Geophysical Attributes of Natural Gas Hydrates

Geology 420: Spring 2014

Geophysical Attributes of Natural Gas Hydrates By: Shanna Mason

Geophysical Attributes of Natural Gas Hydrates

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TABLE OF CONTENTS

LIST OF FIGURES………………………………………………………………………….………2

ABSTRACT……………………………………………………………….………………………….3

INTRODUCTION…………………………………………………….…………….………………4

SEISMIC WAVE PROPAGATION IN GAS HYRATES…………………………………6

GEOPHYSICAL ANOMALIES OF GAS HYDRATE BEARING SEDIMENTS….10

BOTTOM SIMULATING REFLECTOR……………………………………………10

AMPLITUDE ANOMALIES……………………………………………………………13

ANOMALIES CAUSED BY GAS FLOW…………………………………………..14

CONCLUSION……………………………………………………………………………………..15

REFERENCES CITED…………………………………………………………………………….18


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LIST OF FIGURES

Figure 1. Gas Hydrate structures (Makogon, 1981)…………………………………….........20

Figure 2. Phase boundary diagram (Kvenvolden and McMenamin,

1980)………………………………………………………………………………………………….…….………..20

Figure 3. Conoco Milne Point Unit D-‐1 (Lee et al., 2009)…………………….….…………..21

Figure 4. Biot-‐Gassman Theory (Lee et al., 2009)…………………………..……………………21

Figure 5. Velocities of Gas Hydrate bearing sediments with respect to water

porosities (Lee and Collett, 2000)………………………………………………………..……………..22

Figure 6. P-‐wave and S-‐wave velocities (Sava and Hardage, 2009)……………………..23

Figure 7. P-‐wave attenuation (Chand and Minshull, 2004)………………………………….23

Figure 8. 12-‐fold seismic reflection profiles (Shipley et al., 1979)……………………….24

Figure 9. Seismic reflection profile of the Outer Blake Ridge area (Chand and

Minshull, 2003)…………………………………………………………………………………………………..25

Figure 10. Gas Chimneys in a seismic section of the Outer Blake Ridge area

(Holbrook et al., 2002)……………………………………………………………………………………….25

Figure 11. Blake Ridge seismic section, line R38 (Holbrook et al., 2002)…………….26


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Abstract

Since their discovery, natural gas hydrates have been postulated as a potential source of

harvestable energy. The importance in the research conducted on natural gas hydrates does

not stop there. Gas hydrates also pose a problem when drilling in areas that contain sediments

bearing them. Luckily, the nature of natural gas hydrates enables them to have certain

indicative geophysical attributes present in reflection seismic surveys. It is these attributes

upon which this paper will focus on.

First, we will take a look at the structure of natural gas hydrates and their occurrence in nature.

This will lead us into the elastic properties of gas hydrates and the way they alter the elastic

properties of the sediments they form in. Elastic properties determine the way that seismic

waves move through a rock unit, specifically the seismic velocities of both primary and

secondary waves. The way these waves move through rock units containing gas hydrates

determines what is shown in a seismic reflection survey. These important geophysical

attributes include; the bottom-‐simulating reflector, amplitude anomalies, and anomalies

caused by gas or fluid flow. Each geophysical attribute specific to gas hydrates can be explained

according to the elastic properties of the containing sediment and the way seismic waves move

through them.


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Introduction

Gas hydrates are crystalline inclusion compounds, or clathrates, in which a lattice of

water molecules traps gas molecules. These special water molecules have hydrogen bonds that

cause the water molecules to align in regular orientations, giving them their lattice-‐like

structure (Carroll, 2003). During crystallization gas molecules are forced into a much smaller

volume than would occur in a gas or liquid state, increasing their density (Max and Lowrie,

1996). The specific structure formed by the water molecules is dependent on the gas molecules

that were in contact with the water during crystallization. The amount of gas trapped within the

lattice depends on the external temperature and pressure conditions present during

crystallization (Makogon, 1981). Natural gas hydrates are characterized by two structures,

structure 1 and structure 2, that occur together as a unit cell. The amount of concentration is

termed hydrate saturation (See Figure 1a and 1b, Makogon, 1981).

The formation of a gas hydrate requires three things: a hydrate former, which in nature

consists of mostly methane, with trace amounts of other hydrocarbons thrown in. Certain

temperature and pressure conditions must also be present, preferably low temperatures under

high pressure. The last thing needed is just the right amount of water molecules present, not

too few and not too many (Carroll, 2003). Conditions of hydrate formation are favorable in

permafrost and deep oceanic settings. The source of natural gas in these regions appears to be

either in the sediments themselves or trapped beneath “caps” of gas hydrates. Hydrates can

also form freely in water, however unless they attach to sediment they simply float to the

surface and dissipate (Max and Lowrie, 1996). The Gas Hydrate Phase Diagram more accurately


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describes the conditions necessary to hydrate formation (See Figure 2, Kvenvolden and

Mcmenamin, 1980). The hydrate-‐gas phase boundary shown is accurate for a pure methane

system. If NaCl is present it lowers the temperature of hydrate formation, shifting this

boundary to the left. If other types of hydrocarbons are present the temperature increases, and

this boundary shifts to the right (Kvenvolden and Mcmenamin, 1980).

Gas hydrates occur in a zone of thermodynamic equilibrium that parallels the sea floor

and extends to a certain depth, dependent on the geothermal gradient of the region (Max and

Lowrie, 1996). The Gas Hydrate Stability Zone (GHSZ), shown in the shaded region of the

diagram in Figure 2, is an area beneath the surface in which heat convection from below

stabilizes in temperature by dissipating into the sea. Leaving a zone of stable temperature and

pressure in which hydrates can form (Max and Lowrie, 1996).

Gas hydrates occur physically in nature four ways; (1) As residents in the pore space of

coarse-‐grained rocks, (2) solid unit cells filling fractures in rock, (3) disseminated in fine-‐grained

rocks, (4) a massive solid geological unit composed mainly of gas hydrates with a small amount

of sediment (Collett, 1929). In the last two cases the gas hydrates act as part of the sediment

framework. However, gas hydrates have been shown to preferentially form either with or

around existing fractures or in the pore space of sand-‐rich reservoirs. This is due to the lower

capillary pressure present in the pore space of coarse-‐grained sediment, or secondary pore

spaces, that permits the migration of gas hydrates (Collett, 1929).


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Seismic Wave Propagation in Gas Hydrates

To understand the importance of the way seismic waves travel through gas hydrate

bearing sediments, it is beneficial to first discuss the basic properties of seismic waves. . The

elastic modulus of a medium quantitatively describes its ability to withstand stress and strain.

Seismic waves are elastic strain energy emitted from a seismic source through a certain type of

medium (Kearey and Brooks, 1984). As these waves move through a medium energy carried

with them is spread out in a circle. Due to this geometrical spreading, energy decreases along

the ray path during propagation. Energy is also lost in this ray path according to the elastic

properties of the rock units it is moving through. Energy along the ray path decreases until the

ray eventually stops. This gradual decrease in energy of seismic waves is called attenuation

(Kearey and Brooks, 1984).

There are two types of seismic waves, Compressional (P-‐waves) and Shear waves (S-‐

waves). P-‐waves propagate in the direction of wave travel, changing both the volume and

shape of the medium it is travelling through. S-‐waves travel by pure shear strain, perpendicular

to the direction of wave travel, and only affect the shape of the medium. The velocity of a

seismic wave is determined by (Kearey and Brooks, 1984):

𝑉𝑒𝑙𝑜𝑐𝑖𝑡𝑦 = 𝐸𝑙𝑎𝑠𝑡𝑖𝑐 𝑀𝑜𝑑𝑢𝑙𝑖 𝑜𝑓 𝑀𝑎𝑡𝑒𝑟𝑖𝑎𝑙𝐷𝑒𝑛𝑠𝑖𝑡𝑦 𝜌 𝑜𝑓 𝑀𝑎𝑡𝑒𝑟𝑖𝑎𝑙

The elastic properties of gas hydrates can be described as a function of four things; (1)

elastic properties of the host sediment, (2) elastic properties of pure gas hydrates, (3) hydrate

saturation, and (4) the way that the hydrates are distributed in the sediment (Sava and


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Hardage, 2009). The studies discussed in the following section describe how elastic properties,

and the ways in which seismic waves propagate through gas hydrates, enable us to determine

areas containing gas hydrate bearing sediments in seismic reflection surveys.

In arctic settings, gas hydrate saturation increases proportionally to sediment porosity.

This was indicated by the findings at the Conoco Milne Point Unit D-‐1 well, on the North Slope

of Alaska and modeled by Lee et al. in “Seismic-‐attribute Analysis for Gas-‐hydrate and Free-‐gas

Prospects on the North Slope of Alaska (2009).” The area where this well is located contains

sediment characterized by unconsolidated sandstones with interbedded siltstones and shale.

The geothermal gradient restricts the GHSZ to approximately 850-‐1000 meters. Figure 3a shows

the P-‐wave velocity, density porosity, and clay content for sediments within the GHSZ at the

Conoco Milne Point Unit D-‐1 well. The data shows that sand sediments have low P-‐wave

velocities and high density porosities. This was used to estimate a gas hydrate saturation in the

sandstones of 80%. The silt and shale sediments were shown to have higher P-‐wave velocities

and lower density porosity. Lee and Collett used the results obtained from this well to obtain

the Reservoir and Seal model shown in Figure 3b. Then, using the Biot-‐Gassman theory and the

Reservoir and Seal model in Figure 3b, they calculated seismic velocities in gas hydrates against

the percentage of hydrate saturation (See Figure 4, Lee et al., 2009). The Biot-‐Gassman theory

relates the elastic moduli of gas hydrates with the pressure present to estimate the

compressional and shear wave velocities (Lee et al., 2009).

Lee and Collett studied seismic velocities in gas hydrate bearing sediments of varying

porosity in a Canadian well named Mallik 2L-‐38 (2000). Using a three-‐phase weighted equation

for both P and S-‐waves, they calculated normal trend curves for both gas hydrate sediments


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and non-‐gas hydrates. These trend curves were then compared from data obtained from Mallik

2L-‐38. Figure 5 shows the calculated normal trend and the data set obtained from the well (Lee

and Collett, 2000). On comparison, the data varies slightly from the calculated trend. The

authors attribute this to differing concentrations of gas hydrates present in the sediment.

When this is accounted for in calculations, the velocities mimic the normal trend curve. The

overall relationship described by the trend curve indicates that as velocity increases, porosity

decreases (Lee and Collett, 2000).

Diana Sava and Bob Hardage created rock physics models in an attempt to be able to

infer gas-‐hydrate concentrations from seismic measurements. They describe four rock physics

models, each model varying in the way that gas hydrates occupy sediment. To calculate

velocities in each case the authors determine at which point each model meets the critical

porosity. They define this as the point at which grains in sediment cease to float in a grain

matrix and become part of the sediment framework. Each model uses different calculations for

gas hydrate concentrations above and below the critical porosity. These models were then

compared to published laboratory studies in confirmation of their methods (Sava and Hardage,

2009).

The four models outlined by Sava and Hardage are: Model A assumes hydrates act as a

part of the load-‐bearing frame of a sediment. Model B assumes gas hydrates are disseminated

throughout the rock but occupy the pore space and have no effect on the frame. Model C

assumes hydrates to occur in layers between unconsolidated water-‐saturated marine sediment.

Model D assumes hydrates to also occur in layers, but in this model the gas hydrates occupy

99% of the pore space, with a small amount of sediment in between (Sava and Hardage, 2009).


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The results of the calculations used show P and S-‐wave velocities as they relate to gas hydrate

concentration in each of the four models (See Figure 6, Sava and Hardage, 2009). Results

showed that both P and S-‐wave velocities increase with gas hydrate concentration. However,

the amount of increase varies in each model. P-‐wave velocity increased the highest in models A

and D, and the smallest increase occurred in model C. S-‐wave velocity increased insignificantly

in model B, when the gas hydrates were not connected to the mineral frame. They also found

that the pore-‐filling model, model B was only physically realistic in areas of small gas hydrate

concentrations. At higher concentrations, model A should be considered instead of model B

(Sava and Hardage, 2009). Furthermore when the velocities are plotted as a ratio, in all models

except for B, the ratio decreases with gas hydrate concentration. The difference that occurs in

model B is due to the P-‐wave velocity increasing while the S-‐wave velocity stays relatively

constant (See Figure 6, Sava and Hardage, 2009). A large increase in S-‐wave velocity only occurs

in small gas hydrate concentrations when the wave is propagating with its displacement vector

polarized parallel to the layers, as in model D. This indicates that in similar environments we

may be able to deduce gas hydrate concentrations from S-‐wave information (Sava and Hardage,

2009).

In “Effect of Hydrate Content on Seismic Attenuation,” Chand and Minshull relate

seismic properties of hydrate bearing sediments to porosity, mineralogy, micro-‐structure, clay

particle anisotropy and hydrate saturation (2004). The forward model they produced predicted

both P and S wave attenuation to increase with hydrate saturation and frequency (See Figure 7,

Chand and Minshull, 2004). This is odd because gas hydrates should increase the strength of

sediments and therefore decrease attenuation. The model was then compared with data


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observed at the Mallik 2L-‐38 well in the Mackenzie Delta, Canada. The data was used as

evidence to support their theory that this increase in attenuation can be attributed to a

difference in permeability between the host sediments and the amount of fluid flow in the rock.

From these studies it is obvious that the formation of gas hydrates within a sediment

changes the sediment’s elastic properties. Especially if the gas hydrates are cementing the

sediment grains. Both primary and secondary seismic velocities increase, depending on the

amount of gas hydrate saturation, primary velocity most notably (Lee and Collett, 2009). Gas

hydrates have also been shown to prefer sediments with large pore spaces, or as the filling of

large secondary pore spaces, such as joints and fractures (Chand and Minshull, 2003).

Understanding the elastic behavior caused by gas hydrate formation in marine sediments

allows us to explain the various anomalies found in seismic reflection surveys.

Geophysical Anomalies of Gas Hydrate Bearing Sediments

Bottom-‐Simulating Reflector

The most notable method for determining the presence of gas hydrates in a seismic section is a

Bottom Simulating Reflector (BSR). The interpretation of the BSR is dependent upon the

knowledge of how seismic waves propagate through gas hydrates and their relation to

surrounding mediums. A seismic reflection survey measures the amount of energy that is

reflected at an interface in the subsurface, this energy is described quantitatively as a fraction

termed the Reflection Coefficient and is basically the ratio of acoustic impedance between

layers (Kearey and Brooks, 1984):


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𝑅𝑒𝑓𝑙𝑒𝑐𝑡𝑖𝑜𝑛 𝐶𝑜𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑡 = 𝐴𝑚𝑝𝑙𝑖𝑡𝑢𝑑𝑒 𝑅𝑒𝑓𝑙𝑒𝑐𝑡𝑒𝑑𝐴𝑚𝑝𝑙𝑖𝑡𝑢𝑑𝑒 𝐼𝑛𝑐𝑖𝑑𝑒𝑛𝑡

= 𝑉𝑒𝑙𝑜𝑐𝑖𝑡𝑦!"#$% !𝐷𝑒𝑛𝑠𝑖𝑡𝑦!"#$% ! − 𝑉𝑒𝑙𝑜𝑐𝑖𝑡𝑦!"#$% !𝐷𝑒𝑛𝑠𝑖𝑡𝑦!"#$% !𝑉𝑒𝑙𝑜𝑐𝑖𝑡𝑦!"#$% !𝐷𝑒𝑛𝑠𝑖𝑡𝑦!"#$% ! + 𝑉𝑒𝑙𝑜𝑐𝑖𝑡𝑦!"#$% !𝐷𝑒𝑛𝑠𝑖𝑡𝑦!"#$% !

𝑾𝒉𝒆𝒓𝒆:

𝑉𝑒𝑙𝑜𝑐𝑖𝑡𝑦!"#$% !𝐷𝑒𝑛𝑠𝑖𝑡𝑦!"#$% ! = 𝑨𝒄𝒐𝒖𝒔𝒕𝒊𝒄 𝑰𝒎𝒑𝒆𝒅𝒂𝒏𝒄𝒆 𝒐𝒇 𝑳𝒂𝒚𝒆𝒓 𝟏

𝑉𝑒𝑙𝑜𝑐𝑖𝑡𝑦!"#$% !𝐷𝑒𝑛𝑠𝑖𝑡𝑦!"#$% ! = 𝑨𝒄𝒐𝒖𝒔𝒕𝒊𝒄 𝑰𝒎𝒑𝒆𝒅𝒂𝒏𝒄𝒆 𝒐𝒇 𝑳𝒂𝒚𝒆𝒓 𝟐

𝑉𝑒𝑙𝑜𝑐𝑖𝑡𝑦!"#$% !𝐷𝑒𝑛𝑠𝑖𝑡𝑦!"#$% ! > 𝑉𝑒𝑙𝑜𝑐𝑖𝑡𝑦!"#$% !𝐷𝑒𝑛𝑠𝑖𝑡𝑦!"#$% != 𝑷𝒐𝒍𝒂𝒓𝒊𝒕𝒚 𝒐𝒇 𝒘𝒂𝒗𝒆 𝒖𝒏𝒄𝒉𝒂𝒏𝒈𝒆𝒅

𝑉𝑒𝑙𝑜𝑐𝑖𝑡𝑦!"#$% !𝐷𝑒𝑛𝑠𝑖𝑡𝑦!"#$% ! < 𝑉𝑒𝑙𝑜𝑐𝑖𝑡𝑦!"#$% !𝐷𝑒𝑛𝑠𝑖𝑡𝑦!"#$% != 𝑷𝒐𝒍𝒂𝒓𝒊𝒕𝒚 𝒐𝒇 𝒘𝒂𝒗𝒆 𝒓𝒆𝒗𝒆𝒓𝒔𝒆𝒅

The BSR is a reflection of a chemical phase boundary, it marks the interface of either

high velocity gas hydrate bearing sediments over sediments of normal velocity, or between gas

hydrate bearing sediments and low velocity free gas trapped beneath. This appears in a seismic

section to run parallel to the bedding plane cross-‐cutting the strata (Chand and Minshull, 2003).

BSRs can appear in a seismic section in multiple environments, but one of the key

determinations of gas hydrate bearing sediments is a polarity reversal (Holbrook et al., 2002). In

most environments, the BSR will have the same polarity as the sea floor reflection. However

when gas hydrates are present, polarity above and below the BSR are reversed (Sunjay et al.,

2011).

The occurrence of a BSR beneath the subsurface in a GHSZ is the most important

geophysical indicator of the presence of gas hydrates. However, gas hydrates can be present

without a BSR. In the case where free gas is trapped beneath, a BSR must be indicated in a


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seismic section. This is due to the fact that hydrate bearing sediments must sufficiently stiffen

the medium to trap the gas (Sunjay et. al., 2011).

Using seismic methods when quantifying gas hydrate saturation depends on the

assumption that primary velocity increases with hydrate saturation above the BSR. More so

than the normal velocity increase that occurs in sediment due to compaction. Below the BSR,

velocity is assumed to decrease due to the presence of free gas or the absence of hydrate

bearing sediments. Secondary velocity is assumed to incrementally increase, but only

noticeably if the hydrate acts as a cement in the sediment (Chand and Minshull, 2003).

The initial confirmation of a BSR’s appearance that indicated the presence of gas

hydrates stems from seismic reflection surveys completed on the Blake Outer Ridge, in the

Atlantic Ocean off of the East Coast of the United States. Geophysicists at the Lamont-‐Doherty

Geological Observatory noticed that BSRs in seismic surveys of this area appeared to cross-‐cut

strata in multiple areas (Kvenvolden, 1983). Later Sonobuoy measurements conducted by

Bryan, confirmed the relation of BSRs observed in the Blake Outer Ridge to gas hydrate bearing

sediments (Holbrook et al., 2002).

The BSR in the Blake Outer Ridge occurs at 0.6 seconds sub-‐bottom depth on the flank

of the Ridge, and has been determined the base of the gas hydrate formation in this area (See

Figure 8a, Shipley et al., 1979). Landward dipping bedding plane reflections are also seen in

seismic sections, composed of silty clay beds intersecting the BSR (Shipley et al., 1979). The high

amplitude calculated in this area led Bryan to suggest that the medium stiffness could be high

enough to trap free gas beneath (Shipley et al., 1979).


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Kvenvolden and Barnard outline the three types of geophysical evidence present in the

Blake Outer Ridge that indicate gas hydrate bearing sediments in, “Hydrates of Natural Gas in

Continental Margins (1980).” The first is the presence of a BSR, the second being sonobuoy

measurements indicating higher velocity above and lower velocity below the acoustic reflector,

and lastly a polarity reversal above and below the BSR. The reflector has been shown to cut

across the strata and parallel the seafloor, with an interestingly high P-‐wave velocity above the

reflector (Kvenvolden and Barnard, 1980).

Since the discovery at the Blake Outer Ridge, multiple BSRs have been attributed to gas

hydrate bearing sediments around the world. In a seismic section off the coast of Panama, a

BSR was discovered at 0.4 seconds sub-‐bottom depth (See Figure 8b, Shipley et al., 1979).

Offshore of the Nicoya Peninsula in Costa Rica occurs a BSR in less than 1,000 meters water

depth, similarly in the American Trench south of Acapulco, Mexico (See Figure 8c and 8d,

Shipley et al., 1979).

Amplitude Anomalies

Depending on gas hydrate concentration, seismic reflectance can either be enhanced or

suppressed. It has been shown that gas hydrates tend to form in more porous strata, which has

less velocity. Once formed, the gas hydrates raise the velocity of the more porous strata,

relative to the higher velocity layers that surround it. This occurs mostly in small gas hydrate

concentrations, reducing the impedance contrast between the hydrate bearing sediments and

the surrounding medium. This is termed “blanking” when seen on a seismic section. At high gas

hydrate concentrations, gas hydrate bearing sediments can have a significantly higher velocity


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than the surrounding medium, enhancing seismic reflectance (See Figure 9, Holbrook et al.,

2002). These are shown as “bright spots” or enhanced reflections and are most commonly from

free gas dispersed in sediments, which scatters acoustic energy, resulting in this anomaly’s

appearance in a seismic section (Popescu et al., 2007).

Anomalies Caused by Gas and Fluid Flow

Other disturbances appearing in a seismic reflection survey are narrow zones of low reflectivity

that can be attributed to the seepage of gas or the movement of sediment below the surface

(Popescu et al., 2007). In both cases, heat is drawn up from below cooling the temperature

beneath the surface. Because gas hydrates can only form in a stable environment of certain

temperature and pressure ranges, the GHSZ is shifted when these disturbances occur. When

the temperature beneath the surface shifts enough and the GHSZ changes, so does the location

of the BSR. During seismic reflection surveys, if this phase boundary shift was recent, a

spurious BSR may be indicated in the seismic section. This is called a Paleo-‐BSR (Chand and

Minshull, 2003). During a GHSZ phase shift gas hydrates destabilize, causing a rapid release in

gas. This is evident as chimneys in seismic sections (See Figure 10 and 11, Holbrook et al., 2002).

Water seepage could also be the cause. As gas hydrates form, water in the pore space of the

sediments is pushed out causing shifts in the subsurface. This can also be attributed to some of

these disruptions seen in seismic surveys (Chand and Minshull, 2003).


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Conclusion

(1) Gas hydrate bearing sediments actively change the elastic properties of the sediments

they form in. Especially in a case where the gas hydrates are acting as the cement of the

sediment. This change in elastic properties is reflected in the behavior of seismic waves

as they move in gas hydrate bearing sediment.

(2) Gas hydrates have been show to preferentially form in two cases; in the pore space of

coarse-‐grained sediment, or either with or around secondary pore spaces such as joints

and fractures. In any sediment they occupy, gas hydrate saturation increases with

porosity.

(3) Both P and S-‐wave velocities increase with gas hydrate saturation. P-‐wave velocity

increases the most with gas hydrate saturation. S-‐wave velocity changes minimally,

unless the gas hydrates are part of the framework of the sediment.

(4) The behavior of seismic waves in gas hydrate bearing sediments depends on how they

occur in nature. Understanding these behaviors enable us to determine the anomalies

that occur in a seismic reflection survey as indicative of the presence of gas hydrate

bearing sediments.

(5) Seismic attenuation increases with gas hydrate saturation instead of decreasing. This is

due to the difference in permeability between the host sediments and the amount of

fluid flow present.

(6) The presence of a BSR in an area that can be classified as the GHSZ is the most

important indicator of the presence of gas hydrate bearing sediments. A BSR was first


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noticed in seismic sections taken at the Blake Outer Ridge to be cross cutting strata. It

was not until later that studies confirmed its relation to the presence of gas hydrates.

Since this discovery, multiple BSRs have been found and correlated to the presence of

gas hydrates.

(7) Depending on the strata in which they formed, gas hydrates can either suppress or

enhance reflectance in a seismic survey. At small gas hydrate concentrations “blanking”

occurs. When the subsurface contains sediments with high concentrations of gas

hydrates, “bright spots” occur on the seismic section.

(8) Multiple other anomalies seen in seismic section can be attributed to gas or fluid flow

within the sediment caused by gas hydrates. These include chimneys, pipes, narrow

zones of low reflectivity, paleo-‐BSRs, etc. If the GHSZ shifts, gas hydrates destabilize and

release gas. Also during formation of gas hydrates in sediment, the water already

present in the pores is pushed out. This causes the disruption of normal subsurface

sedimentary structures.


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References Cited

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Chand, S., and T. Minshull, 2003, Seismic Constraints on the Effects of Gas Hydrate on Sediment Physical Properties and Fluid Flow: A Review: Geofluids, v. 3, p. 275-‐289.

Chand, S., and T. Minshull, 2004, The Effect of Hydrate Content on Seismic Attenuation: A Case Study for Mallik 2L-‐38 well data, Mackenzie Delta, Canada: Geophysical Research Letters, v. 31, L14609, p. 1-‐4.

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Holbrook, W. S., A. R. Gorman, M. Hornbach, K. L. Hackwith, and J. Nealon, 2002, Seismic Detection of Marine Methane Hydrate: The Leading Edge, July 2002, P. 686-‐689.

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Kvenvolden, K. A., and M. A. McMenamin, 1980, Hydrates of Natural Gas: A Review of Their Geologic Occurrence: Geological Survey Circular, 825, 6 p.

Kvenvolden, K. A. and L. A. Barnard, 1983, Hydrate of Natural Gas in Continental Margins, in Watkins, J. S. and C. L. Drake, eds., in Studies in Continental Margin Geology: AAPG Memoir 34, p. 631-‐640.

Lee, M. L., and T. S. Collett, 2000, Elastic Properties of Gas Hydrate-‐Bearing Sediments: Geophysics, v. 66, no. 3, p. 763-‐771.

Lee, M. W., T. S. Collett, and T. L. Inks, 2009, Seismic-‐Attribute Analysis for Gas-‐Hydrate and Free-‐Gas Prospects on the North Slope of Alaska, in T. Collett, A. Johnson, C. Knapp, and R. Boswell, eds., Natural Gas Hydrates: Energy Resource Potential and Associated Geologic Hazards: AAPG Memoir 89, P. 541-‐554.

Makogon, Y. F., 1981, Hydrates of Natural Gas: Tulsa, Oklahoma, PennWell Books, 237 p.

Max, M. D., and A. Lowrie, 1996, Oceanic Methane Hydrates: A “Frontier” Gas Resource: Journal of Petroleum Geology, v. 19, no. 1, p. 41-‐56.

Sava, D., and B. Hardage, 2009, Rock-‐Physics Models for Gas-‐Hydrate Systems Associated with Unconsolidated Marine Sediments, in T. Collett, A. Johnson, C. Knapp, and R. Boswell, eds., Natural Gas Hydrates: Energy Resource Potential and Associated Geologic Hazards: AAPG Memoir 89, p. 505-‐524.


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Shipley, T. H., M. H. Houston, R. T. Suffler, F. Jeanne Shaub, K. J. Mcmillen, J. W. Ladd, and J. L. Worzel, 1979, Seismic Evidence for Widespread Possible Gas Hydrate Horizons on Continental Slopes and Rises: AAPG Bulletin, v. 63, no. 12, p. 2204-‐2213.

Sunjay, M. Banerjee, and N. P. Singh, 2011, Geophysical Techniques for Exploration and Production of Gas Hydrate: Proceedings of the 7th International Conference on Gas Hydrates, Edinburgh, Scotland, July 17-‐21, 3 p.


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Figure 1: A: Gas Hydrate Structures: a) form with 14 faces, Structure 1, b) Pentagonal Dodecahedron, c) form with 16 faces, Structure 2. B: Unit Cells: a) Structure 1, b) Structure 2. (from Makogon, 1981)

Figure 2: Phase Boundary Diagram showing free methane gas and methane hydrate for a fresh water-‐pure methane system. Addition of NaCl to water lowers temperature of hydrate formation, in effect shifting gas-‐hydrate curve to the left. Addition of other hydrocarbons raise the temperature of hydrate formation, shifting the curve to the right. Therefore, impurities in natural gas will increase area of hydrate stability field (from Kvenvolden and Mcmenamin, 1980).


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Figure 4: Modeled velocities using the modified Biot-‐Gassman theory in the Lee, Collett, and Inks study. A) Modeled seismic velocities with respect to gas hydrate saturation, in sediment with 38% porosity and 10% clay content. B) Modeled seismic velocities with respect to gas saturation, in a sediment with 38% porosity and 10% clay content (from Lee et al., 2009).

Figure 3: A) Graph showing measured P-‐wave velocities, porosities, and clay contents at depths between 2500-‐3000 ft. Data obtained at the Conoco Milne Point Unit D-‐1 well on the North Slope of Alaska. B) Reservoir and Seal model obtained from well data and used for further model calculations (from Lee et. al., 2009).


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Figure 5: Velocities of gas hydrate bearing sediments with respect to water porosities. Velocities calculated in the study by Lee and Collett (2000) are shown, as well as data collected from the Mallik 2L-‐38 well (from Lee and Collett, 2000).


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Figure 7: Top: P-‐wave attenuation by Chand and Minshull (2004). Modeled in sediment with equal amounts of clay and quartz as a function of frequency (Hz) and gas hydrate saturation. Bottom: Predicted S-‐wave attenuation. Modeled in the same sediment as top (from Chand and Minshull, 2004).

Figure 6: Top Left: P-‐wave velocity as a function of the volumetric fraction of gas hydrate (Cgh) in sediments of pure quartz for the four models discussed in the Sava and Hardage study (2009) Model C, Dotted line represents layers of pure gas hydrates producing slow P-‐waves, solid line represents fast P-‐waves. Model D, layers of disseminated, load-‐bearing gas hydrates producing slow P-‐waves (dotted line), and fast P-‐waves (solid line).

Bottom Left: S-‐wave velocity plotted in the same way as the top left. Model C, S-‐ waves with slow polarization (dotted line), S-‐waves with fast polarization (solid line). Model D, same as above but shows slow and fast S-‐wave polarization on solid and dotted line, respectively.

Top Right: P-‐wave/S-‐wave ratio as a function of the volumetric fraction of gas hydrate (Cgh) in sediments of pure quartz for the four models in the study discussed by Sava and Hardage (2009). Models C and D same Ratio of P and S-‐waves indicated above. (from Sava and Hardage, 2009).


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Figure 8a: 12-‐fold multichannel seismic reflection profile from the crest and eastern flank of the Blake Outer Ridge.

8b: 12-‐fold seismic reflection profile from offshore of the Pacific Ocean in Panama.

8c: 24-‐fold seismic reflection profile from the Nicoya Peninsula, Costa Rica.

8d: 24-‐fold seismic reflection profile of Middle America Trench, south of Acapulco, Mexico (from Shipley et al., 1979).


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Figure 9: Single channel seismic reflection profile of the Outer Blake Ridge area, indicating a blanking zone and the BSR (from Chand and Minshull, 2003).

Figure 10: Chimneys evident in a seismic section taken at the Blake Out Ridge, line 3D-‐03 (from Holbrook et al., 2002).


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Figure 11: Seismic section taken at the Blake Outer Ridge, line R38. Shows high-‐amplitude reflections and a disrupted BSR attributed to movement of free gas beneath the surface due to gas hydrates (from Holbrook et al., 2002).

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Geophysical Attributes of Natural Gas Hydrates

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