust as north south trending
prism by the process of reverse faulting, thrusting and
folding (Curray 2005). The study areas (A and B) corre-
spond to the deepwater part of the fore-arc sub-basin of
Andaman basin and are shown in Fig. 1 on the regional
tectonic map of Andaman basin.
Data and methods
The study area in the Andaman deepwater basin is covered
by 2D and 3D seismic surveys. Area A is covered by
162-fold 2D seismic survey with Common Depth Point
(CDP) interval 12.5 m while Area B is covered by 62-fold
3D seismic survey with a grid of 12.5 9 25 m.
Standard as well as special 3D seismic data processing
scheme, such as pre-stack time and depth migration, very
close grid migration velocity analysis, amplitude, phase
and frequency attributes were performed. Coherence
inversion, model-based and grid-based tomography of 3D
seismic data has been carried out to obtain the best possible
depth image as well as interval velocity-depth model. AVO
curves for the BSRs are generated to see if there is any
correlation between the presence of gas below the hydrate
layer and amplitude increase with offset. Table1 shows the
processing scheme and parameters for 2D/3D data pro-
cessing and pre-stack depth migration processing. Analysis
of seismic data reveals BSR like reflection events on the
seismic sections. To study the characteristics of observed
BSR-like features on seismic sections, special processing
efforts were made to highlight the BSR. Detailed velocity
analyses on pre-stack time and pre-stack depth migration
gathers have been carried out to obtain the best estimate of
root mean square (RMS) and interval velocity fields.
Coherency inversion of seismic data was used to derive the
interval velocity model for the 3D data and thereafter
model-based tomography is used to update the velocity
field. However, some of reflection events between the
mapped horizons were not flat. As a result, residual depth
Fig. 1 Tectonic map of Andaman Basin showing study areas.Stars
indicate drilled well locations. Green dotindicates the location of gas
hydrate accumulation confirmed under NGHP-01
Mar Geophys Res (2013) 34:116 3
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move-out analysis in vertical mode in a grid of
250 9 250 m followed by grid-based tomography tech-niques was implemented to obtain the final interval
velocity model in the depth domain. The resulting final
velocity model was used for migration of data in the depth
domain and it was observed that the reflection events were
now flat verifying that the velocity model was optimum.
The amplitude versus offset analysis of pre-stack time and
pre-stack depth migration gathers at representative loca-
tions was carried out to investigate the nature of the BSR
on seismic sections from the Andaman deepwater basin.
Results and discussion
Seismic sections from the Andaman area (Fig. 2a, b) show
BSRs at 200800 ms below the sea bottom. These BSR
events are distinct and have characteristic features of a
classical BSR, such as mimicking the sea floor, polarity
reversal, cross-cutting the lithological boundaries, and
blanking above and below the BSR. There is an abundance
of BSRs seen in the seismic sections within the deep water
of Andaman basin, at some places as shallow as 200 ms
below the seafloor, while at other places it is quite deep, at
about 800 ms below the seafloor. The non-systematic dis-tribution of BSRs at different depth horizons reflects the
varying PT regime at different locations in the area. The
variation in BSR depths is significant and may be attributed
to the high sedimentation and erosion of ridges, which
is still continuing in the area, and thereby suggests a
dynamic hydrate system in the Andaman deep water areas.
A dynamic hydrate system with significantly varying base
of the hydrate stability has also been reported by Hornbach
et al. (2008) based on the 3D seismic data analysis in the
Blake Ridge hydrate province. Gas hydrate stability zone
thickness in the area has been calculated to be of the order
of 200650 m, which is in agreement with those proposedby Rastogi et al. (1999), computed on the basis of geo-
thermal gradient, seabed temperature and bathymetry data.
The BSR is discontinuous with varying amplitudes, and at
several places very high amplitudes are observed (Fig. 2a).
The reflection strength may be attributed to the saturation
of gas hydrate. In some places, the BSR is observed at very
shallow depths *200 m below the sea floor. This is as a
result of high thermal gradient in the area. The thermal
gradient derived from the well data is of the order of
Table 1 Seismic data processing scheme and parameter
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is estimated to be at 460 m depth below the sea floor in this
part of the basin, which is more than 200 km away from
the ocean floor spreading center. In Fig. 3, the estimated
gas hydrate stability zone thickness is calculated graphi-
cally by superimposing hydrothermal and geothermal gra-
dients (52 C/km) obtained from the well data on the
average gas hydrate stability curve crystallizing from
87.7 % methane and rest C2?gases (Sloan1998). We alsoassumed averaged hydrostatic pore-pressure gradient
(10 MPa/km) to calculate relationship between sediment
depth and pressure. The calculated gas hydrate stability
zone thickness may be considered approximate since sev-
eral factors cannot be fully accounted. However, the BSRs
on the seismic sections at the drilled well locations match
perfectly with the depths of hydrate layer encountered in
the wells. Figure4 shows the estimated gas hydrate sta-
bility zone thickness as 490 m below the seafloor at a
location in Area B which is more than 200 km away from
the seafloor spreading centre. The drilling result and seis-
mic data interpretation corroborate very well.
Some of the seismic sections in Area B reveal two
distinct BSRs (Fig. 5). The upper BSR is traced as a con-
tinuous reflector stretching over 3 km of length, while the
lower BSR is traced at approximately 50 m below
the upper one at various locations and is localized one. The
upper BSR can be interpreted as an active methane hydrate
BSR and the lower BSR as a residual hydrate-related BSR.
Migration of methane hydrate stability zone from lowerBSR to upper BSR might have happened as a result of sea
water warming and tectonic uplift. Seismic sections in the
Andaman basin show very clearly active erosion of the
ridges below which very prominent BSR are observed.
This erosion would reduce the overburden and may result
in pushing down the base of the gas hydrate stability zone.
However, increased heat flow caused by tectonic activities
in the area would have also lead to an upward movement.
The Andaman Basin is tectonically active and changes in
temperaturepressure conditions are very much expected
as a result of seafloor spreading, subduction of the Indian
plate beneath the Southeast Asian plate and volcanic
Fig. 3 Gas hydrate stability
zone thickness plotted on the
basis of temperature
measurements made while
drilling the well at a location
which is nearly 60 km away
from the spreading center. The
drilling results confirm presence
of gas hydrate at such a shallow
depth in the area. The well
location is shown in Fig.1 by a
red star
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increases rapidly and there is a phase shift at higher
offsets ([3,622 m). Figure7 shows the AVO curves
obtained at three representative locations. Figure7a rep-
resents the AVO response at the location where there is
significant evidence of free gas below the hydrate layer,
such as very low interval velocity and high amplitude
(Fig.8). The AVO anomaly is very strong at such loca-
tions. A very weak AVO anomaly is observed (Fig. 7c) at
the locations where there is no evidence of free gas belowthe hydrate layer (location-C shown on Fig.2b). A
moderate AVO anomaly (Fig.7b) is observed at the
locations where there is some indication of free gas
beneath the BSR, such as lowering of interval velocity as
compared to the background velocity (location-B shown
on Fig. 9). For generation of AVO curves presented in
this study pre-conditioned pre-stack time migration gath-
ers are used. The preconditioning of pre-stack time
migration gathers included careful parameterization for
geometrical spreading correction, amplitude equalization
for each offset, Q-correction, residual move-out correc-
tion, and a band pass filtering. The study of AVO
responses of the BSR at various locations reveal that the
AVO anomaly is mainly as a result of the underlying free
gas rather than as a result of the hydrate layer above the
BSR. Ecker and Lumley (2001) have also reported similar
results in the Blake Outer Ridge through modeling
methane hydrate in sediment overlying a layer of free
methane gas-saturated sediment. The presence of free gas
below the BSR in the Andaman offshore is also reported
by Satyavani et al. (2008).
To obtain the best possible estimate of depth and
interval velocity model, the 3D seismic data of Area B has
been depth-migrated. Coherency inversion followed by
three iterations of model-based tomography has been car-
ried out to get the best estimates of interval velocity model.
Some of the reflection events between interpreted horizons
are not flat, indicating sub-optimal velocity in that zone.
Therefore, vertical residual depth move-out analysis is
carried out in order to flatten all the reflection events in thezone of interest (1,000 m below sea floor). The velocity
model is then updated using grid-based tomography. This
velocity model was used to carry out final depth migration.
Thereafter, it was observed that all the reflection events
were flat, which suggests that the final velocity model was
an optimum one. Figure8shows a representative seismic
depth section (corresponding to Line A, Fig. 10) showing
the base of the hydrate layer at about 1,900 m, with a high
interval velocity of 1,950 m/s. The very low interval
velocity of 1,450 m/s and high amplitude of the reflection
events observed beneath the BSR suggests free gas accu-
mulation below the hydrate layer. Reflection events on the
anticline below the BSR are truncating against the BSR,
which shows that the hydrate layer functions as a seal
preventing the upward movement of gas. Seismic sections
in the area show high amplitude and low frequency events
below the BSR (Fig.5) suggesting gas migration from
depth. The gas might have migrated to a structurally higher
location and have been trapped around the area towards the
south of Line B shown in Fig. 10. The area of possible free
gas below the gas hydrate layer is mapped and shown in
Fig. 5 a Depth slice from pre-stack depth migration data volume at
2,260 m indicating double BSR (Area B). b The seismic section in
depth exactly matches the events. The primary and secondary BSR
are cutting across the layers but secondary BSR is localized one andseems to be relict indicating low order change in the pressure
temperature conditions. Seismic section shows the gas migration path
well below the BSR. In this area the gas hydrate stability zone
thickness is 490 m. 120 m thick sediments have been deposited after
the tectonic event forming the anticline which might have changedthe PT conditions in the area
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Fig.10. Figure9 shows a seismic section (corresponding
to Line B, Fig.10) without any indication of free gas
beneath the hydrate layer as no significant lowering of
interval velocity is observed in the section. The area of the
BSR without evidence of free gas is also mapped and
shown in Fig. 10. The overall seismic signatures derived
from the study of seismic data in Area B of the Andaman
deepwater basin reveal the presence of free gas below the
hydrate layer. The results of two wells drilled in the area
showed the presence of biogenic gas at shallow depths (fewhundred meters below the sea floor). The wells encountered
hydrate layers at depths predicted by the seismic data
analysis. Furthermore, the inferred results are in agreement
with models of conventional gas hydrate formation and
thus support the model for gas hydrate formation and
development of the BSR proposed by Claypool and Kaplan
(1974), which envisaged that methane is generated mi-
crobially from organic matter and hydrate formation takes
place concurrent with sedimentation. Geochemical analysis
of hydrate sediments cored under the National Gas Hydrate
Programme Expedition-1 confirmed the microbial origin of
hydrates found in Andaman basin (Collett et al. 2008a,b).
The study by Briggs et al. (2012) also validate the presence
of sediments composed of*1 % marine derived organic
carbon and biogenic methane in the Andaman Sea.
Down-hole log data analysis
A well was drilled in the study Area B with logging
measurements made while drilling (LWD). The well
resistivity log superimposed on the seismic section is
shown in Fig. 11. The base of the hydrate layer identified
on the basis of resistivity and acoustic transit-time logs
response matches the depth of the base of hydrate layer
inferred from the pre-stack depth migrated seismic section.
High seismic amplitudes on the seismic section at shallow
levels in the figure coincide with high resistivity log
Fig. 6 a Migration velocity analysis panel showing BSR on PSTM
gather. BSR also shows phase change at higher offset ([3,622 m).
b The interval velocity calculated from root mean squire (RMS)
velocity shows increased interval velocity (1,950 m/s) above the
BSR. The red and white lines show velocity trend at the nearby
locations 1 km away, whereas black line indicates RMS velocity
picked at the location. Highest semblance value zone is indicated by
red in the figure
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responses indicating secondary hydrate accumulation or
free gas accumulation. This gas may be the dissociated
hydrate gas accumulated and trapped in the suitable res-ervoir rock at a depth of about 1,600 m below the sea level.
Minor faults seen on the seismic sections might have
worked as conduits for the gas migration from deeper
depths.
The responses of the gamma ray (c), P-wave-velocity
(converted from transit-time), resistivity and bulk density
logs in the zone of interest are shown in Fig.12. The
zone can be divided into four units. Unit 1 is character-
ized by relatively high natural gamma-ray, low velocity,
low resistivity and high density. This unit seems to be
shale, devoid of gas hydrate as indicated by higher
gamma values. Unit 2 is characterized by increasedvelocity from 1,650 to 1,950 m/s, a lower natural gamma
value indicating greater sand percentage than Unit 1. The
resistivity log shows an increase from 1.2 to 2.2 Xm. Unit
2 also shows a slight decrease in bulk density from 1.9 to
1.7 gm/cc in the depth interval of 1,8651,895 m. The
resistivity and velocity logs of the Unit 2 indicate the
presence of gas hydrate over a depth interval between
1,865 and 1,895 m. The velocity log is selected to pre-
cisely locate the boundary between Unit 2 and Unit 3.
This acoustic velocity boundary does not exactly match
the decrease in the resistivity observed near the base of
Unit 2. The discrepancy of about 15 m is likely as a resultof the presence of significant amounts of free gas below
the deepest gas hydrate occurrence. 3D pre-stack depth
migration of seismic data has also predicted the free gas
below the gas hydrate (Fig. 8). The interval velocities
estimated by the pre-stack depth migration of 3D seismic
data in the gas hydrate zone and in the free gas zone
below the hydrate layer are of the order of 1,950 and
1,500 m/s respectively. The log data largely validated the
velocity profile obtained by pre-stack depth migration
across the hydrate zone. In Unit 4, the bulk density
decreases from 1.7 to 1.28 g/cc in its upper part
(1,9101,930 m), without any significant decrease in thevelocity and increase in the resistivity. Although, there is
sudden low natural gamma value at a depth of 1,930 m.
Therefore, Unit 4 seems to be devoid of free gas, or
hydrate, and lowering of bulk density may be attributed to
the change in lithology. The electrical resistivity and
acoustic transit-time logs indicate the presence of gas
hydrate at the depth interval between 1,865 and 1,895 m.
The resistivity log data can be used to quantify the
amount of gas hydrate in the sedimentary section. The
Fig. 7 AVO curves generated from the analysis of PSTM data.
aStrong AVO anomaly where free gas below the BSR is indicated by
the velocity and amplitude anomalies. b Moderate AVO anomaly
where there is some seismic indication for free gas below the BSR.
c Low or no AVO anomaly where there is no indication for free gas
below the BSR. The red curve corresponds to the amplitudes of the
raw data. Thedashed light blueand cyan curves are theoretical AVO
curves that are fitted to the data. The light blue curve corresponds to
the Aki and Richards (2002) curve. The cyan curve is the Shuey
(1985) curve. The amplitudes on the y axis are normalized to 100. The
angle on the x axis is incidence angle in degree
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resistivity log data in Unit 2 shows an increase in the
resistivity from 1.1 to 2.2 Xm in the hydrate layer.
Gas hydrate, like ice, acts as an electrical insulator. The
presence of gas hydrate (or free gas) increases the resis-
tivity of the host rock. Using the assumption that the high
resistivity above the BSR is caused by the presence of gas
hydrate in the pores (i.e., pores are filled either with water
or with gas hydrate), gas hydrate saturation can be esti-
mated by using Archies equation (1942), as proposed by
Lu and McMechan (2002):
S 1 R0=Rt 1n 1
where R0 is resistivity of formation saturated with water
which can be estimated as the background resistivity which
in this case is taken as 1.1 Xm. Rt is the measured resis-
tivity in the hydrate zone which is 2.2 Xm. For hydrated
clastic sediments, the value ofncan be taken as n 1:9386(Pearson et al. 1983).
Using the above parameters, the hydrate saturation is
estimated to be on the order of about 30 %. The exponent
Fig. 8 Pre-stack Depth Migrated section (Line-A shown in Fig. 10) overlain by Interval velocity section showing BSR. Very low Interval
velocity below BSR indicates free gas below the gas hydrate layer which is working as barrier for further upward migration of the gas
Fig. 9 Pre-stack Depth
Migrated section (Line-B shown
in Fig.10) overlain by Interval
velocity section showing BSR
and the location for AVO
analysis presented in Fig. 7b
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n in the above Eq. (1) is empirical and can, therefore,
introduce an error in hydrate concentration estimates. The
critical factor in estimation using the resistivity log is thechoice of baseline indicating hydrate-free sediments, which
is dependent upon the pore water salinity.
Resource assessment
To estimate the volume of hydrate-bound gas at specific
site, the areal extent of hydrate occurrence, the thickness of
the gas hydrate stability zone, the gas hydrate
concentration in the sediment and the gas hydrate yield are
required parameters. Delineation of gas hydrate accumu-
lation in Area B is done by mapping the BSR over the area.Figure10 shows the areal extent of the BSR. The total area
is estimated to be on the order of 25 km2. The average
thickness of the gas hydrate stability zone is estimated to be
400 m, but based on the log data analysis it is concluded
that gas hydrate is present in a 30-m-thick layer above the
BSR throughout the area. Resistivity logs provide estimates
of hydrate saturation at 30 %. The well encountered mainly
claystone and siltstone at depths well below the BSR.
Therefore, the average porosity of the sediments containing
Fig. 11 Resistivity Log
superimposed on pre-stack time
migrated seismic section shows
gas hydrate zone. There is an
increase in resistivity from
background value of 1.1 to 2.2
Xm in the hydrate zone. Fluid
expulsion paths are also seen.
The high resistivity in theshallow zone at 1,730 ms
indicates possible secondary
hydrate accumulation
Fig. 10 Bathymetric map
showing estimated area of
hydrate accumulation in Area-B
based on the presence of BSR.
Contour interval is 40 m. The
location of the well is indicated
by a star. Line-A indicates
location of the seismic section
shown in Fig.8. Line-B
indicates location of seismic
section shown in Fig. 9
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Fig. 12 Well Logs from a well in Andaman area marked by a blue star in Fig.1. Unit-2 marks the zone of gas hydrate. Hydrate layer is
characterized by higher velocity, lower density and higher resistivity than the hydrate-free background zone
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hydrates in the area can be estimated to be about 50 %
without any loss of generality. Kvenvolden and Claypool
(1988) estimated the global volume of hydrate-bound
methane at 40 9 1015 m3 assuming average thickness of
the gas hydrate stability zone as 500 m and the average
porosity of the sediment as 50 %. For the purpose of this
study, the gas hydrate yield is estimated to be of the order
of 150 m3 of hydrocarbon gases. Gas hydrate yield isdefined as the volume of gas held in gas hydrate (Sloan
1998). One cubic meter of pure methane gas hydrate
contains 172 m3 of methane at standard temperature and
pressure if the gas hydrate structure is completely filled
with methane (unrealistic in the natural environment) and
139 m3 of methane if only 70 % of lattice cages are
occupied (Collett 1995). The total volume of hydrate-
bound gas (V) is estimated using the following equation.
V Areal extentGHSZ thicknessHydrate saturationPorosityGas hydrate yield
2The total hydrate bound gas in Area B is estimated to be on
the order of 1.8 9 1010 m3 at standard temperature and
pressure. This gas hydrate resource is comparable (by
volume) with the reserves within large or major conven-
tional gas fields.
Conclusion
In this paper, 2D and 3D seismic-reflection profiles, as well
as well log data from the Andaman deepwater basin area,are analyzed to examine the bottom-simulating reflectors
associated with a gas hydrate accumulation, and to conduct
the gas hydrate resource assessment of the area. Conven-
tional 2D and 3D data processing, as well as special pro-
cessing, such as a velocity and amplitude study (VAMP),
coherency inversion, model-based and grid-based tomog-
raphy of seismic data have been carried out to examine the
characteristics of the observed BSRs and investigate whe-
ther these BSRs are associated with gas hydrate accumu-
lation in the area. The results from seismic data are
correlated with the well log results to substantiate the
seismic data interpretation.Seismic data analysis reveals the presence of BSR
showing all the characteristics of a classic BSR associated
with gas hydrate formation in the area. The 2D and 3D
seismic sections reveal that at several places the amplitude
of BSR is large coupled with a high interval velocity of the
order of 1,950 m/s just above the BSR, whereas at some
other places the amplitude of the BSR is small and
accompanied by an interval velocity of the order of
1,750 m/s. The variation of the base of gas hydrate stability
zone in the Andaman area (200650 m) suggests a
dynamic hydrate system in Andaman Basin. The presence
of double BSRs on seismic sections within Area B also
suggests changes in PT conditions in the Pleistocene.
There is a sizeable velocity inversion across the BSR
(1,9501,650 m/s) in Area B. The areas with low velocity
(on the order of 1,450 m/s) below the BSR and high
amplitudes indicate a free/dissociated gas deposit beneaththe gas hydrate layer. The amplitude variation with offset
analysis of the BSR data show an increase in amplitude
with offset; however, the AVO anomaly is more pro-
nounced at places having low velocity below the BSR
level, suggesting a possible free gas deposit beneath the
hydrate layer.
Pre-stack depth migration of 3D seismic survey in Area
B of the Andaman deepwater basin reveals a pool of free
gas beneath the hydrate layer. Strong AVO anomalies of
the BSR coinciding with strong velocity inversions further
substantiate the presence of a gas hydrate layer capping a
free gas pool in Area B. The logging results from the welldrilled in Area B confirm the existence of a hydrate deposit
visualized by seismic analyses; however, the well velocity
below the hydrate layer does not fully corroborate with
seismic velocity. The resistivity and acoustic transit-time
log data provides an estimate of hydrate saturation of
nearly 30 % in the 30-m-thick hydrate layer at a depth
interval of 1,8651,895 m. The total hydrate bound gas in
the Area B is estimated to be 1.8 9 1010 m3, a volume
comparable to the reserves in major conventional gas
fields.
Acknowledgments The authors are grateful to the Oil and NaturalGas Corporation Limited (ONGC) for granting permission to use the
data for this study. The authors express their sincere thanks to Shri S.
V. Rao, Director (Exploration), Sri A. K. Dwivedi, Basin Manager,
and Shri S. Panigrahi, General Manager and Head Geophysical Ser-
vices for their support and encouragement for completion of this
study. The authors are also thankful to Dr. Peter D. Clift and anon-
ymous reviewers for their comments and suggestion which really
improved the quality of the paper.
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