A Review of Research on Gas Hydrates in Makran · A Review of Research on Gas Hydrates in Makran...

Bahria University Research Journal of Earth Sciences Vol. 1, Issue 1, June 2016

Page 28 ISSN 2415-2234 © BURJES

A Review of Research on Gas Hydrates in Makran

Muhammad Jahangir Khan, Mubarik Ali

Department of Earth & Environmental Sciences

Bahria University Karachi Campus

Abstract - Makran region has been remained an area of

interest for local and foreign geoscientists for many

reasons. Gas-hydrates are a naturally formed marine

resource of Methane gas and also used as a source of

combustible energy. Makran offshore known for the

occurrence of gas-hydrates, however, their volume and

geometries are not yet clearly developed. This study

comprehends recent research developments on gas-

hydrates in Makran and serve its academic role to

understand the existence of gas-hydrates in Makran region

and its exploitation from virgin offshore areas of Makran

(Pakistan and Iran) with modern technologies and drilling

methods.

Key Words: Makran, Gas-hydrates, Seismic, BSR,

Subduction.

I. INTRODUCTION

The energy demands are rising in this modern era. Futurist

scientists are eyeing on unconventional hydrocarbons.

Globally efforts have been made to explore, exploit and

export hydrocarbons. Gas-hydrates are unique aggregates,

composed of physically and chemically hetrogenous

compounds of water (ice) and low-molecular weight gases

(mostly methane, ethane, propane, butane and some

inorganic origin gases like CO2 and H2S). Crystalline

structure of natural gas-hydrates has multi-face geometry in

which water molecules camouflage the centrally located

methane gas. Exploitation of gas accumulation, for the first

time, found economically viable in tectonically active areas

(which controls the geometry and sub-surface distribution of

gas-hydrates) of Alaska and SE Japan in 2012 and in 2013,

respectively (Ruppel, 2007; Wallmann, 2013; Collett, 2011;

Boswell and Collett, 2011)

The receipe of gas-hydrates consists of several ingrediants

known today like low formation temperature, high formation

pressure, pore water salinity, the gas (either Biogenic or

Thermogenic origion), permeability of medium, marine

sediments or reservoir (to provide the porosity) and active

subduction tectonics. Gas-hydrates are mostly found within

upper few hundred meters of ocean floor sediments where

the water depth exceeds a few hundred meters to maintain

the thermo-barometric stability conditions for gas-hydrates

on the continental margins (figure 1), particularly in

accretionary prisms at active ocean-continent convergent

boundaries (Shipley et al., 1979; Minshull and White, 1989;

Hyndman and Spence, 1992; Kvenvolden, 1993; Xu and

Ruppel, 1999).

Fig 1. Natural settings for gas-hydrates (after Ergun, 2015)

This paper present a review of research conducted on

Makran (North Arabian Sea) portrayed in figure 2.

Fig 2 Makran offshore Areas & discoveries in Arabian Sea.

Inset Map of northern Arabian Sea. (After von Rad et al., 2000)



II. TECTONIC OVERVIEW

Tectonic structure of Makran is complex and is characterized

as on-going subduction of the north-eastern part of Arabian

plate (possibly one of the oldest parts of the Indian Ocean)

which is under thrusting the Afghan block (currently at the

southeastern edge of the Eurasian plate) at a rate of ~4

cm/year since Late-Cretaceous. Makran subduction zone is

similar to other subduction zones of the world such as

Southern Lesser Antilles (Caribbean Sea), Hikurangi (New

Zealand), Cascadia (North America), and Sumatra/Sunda

(Indonesia) where relatively high sediment input dominants

in inter-bedded hemi-pelagites facies associations and

turbidities (Bourget et al., 2010; Mouchot et al., 2010;

Bourget et al., 2011) characterizing the thickest sedimentary

cover approximately 7.5 Km (Smith et al., 2013). The

sedimentary cover in Makran subduction zone has

significant realities, i.e. more thickness even than that of

Potwar basin (Khan, 2015), low subduction angle 4-5 degree

(Smith et al., 2013) with slow to moderate convergence rates

~3-5 cm/year (White, 1979, White and Louden, 1983;

DeMets et al., 1994; Flueh et al., 1989). Makran has

remained an active Margin from Cretaceous to Present

(McCall and Kidd, 1982).

Methane gas stored in offshore environments controlled by

geological and biological elements associated with active

tectonics within and beneath gas-hydrates, which may

provide a major energy source in Makran subduction zone

similar to Alaska and Japan offshore areas bearing gas-

hydrates.

Geologically, a horizon defines the lower boundary

conditions of high-pressure and relatively low-temperature

zone in which gas-hydrates are stable in any region

(Andreassen et al., 1990; Kvenvolden et al., 1993). A simple

geological model for the occurrence of gas-hydrates in

offshore areas illustrated in figure 3.

Fig 3. Simple geological model of gas hydrates under sea-water

III. METHODOLOGY

The objective of this study is based on reviewed analysis of

several collaborative research programs exclusively aimed to

study various dimensions of Makran subduction system and

gas-hydrates. Analysis of research publications based on

interpretation of prior offshore surveys conducted in Makran

provide substantial evidences (physical, geological,

biological and geophysical attributes) in favour of existence

of gas-hydrates in Makran region.

A) Physical Observations

At Makran, physical features of gas-hydrates are directly

observed through seafloor sediments and under-water

photography. Smell and flame signify the release of gas from

trapped gas-hydrates. Potential gas bubbles and escape

marks are captured at sea-floor (figure 4A) in Makran. The

raised seafloor or appearing/disappearing “islands” in

Makran region signify the dynamic plate margin and expose

the potential of released gases. An example of physical

sample of gas-hydrates collected with seafloor sediments

from Oregon offshore region is depicted in figures 4B & 4C.

Fig 4 A. Gas leaking from the sea-floor sediments, Makran

(after Tabrez and Inam, 2012)

Fig 4 B: Gas-hydrates sample found in Alaskan North Slope.

(after Collett, 2011)



Fig 4C: Gas-hydrates with sea-floor sediment of Alaskan North

Slope (after Collett, 2011)

B) Geophysical Exploration: (Seismic)

Geophysical exploration have remarkable success and

contributed significantly in unearthing the anatomy of

complex convergent margin of Makran. Geophysical hunt

for gas-hydrates in Makran based on conventional petro-

physical models (Flueh and Bialas, 1996; Ellouz-

Zimmermann et al., 2007), Multi-channel marine seismic

reflection profiles (Ellouz-Zimmermann et al., 2007; Smith

et al., 2013), Seismic inversion models (Sain et al. 2000),

Amplitude Verses Offset (AVO) modelling (Ojha et al.,

2014) and Seismic velocities analysis (Minshull and White,

1989; Holbrook et al., 1996) indicate the presence of a strong

and widespread horizon i.e. bottom-simulating-reflector

(BSR).

The BSR is an indicative tool in case of gas-hydrates

interpretation based on seismic reflection data. The BSR has

been clearly identified in various regions of Makran offshore

with varying amplitude (Figure 5).

Fig 5 BSR in the Makran accretionary prism

(after Ojha et al., 2004)

High amplitude dipping reflectors below the BSR are

observed on 2D seismic image of Makran offshore (Figure

6a). Moreover, the reflectors dip towards north (landwards)

in the Makran accretionary prism. It is also observed from

Makran offshore seismic profiles that highly reflective

region beneath the BSR may represent alternating gas-rich

and gas-poor strata.

Fig 6 A Seismic line in Makran offshore: Stacked section from

the Makran. “B” marks the base of the zone with very low

velocities and unlabelled arrows mark the base of the LVZ

(after Sain et al., 2014)

Theoretically, P-waves velocity increases with depth due to

over burden pressure and compaction of lithified

sedimentary layers but while passing through gas-hydrates

zone (which acts as speed breaker for P-waves) its velocity

decreased notably from 2.2 km/sec to 1.3 km/s at a depth of

2300 m, below the sea floor (figure 6b). This low-velocity-

zone (LVZ) marks an anomalous seismic zone (vertically) in

subsurface overlying thick sedimentary cover load.

Fig 6 B Low velocity Zone identified by waveform inversion

results for the Makran at CDPs 4375 and 4400 of figure 6A

(after Sain et al., 2014)



The approach of seismic inversion reveals a thick layer of

gas-charged sediments (200–350 m) below the BSR in the

Makran accretionary prism (Ojha, et al 2014). The LVZ

observed in Makran is much thicker than LVZ found in the

abyssal plain close to the toe of the Makran accretionary

wedge (White, 1979; Minshull and White, 1989). The

difference in thickness may be due to a steady supply of gas

as sediments are compacted in the accretionary wedge.

Volumetric calculations of the free gas reserve in Makran

may be estimated by measuring the thickness of LVZ and

related factors (von Rad et al., 2000; Roeser et al., 1997). A

continuous high-amplitude reflection event below the BSR

corresponds to the bottom of a probable connected gas

reservoir whereas below LVZ more discontinues events

could been observed in visual interpretation of seismic

profiles in Makran (figure 7)

Fig 7 BSR with undulating topography and discontinuous

events at bottom, Makran accretionary prism

(after Smith et al., 2013)

Figure 8 presents a case to correlate P-wave velocities from

vertical seismic profiles at sites 995 and 997 with single

channel section on Blake Ridge (Holbrook et al., 1996). The

seismic attributes of BSR in Makran are similar to the BSR

in Blake Ridge (Sain et al. 2000).

Fig 8 BSR with respective velocity model i.e. Low velocity

zone (after Holbrook, 1996)

C) Geological evidences:

The Makran accretionary wedge lies in the vicinity of major

biological productivity in surface water governed by

upwelling of monsoon and sediments transport. It may be a

reason for the development of an oxygen-minimum-zone

(OMZ) illustrated in figure 9 which controls accumulation

rate of the predominantly organic matter in the marine

sediments (von Rad et al., 1996; Edwards et al., 2000). OMZ

conditions are identified in northeastern Arabian sea (von

Rad et al., 1999).

Fig 9 Schematic cross section of the Makran accretionary

wedge (after Von Rad et al, 2000)

Gas hydrates may be distributed in concentrations that vary

both laterally and vertically, and are controlled by

heterogeneities in lithology, permeability, and methane

transport (Ruppel and Kinoshita, 2000; Trehu, 2006; Riedel

et al., 2006; Collett, 2009; Collett, 2011; Malinverno et al.,

2008). The layer of gas-hydrates may form a barrier for fluids

to flow which inhibits sediment consolidation, leading to the

development of excess pore pressures at the BSR and their

decomposition can lead to locally reduced shear strength at

the BSR so they could play an important role during slope

failure (Booth et al., 1994).

Geologically, the mechanism of disassociation of

methane gas in appreciable amount from gas-hydrates in

Makran offshore could happen due to following reasons

alone or in combination.

Upward movement of the base of gas-hydrates stability

field, probably caused by a push during thrusting in

Makran subduction zone (White, 1982; Minshull et al.,

1992)

The high sedimentation rate in Makran (Raza et al.,

1983; Smith et al., 2013) may also lead to rapid upward

movement of the phase boundary relative to the

sediment column (Minshull et al., 1992; Korenga et al.,

1997; Pecher et al., 1998)

Tectonic uplift in Makran (Sain et al., 2000) results in

buildup of elastic stresses or release of elastic strain

energy (tectonic earthquake) in Makran (Khan, 2015)



and under water land- sliding, can put slip-thrust and

transfer its momentum to stimulate and accelerate the

leaking of aqueous-gas i.e. gas-hydrates (Methane).

Figure 10 depicts the geochemical reaction occur in

subsurface (under seawater) associated with gas-hydrates.

Theses multiple conditions contribute in separation of fused

compounds and isolation of gas that occupied the pore spaces

of the marine sediments and sedimentary layer saturated with

integrated gas (mostly methane) stationed under the cap of

BSR.

Fig 10. Geochemical reaction of gas-hydrates and their impact

IV. RESULTS (PRODUCTION OF METHANE GAS)

Globally, efforts have been made to produce methane gas

from gas-hydrates. Production methods reviewed by Collett,

2002. Figure 11 depicts common production models applied

for dissociating or melting in-situ gas hydrates by;

Heating the reservoir beyond hydrate-formation

temperatures - (most expensive)

Injecting an inhibitor such as methanol or glycol into the

reservoir to decrease hydrate stability (most promising).

Depressurization below hydrate equilibrium (most

practical) with thermal injection in well wall- (most

effective).

Fig 11 Harvesting of Methane gas (after Collett, 2002)

Recently, several studies have shown that it may be possible

to produce methane from the gas-hydrates by displacing the

methane molecule in the hydrate structure with carbon

dioxide, thus releasing methane and carbon dioxide (Graue

et al., 2006).

In Makran, the production of gas hydrates could be made

possible by adopting any one of the above described

methods.

A production model (figure 12) of permafrost areas

demonstrates how gas is being released from gas-hydrates

with depressurization method. Initially a test well is drilled

and an electrical submersible pump (ESP) installed above

perforations that depressurising the seabed formation by

lowering the water level in the well surrounding sand grains

are prevented to enter into the well. Hydrates disassociation

from gas-hydrates compounds produce water and gas

(methane). Then after gas-water filtration process allowed

gas to flow to the surface through pipe lines and filtered

water re-injected through a separate water disposal well.

Fig 12 Well completion for getting gas hydrates

V. DISCUSSION

The harvesting of Methane from gas-hydrates is an in-situ

drilling/pumping operation analogous to that used for

transformation of coal-bed by steam into methane gas

(change in physical state of matter).

Practically, the operation of getting the converted methane

gas from a well lithified solid material of geological times is

an engineering challenge. It involves pre- and post-

operational geological and environmental hazards as well as

probable consequential disasters. Any execution plan when

adopted in Makran may create a misbalance in subsurface

geological settings near and/or far coast of Makran.



Methane gas trapped beneath hydrates may also be a

significant drilling hazard in deep waters of Makran: driling

operations affect the ecosystem and sea-floor stability. Too

much gas extraction from gas-hydrates may destabilize the

seabed, trigging huge under water landslide which may cause

tsunamigenic earthquake in Makran region and locally

submarine slope failures due to disassociation of gas-

hydrates (Nixon and Grozic, 2007). Moreover, on

combustion/melting of gas-hydrates release greenhouse

gases into the atmosphere which may impact on climate

change or global warming in coastal areas of Pakistan and

Iran.

The importance of gas-hydrates is also evident from

literature such as:

Gas-hydrates forming an impermeable layer that may act

as a cap rock for hydrocarbons in Makran similar to that

occurring in southeast U.S (Dillon et al. 1980).

Gas-hydrates may indicate hydrocarbons in conventional

petroleum system.

The simulating reflector at bottom of gas-hydrates can be

used to estimate thermal gradients and hence heat flow in

oceanic sediments (Hyndman and Spence, 1992) which

contribute in knowing burial thermal history of

sedimentary basin in Makran.

Gas-hydrates could act as a state indicator of thermal

maturity and migration of hydrocarbons (gas reserve) in

Makran.

VI. CONCLUSIONS

All necessary elements existing with active tectonics

characterises Makran a host of gas-hydrates. It is

evident from sea bed observations and field data of

various offshore scientific surveys conducted in past

decades by several geoscientists / research institutes of

the world, it is apparent that gas-hydrates are present in

Makran.

Analysis of remotely sensed geophysical (seismic) data

provide strong evidence of gas-hydrates in Makran

offshore.

Bottom-simulating-reflector (BSR) is the key indicator

on seismic data that results from strong impedance

contrast at the bottom of gas-hydrate zone. The Makran

accretionary complex shows a distinct bottom-

simulating-reflector, indicating a thick gas-hydrate

bearing horizon between the deformational front and

about 1350 m water depth which seals off the upward

flow of gas-charged fluids (von Rad et al., 2000).

The gas-hydrates were initially described by White

earlier (in 1982 and 1983) in Makran region, the expanse

and abundance of gas-hydrates in offshore Makran was

affirmed by later seismic investigations.

Scientific outcome of the surveys showed a substantial

step ahead for long period accumulation and migration

process of natural gasses available in huge quantities in

‘gas- hydrate’ in marine sediments in Makran offshore.

Interpretation of high resolution seismic reflection data

when integrated with other geological experiments may

increase the existing knowledge about gas-hydrates that

will help in quantification and zonation of methane gas-

hydrate reserves in Makran.

ACKNOWLEDGMENTS

The authors are much obliged to reviewers of this manuscript

for critical review and suggestions in anticipation, we are

also thankful to Mr. M. Tahir for his valuable remarks and

scholastics feedback.

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