GEOLOGICAL SURVEY CIRCULAR 825 

Hydrates of Natural Gas: A Review 

of Their Geologic Occurrence 

Hydrates of Natural Gas: 

A Review of Their Geologic Occurrence 

By Keith A. Kvenvolden and Mark A. McMenamin 

GEOLOGICAL SURVEY CIRCULAR 825 

1980 
United States Department of the Interior 

CECIL D. ANDRUS, Secretary 

Geological Survey 

H. William Menard, Director 

Free on application to Branch of Distribution, U.S. Geological Survey, 1200 South Eads Street, Arlington, VA 22202 
CONTENTS 

Page 

Summary ---------------------------------------------------------------- 1 

Introduction ----------------------------------------------------------- 2 

Gas hydrates in permafrost regions ------------------------------------- 2 

Gas hydrates in oceanic sediments -------------------------------------- 4 

Bottom-simulating reflectors -------------------------------------- ~ 

Other seismic evidence for gas hydrates in oceanic sediments ------ 7 

Selected bibliography -------------------------------------------------- 8 

ILLUSTRATIONS 

Page 

Figure 1. Phase boundary diagram showing free methane gas and methane 

hydrate --------------------------------------------------2 

2. Map showing location of known and inferred occurrences of gas 

hydrates in permafrost regions and in oceanic sediment ---3 

3. Graph of gas hydrate zone shown as intersection of geothermal 

gradient with hydrate-gas phase boundary -----------------4 

4. Diagram showing idealized section of methane gas hydrate zone 

in sediments of outer continental margins ----------------5 

5. Diagrammatic seismic profile of Blake-Bahama Outer Ridge 

showing bottom-simulating reflector ----------------------6 

TABLES 

Page 

Table 1. Geologic, geophysical, and geochemical evidence for gas hydrates beneath permafrost ---------------------------------3 

2. Geologic, geophysical, and geochemical evidence for gas hydrates in the seafloor ------------------------------------5 

m 

Hydrates of Natural Gas: 

A Review of Their Geologic Occurrence 

By Keith A. Kvenvolden and Mark A. McMenamin1 

SUMMARY 

Gas hydrates are a type of inclusion compound or clathrate formed as icelike mixtures of 

gas and water in which gas molecules are trapped 

within a framework or cage of water molecules. 

Large quantities of natural gas (mainly methane) 

can be trapped in sediments in the form of gas 

hydrates. The pressure and temperature conditions for the formation of gas hydrates are found 

in regions of permafrost and beneath the sea in 

outer continental margins and ocean basins. In 

addition to suitable temperature and pressure 

conditions, sufficient concentrations of methane 

must be present for hydrate stability. This 

methane may be produced in sediments by biological processes; however, the extent of this 

production is not fully understood. Methane 

could also migrate from thermochemically altered 

organic matter at depth. 

The presence of gas hydrates in permafrost 

regions has been established at the Messoyakha 

gas field in western Siberia, in two exploratory 

wells in the Mackenzie Delta area of Canada, and 

in a wildcat well on the North Slope of Alaska. 

In the Alaskan well, a pressure core barrel successfully recovered gas hydrate in sediment at 

in situ temperature and pressure. Although gas 

hydrates have now been identified in permafrost 

areas, the geographical extent of these hydrates 

has not yet been determined. 

Geologic, geochemical, and geophysical evidence suggests that gas hydrates can exist in 

many areas beneath the seafloor. Geologic evidence consists of (1) widespread areas where 

pressures and temperatures are suitable for gashydrate stability and (2) recovery of deep-sea 

sediment cores that release gas and cool endothermically when exposed to sea-level temperature 

and pressure. Geochemical observations include 

(1) high concentrations of methane in sediment, 

1

Present address: Department of Geological Sciences, University of California, 

Santa Barbara, CA 93106 

usually accompanied by low concentrations of ethane, and (2) gas released from thawing cores in 

the volumes expected for the decomposition of gas 

hydrate. Geophysical evidence includes (1) seismic anomalies that can be explained by the properties of sediments containing hydrates, (2) high 

acoustic velocity in sediments from suspected 

hydrate zones, and (3) laboratory experiments 

which show that the acoustic velocities of sediment increase when gas hydrate is formed in the 

sediment. 

The main seismic evidence for submarine gas 

hydrates are reflectors that simulate the topography of the seafloor. Such reflectorP appear on 

seismic records from the east coast of North 

America, the North Pacific, the Bering Sea, and 

the Pacific and Atlantic coasts of Central America. They lie anywhere from 100 to 1,100 meters 

below the bottom. Bottom-simulating reflectors 

have been seen only in water deeper than 400 m 

and are generally manifest on seismic records as 

a reflection polarity reversal caused [y an abrupt decrease in seismic velocity. The exact 

relations of bottom-simulating reflectors to gas 

hydrate zones are uncertain. The change in seismic impedance may mark the bottom of th~ gashydrate zone where free methane gas is trapped 

beneath the hydrate layer. Other features on 

seismic records that may indicate the presence of 

hydrates are bottom-paralleling bright spots, 

pagoda structures, and deep-water velocity amplitude features (VAMP's). 

Not all bottom-simulating reflectors may be 

directly related to gas hydrates. For e.xample, a 

bottom-simulating reflector lacking polErity reversal in sediments of the Bering Sea is currently thought to represent a migrating diagenetic 

boundary related to the dissolution of diatoms 

and the induration of claystone. 

If large quantities of gas hydrate are widespread in permafrost regions and in offshore 

marine sediments, they may be potential energy 

resources. The gas within the hydrate itself may 
not be recoverable, but the free gas trapped beneath it may be the important resource. 

INTRODUCfiON 

Gas hydrates are a special kind of clathrate 

formed from a mixture of natural gas and water. 

Clathrates are inclusion compounds in which two 

or more chemical compounds form a stable crystalline mixture. One constituent forms the clathrate framework, a molecular lattice with an ordered geometric pattern. The other constituents 

of the clathrate fill the sites or cages within 

the framework structure. A clathrate framework, 

because it may contain many vacant sites, is a 

nonstoichiometric substance (a substance that 

cannot be accurately described by a single chemical formula). Water, because of its hydrogenbonding properties, is an excellent clathrate 

framework compound. An expanded ice lattice 

forms cages that can contain gas molecules such 

as argon, methane, propane, isobutane, sulfur 

dioxide, carbon dioxide, and hydrogen sulfide. 

The term "gas hydrate" is used here to mean 

a water-methane clathrate that may or may not 

contain ethane, propane, and other hydrocarbons. 

Under suitable pressures, natural gas, which contains mainly methane, can interact with water to 

form a solid icelike mixture, that is, a gas hydrate, at temperatures considerably above the 

freezing point of water (fig. 1). The amount of 

methane needed for hydrate formation depends on 

temperature and pressure. In the methane-water 

system, only methane present in excess of the 

amount soluble in water is available for hydrate 

formation. In an ideally saturated hydrate, with 

all cages filled by methane molecules, the molar 

ratio of methane to water can be nearly 1:6. The 

formula for an ideal stoichiometric methane-water 

clathrate is CH4·s-3/4H2o. One cubic meter of 

this ideal hydrate would contain the equivalent 

of about 170 cubic meters of free methane gas. 

Gas hydrates found in nature contain less gas 

than this because the lattice cages are not completely filled. Nevertheless, gas hydrates in 

reservoir rocks can contain much more methane per 

unit volume than could be contained as free gas 

in the same space. This fact suggests that if 

gas hydrates exist in large quantities in the 

upper few hundred meters of the earth's crust, 

they would constitute a significant potential 

energy resource. Also, because gas hydrates are 

impermeable to free gas, a layer of hydrate could 

trap economically important quantities of free 

gas below it. 

Petroleum exploration drilling in Arctic 

regions has established the occurrence of gas 

hydrate below permafrost (fig. 2). Marine seismic studies and Deep Sea Drilling Project cores 

strongly indicate the presence of gas hydrates 

in some seafloor sediments (fig. 2). This paper 

reviews what is known about these naturally 

occurring gas hydrates. 

2 

GAS HYDRATES IN PERMAFROST REGIONS 

The fact that temperature and pressure conditions beneath permafrost regions fall ¥7ithin 

the stability field of gas hydrates (fig. 3) was 

recognized in the 1940's. Now several deposits 

of gas hydrate are known (table 1). In 1970, 

well logging and formation tests in the ~·,~ssoyakha gas field in western Siberia revealed billions of cubic meters of methane gas frozen as 

gas hydrate (Makogon and others, 1971, 1972; both 

METHANE 

GAS+WATER 

Methane hydrate 

stability field 

Figure 1--Phase boundary diagram showin~: free 

methane gas and methane hydrate (pattern) for 

a fresh water-pure methane system. Addition 

of NaCl to water lowers temperature of hydrate 

formation, in effect shifting gas-hy~rate 

curve to left. Addition of co2 , H2S, c2H6 , or 

c3n8 raises temperature of hydrate formation, 

in effect shifting curve to right. Therefore, 

impurities in natural gas will increase area 

of hydrate stability field. Depth scale is an 

approximation assuming that lithostatic and 

hydrostatic pressure gradients are b0th 0.1 

atmosphere per meter (10.1 kPa/m), but the 

true lithostatic gradient is slightly greater. 

Redrawn after Katz and others (1959). 
Arctic 

Ocean 

Pacific 

Ocean 

Atlantic 

Ocean 

Atlantic 

Ocean 

Pacific 

Ocean 

Figure 2.--Known and inferred accumulations of gas hydrate. Dots on land depict susp~cted or confirmed occurrences of gas hydrates beneath permafrost. Dots in oceans show areas where seismic 

or drilling evidence suggests presence of gas hydrates. See tables 1 and 2 for a listing of 

each site, along with geologic, geophysical, and geochemical evidence for hydrate presence, research organization involved, and pertinent references. 

references cited by Milton, 1976). Methane was 

released from the gas hydrate by injecting methanol into test wells that perforated the hydrate 

zone. The injections of methanol, which serves 

as a hydrate inhibitor, resulted in a large increase in gas productivity from the test wells. 

The parts of the Messoyakha field containing gas 

hydrates are calculated to have 54 percent more 

reserves than would be expected in an equal volume of reservoir rocks filled with free gas. 

Two exploratory wells drilled in permafrost 

of the Mackenzie Delta penetrated shallow sand 

reservoirs containing gas hydrate at depths of 

820 to 1,100 m (Bily and Dick, 1974). The amount 

of formation gas in the drilling mud increased 

significantly during penetration of these sands. 

Although these sands were very porous, their permeability was extremely low. Low permeability 

and pronounced gas release are characteristic of 

hydrate-filled reservoirs. '!he loP permeability 

is thought to result from the plugring of sediment interstices by gas hydrate, and the gas liberated is attributed to hydrate decomposition. 

Other characteristics of the ras hydrate 

zone are visible on well logs (Bily and Dick, 

1974). The hydrate-bearing sands bive a relatively high resistivity. Spontanecus potential 

(SP) curves show very little deflection in hydrate zones in comparison with deflections in 

free-gas and free-water zones. Sor.ic logs show 

Table 1. --Evidence for gas hydrates beneath permafrost 

Numher 

on 

fig. 

2 

Location 

Messoyakha gas field and 

northwestern Siberia, 

U.S.S.R. 

Mackenzie Delta, Northwest 

Territories, Canada 

West Prudhoe Bay, Alaska, 

u.s.A. 

Arctic Archipelago, Northwest Territories, Canada 

Vilyuy Basin, Yakutia, 

u.s.s.R. 

Permafrost 

thickness 

(m) 

Predicted 

depth range 

of hydrate 

(m) 

Geophysical 

evidence 

Geochemical 

evidence 

Research 

organization References 

450-----250-870---------------------- 90 pet. or more meth- --U.S.S.R.------- Makogon and 

ane in gas analysis; others, 1971. 

large volme of gas 

from thawed cores. 

610------820-llOO--- Well logs: small deflection --do.--------- Imperial Oil, 

in SP; high resistivity; Ltd. 

high velocity sonic log; 

over-gage caliper log. 

610---------210-IIDO----- Well logs: high resistivi- --do.------------------Ex-on---------

ties; high velocity sonic 

log; gas kicks in mud logs. 

BUy and Dick, 

1974. 

R. D. Me Iver, 

written commun., 

1979. 

530----- No data 

in print. 

--------------------No data in print------ Dome Petrole...,, Hitchson, 1974. 

utd. 

>500-----do.-----------------------do.-------------u.s.s.R.----- Makogon and 

others, 1972. 

3 
an increase in acoustic velocity, and the caliper 

log indicates over-gage borehole through many 

hydrate-bearing zones. 

The first conclusive confirmation that gas 

hydrates occur in sediments under permafrost was 

obtained from pressure core barrel samples at the 

Arco-Exxon N.W. Eileen State No. 2 wildcat well 

on the north slope of Alaska in the West Prudhoe 

Bay field (R.D. Mciver, written commun., 1979). 

Two pressurized cores were successfully recovered 

and were maintained in the barrel at temperatures 

slightly above 0°C. At the time the cores were 

collected it was uncertain whether the cores had 

contained gas hydrate or only pressurized free 

methane. If the cores had contained free gas at 

in situ temperature and pressure, the pressure in 

l 

t 

'l -

'""" ,, """'""''-----}, . 

I \ ··-- 1 \_ 

I \_ 

1&oo.___ __ _J_ ____ -,L---__J---~---,.L,-----...1~ 

TEMPERA fURE, IN DEGREES CELSIUS 

EXPLANATION 

- - - Geothermal gradient ---Hydrate-gas phase boundary 

Gas hydrate zone 

Figure 3.--Gas hydrate zone, shown as area between intersections of geothermal gradient 

with hydrate-gas phase boundary curve. Within 

this area, pressure and temperature conditions 

are favorable for hydrate formation. Plot 

shows pure methane hydrate zone for an Arctic 

region in which permafrost is 600 m thick and 

enough methane is present to form hydrate. 

Pressure gradient is 0.1 atm/m (10.1 kPa/m). 

Geothermal gradient changes at base of permafrost. Changes in gas composition or water 

salinity will shift phase boundary curve in 

same sense as figure 1. Redrawn after Bily 

and Dick (1974). 

4 

the barrel would have been expected to decrease 

linearly as gas was withdrawn from th~ barrel. 

However, pressure was maintained even after repeated samples were withdrawn. This result could 

only have been caused by the decomposition of 

hydrate after each increment of pressure release 

and gas removal. Immediately after each sample 

was removed and the system closed, pressure began 

to build toward the pressure of hydrate equilibritnn. The sampled gas was 93 percent methane and 

7 percent nitrogen. 

Well logs in the Eileen well al~o indicated 

the hydrate zone. Whereas strong mud.-gas shows 

suggested free gas in the formation, the high 

resistivities and high velocities on the sonic 

log indicated ice or icelike materials. The 

density log also responded as if there were ice 

rather than gas in the formation. Ir combination 

the resistivity, sonic, and mud logs pointed to 

the presence of hydrates. 

In addition to the North Slope, the Mackenzie Delta, and the western Siberian lasin, the 

Arctic Archipelago of Canada and the Vilyuy basin 

of the U.S.S.R. show evidence of gas hydrate 

deposits. Gas-flow data from a well in Winter 

Harbor in the Arctic Archipelago sugrested the 

presence of gas hydrates (Hitchon, 1~74). Makogon and others (1972) reported that the Central 

Vilyuy gas field in Yakutia, U.S.S.R., contains 

gas hydrates beneath permafrost. 

Any sedimentary basin with extersive areas 

of relatively thick (several hundred meters) continuous permafrost may contain potentially commercial occurrences of gas hydrates. Antarctica 

may have large gas hydrate accwnulations, although few data on Antarctic permafrost areas are 

available (Hitchon, 1974). No drilling program 

has been conducted on the Antarctic continent 

except for a few experimental holes, but the land 

temperature profile is such that gas hydrates 

could exist at depths of,more than 1,800 m. 

GAS HYDRATES IN OCEANIC SEDIMENTS 

Pressure and temperature conditions in the 

deep sea fl,oor are within the range of gas hydrate stability (fig. 4), and the possible presence of gas hydrates has been noted in several 

areas (table 2). Observation of deep-sea sediment cores that release large quantities of 

methane suggest that gas hydrates exist in some 

areas beneath the seafloor. In reviews of sediment gas data from the Deep Sea Drilling Project 

(DSDP), Claypool and others (1973; Legs 10-19) 

and Mciver (1974; Legs 18-23) described instances 

where gas evolved from core samples after they 

were taken on deck. Gas evolution S')metimes 

continued for several hours, and the pressures 

generated were occasionally sufficie~t to extrude 

cores from the barrel and rupture sealed containers. The expanding, cooling gas fo~ned ice on 

the exposed cores. The quantity and rate of gas 

evolution could indicate the decomposition of gas 

hydrate, although high concentrations of gas not 

in hydrated form can produce similar results. In 

most instances, the gas was methane with traces 
of ethane. Recent results from DSDP Legs 66 and 

67 provide more evidence for the presence of gas 

hydrates in sediments on the slope of the MidAmerica Trench off Central America (Geotimes, 

1979a, 1979b). On both of these legs, gas-releasing cores were observed. Frozen sediment was 

recovered from the zone of hydrate stability at 

Sites 490, 491, 492, 497, and 498. Gas expansion 

volumes as high as 50 to 1 (R. von Huene, oral 

commun., 1979) suggest that gas hydrates had 

indeed been sampled. 

Methane in ocean sediment may be of either 

biochemical or thermal origin. It is uncertain, 

however, that enough gas to support the formation 

of gas hydrate can be generated by methane-producing bacteria alone. Claypool and Kaplan 

(1974) estimated that biological generation of 

methane in sediment containing 0.5 percent organic carbon near sediment depths of around 1,000 

m (corresponding to 30°C) is 20 mmol per kilogram 

of interstitial water. They also estimated the 

threshold methane concentration for hydrate stability as 58 mmol/kg. Thermocatalytic methane is 

produced by alteration of organic matter at temperatures greater than 50°C, but gas hydrates are 

stable only at relatively low temperatures. For 

gas hydrates to form from thermocatalytic natural 

gas (methane), such gas must migrate upward from 

burial depths greater than 1,500 m into cooler 

regions where it could be incorporated into a gas 

hydrate zone. 

BOTIOM-SIMULATING REFLECTORS 

A bottom-simulating reflector is an anomalous seismic reflector that parallels the topography of the seafloor but lies anywhere from 100 

to 1,100 m below the bottom. Bottom-simulating 

reflectors are most easily recognized on seismic 

records when they cut across other reflectors and 

occur in water depths greater than 400 m. They 

SEA LEVEL 

500 

1000 

ffi 1500 

tu 

:::;;: 2000 ~ 

::r: 2500 

0 

~ 

3000 

3500 

4000 

Sedimentwater interface 

1.5°C 

Assumed geothermal 

gradient= 27.3° C/1000m c==J Hydrate zone undY the ocean 

Figure 4.--Idealized section of gas hydrate zone 

in sediments of outer continental mar.?ins, 

computed for pure methane. Stippled zone is 

potential area of hydrate formation, where 

pressure and temperature conditions are correct for hydrate stability assuming an adequate supply of methane. Presence of other 

gases in methane would increase methane hydrate envelope downward. Increasing pressure 

from height of water column causes increase in 

subbottom depth of gas hydrate zone. Decreasing sediment temperature (down to 1°C in coldest, deepest bottom waters) also decreases 

hydrate subbottom depth. Redrawn fron R.D. 

Mciver, written commun., 1979. 

Table 2.--Evidence for gas hydrates in the seafloor 

Number 

fig. 

2 

10 

II 

12 

13 

14 

Location 

Blake-Bahama Outer Ridge, 

western Atlantic, off 

east coast of U.S.A. 

Bering Sea, North Pacific 

Beaufort Sea, Arctic Ocean 

Western North Atlantic 

We stern Cul f of Mexico 

Northern Colombian and northern Panamanian coasts 

Central America, Pacific 

side 

Western Africa 

Culf of Oman 

Water 

depth 

(m) 

3700 

2000-

4000+ 

40Q2500+ 

250o3800 

1200-

2000 

1500-

3000 

800-

2400 

2000-

5500 

3000 

Predicted sub= Bottombottom depth simulating Other Geochemical 

range of reflectors geophysical evidence 

hydrate (subbottom evidence 

(m) de th m) 

0-1000--------about 500------------- 90 pet. or more methane 

in gas analysis; large 

volume of gas from 

thawed cores. 

Research He fer,.~nces 

organization 

Lamont-Doherty, Markl and others, 

DSDP, and USGS. 1970; Ewing and 

Hollister, 1972; 

Dillon and others, 

1980. 

Q-)1000-------580-670---- VAMP's, )500 m 90 pet. or more methane --DSDP and USGS--- Scholl ard Creager, 

beneath the in gas analysis. 1973; Scholl and 

sea floor. Cooper, 197 8. 

0-700+ ---------100-300-------------------No data in print-----------USGS------------ Grantz ard others, 

1976. 

0-) 1 OOD---------500-600--------------------do. ---------------------Lamont-Doherty--- Tucholke and 

others, 1977. 

0-50D-----------1 00-11 OD---------------------do. ------------------Uni v. of Texas , 

Marine Science 

Institute. 

Shipley and others, 

1979. 

0-800-------10Q-11 00----------------do .-----------------------do .--------------Do. 

0-65D--------100-1100---------------do.---------------------do .-------------Do. 

0-)100D---------------- Pagoda structures --do.-----------------Woods Hole, Mass. Emery, 1974. 

at very shallow 

depth. 

0-800-------------- Bright spots at --do.---------------Cambridge, U.K. 

600-700 m depth. 

5 

White, 1979. 
are generally characterized by reflection polarity reversals and large reflection coefficients 

(Shipley and others, 1979). Gas hydrate zones 

and diagenetic boundaries in sediment have been 

proposed as possible causes of these anomalous 

seismic reflectors. 

Before DSDP Leg 11 in the Blake-Bahama 

region of the Atlantic Ocean, geophysicists of 

the Lamont-Doherty Geological Observatory were 

intrigued by bottom-simulating reflectors on 

their seismic records over the Blake-Bahama Outer 

Ridge (Markland others, 1970; Stoll and others, 

1971). The observation that some of the seismic 

reflectors on their records intersected other 

reflectors and paralleled the seafloor (fig. 5) 

was unexplained. One of the major objectives 

of Leg 11 was to investigate the nature of the 

bottom-simulating reflectors and determine their 

relation, if any, to accumulations of gas hydrate 

beneath the seafloor. The strongest reflector on 

the ridge mimicked the ridge profile at a depth 

of more than 500 m below the seafloor. Samples 

from Sites 102, 103, and 104 of Leg 11 yielded 

mainly methane and traces of ethane (for quantitative gas composition data see Claypool and others, 1973). The strong bottom-simulating reflector was at that time correlated with a distinct 

break in the drilling rate and with a zone of 

nodules of siderite and ankerite (Lancelot and 

Ewing, 1972). Another explanation, suggested by 

Stoll, Ewing, and Bryan (1971), Ewing and Hollister (1972), and Dillon, Grow, and Paull (1980), 

is that the bottom-simulating reflector corresponds to the isotherm that separates a gas environment from a gas hydrate environment. The 

average thermal gradient for the ridge (about 35-

400C/km) permits a gas hydrate to exist at the 

500-m sediment depth of the strong reflector, but 

below that level the temperature would be high 

~ 4 0 

C,) 

L.LJ 

en 

~ 

uJ 

:::iE 

~ L.LJ 

> <( 

a: 

1-

C,) 

:E 

en 

w 

en 

> <( 

3:. 

6 

~ 

6 

7 

8 

-----~-----

----- 0 50 100 KILOMETERS 

Figure 5.--Diagrammatic seismic profile of BlakeBahama Outer Ridge, showing thin, regularly 

spaced reflectors from strata. Bottom-simulating reflector (stippled) transects bedding 

and parallels seafloor and may represent hydrate-to-free-gas transition at bottom of gas 

hydrate zone. Redrawn from Tucholke and others (1977). 

6 

enough that methane could exist only as a gas. 

The interface between gas hydrate above and free 

gas below could provide a velocity contrast that 

is responsible for the bottom-sim•1lating reflector. Furthermore, the apparent acoustic velocity 

through sediment overlying this reflector was 

determined to be about 2 km/s (Lancelot and Ewing, 1972). This value, later co~firmed by independent sonobuoy measurements ( Br:ran, 1974), is 

unusually high for hemipelagic se1iment. 

Laboratory experimental studies provide further evidence linking gas hydrates with the sediments of the Blake-Bahama Ridge. Stoll, Ewing, 

and Bryan (1971) demonstrated that when methane 

mixed with water-saturated sand w~s brought to 

the estimated temperatures and pressures of the 

Blake-Bahama Ridge sediment, the velocity of compressional seismic waves in the sand increased 

from 1.7 to 2.5 km/s. This experimental increase 

in seismic velocity helps explain the unusually 

high velocities observed in hemip~lagic sediment 

of the ridge. 

Since they were first found in the BlakeBahama Ridge (Markl and others, 1970; Ewing and 

Hollister, 1972), other bottom-simulating reflectors have been reported (fig. 3) from the western 

North Atlantic Ocean (Tucholke and others, 1977), 

the Beaufort ~a of the Arctic Ocean (Grantz and 

others, 1976), and the Bering Sec and elsewhere 

in the North Pacific Ocean (Scholl and Creager, 

1973). Shipley and others (1979) described bottom-simulating reflectors in sediments off the 

east coast of the United States, in the western 

Gulf of Mexico, off the northern coasts of Colombia and Panama, and along the Pacific coast of 

Central America from Panama to k.apulco, Mexico. 

From studies of the western North Atlantic, 

Tucholke, Bryan, and Ewing (1977) identified two 

high-amplitude reflecting horizons that ·are conformable with the seafloor and atout 500 to 600 m 

beneath it. These horizons show the following 

characteristics: they cut acros~ bedding-plane 

reflectors in the same manner as the reflector 

horizon observed on the Blake-Bab:tma Ridge; they 

appear to be restricted to areas where sedimentary strata dip landward; and th~ir subbottom 

depth increases with the seafloor depth, a relation consistent with theoretical predictions of 

the configuration of a hydrate zone (fig. 4). A 

zone of gas hydrates may overlie the anomalous 

horizons, and thus the seismic horizons could be 

reflectors representing an impedance contrast 

caused by the downward change from gas hydrate to 

gas in the sediment. Grantz and others (1976) 

identified a strong seismic reflector that mimics 

the bathymetry of the seafloor, 100-300 m beneath 

it, on the continental slope of the Beaufort Sea 

north of Alaska. The bottom-sim1uating reflector 

was identified in about 60 percent of seismic 

profiles obtained in water deeper than 400-600 m. 

Not all bottom-simulating reflectors can be 

directly related to the presence of gas hydrates. 

Although gas hydrates provide one reasonable 

explanation for bottom-simulatin.~ reflectors, 

these acoustic features may also result from 

temperature controlled diagenetic effects. Dur-
ing DSDP Leg 19, Scholl and Creager (1973) noted 

seismic reflectors that tend to parallel the seafloor in some Bering Sea sediments draped on the 

Umnak Plateau, and they coined the acronym BSR 

for the bottom-simulating reflector. At two 

sites, 184 and 185, the reflector was penetrated. 

Although methane was observed in sediment from 

Site 185, no other evidence for gas hydrates was 

noted. Scholl and Creager (1973) attribute this 

reflector to a lithologic transition from hemipelagic diatom ooze to indurated claystone. 

Because of the time-transgressive nature of this 

horizon, this BSR appears to represent some sort 

of migratory diagenetic boundary related to the 

dissolution of diatoms and the formation of claystone. Hein and others (1978) confirmed that 

opal-A is transformed to opal-CT in the temperature range corresponding to a subbottom depth of 

600 m, the depth of the BSR in Bering Sea sediment. Further, because the subbottom depth of 

the BSR on the flanks of Umnak Plateau decreases 

with increasing water depth, this BSR is probably 

not directly caused by gas hydrates (Shipley and 

others, 1979). Nevertheless, gas hydrates may 

still play an important role in the formation of 

this BSR. Claypool and Kaplan (1974) note that, 

in all cases of BSR's described from the Bering 

Sea, the lithologic change is consistent with the 

inferred depth of the isotherm where gas hydrate 

would decompose under the prevailing pressure 

conditions. Claypool and Kaplan (1974) suggest 

at least an indirect link between lithification 

and gas hydrate. Free carbon dioxide and methane 

concentrations determine the pH of interstitial 

water. The pH conditions determine the solubility of lithologic components such as carbonate 

and silica. Gas hydrate formation may influence 

carbon dioxide and methane activities and may 

therefore influence the dissolution and reprecipitation of carbonate and silica cements. 

Thus, some bottom-simulating reflectors are 

not directly related to the presence of gas hydrates, as illustrated above, but bottom-simulating reflectors may not be observed even though 

gas hydrates are present. For example, in areas 

where sedimentary stratification parallels the 

seafloor, the bottom-simulating reflector may not 

be detectable. During drilling on DSDP Leg 67 in 

the Mid-America Trench off Guatemala, gas hydrates apparently were encountered at Sites 497 

and 498, but the seismic records for these areas 

showed no obvious bottom-simulating reflectors 

(R. von Huene, oral commun., 1979). 

Gas hydrate zones in the seafloor may form 

regionally extensive impermeable seals and may 

trap economically important accumulations of 

natural gas, provided sufficient sources for 

methane are present. If gas were to migrate upward and be impeded by hydrate, it would accumulate at the lower boundary of the hydrate zone as 

free gas and would cause a significant decrease 

in seismic velocity at that boundary (Bryan, 

1974). Bottom-simulating reflectors are strongest in the vicinity of ridge crests and tend to 

7 

fade out on the flanks (Mark! and others, 1970), 

a configuration that suggests gas accumulation at 

the crest as in an anticlinal trap. 

OTHER SEISMIC EVIDENCE FOR GAS HYD~ATES 

IN OCEANIC SEDIMENTS 

Seismic features other than bottom-simulating reflectors may be produced by gas hydrates. 

These features include bottom-parallel bright 

spots, pagoda structures, and deep-water velocity 

amplitude features (VAMP's). White (1977) suggested that seismic bright spots (amplitude anomalies) in the Gulf of Oman are caused by natural 

gas accumulations. These subbottom reflectors 

are curved, mimic the topography of the seafloor, 

and transgress local bedding (making these bright 

spots similar to bottom-simulating reflectors, 

but of lesser lateral extent). White (1979) inferred that the gas is held in place not by a 

stratigraphic trap but by an impe~eable gas hydrate layer that follows the seafloor topography. 

Emery (1974) has suggested that pagoda 

structures may be related to gas hydrates, although these are very shallow subbottom seismic 

features, and there is little consensus regarding 

their significance. Geophysical traverses ~~ross 

nearly flat mud bottom at depths between 2,000 

and 5,000 m off western Africa provided ext€nsive 

seismic shallow-penetration recordings at 3.5 

kHz. The recordings reveal the presence of common alternating light and dark triangular S€ismic 

features, or pagoda structures, whose internal 

structure and acoustic properties may, according 

to Emery (1974), be due to local centers of gas 

hydrate induration. 

Velocity-amplitude features (VAMP's) are 

acoustic anomalies showing narrow (1 to 2 km) 

subsurface columns of concave reflection horizons 

called pulldowns, associated with gently arched, 

high-amplitude horizons higher in the sectio~ 

(Scholl and Cooper, 1978). These kinds of acoustic anomalies, common on seismic profiles collected in the Bering Sea, are typically seen in 

flat-lying beds at subbottom depths greater than 

500 m and in deep water around 4,000 m. VAMP's 

are essentially deep-seated "bright spots" u'"~derlain by reflection horizon pulldowns. Gas hydrates may be responsible for VAMP's by cont~ibuting to the restriction of upward-migrating gas 

and fluids. 

It is difficult to detect gas hydrates by 

purely chemical and seismic means. Support for 

the indentification of gas hydrate in seafloor 

sediments will result when cores containing l.ydrates recovered at their subsea temperatures and 

pressures can be analyzed. A pressure core t .'lrrel capable of this task has been designed br 

DSDP but has never been successfully deploye~ in 

a gas hydrate horizon. From the geologic, g~ophysical, and geochemical evidence obtained thus 

far, there is little doubt that gas hydrates are 

an important part of the marine sedimentary 

record. 
SELECTED BIBLIOGRAPHY 

The following is a selected bibliography containing the references cited in this paper along with 

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8 

____ 1951b, Erratum: Suggested structures of 

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10 
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11 

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