Geophysical evidence for gas hydrates in the deep water of...

Geophysical evidence for gas hydrates in the deep water of the SouthCaspian Basin, Azerbaijan

C.C. Diaconescua,b,*, R.M. Kieckheferc, J.H. Knappa,d

aDepartment of Geological Sciences, Cornell University, Ithaca, NY 14853, USAbNational Institute for Earth Physics, P.O. Box MG-2, Bucharest, Magurele, RomaniacChevron Overseas Petroleum Inc., P.O. Box 6046, San Ramon, CA 94583-0746, USA

dDepartment of Geological Sciences, University of South Carolina, Columbia, SC 29208, USA

Received 15 May 2000; received in revised form 6 October 2000; accepted 13 October 2000

Abstract

New 2-D seismic re¯ection data from the South Caspian Sea, offshore Azerbaijan, document for the ®rst time in the deep water (up to

650 m) of this area, the presence of gas hydrates. Geophysical evidence for gas hydrates consists of a shallow (300±500 m below sea¯oor)

zone of pronounced high velocity (,2,100 m/s) as compared with the surrounding sediments (1550±1600 m/s). This zone appears on the

seismic data as a depth-limited (,200 m thick) layer extending down the ¯ank of an elongate structural high, and displays seismic blanking

effects on the sedimentary section. A strong positive-polarity �Rc < 0:123� re¯ector marks the top of this velocity anomaly, and is interpreted

as the top of the gas hydrate layer. Similarly, a high-amplitude �Rc < 0:11�; negative polarity re¯ector coincides with the base of the high-

velocity layer, and is interpreted as the base of the hydrate zone. Both the top and bottom of the hydrate layer approximately parallel the

sea¯oor bathymetry, and cut discordantly across the stratigraphic section, suggesting that the two re¯ectors are thermobaric and not

stratigraphic interfaces. Decreasing amplitude with offset at the base of the gas hydrate layer may indicate the accumulation of free gas

beneath this interface. These gas hydrates fall within the hydrate stability ®eld predicted from thermobaric modeling for the South Caspian

Basin, but typically in thinner layers than would be expected from theoretical calculations. The minimum predicted water depth that allows

hydrate formation is ,150 m, and the maximum predicted thickness of the gas hydrate stability ®eld is ,1350 m. q 2001 Elsevier Science

Ltd. All rights reserved.

Keywords: Gas hydrates; Caspian Sea; Bottom-simulating re¯ector

1. Introduction

Ever since the discovery of the Messoyakha gas ®eld in

West Siberia (Sapir et al., 1973) gas hydrates have attracted

the considerable attention of the scienti®c community. Also

known as methane clathrates or clathrate hydrates of natural

gas, these substances are similar to ice, but are composed of

rigid cages of water molecules that entrap molecules of

hydrocarbon gas (Kvenvolden, 1993; Sloan, 1990, 1998).

Three principal aspects of gas hydrates interest the geologic

community: (1) the potential drilling hazards they pose; (2)

their fuel resource potential; and (3) their possible role in

global climate change (Kvenvolden, 1993, 1995). Recent

estimations indicate that the largest accumulations of

natural gas on Earth are in the form of gas hydrates (Collett,

1994) that occur mainly offshore in deep-water marine sedi-

ments, or in association with permafrost in polar zones

(Kvenvolden, 1993). Gas hydrates occur under speci®c

conditions of pressure and temperature, where the supply

of methane is suf®cient to stabilize the hydrate structure.

Since these hydrates form in relatively low temperature

environments, the geothermal gradient limits hydrate occur-

rence to shallow (,1000 m) regions of the sedimentary

section. Furthermore, composition of the hydrocarbon

gases and pore-water salinity are also important controlling

factors in the gas hydrate stability zone (Sloan, 1990, 1998).

Gas hydrates are best known from geophysical (seismic

re¯ection pro®ling) and geochemical (Deep Sea Drilling

Project) studies (e.g. Collett, 1994; Hyndman & Spence;

1992; Kvenvolden, 1995; Shipley et al., 1979; Lee et al.,

1994; Xu & Ruppel, 1999). The typical seismic signature of

gas hydrate is a high-amplitude re¯ector that approximately

parallels the sea¯oor and deepens with increasing water

depth, giving rise to a bottom-simulating re¯ector (BSR)

Marine and Petroleum Geology 18 (2001) 209±221

0264-8172/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved.

PII: S0264-8172(00)00061-1

www.elsevier.com/locate/marpetgeo

* Corresponding author. Department of Geological Sciences, Earth and

Water Science Building, University of South Carolina, 700 Sumter Street,

Columbia, SC 29208, USA. Tel.: 11-803-777-3272; fax: 11-803-777-

6082.

E-mail address: [email protected] (C.C. Diaconescu).

(Hyndman & Spence, 1992; Shipley et al., 1979; Sloan,

1990). A BSR originates in marine or permafrost environ-

ments from the development of a consolidated or ªfrozenº

layer (the gas hydrated sediments) above a zone of uncon-

solidated sediments, frequently containing free gas

(Andreassen, Hart, & MacKay, 1997). High compressional

wave velocity (2000±2,500 m/s) in the gas hydrate sedi-

ments in comparison with the underlying section results in

a negative polarity for the BSR. In addition, the hydrated

layer usually displays ªblankingº effects of the sedimentary

section, i.e. reduced acoustic impedance contrasts caused by

the cementation of the host sediments by the gas hydrate

molecules (e.g. Hyndman & Spence, 1992).

Because the hydrate BSR follows thermobaric rather than

lithologic interfaces, it is usually observed to crosscut other

re¯ectors (stratigraphic layers) on continental slopes (Sloan,

1990, 1998). Previous studies (e.g. Kvenvolden, Barnard,

Brooks, & Wiesenburg, 1981; Kvenvolden, 1993, 1995)

suggested that the presence and location of BSRs are

spatially controlled by the intersection of the geothermal

gradient with the stability curve for gas hydrates coincident

with the BSR. However, recent studies (e.g. Xu & Ruppel,

1999) suggest that the base of gas hydrate occurrences in

marine sediments does not always coincide with the base of

the hydrate stability zone. Similarly, although the top of the

gas hydrate zone in marine sediments usually coincides

with the sea¯oor (e.g. Kvenvolden, 1993), Xu and Ruppel

(1999) argued that this need not always be the case from a

theoretical standpoint.

The Caspian Sea basins of Central Eurasia constitute one

of the major petroleum provinces of the world (Fig. 1),

attracting substantial investment by the international petro-

leum industry since the break-up of the Soviet Union. As

petroleum exploration in the Caspian Sea has recently

moved to deeper water, concern over the potential hazard

posed by gas hydrates has become an important issue. Chev-

ron was among the ®rst companies to establish an explora-

tion lease block in the deep water of the South Caspian Sea.

The Absheron block, named after the nearby Absheron

Peninsula in Azerbaijan, is situated in water depths ranging

from ,150 to 715 m (Fig. 1). The principal motivation for

studying gas hydrates in the South Caspian Sea is driven by

concern over potential geo-hazards during drilling, such as:

(1) uncontrolled release of gas trapped beneath the hydrate

seal; (2) dissociation of gas hydrates due to warming by

drilling ¯uids, which could occur slowly or explosively;

and (3) slope instability since the gas hydrates can affect

the strength of the sediments in which they reside, a process

also likely to play a signi®cant role in sediment transport

(McIver, 1982).

The South Caspian Sea meets the conditions required for

gas hydrate stability, including: (1) supply of natural gas; (2)

low geothermal gradient; and (3) low sea¯oor temperature.

Gas hydrates were previously identi®ed in the South

Caspian Sea, but only in shallow water, and in association

with mud volcanoes (Ginsburg et al., 1992; Ginsburg &

Soloviev, 1994; Yefremova & Zhizhcenko, 1974). In this

paper, we present results from a regional 2D seismic re¯ec-

tion pro®le, collected across the Absheron block as part of

Chevron's exploration program, that indicates the presence

of gas hydrates in the deep water (,700 m) portion of the

basin. Our results draw on the seismic characteristics of gas

hydrates, and represent one of the ®rst examples of buried

gas hydrates in marine sediments. Comparison of the inter-

preted gas hydrate occurrence on the seismic data with the

gas hydrate stability ®eld estimated from thermobaric

modeling of a gas composition speci®c to the study area,

indicates a much thinner depth range than would be

predicted from modeling.

2. Geologic setting of the South Caspian Sea

The South Caspian Sea (Fig. 1) is considered to be a

remnant back-arc basin of the Tethys Sea that evolved adja-

cent to the rapidly uplifting Greater Caucasus Mountains

since the Paleogene (Zonenshain & Le Pichon, 1986;

Zonenshain, Kuzmin, Natapov, & Page, 1990). Several kilo-

meters of Plio-Pleistocene clastic sediments that originated

in the Kura and Volga rivers overlay the Mesozoic and

Paleogene sections. An important part of these sediments

derived from the eroding Lesser and Greater Caucasus

Mountains, which formed as the Arabian Plate collided

with Eurasia, ®rst closing Tethys, and later, the back-arc

basin. A major compressional system with associated

strike±slip structures formed in the process of the South

Caspian Sea closure (Zonenshain & Le Pichon, 1986;

Zonenshain et al., 1990).

The general physiography of the Caspian Sea region can

be roughly divided into three areas. The northern one-third

of the Caspian Sea, north of the Greater Caucasus, rests

upon older Paleozoic platforms of the Eurasian craton

with a thick post-Paleozoic sedimentary cover (Zonenshain

et al., 1990). South of this area, the water depths increase

sharply from 50 to over 700 m. The Absheron Ridge is a

narrow neck of relatively shallow water (50±300 m) that

extends between Baku (Azerbaijan) and Turkmenbashi

(Turkmenistan) to form a bridge between the Great Cauca-

sus compressional orogen and the Kopeh±Dagh strike±slip

fold belt system (Priestley, Baker, & Jackson, 1994). South

of the ridge the water deepens quickly to reach about

1000 m over the South Caspian basin, where our study

area is located (Fig. 1).

The presence of numerous gas-driven mud volcanoes and

active oil and gas seeps suggest that hydrocarbons are form-

ing and migrating within the basin today (Bagirov & Lerche,

1997; Ginsburg et al., 1992; Ginsburg & Soloviev, 1994).

Furthermore, active seismicity in the region (Priestley et al.,

1994) attests that structures and associated hydrocarbon

traps in the shallow section are forming now. In the Caspian

Sea, mud volcanoes are a signi®cant source of methane and

other hydrocarbon gases. This factor, in connection with the

C.C. Diaconescu et al. / Marine and Petroleum Geology 18 (2001) 209±221210

low thermal conductivity of the zones of mixed mud, water,

and gas, make the diapirs cool, creating conditions favorable

for hydrate occurrence in this area.

Rapid subsidence and high sedimentation rates in the

South Caspian Sea (.2.0 km/Ma) have led to low heat

¯ow in the basin. Sea¯oor temperatures of 5.8±6.28C, and

geothermal gradients of 11±178C/km were reported by

several authors for the South Caspian Sea (Ginsburg et al.,

1992; Schoellkopf, & Dahl, 1995; Tagiyev, Nadirov,

Bagirov, & Lerche, 1997). An environmental study

C.C. Diaconescu et al. / Marine and Petroleum Geology 18 (2001) 209±221 211

Fig. 1. Location map of seismic pro®le ABSHERON 2 within the South Caspian Sea, offshore Azerbaijan. A portion of pro®le ABSHERON 2 is displayed as

A±B across the Absheron Block (gray rectangle). Star labeled C indicates location of core sample for hydrocarbon gas geochemical analysis explained in text.

ABSH

ERO

N2

A

B

C

AbsheronPeninsula

50

100

600

500

200

25

300

400

700

0 25 50 km

AMV

AbsheronRidge

PrecaspianBasin

CaspianSea

Azerbaijan

Uzbekistan

Turkmenistan

Kazakstan

Russia

Georgia

Turkey

IranIraqSyria

Armenia

NorthCaspian

Basin

SouthCaspian

Basin

performed within the study area (location C in Fig. 1;

Moukhtarov, 1998) found gases of a mixed bacterial-

thermogenic origin in core samples from three different

locations. The low temperatures, abundance of hydrocarbon

gases and deep water (,1100 m) enhances the probability

of gas hydrate formation.

3. Geophysical evidence of Absheron gas hydrates

Previous studies of gas hydrates (Andreassen et al., 1997;

Dillon, Lee, & Coleman, 1994; Hyndman & Spence, 1992;

Lee et al., 1994; Malone, 1994; Shipley et al., 1979) have

documented the signi®cantly distinctive seismic properties

they exhibit in relation to surrounding sediments. Among

these are: (1) reversed (negative) polarity of the BSR rela-

tive to the sea¯oor re¯ection, caused by the acoustic impe-

dance contrast between (2) the higher compressional (P-)

wave velocity of the hydrate layer with underlying lower P-

wave sedimentary layers, (3) `blanking' effects caused by

reduced acoustic impedances within the gas hydrate cemen-

ted sediments, (4) distinctive AVO effects at the BSR, if the

gas hydrate layer overlays gas sands, and (5) crosscutting

relationship of the BSR with the stratigraphic section on the

continental slope. In the following, the seismic characteris-

tics of the Absheron hydrated sediments are examined with

respect to re¯ection character, re¯ection amplitude, and

rock physics.

3.1. Seismic data acquisition and processing

The ABSHERON 2 pro®le (Fig. 1), collected in the deep

water of the South Caspian Sea, is ,70 km long and was

one of the ®rst deep (20 s) re¯ection pro®les acquired in the

Caspian region. Only the ®rst 2000 ms of the southern half

(,30 km) of the line (A±B in Fig. 1) were selected for

hydrate-related processing. Sea¯oor depth along this portion

of the pro®le varies between 450 and 715 m (Fig. 1).

The seismic processing ¯ow for line ABSHERON 2 is

shown in Table 1. An off-end spread with 204 receivers at

25 m spacing was used in acquisition. Shot spacing was

50 m, and the offset range was 231±5306 m. In order to

optimize the appearance of the hydrate-related seismic char-

acteristics, a true amplitude analysis, with wavelet decon-

volution, was performed. Preservation of the true re¯ection

amplitudes was also required for amplitude variation with

offset (AVO) analysis as well as for possible indication of

hydrate-related re¯ection ªblankingº effects. Accordingly,

no time dependent amplitude scaling was applied to the

data.

3.2. Re¯ection character

The multichannel seismic re¯ection pro®le ABSHERON

2, shown in Fig. 2, displays clear evidence for the presence

of gas hydrates, including the existence of a BSR. The

migrated and depth converted seismic section is shown in

Fig. 2a, whereas the velocity ®eld superimposed over the

migrated time section is shown in Fig. 2b. These seismic

velocities were derived from stacking velocities that were

picked on the CMP (common mid point) gathers using Dix's

(1955) equations. In this paper, for the purpose of re¯ection

character analysis, we refer to the maximum excursion of

the seismic wavelet to the right as a peak, or positive polar-

ity (a re¯ection at an interface resulting from a low-

impedance layer above a high-impedance layer), and the

maximum excursion of the seismic wavelet to the left, as

a trough, or negative polarity (a re¯ection at an interface

resulting from a high-impedance layer above a low-

impedance layer).

The seismic sections in Fig. 2 display a strong, positive

polarity re¯ection from the sea¯oor at 600 ms (450 m below

sea level, or mbsl, at B) to 1000 ms (715 mbsl at A) (see

Fig. 1 for location). Two of the more prominent re¯ectors

observed on these seismic sections can be traced for

,12 km and are labeled Base Absheron Hydrate (BAH)

and Top Absheron Hydrate (TAH). BAH is a continuous,

high amplitude, negative polarity re¯ector at ,1150 mbsl

(,1500 ms) approximately 8 km from A in Fig. 1, and shal-

lowing to ,900 mbsl (,1200 ms) above the Absheron

structural high, situated approximately 20 km from A.

TAH is a strong positive polarity re¯ector (with the same

polarity as the sea¯oor), and approximately parallels BAH

,150±200 m shallower in the section. Both of these re¯ec-

tors approximately parallel the sea¯oor, shallow within the

sedimentary section with decreasing water depth, and termi-

nate laterally with no obvious structural control. Thus, BAH

displays the seismic characteristic of a gas hydrate BSR. It is

important to mention here that neither BAH nor TAH can be

interpreted as primary multiples of the sea¯oor since they

occur at much shorter traveltimes than double the sea¯oor

traveltime.

3.3. Seismic velocity

Seismic velocity is one of the most important diagnostic


Table 1

Processing ¯ow

Demultiplexing

Geometry

Spherical divergence correction (with stacking velocities)

Surface consistent deconvolution (128 ms operator length)

Bandpass ®lter (Ormsby: 4-8-50-60 Hz)

F-X deconvolution (Wiener Levinson)

Velocity analysis

Normal moveout correction

Surface consistent amplitude scaling

Amplitude calibration to Shah Deniz 6 well

Top mute

CMP median stack

Conversion from minimum- to zero-phase wavelet

Migration (With interval velocities derived from stacking velocities)

Time to depth conversion using smoothed interval velocities

C.C

.D

iaco

nescu

eta

l./

Marin

eand

Petro

leum

Geo

logy

18

(2001)

209

±221

213

Fig. 2. Composite display showing: (a) Migrated CMP stack of a portion of line ABSHERON 2 (A±B in Fig. 1). The inferred top (TAH) and base (BAH) of the Absheron hydrate bound an ,200 m thick depth-

restricted hydrate layer situated ,300 mbsf. Positive polarity (peak) re¯ections are shown in black (e.g. sea¯oor and TAH); negative polarity (trough) re¯ections are shown in white (e.g. BAH); (b) Interval

velocity ®eld overlaying the migrated time section of line ABSHERON 2. SF stands for shallow faulting (see text for details). Both seismic sections are vertically exaggerated approximately 8:1.

features of hydrated sediments that distinguishes them from

water-®lled sediments. The P-wave velocities (Vp) of

hydrated sediments typically vary between 2000 and

4000 m/s with the corresponding shear (S-) wave velocity

(Vs) varying between 1400 and 1500 m/s (Anderson, 1992;

Dillon et al., 1994). These velocity ranges are both

considerably higher than seismic velocities of shallow

water saturated sediments.

Since the stratigraphy and structure of the upper 2000 ms

in the study area is fairly simple, a conventional NMO

(normal move-out) stacking velocity analysis was used to

obtain reliable root-mean square (RMS) velocities. Using

Dix's equation (Dix, 1955), the RMS velocities were

converted into an interval velocity model, displayed in

color in Fig. 2b. It was more suitable for this analysis to

display interval velocities in time rather than interval velo-

cities in depth, since the velocity analysis was performed in

time, and the depth-converted section shown in Fig. 2b was

generated from smoothing the velocities laterally. Both

BAH and TAH coincide with the bottom and the top,

respectively, of a shallow, high velocity anomaly

(,2100 m/s) shown in yellow. These velocities are compar-

able with values reported in the literature for hydrated sedi-

ments in other areas (e.g. Andreassen et al., 1997; Singh,

Minshull, & Spence, 1993). The surrounding sediments

display signi®cantly lower velocities of 1550±1600 m/s,

representative for water-®lled sediments. Direct measure-

ments of S-wave velocities were not available for our

study area, but some rough estimates were made from

AVO modeling at BAH, which will be presented in one of

the following sections.

3.4. Seismic ªBlankingº effect

Gas hydrate layers commonly display seismic blanking

effects, i.e. reduced acoustic impedance contrasts within the

hydrated sediments, presumably due to the cementation of

the stratal interfaces by the gas hydrate molecules (Hynd-

man & Spence, 1992; Shipley et al., 1979). Thus, a reduc-

tion of the seismic amplitudes should be expected within the

gas hydrate layer, and is often used as a diagnostic feature.

The blanking effect is proportional to the amount of gas

hydrate that ®lls the host sediments, with more hydrates

resulting in a greater reduction in acoustic impedance (e.g.

Collet, 1994). Blanking occurs within the zone of gas

hydrate occurrence, and there are occasions when it can

be observed even when no BSR is present (Sloan, 1990).

As seen in Fig. 2a, a clear amplitude re¯ection blanking

zone on line ABSHERON 2 coincides with the shallow

high velocity layer, and occurs between BAH and TAH.

This zone with reduced amplitudes relative to the surround-

ing sediments coincides with the shallow high-velocity

anomaly zone visible in Fig. 2b.

3.5. Polarity

The boundary between the sediments containing gas

hydrates and the underlying unhydrated strata is often

very sharp, and characterized on seismic data by a well-

developed high-amplitude, negative polarity re¯ector

(Andreassen et al., 1997; Hyndman & Spence, 1992). The

degree to which this high-amplitude re¯ector, or BSR, is

dependent on the presence of free gas beneath the BSR is

still controversial. Kvenvolden (1993) developed two differ-

ent models, in which the potential accumulation of free gas

is controlled by the mechanism of hydrate formation.

Regardless of the presence or absence of gas sands beneath

the hydrated sediments, the transition from a high velocity

hydrated layer to a lower velocity unhydrated layer under-

neath causes a polarity reversal at the base of the gas hydrate

layer (Dillon et al., 1994; Hyndman & Spence, 1992;

Malone, 1994). Since the polarity of the BSR is an important

factor in the detection of gas hydrates on seismic data, a

detailed wavelet analysis of the BAH re¯ector was

performed.

Line ABSHERON 2 displays the sea¯oor and TAH

re¯ectors as positive polarity events (peak in black), and

the BAH re¯ector as a negative polarity event (trough in

white), as shown in Fig. 2. The onset trough waveform for

BAH indicates that this re¯ection results from a boundary of

a high impedance layer above a low impedance layer. The

CMP gather displayed in Fig. 3a (at ,13 km from A) shows

very clearly the positive polarity re¯ection from the sea¯oor

at ,870 ms, the positive polarity re¯ection from TAH at

,1290 ms, and the negative polarity re¯ection from BAH at

,1430 ms. In order to compare the re¯ection polarity of the

sea¯oor with that of the BAH (the inferred hydrate BSR),

the seismic wavelet information was extracted from both

these re¯ectors (Fig. 3b,c). The basic wavelet extracted

from the sea¯oor is shown in Fig. 3b, and the wavelet

extracted from BAH, and rotated with 1608 to match the

sea¯oor wavelet, is displayed in Fig. 3c. The BAH re¯ection

has a dominant frequency of 26 Hz with a bandwidth of 10±

12 Hz. Cross-correlation of these two basic wavelets shows

that the phase difference between them is 1608, which indi-

cates that the sea¯oor and BAH re¯ections have approxi-

mately opposite polarities.

3.6. AVO analysis

Analysis of amplitude variation with offset, or AVO

analysis, is based on variation of the P-wave re¯ection coef-

®cient with the angle of incidence at an interface separating

two media (Castagna, Swan, & Hoster, 1998). Such analysis

has traditionally been used as diagnosis for the presence of

gas sands beneath shales on the basis of increasing ampli-

tude with large offsets. More recently, Castagna et al. (1998)

have shown that AVO gas sands can exhibit either an

increasing or decreasing response depending on the proper-

ties of both the gas sand and the overlying uncharged

stratum. Thus, in order to convey additional information

regarding lithology and ¯uid-type variations, these authors

developed AVO crossplots as an alternative to the classic


amplitude versus offset (or angle of incidence) plots

(Castagna & Swan, 1997; Castagna et al., 1998; Foster,

Keys, & Reilly, 1997; Simm et al., 2000).

In most studies of gas hydrates (e.g. Andreassen et al.,

1997; Bangs, Dale, & Golovchenko, 1993; Ecker, Dvorkin,

& Nur, 1998) the BSR results from a strong acoustic impe-

dance contrast caused by free gas trapped beneath the gas

hydrate layer. It is still a controversial subject whether the

BSR occurs as a result of accumulation of free gas under-

neath the hydrated layer (Bangs et al., 1993), or if the

presence of the hydrated layer itself is enough to produce

an increased seismic impedance relative to the hydrate-free

underlying sediment and produce bright, phase-reversed

re¯ections (Hyndman & Spence, 1992). However, AVO

studies infer that an increase in the seismic amplitudes

with offset of the BSR (Andreassen et al., 1997; Bangs

et al., 1993) is indicative of presence of free gas underneath

the hydrated layer.

In order to evaluate the presence of free gas accumulation

beneath the gas hydrate layer on line ABSHERON 2, an

AVO analysis at BAH was performed. The approach used

here is based on AVO crossplotting (Castagna & Swan,

1997; Castagna et al., 1998), and bears on Shuey's approx-

imation to the Knott±Zoeppritz equations (Shuey, 1985)


(a)

900

1000

1100

1200

1300

1400

1500

Tim

e(m

s)

Seafloorwavelet

(+)

TAH (+)

Offset (m)250 1050

BAH (-)

(c)

0 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40

0.0

0.5

1.0

-1.0

-0.5

Rel

ativ

eam

plitu

deWavelet width (s)

0 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40

0.0

0.5

1.0

-1.0

-0.5

Rel

ativ

eam

plitu

de

Wavelet width (s)

(b)

(a)

S

0.2 0.4 0.6 0.8 1.0

0.2

0.4

0.6

0.8

1.0B

-1.0 -0.8 -0.6 -0.4 -0.2

-1.0

-0.8

-0.6

-0.4

-0.2

S

B

III

III IV

0.4

0.3

0.2

0.1

0

-0.1

-0.2

-0.3

-0.4

10 20 30

BAH Reflection Coefficient= - 0.115

Angle of Incidence (o)

Am

plitu

de

(d)0.4

0.3

0.2

0.1

0

-0.1

-0.2

-0.3

-0.4

10 20 30

Seafloor Reflection Coefficient= 0.198

Angle of Incidence (o)Am

plitu

de

0.4

0.3

0.2

0.1

0

-0.1

-0.2

-0.3

-0.4

10 20 30

TAH Reflection Coefficient= 0.123

o)

Fig. 3. (a) Amplitude corrected CMP gather (km 14 in Fig. 2) for a 0±308 range of angle of incidence at the depth of the BAH re¯ector, showing the BAH

wavelet becoming less negative with increasing offset; (b) Sea¯oor wavelet; (c) BAH wavelet rotated 1608 to match the phase of the sea¯oor wavelet; and

(d) AVO stacked re¯ection amplitude (S) and gradient (B) crossplot at BAH. The BAH response indicates that both P- and S-wave velocities decrease below

BAH, as expected from an interface which separates rigid, hydrated sediments from gas sands.

that is valid for angles of incidence of 0±308 (Fig. 3d). The

four quadrants displayed in Fig. 3d are different with respect

to the variation of the intercept and gradient, and the classic

shales overlying gas sands plot in quadrant III. The repre-

sentation displayed in Fig. 3d is based on the following

equation:

R�u� < S 1 B sin2 u �1�where

S <Vp2

r2 2 Vp1r1

Vp1r1 1 Vp2

r2

; B � S 2 2Rs;

and

Rs � Vs2r2 2 Vs1

r1

Vs1r1 1 Vs2

r2

where R is the re¯ection coef®cient, u the angle of inci-

dence, S the AVO intercept, and B the AVO gradient. The

AVO intercept (S) is an approximation of the normal inci-

dence stacked re¯ection amplitudes at a given CMP,

whereas the AVO gradient (B) is a measure of the offset-

dependent re¯ectivity (Castagna & Swan, 1997; Castagna et

al., 1998). At zero offsets, the amplitude depends only on the

P-wave velocity and density, whereas at far offsets, ampli-

tudes are dependent on the S-wave velocity and density as

well.

The ®rst-order information on the amplitude variation

with offset at BAH is provided by the CMP gather shown

in Fig. 3a, which shows the BAH re¯ector with decreasing

AVO (the negative re¯ection coef®cient becoming less

negative at far offsets). Fig. 3d shows the seismic re¯ection

amplitudes sampled along BAH plotted as a function of B

and S. Most of these values fall in quadrant II, indicating

that BAH is an interface with a negative AVO intercept (S),

and a positive AVO gradient (B). According to Castagna et

al. (1998) this AVO pattern indicates decreasing AVO

(decreasing re¯ection magnitude versus offset) at the

re¯ecting interface. Furthermore, the inset in Fig. 3d

shows that the average near-vertical re¯ection coef®cient

for BAH is 20.115, and its magnitude decreases at far

offsets. This re¯ection coef®cient is more than half the

magnitude of the near-vertical re¯ection coef®cient at the

sea¯oor, estimated to be ,0.198 (Fig. 3d), and it is similar

to other re¯ection coef®cients for BSRs worldwide (e.g.

Ecker & Lumley, 1993; Shipley et al., 1979). Similarly,

the near-vertical re¯ection coef®cient at TAH is 0.123

(Fig. 3d).

One of the more interesting results of the AVO crossplots

(Castagna & Swan, 1997; Castagna et al., 1998) is that gas

sands with lower Vs than the overlying formation could

exhibit decreasing AVO, and plot in quadrant II. The

media identi®ed by Castagna et al. (1998), which might

produce this behavior, are hard shales (siliceous or calcar-

eous), siltstones, carbonates, or tightly cemented sediments.

Therefore, for layers 1 and 2 where Vp1. Vp2

, and r1 ù r2

amplitudes will plot in quadrant II or III of Fig. 3d. The

change of Vs across the analyzed interface in¯uences the

plotting of the data in either quadrant II (positive gradient)

or quadrant III (negative gradient). If Vs1# Vs2

; the data fall

into quadrant III, and if Vs1. Vs2

; the data will plot in

quadrant II. The AVO modeling at BAH indicates that

Vs1< 800 m=s and Vs2

< 360 m=s �DVs � 440 m=s�;whereas Vp1

< 2100 m=s and Vp2< 1550 m=s �DVp �

550 m=s�; while the density is considered to be constant �r �1:848 g=cm3� across BAH, therefore both the P- and S-wave

velocities are higher in the hydrated layer than in the under-

lying sediments. There are earlier studies (e.g. Lee et al.,

1994) which found out that there is no major change in

density at the BSR. According to Castagna et al. (1998),

this is a case in which decreasing AVO at BAH suggests

accumulation of free gas underneath BAH (class IV gas

sands), and the gas hydrated layer plays the role of the

tightly cemented sediments. The porosity of the hydrated

layer derived from the AVO modeling is 48%, and ,20%

of the pore space is ®lled with gas hydrates.

4. Thermobaric modeling

Theoretically determined phase equilibria clearly distin-

guish natural gas hydrates from water ice, and can be used to

calculate the temperature and pressure at which hydrates

form from a given gas composition (Sloan, 1990, 1998).

Of the different equations developed for gas hydrate

phase-equilibria, this study employed the three-phase equi-

librium analysis based on a statistical thermodynamic deter-

mination of the distribution of the guest particles in the

hydrate structure (Sloan, 1990; Van der Waals & Platteeuw,

1959). This approach provides a comprehensive means of

correlation and prediction of all the hydrate equilibrium

regions of the phase diagram, without separate prediction

schemes for two-phase regions, three-phase regions, etc.

(Sloan, 1990, 1998).

The gas hydrate phase equilibrium diagrams calculated

for the Absheron area illustrate how variations in formation

temperature, pore pressure and geothermal gradient affect

the thickness of the gas hydrate stability zone (Fig. 4).

Sea¯oor temperature was considered to be 5.858C, regard-

less of the water depth, which is toward the lowest tempera-

ture reported for the study area (5.8±6.28C; Schoellkopf &

Dahl, 1995). Note that a 18C change in the sea¯oor tempera-

ture will shift the gas hydrate stability ®eld by at most a few

tens of meters. A hydrostatic pore-pressure gradient of

0.1 atm/m was assumed for calculating the depth scale

(Kvenvolden, 1993).

Intersection of the gas hydrate stability curves with the

sea¯oor isotherm of 5.858C denotes the minimum water

depth at which gas hydrates are stable for a given hydro-

carbon gas composition (Fig. 4). The depth at which the

geothermal gradient intersects the gas hydrate stability

curve marks the predicted base of the gas hydrate stability


®eld (Kvenvolden et al., 1981; Kvenvolden, 1993). Geother-

mal gradients considered in this study (11±178C/km) were

derived from surface heat ¯ow studies (Schoellkopf & Dahl,

1995; Tagiyev et al., 1997), and were hung from three

different water depths. The shallowest (150 m) represents

the minimum water depth for formation of gas hydrates in

the study area, and water depths of 475 and 690 m corre-

spond to the updip and downdip termination of the inter-

preted gas hydrate layer on line ABSHERON 2 (Fig. 2a).

Gas hydrate stability curves in Fig. 4 are calculated for a

system of pure water (zero salinity) and hydrocarbon gas for

three different gas compositions including pure methane (1)

and measured gas samples from within the study area (2 and

3). The gas compositions for gas hydrate stability curves (2)

and (3) are listed in Fig. 4, and represent compositional

analyses of sediment gases liberated from a core sample

taken at location C in Fig. 1 (Moukhtarov, 1998). Curve

(2) corresponds to the gas composition sampled in the

lower part of the core, whereas curve (3) represents the

gas composition sampled 57 cm higher, in the upper part

of the core.

The minimum water depth at which gas hydrates may

become stable within the study area is ,150 m, based on

stability curve (3) (Fig. 4). Note that at this water depth,

methane hydrate does not form because the geothermal

gradient does not intersect the methane hydrate stability


De

pth

(m)

Pressure

(atm)

3

Ice line

100% metha e1 n

Seafloortemperature

=5.85oC

Water depth forgas hydrate

occurrence onABSHERON 2

2

1

1000

1500

2000

2500

500

0 0

50

100

150

200

250

80.00% methane1.87% ethane2.89% propane0.44% iso-butane1.92% n-butane

12.88% heavyhydrocarbons

3

94.87% methane0.63% ethane0.83% propane0.14% iso-butane2.80% octane0.62% ethylene0.11% propylene

2

+ NaCl + CO2, H2S

Water depth:

150 m475 m690 m

Temperature( C)o

0 4 8 12 16 20 24

Fig. 4. Generalized phase equilibrium diagram for a system of pure water and different hydrocarbon gas compositions based on pure methane (1) and measured

gas compositions from the study area (2 and 3; at coring location C in Fig. 1). Hydrate stability ®elds are calculated for geothermal gradients of 118C/km (plain

lines) and 178C/km (dashed lines), for water depths of 150, 475 and 690 m. The base of the gas hydrate stability ®eld is positioned at the depth at which the

geotherm crosses the gas hydrate stability curves. In all cases, the observed Absheron hydrate occurrence falls within the modeled stability ®eld, but typically

at shallower depths and in thinner layers. Depth scale assumes a pore-water hydrostatic pressure gradient of 0.1 atm/m. Arrows show direction toward which

the gas hydrate stability curves move if NaCl (left) and/or CO2 or H2S (right) are added to the system.

(curve 1). For a water depth of 150 m, gas composition (3)

which suggests a thermogenic origin (with ,20% of heavier

hydrocarbon gases than methane), could be stable as a

hydrate from the sea¯oor and could reach thicknesses of

600 m (for a geothermal gradient of 178C/km) to 1150 m

(for a geothermal gradient of 118C/km). Gas composition

(2) which is richer in methane (,95%) would not form gas

hydrates at the sea¯oor, but deeper in the sediments, starting

at ,275 mbsl (for a geothermal gradient of 178C/km) and

could reach thicknesses as large as ,950 m. Small additions

of heavier natural gas components such as ethane, propane,

isobutane and/or CO2 or H2S cause an increase of the


0 4 8 12 16 20

321

1500

0

1000

2000

500

0

50

100

150

200

(a)

Seafloor

0 4 8 12 16 20

321

1500

0

1000

2000

500

0

50

100

150

200

(b)

Seafloor

1500

0

1000

2000

500

0

50

100

150

200

0 4 8 12 16 20

321

(c)

0 4 8 12 16 20

3

Ice line

Seafloortemperature

=5.85oC

21

1500

0

1000

2000

500

0

50

100

150

200

(d)

Seafloor

Temperature (oC)

Dep

th(m

)

Pressure

(atm)

Temperature (oC)

Dep

th(m

)

Pressure

(atm)

Temperature (oC)

Dep

th(m

)

Pressure

(atm)

Temperature (oC)

Dep

th(m

)

Pressure

(atm)

11 oC/km

Seafloortemperature

=5.85oC

Ice line

Seafloor

17 oC/km

Ice line

Seafloortemperature

=5.85oC

Ice line

Seafloortemperature

=5.85oC

17 oC/km

11 oC/km

Fig. 5. Theoretical phase equilibrium diagrams from Fig. 4, showing predicted gas hydrate stability ®elds for: (a) water depth of 475 m, and geothermal

gradient of 178C/km; (b) water depth of 475 m, and geothermal gradient of 118C/km; (c) water depth of 690 m, and geothermal gradient of 178C/km; and (d)

water depth of 690 m, and geothermal gradient of 118C/km. The predicted gas hydrate stability ®eld is shown in shades of gray, where light gray corresponds

to curve 1, medium gray to curve 2, and dark gray to curve 3. Hachured area shows the gas hydrate occurrence zone as determined from the ABSHERON 2

pro®le.

hydrate stability region due to a displacement of the phase

boundary toward higher temperatures and lower pressures

(Kvenvolden, 1993, 1995). Given the known geochemical

and geothermal constraints from the South Caspian Sea, the

predicted thickness of the gas hydrate stability ®eld varies

between 0 and 1350 m. Lower geothermal gradients and

heavier hydrocarbon gases would only serve to enlarge the

gas hydrate stability ®eld.

5. Discussion

Seismic evidence for gas hydrate occurrences in the

South Caspian Sea argue for the presence of buried gas

hydrates that form well beneath the sea¯oor (300±

350 mbsf, meters below sea¯oor). Thermobaric modeling

of the gas hydrate stability ®eld (Fig. 4) suggests that gas

hydrates should be stable from the sea¯oor to depths largely

controlled by the geothermal gradient and the gas composi-

tion. Development of gas hydrates in layers as thick as

,200 m fall within the predicted gas hydrate stability

®eld (Figs. 4 and 5). Fig. 5a and b display gas hydrate

stability ®elds for water depth of 475 m, which is the shal-

lowest for gas hydrate occurrence on line ABSHERON 2

(Fig. 2). These ®gures are different with respect to the

geothermal gradients, which are 17 and 118C/km, respec-

tively. Similarly, Fig. 5c and d display gas hydrate stability

®elds for water depth of 690 m, which is the deepest where

gas hydrates occur on line ABSHERON 2 (Fig. 2). The

zones shown in hatch represent the gas hydrate occurrence

zones from line ABSHERON 2, and how these ®t the gas

hydrate predicted from thermobaric modeling. The shallow-

est gas hydrate occurrence on line ABSHERON 2 lies

between 800 and 940 mbsl (for water depth of ,475 m),

and the deepest gas hydrate occurrence in the section,

between 1000 and 1150 mbsl (for water depth of

,690 m). As seen in Fig. 5(a)±(d) the gas hydrate zone

interpreted with seismic methods ®ts within the gas hydrate

predicted with the phase equilibrium diagrams. The only

exception to this is shown in Fig. 5a, where the gas hydrate

occurrence is below, and not within the predicted methane

stability ®eld corresponding to curve (1). Even though, the

high sedimentation rate of the South Caspian Sea may

ensure the preservation of suf®cient shallow organic matter

for clathrate production in situ, this shows that the gas

composition in the study area might contain heavier hydro-

carbon gases as displayed by the phase equilibrium curves

(2) and (3). Location of line ABSHERON 2 relatively close

(,6±8 km) to a mud volcano (Fig. 1; Absheron mud

volcano) enhances the chance for production of thermo-

genic gas in our study area.

As known from previous studies (e.g. Kvenvolden, 1993,

1995; Sloan 1998), heavier hydrocarbon gases enlarge the

gas hydrate stability ®eld. The present study once again

demonstrates that the thickness of the gas hydrate stability

®eld, as predicted from the thermobaric modeling, is very

sensitive to variation in the geothermal gradient, seawater

depth, and hydrocarbon composition. Each of these factors

can affect the thickness of the stability ®eld by a few

hundred meters. Decrease of the geothermal gradient and

increase in water depth result in increased thickness of the

hydrate stability ®eld. Pore-water salinity and sea¯oor

temperature are also important in the gas hydrate thickness

¯uctuations, but to a lesser extent. Pore water salts in

contact with gas during gas hydrate formation can reduce

the crystallization temperature by about 0.068C for each part

per thousand of salt (Kvenvolden, 1995). Thus, pore water

salinity similar to that of seawater (,35 permil) reduces the

crystallization temperature by 2.18C and the thickness of the

gas hydrate stability zone by about 200 m (Collet, 1994).

The Caspian Sea salinity is low (,10 ½; Ginsburg

and Soloviev, 1994) therefore this might thin the phase

equilibrium by only a few tens of meters compared to

fresh water.

The gas hydrates mapped on the ABSHERON 2 pro®le

occur on the western ¯ank of the Absheron mud volcano

(Fig. 1). Given the thermogenic nature of gas seen in shal-

low piston cores (C in Fig. 1), the mud volcano represents a

likely source for gas in these hydrates. Additionally, chaotic

mud ¯ows and breccias emanating from the mud volcano

may account for the lack of stratigraphic de®nition within

the shallow section south of the Absheron high (Fig. 2). We

cannot rule out the possibility that the lateral and depth-

limited extent of the gas hydrates is in some way affected

by the presence of such mud ¯ows. Analysis of this relation-

ship awaits detailed examination of 3D data in the region.

Alternatively, these seismic data may lend support for the

theory developed by Xu and Ruppel (1999) that the gas

hydrate occurrence zone does not necessarily coincide

with the gas hydrate stability zone predicted from thermo-

baric modeling. A plot of the gas hydrate occurrence with

the predicted stability ®eld (Fig. 5) shows that the hydrates

lie consistently within the hydrate stability zone, but within

a much narrower depth range for all conditions modeled.

According to Xu and Ruppel (1999), this phenomenon

appears to be primarily controlled by the mass fraction of

hydrocarbon gases in bottom water as well as the gas and

¯uid migration rates. The stability of gas hydrates at the

sea¯oor is possible only where there is a constant supply

of gas at suf®cient pressure. The occurrence of gas hydrates

in well-de®ned layers well beneath the sea¯oor could be a

result of the fact that the free gas underneath the hydrate-

cemented sediment may occur as patches of gas-®lled sedi-

ment with variable thickness. The gas hydrates within the

Absheron block develop in relation to the structural high

shown in Fig. 2, suggesting that their occurrence might be

due to larger supply of gas from underneath. There is some

relief on BAH, which does not perfectly parallel the sea¯oor

re¯ection, possibly due to an undocumented thermal

anomaly on top of the structural high or to spatially irregular

gas supply.

Due to the high sedimentation rate in the South Caspian


basin (average of ,2 km/Ma for the past 5 Ma; Devlin et al.,

1999), the isotherms migrate downwards as the sediment

surface builds up. The trapped free gas at the base of the

gas hydrate layer (indicated by the AVO analysis) may

penetrate the hydrate-cemented zone either by diffusion or

along pathways provided by small faults (Dillon et al.,

1994), and it is rapidly converted to hydrate, causing a

higher concentration of hydrate in the lower part of the

hydrate stability zone. This might be the reason for higher

velocities (as shown in tones of red) within the gas hydrate

zone in Fig. 2, in the vicinity of BAH. Also, as an effect

of the high sedimentation rates, the phase-boundary

conditions change rapidly, possibly causing the variation

in the character of the BAH re¯ector.

6. Conclusions

Gas hydrate cemented sediments are for the ®rst time

documented in the deep water of the South Caspian Sea,

where they form in layers well below the sea¯oor, charac-

terized by sharply de®ned upper and lower boundaries

within the host sediments. These gas hydrate layers occur

at the temperature and pressure conditions that mark the

hydrate phase boundary, but in much thinner layers than

predicted from thermobaric modeling of the hydrate

stability zone.

This study shows that gas hydrates of the South Caspian

Sea occur not only on the top of mud volcanoes as

previously studied (Ginsburg et al., 1992; Ginsburg & Solo-

viev, 1994), but also in a fairly undisturbed sedimentary

section. This is the ®rst time, to our knowledge, that a strong

and clear seismic BSR (BAH in this study) has been

observed in the Caspian Sea at a fairly large depth

(,300±500 mbsf), and its occurrence correlates with calcu-

lations of the hydrate stability ®eld from thermobaric

modeling using speci®c geochemical and geothermal data

for the study area.

Thermobaric modeling suggests that the gas hydrates

may be stable in the South Caspian Sea offshore Azerbaijan

in water depths as shallow as 150 m (in case of thermogenic

gas such as curve (3) in Fig. 4), and 460 m for biogenic gas

(for pure methane). These minimum depths for gas hydrate

formation in the study area are much shallower than those

reported for other gas hydrate sites worldwide (e.g. Dillon

et al., 1994; Field, & Kvenvolden, 1985; Lee et al., 1994).

The maximum thickness of the predicted hydrate stability

®eld is 1350 m (Figs. 4 and 5). These results indicate poten-

tial occurrence of gas hydrates in depth-restricted zones well

beneath the sea¯oor (,300±350 mbsf), suggesting that the

actual gas hydrate occurrence zone is thinner and falls

within the gas hydrate stability zone predicted by thermo-

baric modeling, but typically at shallower depths and in

thinner layers (Fig. 5). This is a con®rmation of the theore-

tical model introduced by Xu and Ruppel (1999), according

to which it is very likely that the gas hydrate occurrence

does not coincide with the gas hydrate predicted with the

phase equilibrium diagrams, but it is rather positioned

within the latter.

Acknowledgements

Many thanks are due to Chevron Overseas Petroleum Inc.

(USA), SOCAR (Azerbaijan), and Total (France) for

providing access to the seismic data and for permission to

publish them. Here, special acknowledgments are directed

to John A. Connor, Rukhsara Gulieva, Keith Kvenvolden

and Alan Cooper for their support during this study. Caspian

Geophysical collected the seismic pro®le analyzed in this

paper. Jorge Mendiguren (COPI) provided the software

used for the AVO analysis. Larry Brown offered useful

comments on earlier versions of the manuscript, and Neil

Piggott provided a constructive review. Acknowledgment

is made to the donors of The Petroleum Research Fund,

administered by the ACS, for partial support of this

research. Contribution #1 from the Tectonics and Geo-

physics Lab at USC.

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