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

Earth and Planetary Science Letters 222 (2004) 829 – 843 www.elsevier.com/locate/epsl

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. Milkov a,*, Gerald R. Dickens b, George E. Claypool c, Young-Joo Lee d, Walter S. Borowski e, Marta E. Torres f, Wenyue Xu g, Hitoshi Tomaru h, Anne M. Tre´hu f, Peter Schultheiss i a

BP America, Exploration and Production Technology Group, Room 15.122, Westlake Building, 501 Westlake Park Boulevard, Houston, TX 77079, USA b Department of Earth Science, Rice University, Houston, TX 77005, USA c 8910 West 2nd Avenue, Lakewood, CO 80226, USA d Korea Institute of Geoscience and Mineral Resources, Daejeon, 305-350, South Korea e Earth Sciences Department, Eastern Kentucky University, Richmond, KY 40475, USA f College of Oceanic and Atmospheric Science, Oregon State University, Corvallis, OR 97331, USA g School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA 30332, USA h Department of Earth and Planetary Science, University of Tokyo, Tokyo 113-0033, Japan i GEOTEK, 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

* Corresponding author. Tel.: +1-281-366-2806; fax: +1-281-366-7416. E-mail address: [email protected] (A.V. Milkov). 0012-821X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2004.03.028

830

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

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 Gas hydrate and free gas bubbles can form in the pore space of deep marine sediment when concentrations 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 distributions of gas hydrate and free gas in marine sediment sequences. From a thermodynamic perspective, these distributions should primarily depend on gas

concentrations and three environmental conditions: pressure, temperature, and the activity of water (aw), the latter a parameter measuring the effective concentration 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-

Fig. 1. Diagrams illustrating the widely accepted concept of stratified natural gas hydrate systems and how pore water salinity may modify these systems. (a) Occurrence and boundaries of various gas phases (dissolved, free, and gas hydrate) depend on the variations of energy, fluid, and methane fluxes [7,42]. Methane concentration in pore water defines the occurring phase. When methane concentration in pore water exceeds the solubility of methane, gas hydrate is present above the base of the GHSZ and free gas is present below the base of the GHSZ. Only dissolved gas is present in the system when methane concentration in pore water is below solubility. (b) The salinity of pore water significantly affects gas hydrate stability conditions [21] (thin solid lines) and the solubility of methane [36] (thin dashed line). The calculations are made for pressure (assumed to be hydrostatic) and temperature (based on the geothermal gradient 55 jC km 1) at Site 1249 (thick line). Note that the phase distribution as presented in (a) occurs only at salinity 35 g kg 1. The GHSZ thins when the salinity of pore water increases to 70 g kg 1, and is not present at salinity 140 g kg 1.


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

831

(S > 60 g kg 1; Cl> 1000 mM) pore water at shallow 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 conventional 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 pressurized 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

Fig. 2. ODP Sites 1249 and 1250 on Hydrate Ridge, offshore of Oregon (OR), northwest United States. (a) Tectonic setting of Hydrate Ridge showing the ridge located within the Cascadia accretionary complex where the Juan de Fuca plate subducts beneath North American plate. (b) Bathymetric map [43] showing detailed location of ODP Sites 1249 and 1250, and cross-section of Fig. 9.

832


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 accretionary ridge on the Cascadia margin, f100 km west of the Oregon coast (Fig. 2). Gas vents, authigenic 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 elsewhere, 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 hydrate 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 hydrate 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


833

20 mbsf, and exceeds 90% of pore space across some cm-scale intervals [17]. Downhole temperatures at Site 1249 steadily increase 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 elevated concentrations of dissolved constituents significantly decrease the stability of gas hydrate at a given pressure and temperature [3,19 – 21], although with the measured geotherm and assumed hydrostatic pressure, 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 minimum estimates.

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 diameter), and an outer chamber, which holds f2000 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 hydratebearing 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, incremental release of gas from the PCS at constant temperature

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.

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 equilibrium 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

834


pressure until dissociation ceases, and (2) gas hydrate dissociation releases fresh water, which increases the stability of gas hydrate at given pressure and temperature 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 of f0 jC. After initial temperature equilibration, a manifold was connected to the PCS, allowing 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 increment 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 multivalve, 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 contamination. 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 degassing experiment, ice was removed, the PCS was 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 concentration ( 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 detailed 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

835

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 temperature, 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-

Fig. 6. Observed volume – pressure and volume – time relations for Core 204-1249F-4P. Pressure decreased relatively slowly during release of the first f 70 l of gas, presumably because dissociating gas hydrate released gas and fresh water within a closed container (Fig. 5). Pressure decreased more rapidly as the last f 25 ml escaped, apparently because only headspace free gas (from decomposed gas hydrate) and dissolved gas was present inside of the PCS.

836


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

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

Fig. 7. Composition of gas released from Core 204-1249F-4P showing three distinct stages of degassing. A mixture of gases from free gas and gas hydrate were likely released during Stage 1. Gases from gas hydrate and dissolved gas were probably released during Stage 2 and Stage 3. Note that concentrations of more soluble gases (e.g., CO2) increase at the end of Stage 3. Note also the linear (r2 = 0.86) decrease in C3 concentrations during Stage 1, strongly indicating mixing between C3-rich free gas, and C3-poor gas evolving from gas hydrate.


(C4) and pentane (C5) were minor components, although 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 CO 2 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 1235 ml (or f85% 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 hydrate), 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 CH 4 within 1235 ml at in situ conditions.

837

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 contained 520 ml of gas hydrate (f42% porosity) if it occurred as a ‘‘typical’’ structure I hydrate (qGH = 0.91 g/cm3; stoichiometry of CH46H2O). 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 structure II gas hydrate (stoichiometry of CH45.67H2O), 570 ml of gas hydrate existed at in situ conditions (f46% porosity). However, these gas hydrate abundances 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 pressurized cores are first recovered [25,33]. The measured 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

838


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 therefore 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 interpretation. 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 2041249F-4P began releasing significant quantities of gas at f4.2 MPa (Fig. 6), which suggests a starting pore

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 204-1249F4P 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 composition 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 understanding 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

Fig. 8. In situ temperature and pressure at 14 mbsf at Site 1249 compared to the hydrate stability boundaries for pore water salinities 0 – 150 g kg 1 (calculated using CSMHYD [21] and consistent with experimental results [37] for a salinity of 100 g kg 1). Temperature was measured during Leg 204 at 16.5 mbsf, and the range shown here includes uncertainties in this measurement and extrapolation based on the regional temperature gradient. Pressure is assumed to lie between hydrostatic and lithostatic, with the density of the overlying sediment constrained by measurements made during Leg 204 [17]. Gas hydrate and free gas are predicted 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 surrounding waters are too saline. Core 204-1249F-4P presumably lacked SO42 before 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 204-1249F4P (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 composition. 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

839

from freshening of pore water within the inner chamber (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 2041249F-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 degassing (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 linearly (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. Assuming this source was free gas, it would occupy f76 ml at in situ pressure and temperature, after accounting for methane compressibility. This is within the amount predicted from pore water mass balance considerations.

840


8. Mechanism for co-existence of shallow gas hydrate and free gas Gas hydrate, free gas and dissolved gas likely coexist 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,

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 sediment and removal of dissolved ions, or when both processes operate together.

9. Conclusions and implications

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.

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 coexistence may occur in shallow sediment elsewhere on Hydrate Ridge, and in other regions of high upward methane flux [40]. This has several implications 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 coexist, 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 overestimate 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 during gas transport [9,10]. For example, Suess et al. [10] suggest that free methane moves through shallow sediment of northern Hydrate Ridge along channels lined with gas hydrate that has incorporated 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]

841

References [1] A.V. Milkov, R. Sassen, Economic geology of offshore gas hydrate accumulations and provinces, Mar. Pet. Geol. 19 (2002) 1 – 11. [2] G.R. Dickens, Rethinking the global carbon cycle with a large, dynamic and microbially mediated gas hydrate capacitor, Earth Planet. Sci. Lett. 213 (2003) 169 – 183. [3] G.R. Dickens, M.S. Quinby-Hunt, Methane hydrate stability in pore water: a simple theoretical approach for geophysical applications, J. Geophys. Res. 102 (1997) 773 – 783. [4] B.A. Buffett, Clathrate hydrates, Annu. Rev. Earth Planet. Sci. 28 (2000) 477 – 507. [5] G.R. Dickens, C.K. Paull, P. Wallace, ODP Leg 164 Scientific Party, Direct measurement of in situ methane quantities in a large gas hydrate reservoir, Nature 385 (1997) 426 – 428. [6] M.K. Davie, O.Y. Zatsepina, B.A. Buffett, Methane solubility in marine hydrate environments, Mar. Geol. 203 (2004) 177 – 184. [7] W. Xu, C. Ruppel, Predicting the occurrence, distribution, and evolution of methane gas hydrate in porous marine sediments, J. Geophys. Res. 104 (1999) 5081 – 5095. [8] P.B. Flemings, X. Liu, W.J. Winters, Critical pressure and multiphase flow in Blake Ridge gas hydrates, Geology 31 (2003) 1057 – 1060. [9] E. Suess, G. Bohrmann, D. Rickert, W.F. Kuhs, M.E. Torres, A. Tre´hu, P. Linke, Properties and fabric of near-surface methane hydrates at Hydrate Ridge, Cascadia margin, Fourth International Conference on Gas Hydrates, Proc., Yokohama, 2002, pp. 740 – 744. [10] E. Suess, M.E. Torres, G. Bohrmann, R.W. Collier, J. Greinert, P. Linke, G. Rehder, A. Tre´hu, K. Wallmann, G. Winckler, E. Zuleger, Gas hydrate destabilization: enhanced dewatering, benthic material turnover and large methane plumes at the Cascadia convergent margin, Earth Planet. Sci. Lett. 170 (1999) 1 – 15. [11] M.D. Tryon, K.M. Brown, M.E. Torres, Fluid and chemical flux in and out of sediments hosting methane hydrate deposits on Hydrate Ridge, OR, II: hydrological processes, Earth Planet. Sci. Lett. 201 (2002) 541 – 557. [12] K.U. Heeschen, A.M. Tre´hu, R.W. Collier, E. Suess, G. Rehder, Distribution and height of methane bubble plumes on the Cascadia Margin characterized by acoustic imaging, Geophys. Res. Lett. 30 (2003) (doi:10.1029/2003GL016974). [13] G. Bohrmann, J. Greinert, E. Suess, M. Torres, Authigenic carbonates from the Cascadia subduction zone and their relation to gas hydrate stability, Geology 26 (1998) 647 – 650. [14] M. Riedel, G.D. Spence, N.R. Chapman, R.D. Hyndman, Seismic investigations of a vent field associated with gas hydrates, offshore Vancouver Island, J. Geophys. Res. 107 (2002) (doi:10.1029/2001JB000269). [15] W.T. Wood, J.F. Gettrust, N.R. Chapman, G.D. Spence, R.D. Hyndman, Decreased stability of methane hydrates in marine sediments owing to phase-boundary roughness, Nature 420 (2002) 656 – 660. [16] A.M. Tre´hu, D.S. Stakes, C.D. Bartlett, J. Chevallier, R.A. Duncan, S.K. Goffredi, S.M. Potter, K.A. Salamy, Seismic

842

[17]

[18]

[19]

[20] [21] [22]

[23]

[24]

[25]

[26]

[27]

[28]

A.V. Milkov et al. / Earth and Planetary Science Letters 222 (2004) 829–843 and seafloor evidence for free gas, gas hydrates, and fluid seeps on the transform margin offshore Cape Mendocino, J. Geophys. Res. 108 (2003) (doi:10.1029/2001JB001679). A.M. Tre´hu, G. Bohrmann, F.R. Rack, M.E. Torres, et al, Proc. Ocean Drill. Program, Initial Rep. 204 (2003) [CDROM]. Available from: Ocean Drilling Program, Texas A and M University, College Station TX 77845-9547, USA). A.M. Tre´hu, G. Bohrmann, F.R.,Rack, T.S. Collett, D.S. Goldberg, P.E. Long, A.V. Milkov, M. Riedel, P. Schultheiss, M.E. Torres, N.L. Bangs, S.R. Barr, W.S. Borowski, G.E. Claypool, M.E. Delwiche, G.R. Dickens, E. Gracia, G. Guerin, M. Holland, J.E. Johnson, Y-J. Lee, C-S. Liu, X. Su, B. Teichert, H. Tomaru, M. Vanneste, M. Watanabe, J.L. Weinberger, Three-dimensional distribution of gas hydrate beneath southern Hydrate Ridge: constraints from ODP Leg 204, Earth Planet. Sci. Lett. 222 (2004) 845 – 862 (doi:10.1016/j.epsl.2004.03.035). J.L. de Roo, C.J. Peters, R.N. Lichtenthaler, G.A.M. Diepen, Occurrence of methane hydrate in saturated and unsaturated solutions of sodium chloride and water in dependence of temperature and pressure, AIChE J. 29 (1983) 651 – 657. Y.P. Handa, Effect of hydrostatic pressure and salinity on the stability of gas hydrates, J. Phys. Chem. 94 (1990) 2651 – 2657. E.D. Sloan, Clathrate Hydrates of Natural Gases, 2nd ed., Marcel Dekker, New York, NY, 1998, 705 pp. C.K. Paull, W. Ussler III, History and significance of gas sampling during the DSDP and ODP, in: C.K. Paull, W.P. Dillon (Eds.), Natural Gas Hydrates: Occurrence, Distribution, and Detection, AGU Geophys. Monogr., vol. 124, 2001, pp. 53 – 66. R. Hesse, Pore water anomalies of submarine gas-hydrate zones as tool to assess hydrate abundance and distribution in subsurface: what have we learned in the past decade? Earth-Sci. Rev. 61 (2003) 149 – 179. P.K. Egeberg, G.R. Dickens, Thermodynamic and pore water halogen constraints on gas hydrate distribution at Site 997 (Blake Ridge), Chem. Geol. 153 (1999) 53 – 79. G.R. Dickens, P.J. Wallace, C.K. Paull, W.S. Borowski, Detection of methane gas hydrate in the pressure core sampler (PCS): volume – pressure – time relations during controlled degassing experiments, in: C.K. Paull, R. Matsumoto, P.J. Wallace, W.P. Dillon (Eds.), Proc. Ocean Drill. Program, Sci. Results, vol. 164, 2000, pp. 113 – 126. H. Sahling, D. Rickert, R.W. Lee, P. Linke, E. Suess, Macrofaunal community structure and sulfide flux at gas hydrate deposits from the Cascadia convergent margin, NE Pacific, Mar. Ecol., Progr. Ser. 231 (2002) 121 – 138. M.E. Torres, J. McManus, D.E. Hammond, M.A. de Angelis, K.U. Heeschen, S.L. Colbert, M.D. Tryon, K.M. Brown, E. Suess, Fluid and chemical fluxes in and out of sediments hosting methane hydrate deposits on Hydrate Ridge, OR, I: Hydrological provinces, Earth Planet. Sci. Lett. 201 (2002) 525 – 540. A.M. Tre´hu, M.E. Torres, G.F. Moore, E. Suess, G. Bohrmann, Temporal and spatial evolution of a gas hydrate-bearing accretionary ridge on the Oregon continental margin, Geology 27 (1999) 939 – 942.

[29] J.E. Johnson, C. Goldfinger, E. Suess, Geophysical constraints on the surface distribution of authigenic carbonates across the Hydrate Ridge region, Cascadia margin, Mar. Geol. 202 (2003) 79 – 120. [30] A. Tre´hu, N.L. Bangs, M.A. Arsenault, G. Bohrmann, C. Goldfinger, J.E. Johnson, Y. Nakamura, M.E. Torres, Complex subsurface plumbing beneath southern Hydrate Ridge, Oregon continental margin, from high-resolution 3D seismic reflection and OBS data, Fourth International Conference on Gas Hydrates, Proc., Yokohama, 2002, pp. 90 – 96. [31] J.M. Gieskes, T. Gamo, H. Brumsack, Chemical methods for interstitial water analysis aboard JOIDES Resolution, Ocean Drill. Program, Tech. Note 15 (1991) 1 – 60. [32] T.L. Pettigrew, Design and operation of a wireline pressure core sampler, Ocean Drill. Program, Tech. Note vol. 17,Ocean Drilling Program, College Station, TX, 1992. [33] G.R. Dickens, D. Schroeder, K.-U. Hinrichs, The Leg 201 Scientific Party, The pressure core sampler (PCS) on ocean drilling program leg 201: general operations and gas release, in: S.L. D’Hondt, B.B. Jorgensen, D.J. Miller, et al (Eds.), Proc. Ocean Drill. Program, Initial Rep., vol. 201 2003, pp. 1 – 22 ([Online]. Available from the World Wide Web: . [Cited 2004-03-07]). [34] A.V. Milkov, G.E. Claypool, Y.-J. Lee, W. Xu, G.R. Dickens, W.S. Borowski, The ODP Leg 204 Scientific Party, In situ methane concentrations at hydrate Ridge offshore Oregon: new constraints on the global gas hydrate inventory from an active margin, Geology 31 (2003) 833 – 836. [35] A. Pimmel, G. Claypool, Introduction to shipboard organic geochemistry on the JOIDES Resolution, Ocean Drill. Program, Tech. Note 30 (available from World Wide Web: ), 2001, pp. 1029. [36] W. Xu, Phase balance and dynamic equilibrium during formation and dissociation of methane gas hydrate, Fourth International Conference on Gas Hydrates, Proc., Yokohama, 2002, pp. 195 – 200. [37] T. Maekawa, S. Itoh, S. Sakata, S. Igari, N. Imai, Pressure and temperature conditions for methane hydrate dissociation in sodium chloride solutions, Geochem. J. 29 (1995) 325 – 329. [38] D.R. Lide, CRC Handbook of chemistry and physics, 83rd edition, CRC Press, Cleveland, OH, 2002, 2664 pp. [39] M.J. Whiticar, E. Suess, Characterization of sorbed volatile hydrocarbons from the Peru margin, Leg 112, Sites 679, 680/681, 682, 648, and 686/687, in: E. Suess, R. von Huene, et al (Eds.), Proc. Ocean Drill. Program, Sci. Results, vol. 112, 1990, pp. 527 – 538. [40] R. Sassen, S.T. Sweet, A.V. Milkov, D.A. DeFreitas, M.C. Kennicutt II, Thermogenic vent gas and gas hydrate in the Gulf of Mexico slope: is gas hydrate decomposition significant? Geology 29 (2001) 107 – 110. [41] T.S. Collett, A review of well-log analysis techniques used to assess gas-hydrate-bearing reservoirs, in: C.K. Paull, W.P. Dillon (Eds.), Natural Gas Hydrates: Occurrence, Distribution, and Detection, AGU Geophys. Monogr., vol. 124, 2001, pp. 189 – 210.

A.V. Milkov et al. / Earth and Planetary Science Letters 222 (2004) 829–843 [42] C. Ruppel, M. Kinoshita, Fluid, methane, and energy flux in an active margin gas hydrate province, offshore Costa Rica, Earth Planet. Sci. Lett. 179 (2000) 153 – 165. [43] D.A. Clague, N. Maher, C.K. Paull, High-resolution multibeam survey of Hydrate Ridge, offshore Oregon, in: C.K. Paull, W.P. Dillon (Eds.), Natural Gas Hydrates: Occurrence, Dis-

843

tribution, and Detection, AGU Geophys. Monogr., vol. 124, 2001, pp. 297 – 303. [44] Y.F. Makogon, Special Characteristics of the Natural Gas Hydrate Fields Exploitation in the Zone of Hydrate formation, TsNTI MINGASPRPOMa, Moscow, (1966) 17 pp. (in Russian).