Co-existence of gas hydrate, free gas, and brine within the regional gas hydrate stability zone at Hydrate Ridge (Oregon margin): evidence from prolonged degassing of a pressurized

www.elsevier.com/locate/epsl

Earth and Planetary Science Letters 222 (2004) 829–843

Co-existence of gas hydrate, free gas, and brine within the regional

gas hydrate stability zone at Hydrate Ridge (Oregon margin):

evidence from prolonged degassing of a pressurized core

Alexei V. Milkova,*, Gerald R. Dickensb, George E. Claypoolc, Young-Joo Leed,Walter S. Borowskie, Marta E. Torresf, Wenyue Xug, Hitoshi Tomaruh,

Anne M. Trehuf, Peter Schultheissi

aBP America, Exploration and Production Technology Group, Room 15.122, Westlake Building, 501 Westlake Park Boulevard,

Houston, TX 77079, USAbDepartment of Earth Science, Rice University, Houston, TX 77005, USA

c8910 West 2nd Avenue, Lakewood, CO 80226, USAdKorea Institute of Geoscience and Mineral Resources, Daejeon, 305-350, South KoreaeEarth Sciences Department, Eastern Kentucky University, Richmond, KY 40475, USA

fCollege of Oceanic and Atmospheric Science, Oregon State University, Corvallis, OR 97331, USAgSchool of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA 30332, USA

hDepartment of Earth and Planetary Science, University of Tokyo, Tokyo 113-0033, JapaniGEOTEK, Daventry, Northants NN11 5RD, UK

Received 17 July 2003; received in revised form 12 March 2004; accepted 22 March 2004

Abstract

Standard scientific operations on Ocean Drilling Program (ODP) Leg 204 documented a horizon of massive gas hydrate and

highly saline pore water f0–20 m below the southern summit of Hydrate Ridge offshore Oregon. The sediment zone lies near

active seafloor gas venting, raising the possibility that free gas co-exists with gas hydrate in shallow subsurface layers where

pore waters have become too saline to precipitate additional gas hydrate. Here we discuss a unique experiment that addresses

this important concept. A 1-m-long pressurized core was retrieved from f14 m below sea floor at Site 1249 and slowly

degassed at f0 jC in the laboratory over f178 h to determine in situ salinity and gas concentrations in the interval of massive

gas hydrate. The core released f95 l of gas (predominantly methane), by far the greatest gas volume ever measured for a 1 m

core at ambient shipboard pressure and temperature conditions. Geochemical mass-balance calculations and the pressure of

initial gas release (4.2 MPa) both imply that pore waters had an in situ salinity approaching or exceeding 105 g kg� 1, the

approximate salinity required for a gas hydrate–free gas–brine system. Relatively high concentrations of propane and higher

hydrocarbon gases at the start of core degassing also suggest the presence of in situ free gas. Gas hydrate, free gas and brine

likely co-exist in shallow sediment of Hydrate Ridge. Near-seafloor brines, produced when rapid gas hydrate crystallization

0012-821X/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.epsl.2004.03.028

* Corresponding author. Tel.: +1-281-366-2806; fax: +1-281-366-7416.

E-mail address: [email protected] (A.V. Milkov).

A.V. Milkov et al. / Earth and Planetary Science Letters 222 (2004) 829–843830

extracts large quantities of water, impact the distribution and cycling of gas and gas hydrate in this region and perhaps

elsewhere.

D 2004 Elsevier B.V. All rights reserved.

Keywords: methane; gas hydrate; free gas; brine; hydrocarbons; pressure core; Leg 204; Site 1249; Hydrate Ridge

1. Introduction concentrations and three environmental conditions:

Gas hydrate and free gas bubbles can form in the

pore space of deep marine sediment when concen-

trations of low molecular weight gases, typically

methane (CH4), surpass saturation. These naturally

occurring phases command our attention because they

may constitute a future energy resource [1] and a

significant component of the global carbon cycle [2].

Crucial to many investigations are the vertical dis-

tributions of gas hydrate and free gas in marine sedi-

ment sequences. From a thermodynamic perspective,

these distributions should primarily depend on gas

Fig. 1. Diagrams illustrating the widely accepted concept of stratified natur

systems. (a) Occurrence and boundaries of various gas phases (dissolved,

methane fluxes [7,42]. Methane concentration in pore water defines the occ

solubility of methane, gas hydrate is present above the base of the GHSZ and

present in the system when methane concentration in pore water is below so

stability conditions [21] (thin solid lines) and the solubility of methane [36]

be hydrostatic) and temperature (based on the geothermal gradient 55 jCpresented in (a) occurs only at salinity 35 g kg� 1. The GHSZ thins when th

salinity 140 g kg� 1.

pressure, temperature, and the activity of water (aw),

the latter a parameter measuring the effective concen-

tration of water and inversely related to salinity [3].

Most descriptions of marine gas hydrate systems have

assumed steady increases in subsurface pressure and

temperature, and pore water salinity close to that of

seawater. This has led to the widely accepted concept

that natural gas hydrate systems are stratified [4–7],

with an upper gas hydrate stability zone (GHSZ) and a

lower free gas zone (FGZ) separated at a depth where

pressures and temperatures on the geotherm intersect

those on a dissolved gas–gas hydrate–free gas equi-

al gas hydrate systems and how pore water salinity may modify these

free, and gas hydrate) depend on the variations of energy, fluid, and

urring phase. When methane concentration in pore water exceeds the

free gas is present below the base of the GHSZ. Only dissolved gas is

lubility. (b) The salinity of pore water significantly affects gas hydrate

(thin dashed line). The calculations are made for pressure (assumed to

km� 1) at Site 1249 (thick line). Note that the phase distribution as

e salinity of pore water increases to 70 g kg� 1, and is not present at

A.V. Milkov et al. / Earth and Planetary Science Letters 222 (2004) 829–843 831

librium curve for seawater (Fig. 1). Depth intervals

within these zones may not contain their respective

phases because of insufficient gas concentration [5–7].

However, this simple view does preclude free gas

above gas hydrate (i.e., in the GHSZ) without invoking

kinetic arguments and multi-phase fluid flow [4,7,8].

Several recent studies have suggested significant

amounts of free gas within the GHSZ. This idea has

received its strongest push from investigations along

the Cascadia margin off western North America

(Fig. 2). Here, free gas within the GHSZ has been

inferred from gas bubble trains emanating above the

seafloor [9–12], ‘‘globular’’ fabrics within recovered

gas hydrate specimens [13], and acoustic ‘‘wipe-outs’’

on seismic profiles [14,15]. Apparently, either: (1)

kinetic effects prevent the precipitation of gas hydrate

at favorable stability conditions despite excess gas, or

(2) environmental conditions vary over short distances

so that the GHSZ has a complicated volume. For

example, the upward migration of warm fluids could

allow free gas to exist much shallower than expected

from regional geotherms [15,16], or free gas could be

separated from water along channels [10].

Ocean Drilling Program (ODP) Leg 204 drilled a

series of boreholes through the GHSZ of southern

Hydrate Ridge on the Cascadia Margin (Fig. 2)

[17,18]. One intriguing finding was abundant gas

hydrate (>30–40% of porosity) and high salinity

Fig. 2. ODP Sites 1249 and 1250 on Hydrate Ridge, offshore of Oregon (

showing the ridge located within the Cascadia accretionary complex where

Bathymetric map [43] showing detailed location of ODP Sites 1249 and

(S>60 g kg� 1; Cl>1000 mM) pore water at shal-

low depths ( < 20 m) below the summit of Hydrate

Ridge (Fig. 3) [17,18]. Because relatively small

increases in salinity can dramatically decrease the

stability of gas hydrate [3,19–21], it is possible that

free gas could occur with gas hydrate and brine in

horizons within the regional GHSZ (Figs. 1 and 3).

Co-existence of gas hydrate, free gas and brine does

happen in laboratory experiments [3,19], and has

been suggested as a means to move free gas

through the GHSZ [8]. Evaluating such co-existence

in nature is difficult, however, because in situ

salinity and gas concentrations are hard to quantify

in gas hydrate bearing sediment using standard

techniques. Dissociation of gas hydrate during con-

ventional core retrieval releases gas, which escapes

[5,22], and fresh water, which decreases pore water

salinity [23,24].

In this paper, we present and discuss data acquired

from a pressurized core that was retrieved during Leg

204 from the high salinity horizon at Hydrate Ridge.

The core was collected and examined because, in

theory, in situ salinity and gas concentrations can be

determined through slow, controlled degassing of pres-

surized cores [25]. Our data suggests that gas hydrate,

free gas and brine indeed co-exist near the seafloor.

Depth intervals containing hypersaline pore waters,

perhaps produced during gas hydrate formation, can

OR), northwest United States. (a) Tectonic setting of Hydrate Ridge

the Juan de Fuca plate subducts beneath North American plate. (b)

1250, and cross-section of Fig. 9.

Fig. 3. Chloride concentrations at ODP Site 1249, where a horizon of massive gas hydrate and highly saline pore water exists at 0–20 mbsf. In

this interval, standard whole-round intervals, and selected wet- and dry-looking samples have Cl� concentrations as high as 1008, 1039, and

1368 mM, respectively [17]. However, all these samples contain fresh water from dissociated gas hydrate. The Cl� concentration of in situ pore

water in core 1249F-4P was at least 1650 mM and probably higher. Dashed line indicates Cl� concentration in seawater (559 mM).


explain the presence of free gas within the regionally

based GHSZ at Hydrate Ridge and other localities.

2. Geological setting

Hydrate Ridge is a 25-km-long, 15-km-wide ac-

cretionary ridge on the Cascadia margin, f100 km

west of the Oregon coast (Fig. 2). Gas vents, authi-

genic carbonate buildups, gas hydrate outcrops, and

chemosynthetic communities on the seafloor attest to

an area of gas-charged sediment where fluids and

gases migrate in the subsurface [9,10,26,27]. Seismic

reflection surveys across the ridge also reveal a strong

bottom-simulating reflector (BSR), which as else-

where, has been interpreted as marking an interface

between overlying sediment with gas hydrate and

underlying sediment with free gas [28].

In 2002, ODP Leg 204 drilled nine sites on and

around the southern summit of Hydrate Ridge to

characterize the amount and distribution of gas hy-

drate and free gas in this region [17,18]. Of these sites,

Site 1249 (Fig. 2) was cored to f90 m below sea

floor (mbsf) at 778 m below sea level (mbsl) in an

area of high seafloor reflectivity and gas venting [29],

and where shallow free gas had been inferred from gas

hydrate fabric [13]. Seismic images made prior to

drilling also showed a 30-m-thick interval of chaotic,

strong reflectivity immediately below the seafloor,

which was interpreted as representing massive gas

hydrate [30]. Although the BSR at this location

(f115 mbsf) was not penetrated for safety reasons,

drilling confirmed the presence of massive gas hy-

drate at shallow depths, with direct observations of

core and various proxy indicators suggesting that gas

hydrate fills >30–40% of pore space between f0 and


20 mbsf, and exceeds 90% of pore space across some

cm-scale intervals [17].

Downhole temperatures at Site 1249 steadily in-

crease from f5 jC at the seafloor to f9.5 jC at 90

mbsf, as might be expected from regional geotherms

[17,18]. Assuming a hydrostatic pressure gradient of

f0.01 MPa/m, the interface between the GHSZ and

FGZ should occur at f115 mbsf (Fig. 1b), which is

consistent with seismic interpretations and the overall

idea of stratified gas hydrate systems.

An unanticipated finding at Site 1249 (and to a

lesser degree at Site 1250; Fig. 2) was high salinity

pore water at shallow depth (Fig. 3). Using traditional

shipboard methods for obtaining interstitial waters in

sediment from ODP boreholes [31], analyzed pore

water salinity and chlorinity reached 62 g kg� 1 and

1008 mM, respectively, at f7 mbsf [17]. ‘‘Wet’’ and

‘‘dry’’ samples selected from cores also rendered Cl�

concentrations exceeding 1300 mM [17]. These ele-

vated concentrations of dissolved constituents signif-

icantly decrease the stability of gas hydrate at a given

pressure and temperature [3,19–21], although with

the measured geotherm and assumed hydrostatic pres-

sure, by an amount that precludes free gas (Fig. 1).

However, pore waters were collected at f1 atm and

>15 jC from conventional cores, so that they include

some amount of fresh water released from gas hydrate

dissociation during sediment recovery. Pore water

salinity and chlorinity at Site 1249 determined using

standard techniques [17] must be considered mini-

mum estimates.

Fig. 4. The ODP Pressure Core Sampler (PCS) and degassing

components used on Leg 204 [17,34]. PCS tool is drawn to scale;

manifold, bubbling chamber, and recording system are not drawn to

scale.

3. Methods

The ODP Pressure Core Sampler (PCS) is designed

to recover a short sediment core, including pore water

and gas, at in situ pressure [32,33]. The tool (Fig. 4)

consists of an inner core barrel, which ideally collects a

1465 cm3 sediment core (1 m long� 4.32 cm diame-

ter), and an outer chamber, which holdsf2000 cm3 of

seawater pumped down the borehole [25].

Prior to Leg 204, the PCS had been successfully

used to study in situ gas concentrations in hydrate-

bearing sediment at ODP Sites 994–997 on the Blake

Ridge [5,25], and at ODP Site 1230 on the Peru margin

[33]. This work has demonstrated that slow, incremen-

tal release of gas from the PCS at constant temperature

yields characteristic degassing curves in terms of gas

volume and pressure (Fig. 5). The shapes of these

curves depend on the amount, type and phase of gas

within the tool, as well as salinity. For marine sediment

cores containing gas hydrate, the gas volume released

from the PCS increases with relatively small decreases

in pressure once pressure has dropped below equilib-

rium conditions for gas hydrate stability. The reason for

this behavior is twofold: (1) gas hydrate dissociation

releases free gas, which, in a closed container, increases


pressure until dissociation ceases, and (2) gas hydrate

dissociation releases fresh water, which increases the

stability of gas hydrate at given pressure and temper-

ature conditions. Theoretically, the pressure at the start

of this ‘‘hydrate degassing’’ should depend on the

initial salinity within the tool (Fig. 5) [25], which could

be checked through measurements of water chemistry.

However, for logistical reasons, all PCS cores collected

to date have been degassed too quickly to establish an

accurate salinity [25,33].

The PCS was successfully deployed 30 times on

Leg 204 [34]. Upon collecting all these cores, the PCS

was placed into an ice bath to maintain an interior

temperature off0 jC. After initial temperature equil-

ibration, a manifold was connected to the PCS, allow-

ing step-wise release of pressure and gas (Fig. 4). Splits

of gases were incrementally collected by bubbling gas

into a chamber consisting of an inverted graduated

cylinder and a plexiglass tube filled with a saturated

NaCl solution (Fig. 4). The volume of each gas incre-

ment was measured, and aliquots were then sampled

for their gas composition (C1–C6 hydrocarbon gases,

CO2, N2, and O2). These analyses were made on board

of the JOIDES Resolution using an HP 6890 multi-

valve, multicolumn gas chromatograph equipped with

both thermal conductivity (TCD) and flame ionization

(FID) detectors [35]. Hydrocarbon gas concentrations

are reported here as parts per million (ppm) by volume

of total hydrocarbon gases excluding air contamina-

tion. The estimated detection limit for hydrocarbons

and CO2 was 5 ppm, and the reproducibility of gas

concentrations was F 5%. At the end of each degass-

ing experiment, ice was removed, the PCSwas warmed

to f20 jC, and a final volume of gas was collected.

Dissolved gas remaining in the tool after warming was

not measured, but was probably a very small volume

considering the low solubility of methane at surface

conditions. After complete degassing, the PCS was

disassembled, and water and sediment were collected

Fig. 5. Theoretical degassing of a pressurized sediment core at

isothermal conditions (0 jC) as plotted on (a) a temperature–

pressure phase diagram, and (b) a volume–pressure diagram

(modified from [25,44]). Phase diagram is for the pure methane–

water system at various salinities (0–140 g kg� 1), and shows fields

with free gas (FG), gas hydrate (GH), water (W, including the

dissolved gas) and ice (I). In general, as a core is degassed from an

initial high pressure (point A; here at f 7.9 MPa), the pressure–

volume path of degassing depends on the total amount of gas

present and salinity [25,44]. If the concentration of methane in pore

water is equal to solubility, then the pressure curve follows the free

gas pathway AF. If the concentration of methane is less than the

solubility, then the pressure curve follows the dissolved gas

pathways (e.g., AB). The pressure follows pathway ADDVH if

gas hydrate occurs in the pore space (water salinity 35 g kg� 1) and

methane is released from the PCS at the rate below the rate of

methane release from dissociating gas hydrate. If the rate of

methane release from the PCS is greater than the rate of methane

release from dissociating methane hydrate, then the pressure follows

curve ADH. Gas hydrate starts to decompose at 4.2 MPa (point C) if

initial pore water salinity is 105 g kg� 1, but at 2.8 MPa (point E) if

salinity is 0 g kg� 1. The slopes CCV and DDV occur because the

mixing of pore water with fresh water from dissociated gas hydrate

decreases the equilibrium gas hydrate pressure.


from both the outer chamber and the inner core barrel.

When analyzed, the Cl� (F 3 mM) and SO42� con-

centration (F 0.5 mM) of water was determined using

standard shipboard techniques [17].

Total volumes of gas released from Leg 204 PCS

cores and in situ methane concentrations have been

reported previously [17,34]. In this paper, we present

and discuss the degassing of PCS Core 204-1249F-4P,

recovered from 13.5 to 14.5 mbsf at Site 1249 in the

sediment zone of chaotic reflectivity, massive gas

hydrate and high salinity pore water. Unlike other

PCS cores collected during Leg 204 [34] and previous

ODP legs [25,33], this core was degassed for over a

week, a sufficiently long duration to construct a de-

tailed volume–pressure curve. This core was also

unique because it contained an extremely high amount

of natural gas.

4. Experimental results

The first pressure recorded for Core 204-1249F-4P

was 12.97 MPa on the rig floor (Fig. 6). This exceeds

Fig. 6. Observed volume–pressure and volume– time relations for Core 2

the first f 70 l of gas, presumably because dissociating gas hydrate relea

decreased more rapidly as the last f 25 ml escaped, apparently because on

gas was present inside of the PCS.

the f7.9 MPa expected given recovery depth (792

mbsl) and a hydrostatic pressure gradient. While

moving the PCS to the laboratory (f25 min), the

pressure within the PCS rose to 16.24 MPa. Pressure

often increases within PCS cores when first retrieved

because the tool holds a headspace, because gas

pressure within a closed container depends on tem-

perature, and because ambient temperature usually

surpasses in situ temperature [25,33]. However, the

large, rapid pressure rise for Core 204-1249F-4P is

unprecedented [17,25,33,34], and probably indicates

expanding free gas, either in situ or from decomposed

gas hydrate as the tool warmed.

Core 204-1249F-4P was placed into an ice bath

f25 min after recovery, where it slowly equilibrated

to f0 jC over f570 min. During this time, pressure

dropped nearly logarithmically to f8.6 MPa. Such

pressure decay occurs in all PCS cores, and reflects

decreasing internal gas pressure with cooling

[17,25,33,34].

After equilibrating at f0 jC, the core was

degassed for 10,700 min (178.3 h). This degassing

(Fig. 6) involved step-wise release of 106 gas incre-

04-1249F-4P. Pressure decreased relatively slowly during release of

sed gas and fresh water within a closed container (Fig. 5). Pressure

ly headspace free gas (from decomposed gas hydrate) and dissolved


ments varying from 20 to 1300 ml at laboratory

conditions (P= 0.1 MPa, T =f22 jC) [17]. The

cumulative volume of gas released was 95,110 ml,

by far the greatest quantity of gas emitted from the

PCS to date (e.g., the most gas released from a PCS

core on the Blake Ridge was 7485 ml [25]). Gases

released from the core were mixtures of air (N2 and

O2), CH4, CO2, and C2 + hydrocarbon gases (Fig. 7),

such that the total volume comprised 590 ml of air (as

indicated by N2 and O2,) and 94,520 ml of natural

Fig. 7. Composition of gas released from Core 204-1249F-4P showing thr

gas hydrate were likely released during Stage 1. Gases from gas hydrate an

Note that concentrations of more soluble gases (e.g., CO2) increase at t

concentrations during Stage 1, strongly indicating mixing between C3-ric

gases (hydrocarbons and CO2). Most of the latter

(94,210 ml or 99.67%) was CH4.

Gas evolving from Core 204-1249F-4P changed

composition with increasing total volume, defining

three general degassing stages (Fig. 7). During release

of the first f18.5 l of gas, concentrations of methane

(C1, 998,800–997,400 ppm) and propane (C3, 70–29

ppm) decreased while concentrations of ethane (C2,

720–1450 ppm) and carbon dioxide (CO2, 400–1300

ppm) increased. Higher hydrocarbons such as butane

ee distinct stages of degassing. A mixture of gases from free gas and

d dissolved gas were probably released during Stage 2 and Stage 3.

he end of Stage 3. Note also the linear (r2 = 0.86) decrease in C3

h free gas, and C3-poor gas evolving from gas hydrate.


(C4) and pentane (C5) were minor components, al-

though their concentrations generally decreased with

degassing. During the second stage, when the next

f61 l of gas were released, gas composition stayed

remarkably stable (mean concentrations C1 = 997,100

ppm, C2 = 1490 ppm, C3 = 29 ppm, and CO2 = 1390

ppm). As the last f15 l of gas were released, the

concentration of CO2 consistently increased to

f15,000 ppm. Concentrations of C2 and C3 also

increased to 2180 and 47 ppm, respectively, whereas

the concentration of C1 decreased to 982,600 ppm.

Water in the outer chamber after complete degassing

had a Cl� concentration of 678 mM (SO42� was not

measured). The inner core barrel contained a slurry of

water and sediment. The water had Cl� and SO42�

concentrations of 825 and 14.2 mM, respectively, and

the sediment appeared similar to that in surrounding

cores collected by other conventional coring. The dry

weight of sediment in this slurry was 620 g, which

corresponds to a volume of 230 ml assuming an

appropriate sediment density of 2.7 g cm� 3. Given a

full 1.00 m recovery, this implies that gas hydrate,

water, and gas filled 1235ml (orf85% porosity) of the

core at in situ conditions. This is consistent with

porosity estimates for this interval based on Site 1249

well-log data [17]. However, although many PCS cores

recovered during Leg 204 were full or nearly full (after

accounting for volume loss from dissociated gas hy-

drate), we cannot be sure that Core 204-1249F-4P was

indeed 1.00 m long at in situ conditions. We discuss the

ramifications of an incomplete core below.

5. Estimates of in situ methane abundance and gas

hydrate content

Using established PVT relationships for gases,

f3.9 mol CH4 degassed from Core 204-1249F-4P.

Theoretical methane solubility between dissolved gas

and gas hydrate is f58 mM at in situ pressure (f7.9

MPa), in situ temperature (f6 jC), and measured

pore water salinity (66 g kg� 1) for the depth of this

core [36]. This solubility is relatively insensitive to

salinity [36], and limits the maximum amount of

dissolved CH4 in the core to f0.07 mol (the quantity

if water occupied all available space). Gas hydrate and

free gas collectively held >3.8 mol CH4 within

1235 ml at in situ conditions.

The volume of gas hydrate within the core can be

readily estimated from CH4 abundance given an

assumption of no free gas, and knowledge of gas

hydrate density (qGH) and crystal structure [5,25,34].

Applying this reasoning, Core 204-1249F-4P con-

tained 520 ml of gas hydrate (f42% porosity) if it

occurred as a ‘‘typical’’ structure I hydrate (qGH =

0.91 g/cm3; stoichiometry of CH4�6H2O). However,

average qGH of gas hydrate specimens from Hydrate

Ridge has been measured at 0.79 g/cm3 [9], and the

small but significant amounts of propane released

from Core 204-1249F-4P (Fig. 7) may suggest that

gas hydrate occurred as structure II. If the CH4

resided only in dissolved gas and low-density struc-

ture II gas hydrate (stoichiometry of CH4�5.67H2O),

570 ml of gas hydrate existed at in situ conditions

(f46% porosity). However, these gas hydrate abun-

dances are too high if free gas co-existed at depth.

6. Estimates of in situ salinity

Numerous experiments have shown that the outer

chamber of the PCS contains only borehole water

(i.e., surface seawater; f559 mM Cl�) when pres-

surized cores are first recovered [25,33]. The mea-

sured 678 mM Cl� concentration in outer chamber

water, therefore, implies incursion of high salinity

water from the inner core barrel during degassing

and disassembling of the PCS. Simple mixing of

2000 ml seawater and 1235 ml of inner core barrel

fluid with an initial Cl� concentration of 1020 mM

would give the measured Cl� concentrations for both

the outer and inner chambers at the end of degassing.

However, pore space within the inner core barrel must

have contained gas hydrate, so in situ pore water must

be more saline than 1020 mM.

Chloride within the inner chamber after complete

degassing reflects a mixture of borehole water, initial

pore water of high Cl� concentration, and water

originally in gas hydrate and presumably lacking

Cl� [23,24]. Assuming no free gas, in situ pore water

Cl� can be readily estimated from the measured Cl�

concentrations, the total available space, and the

original amount of gas hydrate. Core 204-1249F-4P

had f715 ml of pore water with 1630 mM Cl� and

412 ml of fresh water bound in gas hydrate at in situ

conditions if it began with 520 ml of structure I gas


hydrate. Alternatively, the core had f665 ml of pore

water with 1665 mM Cl� and 389 ml of fresh water

bound in gas hydrate at in situ conditions if it began

with 570 ml of low density structure II gas hydrate. In

either case, in situ Cl� greatly exceeds Cl� measured

by conventional techniques (Fig. 3). Note also that

consideration of a free gas component at in situ

conditions would increase the inferred in situ pore

water Cl� because it would occupy space and there-

fore decrease the amount of available water.

High pore water Cl� concentration should signify

pore water with low aw and high salinity [3]. Indeed,

assuming a conservative relationship between the three

variables (and other caveats above), original pore water

within Core 204-1249F-4P must have had a salinity

>102 g kg� 1. The degassing curve (Fig. 6) provides an

important, independent means to assess this interpre-

tation. At 0 jC, the temperature of degassing, gas

hydrate should begin to dissociate and release large

volumes of gas at f3 MPa if surrounding pore water

has a salinity of 35 g kg� 1 [25]. However, Core 204-

1249F-4P began releasing significant quantities of gas

at f4.2 MPa (Fig. 6), which suggests a starting pore

Fig. 8. In situ temperature and pressure at 14 mbsf at Site 1249 compared

kg� 1 (calculated using CSMHYD [21] and consistent with experimental r

during Leg 204 at 16.5 mbsf, and the range shown here includes uncerta

temperature gradient. Pressure is assumed to lie between hydrostatic and l

measurements made during Leg 204 [17]. Gas hydrate and free gas are pre

water salinity f105 g kg� 1, in good agreement with

the chloride mass-balance calculations.

Initial pore water salinity may, in fact, have been

>105 g kg� 1 for three reasons (beyond consideration

of free gas). First, we assumed that Core 204-1249F-

4P recovered a full 1-m-long core. Previous PCS

operations indicate that this is not always the case

[25,34]. Second, a small amount of sediment may

have been lost during PCS disassembly. Third, it was

observed that water escaped from the PCS during each

gas release. Although the volume and chemical com-

position of the released water was not measured, we

infer that some dissolved ions escaped from the PCS

during the degassing and was not accounted for in

mass-balance calculations. Any of these factors would

result in underestimated in situ salinity. For example,

if only a 0.95 m core was recovered, in situ salinity

may have approached 120 g kg� 1.

An in situ pore water salinity at or above 105 g

kg� 1 has fundamental implications toward our under-

standing of gas hydrate and free gas in shallow

sediment of Hydrate Ridge. At the in situ pressure

and temperature conditions of Core 204-1249F-4P, a

to the hydrate stability boundaries for pore water salinities 0–150 g

esults [37] for a salinity of 100 g kg� 1). Temperature was measured

inties in this measurement and extrapolation based on the regional

ithostatic, with the density of the overlying sediment constrained by

dicted to co-exist at these conditions for salinity of 105–115 g kg� 1.


methane hydrate–free methane gas–brine mixture

should occur between 105 and 115 g kg� 1 [21,37]

(Fig. 8). This is the point where gas hydrate can no

longer incorporate excess methane because surround-

ing waters are too saline.

Core 204-1249F-4P presumably lacked SO42� be-

fore degassing because it came from below the sulfate

reduction zone where pore water SO42� is zero. The

SO42 � concentration of the inner core barrel after

degassing thus reflects borehole water contamination

of the original, collective volume of pore water and

hydrate water. For the two cases stated above, where

all excess gas resides in gas hydrate, there would have

been 1127 and 1054 ml of total SO42� depleted water

within the tool before degassing. A specific, partial

mixing of these volumes and 2000 ml of borehole

water renders the observed Cl� concentrations

(above), but necessitates inner core barrel dissolved

SO42 � concentrations of 12.8 and 13.2 mM after

degassing, rather than the measured 14.2 mM. The

simplest explanation is that the inner core barrel

contained 50–150 ml less water than expected from

gas and Cl� mass balance calculations alone because

free gas occupied space at depth.

7. Evidence of free gas

The first increment of gas collected from the PCS

usually contains air trapped during deployment

[25,33], and this was observed for Core 204-1249F-

4P (Fig. 7). Following this, gas composition should not

vary significantly during degassing if only gas hydrate

and dissolved gas are present in the core. Only toward

the end of the experiment, when all gas hydrate has

dissociated, should gas composition change as various

gases come out of solution according to their solubility.

The second and third stages of degassing (Fig. 7)

generally conform to expectations for a core with

abundant gas hydrate and dissolved gas. During the

second stage of degassing, the core released f61

l (f2.5 mol) of gas. This gas was predominantly CH4

(>99.7%) and had a very uniform overall gas compo-

sition. Moreover, during this degassing stage, pressure

only decreased slightly (Fig. 6). These observations

are consistent with steady dissociation of 340 to 380

ml of methane hydrate (depending on qGH), with the

drop in pressure over time (and volume) resulting

from freshening of pore water within the inner cham-

ber (Fig. 5). Gas increments collected during the third

stage of degassing are relatively enriched in CO2, C2,

and C3, and relatively depleted in C1 (Fig. 7). This

observation is consistent with gas release from water

because CO2, C2, and C3 are more soluble than C1 at

low temperature and pressure [38]. It should be

recognized that the 15 ml (f0.6 mol) of gas released

during stage 3 exceeds the amount that can be

dissolved in 2900 to 3100 ml of water (the total

quantity within the tool including pore water, hydrate

water and borehole water) at in situ or laboratory

conditions. At low pressure, Core 204-1249F-4P

undoubtedly contained a large headspace (100–270

ml), which formed after evacuation of free gas at high

pressure and dissociation of gas hydrate.

In contrast, gas composition varied systematically

as the first 18.5 l of gas were released from Core 204-

1249F-4P (Fig. 7). Of particular interest, gas released

during this first stage is initially depleted in C2 and

CO2, and enriched in C1, C3, C4, and iC5 relative to

gas released during stage 2. Indeed, C4 + components

were not detected during the second stage of degass-

ing (Fig. 7). These changes in gas composition are

significant given the analytical precision, and are best

explained by mixing of gases from two distinct

sources, most likely decomposed gas hydrate and free

gas. Although small amounts of gas can be desorbed

from sediment [39], the sheer volume of gas precludes

this as a plausible source.

Propane (C3) concentrations decrease nearly linear-

ly (r2 = 0.86) during the first stage of degassing (Fig. 7).

A simple mixing model based on C3 variation was

therefore used to estimate the contribution of gas from

the two sources during stage 1 degassing. For one

source, we assumed a C3 concentration of 29 ppm,

the average composition of gas evolved during stage 2

degassing and presumably from gas hydrate. For the

other source, we assumed a C3 concentration of 69

ppm, the composition of the second gas increment.

Based on this mixing model, we estimate that 10.7

l (f0.44 mol) of gas came from decomposed gas

hydrate during stage 1 degassing, while 7.8 l (f0.32

mol) of gas was derived from the other source. Assum-

ing this source was free gas, it would occupyf76ml at

in situ pressure and temperature, after accounting for

methane compressibility. This is within the amount

predicted from pore water mass balance considerations.


8. Mechanism for co-existence of shallow gas

hydrate and free gas

Gas hydrate, free gas and dissolved gas likely co-

exist with hypersaline pore water in shallow sediment

at Site 1249. Considering all information gathered from

Core 204-1249F-4P, sediment pore space between 13.5

and 14.5 mbsf had about 40% gas hydrate, 50% pore

water of >105 g kg� 1 salinity, and 10% free gas.

We propose a straightforward mechanism for this

finding (Fig. 9). Gas hydrate formation extracts

methane and water, but excludes dissolved ions to

surrounding pore water [23,24]. Thus, pore water

salinity rises with gas hydrate precipitation [24].

Unless diffusion or advection remove dissolved ions,

Fig. 9. Possible mechanism for co-existence of gas hydrate, free gas

and brine in shallow sediment at Hydrate Ridge as viewed at large

and small scales. (a) Schematic of cross-section A–AV (Fig. 2)

showing an inferred distribution of gas hydrate (diamonds) and free

gas (circles), and the inferred rapid upward transport of free gas

from depth along Horizon A [17], through the regional gas hydrate

stability zone (GHSZ), to the seafloor and the water column. (b) The

high gas flux (solid arrows) induces rapid gas hydrate formation,

which fills pore space and initiates the flow of dissolved ions

(broken arrows) away from gas hydrate. (c) The filling of pore space

also decreases sediment permeability, which decreases the removal

of dissolved ions. If gas hydrate formation is faster than removal of

dissolved ions, pore waters eventually become too saline for further

gas hydrate formation. Free gas can now co-exist with gas hydrate

and brine.

gas hydrate formation will continue with a supply of

gas until pore water salinity is so high that the

fugacity of gas in water equals the fugacity of free

gas [20]. At this point, a gas hydrate–free gas–brine

equilibrium is reached [20], and excess gas enters

free gas rather than gas hydrate. This is probably the

situation at Site 1249, although its exact cause

remains unclear. Co-existence of gas hydrate, free

gas and brine could occur when high gas flux drives

rapid gas hydrate formation, when excessive gas

hydrate precipitation decreases permeability of sedi-

ment and removal of dissolved ions, or when both

processes operate together.

9. Conclusions and implications

Core 204-1249F-4P was retrieved from a shallow

interval of massive gas hydrate and high salinity

pore water on the southern summit of Hydrate

Ridge. This core offers a unique opportunity to

assess the natural co-existence of gas hydrate, free

gas and hypersaline pore water, a phenomenon only

known previously from laboratory experiments [19].

All information obtained from Core 204-1249F-4P

indicates that it indeed held gas hydrate, free gas

and brine together at in situ conditions. Such co-

existence may occur in shallow sediment elsewhere

on Hydrate Ridge, and in other regions of high

upward methane flux [40]. This has several impli-

cations toward our general understanding of marine

gas hydrate systems.

First, gas hydrate, free gas and salinity each

affect physical properties of bulk sediment (e.g.,

electrical resistivity and acoustic velocity). Thus, in

regions where gas hydrate, free gas and brine co-

exist, the combined affects of multiple variables

need consideration when applying certain proxy

methods to quantify gas hydrate abundance. For

example, electrical resistivity logs have been used

to estimate the amount of water and gas hydrate

within the GHSZ, invariably assuming an Archie

relationship, an absence of free gas, and moderate

to low salinity [41]. This approach would overesti-

mate the amount of gas hydrate if free gas was

present. Preliminary shipboard interpretations of

electrical resistivity logs at Site 1249, based on

the aforementioned assumptions, suggest that sedi-


ment pore space at 14 mbsf holds f65% gas

hydrate and f35% water [17]. This is much

different than suggested above, arguably because

the initial resistivity log interpretation did not

include the appropriate amount of free gas and

dissolved ions.

Second, high-salinity brines may provide a means

to transport free gas through water-bearing shallow

sediment. A growing body of evidence suggests that

free gas moves through the GHSZ on Hydrate

Ridge [9–13] and elsewhere [8,14–16,40]. Where

this phenomenon has been discussed for Hydrate

Ridge, various authors have typically invoked a

special circumstance of gas–water segregation dur-

ing gas transport [9,10]. For example, Suess et al.

[10] suggest that free methane moves through shal-

low sediment of northern Hydrate Ridge along

channels lined with gas hydrate that has incorporat-

ed all surrounding water. We cannot dismiss these

ideas, but note that they are inconsistent with

macroscopic observations indicating abundant water

in shallow sediment of Hydrate Ridge. Even the

thin sediment intervals with the highest gas hydrate

content at Site 1249 have large amounts of water

according to well logs [17] and our information

from Core 204-1249F-4P. Rather than arguing for

segregation of free gas and water within the GHSZ,

we hypothesize that free gas can move through the

regional GHSZ in association with high salinity

water (Fig. 9).

Acknowledgements

This research used data provided by the Ocean

Drilling Program (ODP). The ODP is sponsored by

the U.S. National Science Foundation (NSF) and

participating countries under management of Joint

Oceanographic Institutions (JOI). Funding for this

research was provided by the U.S. Science Support

Program and the U.S. Department of Energy. We

particularly thank D. Schroeder and K. Grigar for

help with PCS modifications and operations. Both

AVM and GRD are thankful to F. Rack for the

invitation to participate in the ODP Leg 204. We

thank K. Gering and an anonymous reviewer whose

constructive criticisms significantly improved this

manuscript. [BOYLE]

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Documents

Co-existence of gas hydrate, free gas, and brine within the regional gas hydrate stability zone at Hydrate Ridge (Oregon margin): evidence from prolonged degassing of a pressurized