oceanic methane hydrates-a frontier gas resource

y /V04o, 4 H . V.

Journal of Petroleum Geology, vol. 19(1), January 1996, pp. 41-56 41

OCEANIC METHANE HYDRATES:A "FRONTIER" GAS RESOURCE

M. D. Max* and A. Lowrie**

Methane hydrates'are ice-like compounds consisting of natural gas (mainly methane)and water, whoqse- ry 1 alstructure effectively compresses the methane: each cubic metreof hydrate can yield over 150 cu.mrof methane. Hydrates "cement" sediments and impartconsiderable mechanical strength; they fill porosity and restrict permeability. Bothbio'genic and ,then 'nl cnic methane have been recovered from hydrates.

Hydrates occur in permafrost regions (including continental shelves), and are stablein ocean-floor sediments below water depths of about 400.m in the "Hydrate StabilityZone" (HSZ). This is a surface-parallel zone of thermodynamic equilibrium that extendsdown from the sediment surface to a depth deternined by 'temperature, pressure and localheat flow. Methane, and water are stable below the HSZ.

Although the ecortomic recovery of hydrates has taken plac'e in Arctic regions, oceanichydrates offerfar greaterpotential asan energy resource. A variety of trapsfor methanegas 'can be formed 'by' oceanic hydrates. In addition to the gas within the hydratesthemselves, simple g'as traps in closures beneath the HSZ in the vicinity of bathymetrichighs, and complex traps involving both hydrate andstructural/stratigraphic components,have been observed.

It has been estimated thatat least twice as much combustible carbon occurs associatfedwith methane hydrates as in all otherfossilfuels on Earth. The evaluation of methanein, and associated with, oceanic hydrates therefore constitutes a major energy explorationfrontier.

INTRODUCTION

Hydrates (also known by their chemical species name of "clathrates") are ice-likecompounds which are stable both at very low temperatures in permafrost regions, and.also in the low temperature-high pressure regimes presentin the deep oceans (Kvenvolden,1993a). Hydrocarbon gases (mainly methane) and fluids are thermodynamically stabilized

* Naval Research Laboratory (NRL), Washington, DC 20375. Temporarily at NATOSACLANTCEN (Underwater Research Centre), I-19138 La Spezia, Italy.Note. This paper does not necessarily represent any view of NATO, or NRL.** 230 F.Z Goss Road, Picayune, Mississippi 39466-9707, USA.

ri Oceanlic '71e'1hanLe- hydrates

in gas hydrates as a result of hydrogen bonding within a crystalline lattice provided bywater molecules (Kuuskraa et al., 1983; Pearson et al., 1983; Kvenvolden et al., 1984).The crystallization process forces methane molecules into a much smaller volume thanthey would occupy as a gas or dissolved in a fluid. Hydrates hold up to 164 cu. m of gasper cu. m of hydrate, and 0.8 cu. m of water (Kvenvolden, 1993a). Perhaps 150 cu. m ofgas will be released from one cubic metre of hydrate, because some of the molecular sitesavailable for methane in the hydrate lattice will be unoccupied, or will be filled withanother gas or hydrocarbon.

Gas occurring in the hydrates is derived from buried organic matter that has beenaltered microbially or thermally to produce methane. Carbon isotopes in hydrates from17 localities (offshore the SE USA, offshore Peri, in the Peru-Chile Trench, offshoreNorth California, in the Gulf of Mexico, offshore Guatemala, in the Middle AmericaTrench, and in the Black and Caspian Seas) were analysed, and were found to be dominatedby '2C (i.e. hiahly-negative &'3C), indicating a biogenic source (Kvenvolden, 1993a).Those samples which were significantly enriched in !3C (i.e. less negative 6'3C) werecommonly accompanied by small amounts of isotopically-heavy CO,, ethane, propane,i-butane, and n-butane; these are generally regarded as indicating a thermogenic source.Hydrates therefore represent an important potential energy resource (Max et al., 1991).This resource, consisting of both gas within the hydrates themselves, and also that trappedin related gas accumulations, has been estimated to be equivalent to twice the combustiblecarbon held in all other fossil fuels (Kvenvolden and Cooper, 1987; Kvenvolden, 1993b).Originally, gas hydrates were thought to be well developed' only in permafrost regionsin the North American and Russian Arctic (Collett, 1983; Makogon, 1988), where theywere first recognized in a natural environment. (Previously, hydrates had become knownto the petroleum industry becaus-e'thicy formed in hydrocarbon pipelines and cloggedthem). In 1972, Arco/ Exxon recovered a pressurized specimen of gas hydrate from adepth of 666 m from an exploration well in Prndhoe Bay, Alaska (Collett,1983). Hydrateshave since been identified in other Arctic permafrost'reg'ions, both on land and oncontinental shelves. In the Barents Sea, however, hydrates are thought to have formedon shallow shelves independent of permafrost (Soiheim and Larsson, 1987; Lov0oet al.,1990). -

The recognition of gas hydrates within oceanic sediments was a major contribution oftheDeep Sea Drilling Project, and confirmed previous, tentative identifications of hydratesmade on seismic reflection records (Bryan and Markl, 1966; Markl et al., 1970; Hollisterand Ewing, 1972). This correlation of seismic response to'the'presence of gas hydrateshas allowed further identifications of hydrates to be made (Tucholke et al., 1977; Paulland Dillon, 1981; Kvenvolden and -Barnard,- 1983'; Kvenvolderi and McDonald, 19855)'.'Shallow oceanic sediments, which are presumably compacted only slightly, have notbeen explored for hydrocarbons because the existence of a "lithified" cap rock near thesea bed can not be envisaged within a conventional geological framework. A conventional,impermeable cap rock, overlain by sufficient lithostatic load to permit a conventionalreservoir rock to maintain pressure, does not exist in the case of hydrate and trapped gasdeposits. The role of the reservoired hydrocarbons is taken first by the gas held withinthe hydrates themselves, and secondly, by the gas trapped below the base of the impermeablehydrate "blanket".

The source of the gas in hydrates appears to located within the sediments themselves;both shallow, biogenic gas and deeper-sourced, thermogenic gas may migrate upwards,and be trapped within and beneath the hydrates. Hydrate thermodynamics appear toprovide a mechanism for concentrating gas during lengthy periods of sedimentation.The purpose of this paper is to describe oceanic methane hydrates and the geologicalsettings in which they are found, and to assess their potential for development as a newmethane gas resource. Accumulations of gas hydrates in deep-sea sediments are entirely

r

M. D. Mar and A. Lowrie 43

10I>-

methane A .gas lice g methane gas

& water

12:tQ-

100 M

_j W W GASECD -..-. I DCO HYDRATE

E ~~~~ PHASE2 5~~~~OUNDARY

- TEMPERATURE (°C)

I...~ ~ ~ .- .

Fig. 1. Gas hydrate phase diagram, showing the stability fields of the water-ice-methane-hydratesse.(Froim Kvenu~volde e ~l an' -cM 3Aiile180"'" C611ett; 1983j h rsneo Oethaneand popane with methane in he hdirteWifil-ha'v~e he"'e'ffect of shifting 'thehyrtpas

boundar tthriht thus, increasing the'.P-.f~ie~ld within 'whichethdii -hy'dr'at6 is'sta;ble.'Thierm:a'lstiimulation vi',,llhave th~e 'effect'of modving'any poiniton the diagram to theiright, while-depressurizationwillmove aniypoint upwar~ds. Inhibitorstimulation, for insta~nce by;~aCIsoluti'on,;'will shift the hydrate phase-boundar curve-towards the. left..Gas will be produced as a result of;-any movement of a P-T-point,'if' its ne oiinis to the right of, or above, the gpts-hydrate phase

- - - - .- - :'-' - - boundary.. ''-.-:

different from conventional gas deposits; thus', n e`w` ex'ploration and exploitation methodsmust be developed. Al'though the economric poetial of oceanic hydrates is far fromestablished, the potential rewards will be great.

OCEANIC GAS'HYDRA:TES AND THE HYDRATE STABILITY ZONE

Where conditions are suitable, oceanic gas hydrates occur.in a surface-parallel zoneof thermodynamic equilibrium (the Hydrate Stability Zone: HSZ) (Figs. 1 and 2), whichextends downwards from the sea floor to a depth defined by the geothermal gradient.Within this zone, the conductive trans fer of heat between the warmer rocks below andthe cold oceanic waters above is in equilibrium within the pressure-temperature field atany particular depth; thus, heat entering the base of the HSZ is conducted.upwards andis dissipated into the sea, thereby maintaining 'conditibn'S of hydrate stability throughoutthe HSZ

Apart from the Arctic, hydrates of natural gas.(mainly methane) and water are stablein the-oceans below water-depths of about.400 in'.. Because the hydrate crystal lattice is.stabilized primarily by pressure (sea-floor temperatures are generally less than +4°C), theHSZ tends to be thicker in deeper waters, where pressures are higher. Hydrates may form'freely within oceanic waters; however, hydrate crystals are lighter than water (S.G. ofabout 0.85), and unless they are bound to sedimentary particles or otherwise held down,they will float upwards and invert to gas.

44 Oceanic methane hydrates

Hydrates can accumulate in any oceanic sediment into which gas has migrated, orwithin which gas has been generated, given the appropriate thermodynamic conditionsHydrates have been identified in sediments on continental slopes and rises. Here, organic-richsediments, deposited principally from turbidity currents, are buried rapidly, preservingtheir methane-generating potential. Hydrates are known to occur in a wide variety of

sediments, whose precise lithology (i.e. whether calcareous, siliceous, or clay-mineralrich) seems to be relatively unimportant. Optimum conditions for the formation of

hydrates are found in thickly-sedimented areas on continental slopes and rises, wherelarge volumes of gas can be generated. Whether produced biogenically or thermally at

greater depths, this gas may migrate upwards into the HSZ. Hydrates in the HSZ may fillthe available porosity, and render an otherwise uncompacted and unconsolidated sedimentvirtually solid. Where present as a "cementing blanket" within sediment, hydrates may

provide an impermeable seal, and therefore trap large volumes of gas.Hydrates can be reworked through a gasification and rehydration process, here termed

the "gas-hydrate conservation cycle", which provides a long-term, nearly steady-statemechanism for concentratirig gas in an area of sediment deposition. Sedimentation on thesea floor would tend to bury the hydrate were it not for the influence of the heat flow frombelow (Fig. 2). As sedimentation proceeds,'the surface of the sediment moves upwardsand the base of the HSZ follows, so that the thickness of the HSZ tends to remain constant.--,-'Thekey to this "conservation cycletois thatIth- as of the HSZ represents a metastable

phase boundary between the gas-hydratea'and gas-and-water fields (Fig. 1), which iscontrolled by pressure and. temperature (possibly'locally modified by geochemicalconditions). Thus, when hy ata se h SZ :becomne unstable, owing to theupw~ard migration of thiszone of stability,..thefy elt" and invert to gas which risesbuoyantly upwards. Apparent fault offsets of- the hydrate-gas contact suggests that the*zorieof equilibration may'be broad (Rowe and Gettrust, 1993 a). IT a thick sedimentarypile,-' gas may be "reworked" -in -this cycle many times.

.This reworking of hydrate'at the base of the5HSZ, combined with.the influx of gasmrigrating from belows-explains why-the base of the HSZ has been observed to be the mostheavily hydrated section.:.The HSZ is therefore potentially stable in the'upper portionsof.the sea floor, even in areas where substantial sedimentation rates occur and where thereis only little or moderate generation of gas. Once formed, gas hydrates will not becomeburied within the sedimentary pile.

THE OCCURRENCE OF HYDRATES

Hydrates have been recognized mainly from multi-channel reflection seismic data andalso from drilling in the Arctic (Grantz and May, 1982; Grantz et al., 1989). Apart fromthese areas of permafrost, they have been identified within sediments on continentalshelves, slopes and rises in many oceanic areas, including the Black Sea, the Gulf of

Mexico and. the Indian Ocean (Kvenvolden and McMenarmin, 1980; Sloan, 1990;Kvenvolden, 1993a). Although oceanic hydrates have mostly been identified in waterdepths of between 500 and 3,000 m, they can also be expected to occur within thick

sedimentary sections at greater depths if the sediments are suitable for the generation ofsubstantial volumes of gas (Max and Lowrie, 1993).

Hydrate "blankets" are increasingly becoming recognized to be widespread, and mayin factbe normal on moderate- to thickly-sedimented continental margins. Their potentialfor trapping and concentrating gas over long periods of time is better than that of normalreservoirs, in which there may be only limited communication between the source andthe trap due to the more complex structural and stratigraphic conditions.

M. D. Mfax and A. Lowrie 45

SL

\ - HYDROTHERMALGRADIENT

GAS I HYDRATEPHASE BOUNDARY

F 3 , SEA FLOOR

4

GAS AND WATER

60 10 20 30

TEMPERATURE (C)

Fig. 2. The thickness of the Hydrate Stability Zone (HSZ, shaded) as a function of the hydrothermaland geothermal gradients (GT). (After Fig. 12 of Kvenvolden and Barnard, 1982). SL, Sea Level.The thickness !of theHSde'S& on the water depth (i.e.pressure), the sea-floor temperature,and the geothermal gradient. The base of? the HSZ Will be located at a depth where the gebthermal

gradient intersects the phase-boundary curve... : .. .... ,: :p a i n .... ... 'th '. . ... .

Types of hydrate gas trapsHydrates may accumulate in two types of trap (Dillon and Paull, 1983). "Simple" traps

occur entirely: within or below the hydrate layer itself; "compouind" traps form from acombination of the hydrate and the geological structure or stratigraphy.

Simple trapsHydrates themselves form traps where there is uneven sea-floor morphology as .a result

either of uneven patterns of sedimentation distribution or of deflation by ocean-bottomcurrents. This type of accumulation is most likely to occur along passive-margin slopesand rises, where thick sedimentary prisms may develop without substantial tectonicdeformation or compaction. They may also be found basinward of a trench associatedwith an active margin. In these areas, the interaction between gravity-driven depositionalprocesses and contour currents may produce graded beds (composed of silts and clays,with occasionally fine sands), which lie parallel with the sea floor.

Faulting in these settings is not likely to be a majorfactor in fluid migration. Potentially,gas may be generated over large areas of the continental slope and even the abyssal plains.The hydrate "cap" may extend for hundreds of miles from the slope (or the base of theslope) where bathymetric cuirminations are most common. Fluids may migrate up-slopebeneath this low-angle cap, theoretically over long distances. The hydrates will be foundin relatively recent sediments (i.e. of late Tertiary and Quaternary ages).

The HSZ tends to maintain a consistent thickness for any particular water depth, givena steady geothermal gradient. Because lateral variations in heat flow in the ocean crusttend to be gradual, the thickness of the HSZ tends to be constant over broad areas. Beneath

46 Oceanic ItethzzL'e lzydr ates

10 KILOMETERS'5 STATUTE MILES'

-4

D) ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~/

..-. ..- .

cAGC ' w

,---BASE OF GAS HYDRATE----W

~~~~~~~~~~~~~~~~~~~~~~~~.,, ,, , , ,,,4,~,,A ,, ,_,,,,L.

-. .... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~.~;

~~~~~~~~~~~~~~~~~~~-5-

TRUE AMPLITUDE

Fia. 3. Discontinuous Bottom-Simulating Reflector (BSR) in 1'nfold multichannel seismic reflectionprofile along slope across the Blake Outer Ridge, on the continental slope of the SE USA. Notethat bedding lies generally parallel to the sea floor, and that the "blanking"? caused by hydratesextends beyond the BSR, suggesting that hydrates are much more widely dispersed than issuggested by-the development of the BSR. Without a well-developed BSR, the strong negative

impedance character is often absent. DSDP drill-hole 533.is nearby.AGC: Automatic Gain Control ob seismic processing output;True Amplitude: data processed to show relative impedance contrasts (highlights strong impedances).

(The figure is Fig. 17b in Max (1990), by T. Edgar (USGS National Center, VA); photo fromW; Dillon (USGS,- Woods Hole, MA).

a bathymetric high, therefore, a matching "arch" at the base of the HSZ can be expectedto form; arched traps have in fact been observed on reflection seismic sections (Fig. 3),and gas here appears to be "ponded" beneath a hydrate layer (Dillon and Paull, 1983).Traps of this nature are analogous to gentle, anticlinal traps in thick stratigraphic sequences,similar to those, for example in the western Arabian Gulf.

M. D. Mcv. and A. Lowrie 47

9'0 1~~~~~~9

Fig. 4. Cartoon of compositehydrate-geological traps on a continental slope (diagrainniaticscale, butwith vertical exaggeration assumed). a. Opposed dip trap.. b. Convergent dip trap; th Hydrateblanket seals the trap above. The impe vious barrier dipping steeply towards the continent can bebasement, impervious strata in the same succession, igneous or diapiric rock, or fault complexes.,

thathave formed seals.

Compound traps

In compound traps, both the local structure and stratigraphy are important in addition,to the hydrate layer.

A gas trap may form where the dip of strata and that of the hydrate layer are opposed(Fig. 4a); the seal is formed partly by hydrate and partly by impermeable strata. This typeof trap resembles a structural trap in which dipping beds are inclined against a fault, oran unconformity trap where the dip of the structure is opposed to that of the plane of theunconformity. Traps may also form where beds dip more steeply than the HSZ (Fig 4b).The hydrate seal is crucial, although gas may also be confined laterally by impermeablestrata or down-faulted basement below an unconformity. This type of trap resembles astratigraphic pinch-out or a trap formed below an impermeable unconformity.

These traps are as likely to form in older sediments that have been brought near thesea-floor into the HSZ as they are in younger sediments, depending on the local geologicalstructure.

Although compound traps have been identified on a passive margin (Dillon et al.,

1993), active margins (especially convergent plate margins) are a more probable setting.There, oceanic crust and sediments are continually subducted into a zone of thermogenicgas generation below the HSZ, which is developed in the overlying accretionary prism.Sediments which might otherwise be too thin to generate gas are emplaced within theaccretionary prism, where their gas-generating potential is enhanced.


Thle SE Coast of the USAAn interesting potential gas accumulation has been identified off the SE coast of the

USA, where diapirs, presumably of salt/gypsurm, penetrate to near the seabed. Paull etal. (1995) identified a probable gas "pocket" in the hydrate blanket immediately abovea diapir, which apparently has introduced a local thermal anomaly. This occurrence isassociated with gas "venting" into the water column from the underlying sediirents, andprobably constitutes a drilling target (Paull, 1995).

'THEEVOLUME OF METHANE PRESENT IN GAS HYDRATESGas-hydrate volumes are uncertain and, with the exception .of a few localities, veryspeculative. Estimates of the volume of methane hydrate present along the continentalmargin of North and Central Amnerica range up to 2,840 Tcf (DOE, 1987); offshore Japanand Timor, up.to 200 Tcf in hydrate and .20 Tcf of free gas may be present (Finlay andKrason, 1986). The volumes of methane present at 13 locations offshore North Americamay be as high as 5,500 Tcf (Malone, 1990).New acoustic-processing techniques for estimating the volumes of gas hydrates present,which have been camred out in an area of about 29,000 sq. km (i.e. approximately the sizeof the State of Maryland) in the Blake Ridge region off South Carolina, suggest that asmuch as 4,000 Tcf may be present (Dillon and..Paull,19.83 Dillon etal., 1980, 1991,1993). Here,-gas isialso trapped beneath ahydrate "blanket" (Dillon andPaull,1983). Theclosure is an irregular ellipse measuring about 18 km x 9 km,.with a vertical gas columnof 100 to 200 m; the '"'reservoir" volume is about 20 cu. kmn. Taking into account a porosityof 10%' (which.ma'y,'bem a low estimate) and the hydrostatic press u're-' at that dep th'eestimated volumeof-inatural ga. is about 600 cii. km (6 x 108 cu: in)', a arge:volume.byany'st'andard. In similar'geological 'situations along'thicky-sedimented margins,

sihnilar-sized accumulations mray also be expected. to'occur.Estimates of the volumes of methane hydrates in Arctic permafrost regions. range-from5x102 to 1.2x106 Tcf; and from 1.1x105 to 2.7x108 Tcf in oceanic sediments worldwide

(Kvenvoldeni, 1993b). -:Glob'al estimates "are converging at'about 10,000 gigatons ofmethane carbon (Potential Gas Committee, 1981; MacDonald, 1990), or about 7x105 Tcf*(2x1 06 cu. m) of methane gas. These estimates do not include estimates of trapped, freegas. Even if the current estimates prove to be orders of magnitude too high,.the methanevolumes associated with oceanic hydrates remain stupendously large. Kvenvolden (1 993b),for example, noted that as little a.,s1 % of the most conservative estimate of the volumeof gashydrates is equal to one half of the World's proved conventional gas reserves.

EXPLORATION STRATEGIES AND METHODS

Exploration for methane gas hydrate accumulations. will be quite different fromconventional hydrocarbon exploration programmues. Factors such as the lithological,sedimentological and geochemical characternstics of an ocean-floor sediment at aparticularsite may not be of primary importance in the search for gas hydrate resources. Otherfactors (such as diagenesis and cementation, brine formation, fluid expulsion, increasesin shear strength, changes in acoustic characteristics, etc.) will probably be more important.Standard basin analyses, including stratigraphic and thermal-history studies, are likelyto be of little use, because hydrates are confined to a near-surface layer which is notrelated to the underlying or older sediments. Time-temperature analyses may likewise beless relevant; if the thermodynamic conditions on the sea floor are suitable for the-captureof gas by hydrates, the exact time of gas generation in relation to the formation ofgeological structures and the deposition of conventional reservoir strata is not important.Underlying sediments may provide a source of thermogenic gas, however, if migrationpaths to the HSZ exist.

MI. D. Max anld A. Lovvrie 49

The principal geophysical tool used in hydrocarbon exploration (multichannel reflectionseismic surveying) will need to be processed in a different manner if the high-resolutionnear-surface seismo-acoustic characteristics of shallow sea-floor sediments are to bestudied Targets deeper than 2 km are not anticipated in the search for hydrate methane.

A first stage in the exploration for gas hydrates would involve a general assessmentof the likelihood of the development of hydrates, and would be equivalent to a generalbasin analysis in conventional exploration. A geological province can be characterizedin terms of its sedimentary thickness and type, and heat flow, and a general indicationof the likelihood of gas generation can be inferred. A map of probable areas for hydrateformation can then be made, and the thickness of the potential HSZ defined. Direct orindirect evidence, using more specific methods, may indicate the presence of hydratesor gas trapped at appropriate target depths. The volume of hydrate within the HSZ canthen be estimated, based on porosity saturation models (e.g., Max and-Lowrie, 1993).Specific exploration targets can then be identified and prioritized.

Seismo-acoustic analysesHydrates effectively cements detrital grains together thus reducing porosity, while also

retaining the ability to anneal fractures and seal secondary porosity which might develop.The presence of hydrates therefore radically alters a sediment's overall seismo-acousticresponse. Acoustic velocity analyses may allow quantification of the volumes of gashydrates present. The determination of the pressure and shear-wave velocities, and theattenuation ch:acteris ofthesedifiientaiy layeras' is th'upper one to 1.5 km 'of theocean floor is fundamental to the identification of hydrates.

The velocity of acoustic waves in hydrates is substantially higher than that of sedimentarysequences without hydrates. Collett (1983) reporttd 'thiat P-wave velocities innaturally-occurring hydrates from drill-holes were between 3.1 km/s and 4:4' km/s.Hydrate-sediment velocities commonly range fr6m 2.3 km/s to 3.2 kmi/s. P-wave velocitiesthrough porous.sediments 'saturated with hydrates may be 60-100% higher than thosethrough the same sediments saturated with free gas (Max, 1989; Rowe and Gettrust,1993b). -

Preliminary quarititative analyses of gas-hydrate volumes in sediments have -beencarried out by Lee et al. (1993), using both true amplitude and interval-velocity analysesof multichannel refl'ectio6n' seismic data. The "blanking" of the normal acoustic structureof the sediment, caused bythe infilling of porosity by the higher-velocity hydrates, yieldsa high-velocity,' low-attenuation, acoustically-transparent zone in which sedimentaryimpedance structures are obscured (Max, 1989; 1990). By modelling different degreesof impedance blanking, and assuming that the different proportions of available porisityhas been cemented by hydrates, quantitative estimates of the volumes of hydrates presenthave been derived. These techniques may only work well at low volumetric fillingfactors. Dvorkin and Nur (1993) found that sediments cemented with a pure water-iceanalogue showed a large increase in both shear- and compressional-wavc velocities;experiments with variable filling of pore space by cement show that velocities-may nearlyreach a maximum with as little as 25% of the pore space filled.

Reflection seismics (i): BSR and impedance blankingThe main indication of the presence of a gas-hydrate layer is a Bottom-Simulating

Reflector (BSR), and this is commonly imaged on seismic-reflection profiles (Paull etal., 1995) (Fig. 3). The BSR is a strong, negative impedance reflector, which is developedat the contact between the low acoustic-velocity gas zone (identified by its high attenuationand scattering, reflection-poor characteristics), and the overlying high acoustic-velocityhydrate. The BSR is most easily identified where it cross-cuts the acoustic structure ofthe sediments (Dillon et al., 1993). Where apparently large volumes of gas are present,"bright spot"-like anomalies are formed.


BSRs were initially regarded as an artefact of seismic processing, or as some inexplicableeffect such as bubble-pulse reverberation, multiple-source reflection, or multiplebottom-bounce reflection, for which no physical basis could be demonstrated. Drillingand velocity analyses proved the existence of methane and hydrates, and demonstratedthat the unexpected seismo-acoustic profile could best be explained by the presence ofhydrates and gas (Hollister and Ewing, 1972).

The BSR is often difficult to identify on seismic records where sedimentary beds lieparallel to the sea floor, because it can be obscured by impedance events within thesediments. In addition, in high-velocity sediments where significant volumes of free gasare absent below the hydrate layer, there may not be a strong phase change, or there mayeven be an increase in seismic velocity (Max, 1989; 1990). Kvenvolden and Bernard(1982), however, noted that in the absence of a BSR, the presence of an appropriatelyhigh-velocity layer near the surface of marine sediments almost certainly indicates thepresence of hydrates.

Reflection seismics (ii): Velocity and Amplitude Structures (VAMPS)This type of structure, which is composed of a restricted "pull-up" immediately. over

a "pull-down" on a reflection seismic record, indicates the presence of gas hydrates andunderlying free gas (Collett, 1993). These structures have been identified in generallyflat-lying sediments in undeformed settings, such as the Bering Sea Basin (Schol. andHart, 1993),-where the base of the HSZ/BSRis difficult toD'pick" within the7seismo-acousticstructure of the sediments. The identification of a VAMPS is most likely in an ocean basinwith extensive, flat-lying sediments where methane hydrates are likely to occur, such asthe Arctic Basin (Max- and Lowrie, 1993).

VAMPS are thought to be caused by hydrate.forming directly over a deep gas. source(Scholl and Hart, 1993). They. may indicate that hydrate deposits may not have mergedinto a continuous bed within the HSZ. They may also offer a cluelo the development ofmore "mature" hydrates, for instance in the Blake Ridge area (Dillon et al., 1993), wherethe hydrate forms a continuous layer of variable thickness and saturation.Non-seismic methods

Exploration for hydrates may involve new geophysical approaches to the study of thesea floor. For example, hydrates induce high electric resistivities, and in this respect,could be distinguished from unhydrated sediments in exploration boreholes on the Arcticcontinental shelves (Pearson etal., 1983). Magneto-telluric methods and high-resolution.sea-floor magnetic and gravity measurements.may also be used to identify hydrates.-Bathymetry and sea-floor morphology

The occurrence of free gas below a hydrate "blanket" can be limited by the extent andgeometry of the hydrate cap; it is important to assess whether the hydrate blanket providesa complete closure, and for this, studies of bathymetry and sea-floor morphology may beimportant. Thus, if a sediment "mound" or plateau is sufficiently large for a bathymetricclosure to be formed at the base of the hydrate, the potential exists for gas to be trapped.Density-driven separation of gas from water beneath the hydrate will take place, in thesame manner as in classical hydrocarbon traps. The size of the trap depends on the extentof the closure, the gas-generating capability of the underlying and nearby sediments, andthe ease of gas/fluid migration within the sediment. -

The presence of "pockmarks" along a sea-floor culmination have been used to inferthe presence of gas beneath a hydrate layer (Vogt et al., 1994). The hydrate layer mayprovide a mechanism for focusing gas migration from a wide area into a specific bathymetricculmination, thus preventing the gas from being vented near its site of generation. Thepresence of sea-floor pockrnarks, however, indicates that at least some proportion of thegas is escaping, rather than being entirely trapped below a hydrate cap.

-VI. D. MLCx acnd A Lowrie 51

EXTRACTION STRATEGIES

Three main extraction methods for recovering methane from hydrates have beentested, one of which is novel and particular to hydrates. All three methods are secondaryrecovery techniques designed to release the gas from the hydrate "blanket". They havebeen pioneered in permafrost terranes - in the Messoyakha (western Siberia) gasfield(Sloan, 1990), and in the Prudhoe Bcy-Kitparulk River field in Alaska (Malone, 1990).Where no gas reservoir exists naturally, one must first be created.

Thermal stimulation

In this method, steam, hot water or another hot fluid is pumped from the surface intothe hydrate layer, whose temperature is raised until the hydrate melts (drill-stem heaterscould also perhaps be used). Although this method has been used successfully onshorein permafrost regions, the heat requirements for its application to the recovery of oceanichydrates are likely to be prohibitive. This is because of the higher pressures on the seafloor; the phase-boundary curve (Fig. 1) indicates that melting will occur at highertemperatures, and will thus require a much greater thermal input. The relatively high heatof fusion/dissociation - at 273'K, this is 54 kJ/mol (Sloan, 1990) - also militatesagainst the large-scale melting of hydrates to obtain methane. Thus, for oceanic hydrate

.deposits,-fithermal.stimulation --would;ipiobably have; too high:, an energy cost to: be:commercially feasible for-long-term gas recovery.Measured temperatures in oceanic hydrates, however, are considerably higher thansea-floor temperatures and the temperatures ofhydrates in continental pennafrostregions.

These range up to 200C (Max, 1990, Fig. 13), which implies that it may be feasible to melthydrates whose temperatures are already elevated if the thermal input for melting issimilar to, or less. than, that of the permafrost hydrates. .

Thermal stimulation, particularly in-place heating, is very suitable for the productionof an initial gas "pocket", and this could prove important if used in combination withdepressurization (sea below).

Inhibitor stimulation

Inhibitor fluids such as brines and alcohols lower the freezing point of hydrates, andwhen pumped downhole, could induce melting. This process is much slower than thermalstimulation, but has the advantage of a lower initial energy input. However, this methodmay be costly. Because of the much higher pressures in oceanic hydrates, it is unlikelythat this method, which has been used in the Messoyakha gasfield and tested in shallowpermafrost hydrates in Alaska, would be effective in shifting the phase-boundary curvesufficiently to allow significant gas recovery (Max, 1990, Fig,. 13).

Depressurization

This is probably the most appropriate method forilarge-scale recovery of hydrates,because.it involves no costly continuing stimulation. By "drawing down" the gas pressurein an existing gas accumulation below a hydrate layer, or from a gas pocket which hasbeen artificially formed by thermal or inhibitor stimulation, the hydrates in contact withthe gas become unstable and invert to gas and water. The pressures within the reservoircan be controlled as a function of gas extraction. The method requires lowering reservoirpressures considerably in the case of oceanic hydrates. There will be a much greaterdifference between the reservoir and ambient pressures in oceanic hydrates than inpermafrost hydrates (Fig. 1), in which the method has been successfully tested.


Drilling

Hydrate gas traps can be accessed either by direct or indirect drilling through the HSZ.Indirect drilling through the geological component of the trap may, however, be preferableto drilling into the gas pocket near its base, especially with respect to a large, simple trap(Fig. 5a). Traps on continental slopes will be in shallower waters than those in the deeper.ocean basins, and because part of the trap may be formed by mechanically-strong rockstrata rather than hydrates, an indirect approach may involve a near-vertical or inclinedhole, with little or no horizontal drilling required (Fig. 5b).

THE STABILITY OF POTENTIAL HYDRATE RESERVOIRS

The presence of hydrates increases a sediment's strength and stability. However, theinteraction between oceanic hydrates aind 'a' stronly-depressurized gas is unknown andis difficult to model. The potential stability of reservoirs of different sizes in relation tohydrate-sediment caps of different characteristics is difficult to estimate, because littleis known about hydrates in oceanic environments. In fact, the association of trapped gaseswith pockmarks (Vogtetal., 1994) indicates that somehydratereservoirs may be metastable,and are unable to contain more than a small volume of gas. Periodic venting of gas appearsto be normal within some hydrate accumulations (Paull, 1995). A number of factors arecritical for deterilmining the characteristics of hydrate-trapped gas reservoirs,'the strengthand stability"of the hydrate cap, and the characteristics of'the sediment in the reservoir,so that an assessment of the gas which may be recovered from the hydrate can be made.

Preliminary~e'xperimental work ini'-aetermi"ningc-the strength of the hydrate seal at thebase of the deposit suggests that as little as '10 m of hydrate cementing the sediment maybe able to support'a'free gas 'colum of 500 m or more, when balanced by hydrostatic andlithostatic pressures (de Boer et al., 1985). --

Hydrate c'ap's in thickly-sedimented'areas, where the sediments may be as old asmrid-Jurassic t'o Cretaceous (such as' the Blake Outer Ridge), can be regarded as "mature",in that the hydrates have moved upwards as sedimentation has proceeded over a considerableperiod of time. The sediments below the mature hydrates have passed through the hydrateconservation cycle described above. Because the process of hydrate formation involvesthe filling of porosity prior to significant compaction, 'the porosity below the maturehydrates may be substantially different from that in unhydrated sediments at depthscorresponding to the base of the HSZ. The porosity and permeability characteristics offine-grained nmarine- sediments- whic' heave -undergone hydration is largely ''unhknown;porosity/permeability may either increase or decrease. In this context, Dillon (pers.comm., 1994) has suggested that a gas-filled reservoir of relatively high porosity in thefine-grained marine sediments off the SE coast of the USA may have relatively lowpermeability, and this could restrict its potential.

When gas evolves from hydrate, it may occupy a greater volume than that occupiedby the hydrate and therefore raise the reservoir pressure. In addition, the water producedthrouo'h the inversionprocess must be accommodated. If the volume of gas evolved fromthe hydrates, in addition to that which has migrated from deeper sources, is significantlygreater than the volume. available, the reservoir may become overpressured, increasingthe potential for a blow-out.

GAS EXTRACTION FROM METHANE HYDRATES: PRESENT AND FUTURE

Natural gas within and below a permafrost layer was originally recognized to be apotential energy resource in Russia, and significant volumes of hydrates have subsequentlybeen identified on both the Russian and North American continental shelves (Hitchon,1974; Kvenvolden and McMenamin, 1980).

M. D. Max and A. Lowrie53

a

Fig. 5. Cartoon of hydrate traps and offset drillingstrat~egy; diagrammatic scale, vertical exaggeration.Drill stem shown by heavy line from ships.

(a) Hydrate culmination trap. Steep penetration of HSZ to the side of gas pocket, and then lateralpoenetration of thie gas reservoir.

(b) Compound hydrate-geological traps. 2a"and 2b show the same indirect drilling method can be.used for traps having up- or down-slope structural components.

CT: Classical geological trap for comparison (in this case resulting from a major unconformity(u) in sedinients on the continental slope.

At the Messoyakha gasfield in western Siberia, methanol and other inhibitor fluidswere injected early in the production'programme, but depressurization is also used at thepresent time. It has been estimated that about' 83 Bcf of gas have been recovered frommethane hydrates at this field since 1969 (Collett, 1993). During the decompressionprogramme, the gas-water interface was maintained at the same position by controllingthe reservoir pressure, effectively increasing reservoir size. Modest production is stillcontinuing. Hydrate reserves at this field are estimated to be about 2.8 Tcf.

No sustained commercial recovery of gas from hydrates has taken place outsideRussia. Test gas recovery from hydrates in the Prudhoe Bay-KuparukRiverfield (Alaska),however, yielded rates similar to those at-Messoyakha. Here, gas hydrates are estimatedto total about 44 Tcf methane (Collett, 1993).

It is likely that depressurization from natural or artifically- stimulated gas pockets nearthe base of the HSZ will provide the main recovery mechanism for deep sea hydrates (Fig.6). Extraction management techniques will also have to'encompass maintenance ofreservoir stability.

Gas hydrates were recovered by piston coring from the upper 2m of sea-floor sedimentsalone the Svalbard (North Atlantic) continental slope during the summer of 1995, in anarea where the presence of hydrates had been predicted by Vogt et al., 1994 (Vogt, pers.commun., 1995).

54 Oceanic methane, hydrates

Future exploration

Gas hydrates in their natural state have proved difficult to study, because they aredifficult and expensive to sample. Hydrates usually disassociate to gas and water duringcore recovery, destroying the hydrate-sediment fabric in the process. Lee 164 of theOcean Drilling Program (Paull, 1995) was scheduled to drill a number of gas hydrateexploration holes in the Cape Fear Slide, the Blake Ridge, and the Carolina Rise off theSE coast of the USA, beginning in November, 1995. Detailed seismic surveys haveprovided very specific drilling targets (Dillon,pers. comm., 1995; Katzman et l., 1994).Pressurized core barrels were. to be'. used and elaborate down-hole geochemical andgeophysical analyses were scheduled to be carried out. Preliminary results from this gashydrate drilling are expected early in 1996.

CONCLUSIONS

Hydrate blankets and underlying gas accumulations appear to provide very largevolumes of potentially-recoverable methane in ocean sediments. Such large potentialenergy resources are commercially attractive. Oceanic methane hydrates constitute a trueexploration frontier. Their preliminary assessment is promising, but almost everythingremains to be done; however, the tools are at hand.

ACKNOWLEDGMENTS

The Author thanks W. Dillon (USGS, Woods Ho'le;MA) for constant help an`d comment,and for the provision of excellent seismic records. R. Malone (Morgantown EhergyTech~nology Center, US Dept.': ,of Ene,,rgyj, provid'ed industrial contacts, and recentinformation about research in tight gas reservoirs and horizontal drilling techniques. K.KvenvoLden, A. Grantz, T. Collett (USGS); A. Solheim (Norsk Polarinstitut); J. Ewing(Lamont-:Doherty Geophysical Observatory) and many other geologists are thanked for-their open discussions.-Journmal'review was by Professor R. Stoneley (lately ImperialCollege, London), whose comments on an earlier draft are acknowledged

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oceanic methane hydrates-a frontier gas resource

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