Anand Prakash, dkk - Evidence of gas hydrate accumulation and its resource estimation in Andaman deep water basin from seismic and well log data.pdf

8/14/2019 Anand Prakash, dkk - Evidence of gas hydrate accumulation and its resource estimation in Andaman deep water basin from seismic and well log data.pdf

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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


N. P. Singh (&)

Department of Geophysics, Faculty of Sciences,

Banaras Hindu University, Varanasi 221005, UP, India


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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

2 Mar Geophys Res (2013) 34:116

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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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http://www.dghindia.org/SedimentaryBasins.aspxhttp://www.dghindia.org/SedimentaryBasins.aspxhttp://www.dghindia.org/SedimentaryBasins.aspxhttp://www.dghindia.org/SedimentaryBasins.aspx


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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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