detection and evaluation of gas hydrates in the eastern nankai trough by geochemical and geophysical...

RESOURCE GEOLOGY, vol. 54, no. 1, 53–67, 2004

53

Detection and Evaluation of Gas Hydrates

in the Eastern Nankai Trough

by Geochemical and Geophysical Methods

Ryo MATSUMOTO, Hitoshi TOMARU and Hailong LU*

Department of Earth and Planetary Science, University of Tokyo, Hongo, Tokyo 113-0033, Japan [e-mail: [email protected]]

* Geological Survey of Canada, Ottawa, CanadaReceived on January 29, 2004; accepted on March 11, 2004

Abstract: Interstitial waters extracted from the sediment cores from the exploration wells, “BH-1” and “MITI Nankai Trough”,drilled ~60 km off Omaezaki Peninsula in the eastern Nankai Trough, were analyzed for the chloride and sulfate concentrations toexamine the depth profiles and occurrence of subsurface gas hydrates. Cored intervals from the seafloor to 310 mbsf were dividedinto Unit 1 (~70 mbsf, predominated by mud), Unit 2 (70-150 mbsf, mud with thin ash beds), Unit 3 (150-250+ mbsf, mud withthin ash and sand), and Unit 4 (275-310 mbsf, predominated by mud). The baseline level for Cl- concentrations was 540 mM,whereas low chloride anomalies (103 to 223 mM) were identified at around 207 mbsf (zone A), 234-240 mbsf (zone B), and 258-265 mbsf (zone C) in Unit 3. Gas hydrate saturation (Sh %) of sediment pores was calculated to be 60 % (zone A) to 80 % (zonesB and C) in sands whereas only a few percent in clay and silt. The total amount of gas hydrates in hydrate-bearing sands was esti-mated to be 8 to 10 m3 of solid gas hydrate per m2, or 1.48 km3 CH4 per 1 km2. High saturation zones (A, B and C) were consis-tent with anomaly zones recognized in sonic and resistivity logs. 2D and high-resolution seismic studies revealed two BSRs in thestudy area. Strong BSRs (BSR-1) at ~263 mbsf were correlated to the boundary between gas hydrate-bearing sands (zone C) andthe shallower low velocity zone, while the lower BSRs (BSR-2) at ~289 mbsf corresponded to the top of the deeper low velocityzone of the sonic log. Tectonic uplift of the study area is thought to have caused the upward migration of BGHS. That is, BSR-1corresponds to the new BGHS and BSR-2 to the old BGHS. Relic gas hydrates and free gas may survive in the interval betweenBSR-1 and BSR-2, and below BSR-2, respectively.

Direct measurements of the formation temperature for the top 170 m interval yield a geothermal gradient of ~4.3°C/100 m. Extrapolation of this gradient down to the base of gas hydrate stability yields a theoretical BGHS at ~230 mbsf, sur-prisingly ~35 m shallower than the base of gas hydrate-bearing sands (zone C) and BSR-1. As with the double BSRs, anoth-er tectonic uplift may explain the BGHS at unreasonably shallow depths. Alternatively, linear extrapolation of the geother-mal gradient down to the hydrate-bearing zones may not be appropriate if the gradient changes below the depths that weremeasured. Recognition of double BSRs (263 and 289 mbsf) and probable new BGHS (~230 mbsf) in the exploration wellsimplies that the BGHS has gradually migrated upward. Tectonically induced processes are thought to have enhanced denseand massive accumulation of gas hydrate deposits through effective methane recycling and condensation. To test the hypo-thetical models for the accumulation of gas hydrates in Nankai accretionary prism, we strongly propose to measure the equi-librium temperatures for the entire depth range down to the free gas zone below predicted BGHS and to reconstruct the waterdepths and uplift history of hydrate-bearing area.

Keywords: chloride anomaly, Nankai Trough, double BSRs, BGHS, uplift, thermal gradient.

1. Introduction

1.1. Background and scope of the study

The Nankai Trough extends ~900 km SW-NE off-shore southwest Japan islands at the convergent marginof the Philippine Sea Plate subducting northwestward tothe Eurasian Plate. Besides being an important area forinvestigation of the accretionary tectonics and earth-quake prediction (e.g., Gieskes et al., 1982; Aoki et al.,1983; Ashi et al., 1995, 2002; Satoh, 2000; Seno, 1977),this active margin has been the focus of gas hydrate

studies for the last two decades. Recent explorationefforts to investigate gas hydrates in the Nankai Troughhave revealed an extensive distribution (~32,000 km2)of high amplitude bottom simulating reflectors (BSRs)(Figs. 1 and 2).

Since the existence of gas hydrates was first suspectedin the continental margin sediments (Lancelot and Ewing,1972), the potential importance of marine gas hydrates asa future natural gas resource has been repeatedly men-tioned in many reports and articles (e.g., Kvenvolden,1998; Kvenvolden and Barnard, 1983; Matsumoto, 1987,1995, 1996). Krason (1992) estimated that 0.42 ~ 4.2 ×

1012 m3 of methane reside ingas hydrates of the NankaiTrough. The estimates arebeing refined by updated datasets (Satoh et al., 1996; Satoh,2000; Matsumoto, 1992,1995). The recoverablereserves of marine gashydrates should be muchlower than the total amount ofsubsurface gas hydrates byone order of magnitude.However they are still signifi-cantly enough to be compara-ble to the current domesticconsumption of 1.9 TCF/year.

The energy security issueand 1997 Kyoto Protcol of 6% reduction of CO2 emissionprompted the Japanese Min-istry of International Trade

R. MATSUMOTO, H. TOMARU and H. L. LU54 RESOURCE GEOLOGY :

TOKAI

SHIKOKU

KYUSHU

KII

EXPLORATORY WELLSMETI NANKAI TROUGH

DISTRIBUTION OF BSRs

OSAKA

NAGOYA

Fig.2

Philippine Sea Plate

Eurasia Plate

km 50 100 150 200 250

Fig. 1 Distribution of BSRs, bathymetry, and the location of the exploratory wells, MITI-Nankai Trough, in the Nankai Trough.

OmaezakiPeninsula

Nankai T

rough

Tenryu River

20km0 10

34°00'

34°20'

34°40'N

33°40'

138°20'E137°40' 138°00'

Tenr

yu C

anyo

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Ryu

yo C

anyo

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200025

00

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600400

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Daini-Tenryu　Knoll

Daiichi-Tenryu　Knoll

Site Survery WellBH-1

BH-2 ~250m < BSR

Spot Coring by HPC

Spot Coring by HPC

Pilot HolePH-1

PH-2Mail Hole MITI Nankai Trough

PSW-2

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~200m

~300m

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LWD

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Wireline LoggingZero offset VSP

PTCS Coring

PTCS and conventional CoringWireline Logging

~250m< BSR

TD=650m

TD=540m

TD=270m

TD=350m

TD=3300m

TD=350m

196 m

232 m

96 m

NANKAI TROUGHMITI

Fig. 2. Location of the two site survey wells and sixexploratory wells in the eastern Nankai Trough offOmaezaki.

and Industry (MITI) (later named the Ministry ofEconomy, Trade and Industry or METI) to launch a fiveyear research and development (R&D) program toexplore marine gas hydrates offshore Japan islands in1995. Under the supervision of METI, Japan National OilCorporation (JNOC) has been conducting the R&D pro-gram including seismic surveys and exploration drillingsoffshore Japan islands, in the MacKenzie Delta, Canada(e.g., Dallimore et al., 1999), and laboratory experiments,and technology development since 1995.

In 1997, site survey wells, BH-1 and BH-2, weredrilled ~60 km off Omaezaki in the eastern NankaiTrough for a geotechnical evaluation and safety assess-ment (Figs. 1 and 2). In 1999 to 2000, JNOC and JapanPetroleum Exploration (JAPEX) drilled exploratorywells in the site survey area as the first integratedefforts of drilling/coring dedicated to explore marinegas hydrate. The program successfully penetratedthrough the BSRs at around 270 mbsf (meter belowseafloor), identified gas hydrate zones, and recoveredgas hydrate-bearing sands with extremely high hydratesaturation (~80 % Sh). Following the initial, successfulfive-year program (1995-2000), METI launched a 16-year program (2001-2016) with the intention of pavingthe road towards commercial production of naturalgases from gas hydrate deposits.

Submarine dives and side scan sonar surveys havefound a large number of methane seeps and related phe-nomena such as carbonate crusts, chemosynthetic com-munities, and mud volcanoes in the Nankai Trough area(Kuramoto, 2001; Tsunogai et al., 2002). Meanwhile,deep-tow seismic surveys by French-Japan joint programfirst identified high amplitude reflectors with reversepolarity at 20 to 40 m below BSRs on the slope of a knollin the eastern Nankai Trough (Foucher et al., 2002). Theycalled this unusual phenomenon a double BSR (orDBSR) and explained that the lower BSRs (BSR-2) wasa relic, phantom reflector representing fossil base of gashydrate stability (BGHS). If this is the case, double BSRsimply a sudden rise of BGHS and consequent dissocia-tion of gas hydrates. Drilling through the double BSRswas expected to provide information to understand theorigin of double BSRs and its implications for thedynamic behavior of subsurface gas hydrates. Gashydrate drilling in 1999-2000 by JNOC/JAPEX providedan invaluable data set to delineate the occurrence andnature of subsurface gas hydrates.

The purpose of this paper is to report scientific resultsof the exploration drillings and seismic surveys for thefirst stage (1995-2000) of the METI research program.Our intent is to present high resolution geochemical datato estimate the amount and occurrence of gas hydrates, tomake a core-log-seismic correlation, and to discuss thestability and dynamic behavior of subsurface gas

hydrates of this active margin in relation to the measuredthermal gradients and ‘double BSRs’ in the study area.

1.2. Geologic setting and seismic data

Geologically and geographically, the eastern NankaiTrough is separated into three zones (Research Group forActive Submarine Faults off Tokai, 1999): fore-arcbasins, outer ridges, and the trough axis. BSRs are wide-spread but discontinuous throughout these zones, show-ing variable amplitude probably depending on tectonicsand evolution of the basins (Baba et al., 2004).

A seismic profile of the 2D seismic line N96-FHshows that the sedimentary sequence is gently dippingnorthwest except for the top ~100 millisecond (msec)section, which is generally horizontal (Fig. 3). BSRs inthe seismograph, which occur at the time-converteddepth of 300 msec (Hato et al., 2004), are linear but dis-continuous, reflecting the lithologic control of the ampli-tude (Tsuji et al., 1998). That is, the amplitude of theBSRs is enhanced in gas hydrate-bearing sandy units,whereas the BSRs are not so conspicuous in hydrate-freesiltstones. Another moderately high amplitude reflectorwith reverse polarity is identified at 50 msec below theBSRs (Fig. 3). These reflectors are referred as doubleBSRs comparable to double BSRs on a slope of the Dai-ichi-Tenryu Knoll (Foucher et al., 2002). Assuming thatthe mean velocity (Vp) of the sediments above the reflec-tors is 1750 m/s (Inamori et al., 2004), the depth of dou-ble BSRs are estimated to be ~263 mbsf (upper BSRs)and ~289 mbsf (lower BSRs), respectively.

1.3. Gas hydrate drilling

Site survey wells, BH-1 and BH-2, were drilled on aflat-topped plateau in a fore-arc basin, 60 km southwest ofthe Omaezaki Peninsula (Figs. 1 and 2), by Russian D/VBavenit (5300 t) in October to November, 1997. Theplateau is approximately 900 to 1000 m deep, bounded bythe Tenryu Submarine Canyon to the northwest, and Dai-ichi- and Dai-ni-Tenryu Knolls with 500 to 600 m eleva-tion to the east and north. Kodaiba Fault at the foot of theknolls runs parallel to the axis of the Nankai Tough,extending toward the Omaezaki Peninsula, Omaezaki sparand Senoumi banks. Uplifting of these topographic highs,which are called an ‘Outer Ridge’, is thought to be relatedto the subduction of the Philippine Sea Plate. MinorRyuyo Canyon between the plateau and the knolls hasbeen abandoned probably because a rapid uplift of theplateau and knolls. Seeps of methane-bearing fluids wereobserved in the Ryuyo Canyon. The slightly low chlorini-ty (526-552 mM) and light carbon methane (-47 to -79 ‰PDB) (Tsunogai et al., 2002) suggest that the fluids arerelated to dissociation of gas hydrates.

Figure 2 shows the location of drill sites and seismiclines A-6 and N96-HF, which were set parallel to the

vol. 54, no. 1, 2004 Detection and Evaluation of Gas Hydrates 55

dip-direction. Drill sites, BH-1 and BH-2, were locatedon a line A-6 with a distance of 196m and were drilleddown to 248.5 and 242.0 mbsf, respectively. Spot-cor-ing by means of 1 and 2 m long hydraulic piston corerswas conducted every 1 to 6 m.

The gas hydrate exploration project included twopilot holes (PH-1, wash-down to 655 m; PH-2, LWD to541 m), main hole (“MITI Nankai Trough”, coring andwire-line logging to 678 m), and three post survey wells(PSW-1, wire-line logging to 355 m; PSW-2, spot cor-ing to 278 m; PSW-3, wire-line logging to 357 m).These holes were drilled in the up-dip side of the sitesurvey wells on the line A-6, about 230 m away fromthe hole BH-2 (Figs. 2 and 3), by a semi-sub type rig,M. G. Hulme, Jr (R&B Falcon Co.) in November 1999to February 2000. The main hole “MITI Nankai Trough”was drilled down to 2355 mbsf, in which the top 678 minterval was dedicated for gas hydrate study and thelower part was for the assessment of conventionalhydrocarbon resources.

1.4. Samples and methods of study

Spot cores from BH-1 and BH-2 were taken at every 1to 6 m from the entire drilling interval down to 248.5 mbsfeither by a 1 m long piston sampler or by a 2 m long smallhydraulic percussion sampler. Two types of temperatureprobe, TEMP and TCPT, of Fugro Co. were deployed inBH-1 and BH-2 to measure the formation temperaturesand to estimate the geothermal gradient in the study area.For the main hole coring, conventional rotary coring by a9 m long core-barrel was employed for the interval of 165to 233 mbsf, and 309 to 327 mbsf. A newly developedpressure-temperature core sampler (PTCS) was deployedfor the interval of high resistivity, believed to be gashydrate zones, in the main hole in order to recover highquality, undissolved samples. The temperatures of coreswere measured by a needle-sensor thermometer soon aftercore retrieval in the core laboratory.

Upon core recovery, sediment cores were immediatelytaken from the barrel and cut into 1 m long sub-cores forinspection. Sediment cores were often intercalated byseveral cm thick soupy horizons (Fig. 4), which are con-sidered to have originated from the dissociation of gashydrates. After recovery, PTCS cores were held to permitcomplete dissociation of gas hydrate and to measure gaspressure. Whole-round core samples, 10 cm long, werecollected every 1 to 2 meters to study water chemistry. Aca. 2 cm thick rind of core samples was skinned off toremove potential contamination of drilling fluids (i.e.,seawater) during core retrieval, then the interstitial waterswere extracted from the “uncontaminated” part of thesamples by means of a Manheim type hydraulic squeez-ing system. Extracted waters were filtered through a 0.45mm disk filter, stored in glass vials, refrigerated, and

shipped to the University of Tokyo. The concentrationsof Cl- and SO4

2- were measured on 40 samples fromBH-1 and 75 samples from the main hole by using aPIA-1000 ion chromatograph (Shimadzu).

1.5. Lithology and geologic age

According to the log-resistivity profiles, a markerhorizon in PSW-1 was correlated to a horizon 9 m high-er in PSW-3, implying that an up-dip shift between BH-1 and the main hole is 31.5 m.

Sediment cores recovered from BH-1 and BH-2 weredominated by dark gray to dark green sandy silt withoccasional thin to moderate sand and a few ash beds. Thesilt was poorly to moderately bioturbated without lamina-tions, and generally enriched in foraminiferal tests. Aremarkable 0.9 m thick, massive medium sandstoneoccurs at 239 mbsf in BH-1. Moderate to high amplitudereflectors at the basal horizons of BH-1 and BH-2 (Fig. 3)may correspond to the sand-mud alternation zone. Mainhole sediments for the interval of 165 to 309 mbsf arealso characterized by dark greenish gray silt and clay withabundant to common foraminifera, while the intercalationof sand and even conglomerate beds increase in frequen-cy down to ~280 mbsf, then suddenly change to massiveclean mud without sand beds for the basal unit. Takinginto consideration of 31.5 m up-dip shift in main hole, thecomposite section of BH-1 and BH-2 and main hole isdivided into four lithologic units as:

Unit 1 (0 to 70 mbsf in BH-1 and BH-2): Dominantdark greenish gray to gray mud with occasional 1 to 2cm thick ash beds. The mud was tightly compactedthroughout, suggesting sub-aqueous erosion of the top-most interval.

Unit 2 (70 to 150 mbsf in BH-1 and BH-2): In addi-tion to thin ash beds, 1 to 5 cm thick sand beds occa-sionally occur. The thickness and frequency of sandbeds tend to increase down-hole. Anomalously cold,soupy intervals in massive mud may indicate the pres-ence of gas hydrates (Fig. 4).

Unit 3 (150 to 250+ mbsf in BH-1 and BH-2; 165 to275 mbsf in the main hole): Ash beds were less com-mon than higher in the section while the frequency andthickness of turbidite sands increased. In particular, ~1m thick massive sand occurred at ~236 mbsf, and ~ 2 mthick sand at 260 mbsf. The depth interval of Unit 3 inBH-1 and BH-2 corresponded to 110 to 210 mbsf.Therefore, the total thickness of Unite 3 was 165 m.

Unit 4 (275 to 310 mbsf in the main hole): This unitlacked sand or ash, and was composed of massive darkgreenish gray mud.

Based on the nanno plankton biostratigraphy andpaleomagnetism (Hiroki et al., 2004), Units 1 to 3 areassigned to the Early Pleistocene (nanno plankton zonesCN15, CN14a, b, and CN13b), and Unit 4 to the Latest


Pliocene (CN13a). Thus, the studied section coveredapproximately 1.8 million years sedimentation.

2. Results

2.1. Sulfate concentration and contamination correction

Sulfate concentrations of near surface mud (27.5mM) was similar to that of seawater (~28 mM), but itrapidly decreased down to 7.7 mM at 5.2 mbsf, 1.8 mMat 11.6 mbsf, and below the level of detection at 15.7mbsf (Fig. 5). The decline of sulfate concentration inocean floor sediments is principally controlled by anaer-


BSR1

BSR21.75 sec

1.50 sec

1.25 sec

(TWT, sec.)

2 km

seafloor

MITI NANKAI TROUGH

PSW-3

WD=945 m

A-6

BSR~270m

BH-1 BH-2

PH-1, 2PSW-1,2Main Hole

PSW-3

1.4

1.3

1.6

1.5

TWT(sec.)

Fig. 3 Seismic profiles N96-FH (A) and A-6 (B) show-ing the stratigraphic relationship of the site surveyholes and the exploratory wells. Note double BSRs onN96-FH and linear but discontinuous BSR on A-6.

8.4

-1.7 -0.4

Fig. 4 Photograph of gas hydrate bearing sediment core.Soupy horizons marked by unusually low temperaturesare thought to have contained gas hydrates. Endothermicdissociation of gas hydrates decreased the temperature ofthe host sediments.

(A)

(B)

obic methane oxidation at the expense of upwardmigrating methane (e.g., Borowski et al., 1996), and thebase of sulfate reduction zone is called as sulfate-methane interface (SMI). Sulfate concentration belowSMI should be zero. However, the interstitial watersfrom the depth interval of 75 to 150 mbsf show variableconcentration of SO4

2-, ranging between 0 and 16 mM.Such a high concentration and high amplitude fluctua-tion within a short interval was probably caused by arti-fact. Because the hydraulic piston coring system injectshigh pressure seawater into the hole, high sulfate anom-alies in deep intervals are likely to have been caused byseawater invasion into the permeable horizons.Assuming that the deeper sediments are free of SO4

2-,and the high sulfate anomalies were caused only by sea-water invasion, the analytical results of Cl- concentra-tion were corrected for the seawater contamination.

2.2. Chloride concentration

Chloride concentrations of interstitial waters of BH-1and the main hole are collectively shown in the his-togram (Fig. 6) and are plotted against the depth belowseafloor (Fig. 7), in which the depth at BH-1 was cor-rected for an up-dip shift in the main hole by subtract-

ing 31.5 m. The top sample in BH-1 is shown at –31.3mbsf in the diagram.

The concentration of the interstitial waters rangedwidely from 103 to 568 mM, while most of the valueswere concentrated within a narrow range at around 540mM (Fig. 6). The chloride concentration decreased by10 mM in the top 30 m thick sediments of Unit 1, thenfell to between 524 and 544 mM (Fig. 7). Low chloridewater, ~512 mM, was recognized in the middle of Unit2, but other samples including silt and sand fell between521and 551 mM. Unit 3 exhibited a wide fluctuation inchloride concentration in the sandstone-bearing zone(170 to 265 mbsf) among which some sandy beds at207 mbsf (anomaly A), 236 to 240 mbsf (anomaly B),245 mbsf, 258 to 265 mbsf (anomaly C) were unusuallylow (103~223 mM) in chloride. It is also interesting torecognize a zone of high chloride concentration (~568mM) closely related with extremely low anomaly zonesat around 240 to 260 mbsf. Chloride concentration inmud-dominant Unit 4 ranges between 490 and 545 mM,20 to 50 mM lower than the seawater value.

2.3. Pore saturation and amount of gas hydrates

2.3.1. Pore saturation: The chloride concentration of


0 5 10 15 20 25 300

50

100

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SULFATE, mM/L (BH-1)

Dep

th, m

bsf

Sea-water contamination

SMI

Fig. 5 The depth profile of sulfate concentrations atthe site survey hole, BH-1. The sharp decrease inSO4

2- concentration within the top 16 meters indi-cates high methane flux from deeper sediments.The unusually high concentration of sulfate indeeper levels may have been caused by sea-watercontamination during coring.

0

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Fig. 6 Histogram showing the range in chloride concen-tration. The chloride concentrations in most of thesamples were around 540 mM/L, whereas some sam-ples were anomalously low in chloride.

interstitial waters is usually kept ataround the seawater level (~546 mM/L)because chlorine is a conservative com-ponent, not usually incorporated in pre-cipitation/ dissolution reactions duringdiagenesis. This has been well docu-mented in a number of DSDP/ODPreports (e.g., Hesse et al., 1985, 2003;Gieskes et al., 1982; Matsumoto, 2000).However, the interstitial water samplesextracted from gas hydrate-bearing sedi-ments are often variably fresher thanseawater. This is explained as a result ofdilution effect by gas hydrates. Gashydrates are composed of water mole-cules and methane. When gas hydratecrystals are formed in salty waters, saltsshould be excluded from the crystalstructures just like water ice. “Interstitialwaters” extracted from sediment coresare not pristine pore waters but a mix-ture of pristine pore waters and purewaters which were originally containedin gas hydrates. Therefore, the chlorideconcentration of the “interstitial waters”of gas hydrate-bearing sediments shouldbe fresher than the ambient pristine porewaters, depending on the amount of gashydrates in sediments (Fig. 8).

Gas hydrate saturation (Sh %) isestimated from the low chloride anom-alies, on the assumption that the gas


Chloride, mM

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bsf

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Chloride, mM

Clay (BH-1)

Silt (BH-1)Sand (BH-1)

Clay (Main hole)

Silt (Main hole)

Sand (Main hole)

Fig. 7 Depth profile of the chloride concentration. The depths of samples fromBH-1 were corrected for up-dip relationship between the main hole. See text fordetails. Clay and silt have concentrations similar to seawater. However,extremely low chloride concentrations occur in sand and silt samples. Yellowzones A, B, and C indicate sand-rich horizons characterized by low-chlorideanomaly.

chlorinity = A chlorinity = A chlorinity << A

gas

CH4

Pore water

Case 1

salltsallt

Sediment particles Gas hydrateDissociationduring coringand sampling

Gas hydrateFormation

OPEN SYSTEM

CH4

Pore water

Low chloride anomaly

subsurface coring 　　laboratory

hydrate

Fig. 8 Conceptual model that explains the use of chloride concentration in extracted pore waters as a proxy for estimating gashydrate saturation in the pore space. The variation of chloride concentration occurs during the formation, recovery, and samplingof subsurface gas hydrates. Waters extracted from the sediment cores are variably depleted in chloride concentration, dependingon the amount of gas hydrates.

hydrate system is open to the ambient pore waters. Thevolume fraction (M) of hydrate-derived-water insqueezed water samples (“interstitial water” samples) iscalculated from the chloride concentration of samples(Ch) and the chloride concentration of gas hydrate-free,ambient sediments (Cb) as,

M = 1- Ch/Cb (1)

Cb, the base line level, is the concentration of unconta-minated, pristine pore waters, but not easily obtained byusual drilling and coring. Therefore, the base line levelis often obtained as a regression curve for gas hydrate-free sediments (Egeberg and Dickens, 1999; Matsumotoand Borowski, 2000; Matsumoto et al., 2000, 2004).For simplicity and convenience, the base line level (Cb)for the Nankai Trough sediments was set at 540 mMbased on the histogram (Fig. 6) and depth profile (Fig.7). Minor anomalies (D = -20 mM) from 0 to 40 mbsfmay not imply the existence of small amounts of gashydrates. Minor anomalies below 140 mbsf in Unit 3probably represent few gas hydrates, as well as themajor and sharp anomalies from 210 to 265 mbsf.Slightly positive anomalies (D = +30 mM) at 240-260mbsf may indicate the local concentration of chlorideexcluded from gas hydrates. Mud and silt in Unit 4 are

generally free of gas hydrates, but apparent negativeanomalies (D = -45 mM) below 305 mbsf may suggestthe formation of gas hydrates, or alternatively that pris-tine pore waters became fresher by progressive dissoci-ation of gas hydrates at the BGHS.

Assuming that the density of gas hydrate is 0.91 g/cm3

(Makogon, 1997), one mole of stoichiometric gas hydrate(CH4•5.75H2O = 119.5 g; 131.3 cm3) is calculated to con-tain 103.5 g (103.5 cm3) of water. This means that 1 cm3

of pure water forms 1.27 cm3 of gas hydrates. Hence, thegas hydrate saturation (Sh %), volume % of gas hydrate insediment pore, calculated as,

Sh = (1.27M × 100) / (1 + 0.27M) (%) (2)

Figure 9 shows the amount of gas hydrate, representedby pore saturations (Sh %) and volume in bulk sediments(vol %). Massive sands in the interval of 235 to 265 mbsfin Unit 3 are highly saturated with gas hydrates with Shof 72 to 82 %, whereas the gas hydrate saturation in adja-cent clays and silts are generally lower, with only 2 to 5% Sh. DSDP/ODP (Kvenvolden, 1983; Jenden andKaplan, 1986) and exploration drillings in the arctic per-mafrost regions (Collett, 1994; Dallimore et al., 1999)have also documented that gas hydrates are preferentiallyformed in coarse-grained sediments. Methane-bearing


0510152025303540-50

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Gas Hydrate, vol.% in sediments

Dep

th,

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f

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0 2 0 40 60 8 0 100Pore Saturation of Gas Hydrate, Sh%

AAAA BBBB

Fig. 9 Amount of gas hydrate in sediments represented by volume % in pore space (pore saturation, Sh%,) (A) and volume %in sediments (vol. %) (B) estimated from the chloride anomaly.

fluids are thought to migrate through permeable sandbeds to form gas hydrates within relatively larger porespace of sand and sandstone.

2.3.2. Amount of gas hydrates: The porosity of near sur-face sediments (1~4 mbsf) in the study area was calculat-ed to be ~55 % from the water content (%) measurements(Matsumoto et al., in prep.). According to the neutron log-ging and core analysis, porosity of the main hole sedi-ments decrease from ~45 % at 165 mbsf to ~40 % at 310mbsf (Akihisa et al., 2002). The volume of gas hydrates iscalculated from hydrate saturation (Sh %) and sedimentporosity (%). As shown in Figure 9, gas hydrates in somesands are observed to occupy 26 to 34 vol % of the sedi-ments, but only 2 to 4 % in clays and silts. Given the totalthickness of 30 m for the ‘hydrated sands’, the totalamount of gas hydrates is estimated to be, (0.26~0.34) ×30 m = 8 ~ 10 m. This gives an approximate estimation ofgas hydrate deposits to be 1.48 km3 CH4 (0.056 TCF) perkm2. Hence, the total amount of gas hydrate methane ofthe eastern Nankai Trough is calculated to be ~1800 TCF(0.056 TCF/km2 × 32,000 km2). The estimate is 10 ~ 100times as much as the early estimates by Krason (1992).

2.4. Core-log-seismic correlation

Wire-line logs have provided high quality resistivityprofiles at the main hole (Fig. 10A and B). Baselineresistivities of water-saturated-mud and sands (R0)showed a slight increase with depth from ~1.3 OHM.M(Unit 1 and Unit 2) to about 1.5-1.7 OHM.M (Unit 3and Unit 4) with sharp spiky high resistivity zones ataround 197 mbsf, 210 mbsf, 238-242 mbsf, and 259-267 mbsf (Fig. 10A). The deeper three zones corre-

spond to the low chloride anomaly zones A, B, and C.The amplitude of the log resistivity anomaly was con-sistent with the intervals of highly gas hydrate saturatedzones B and C, but the hydrate saturation in zone A (11to 18 % Sh) was not well correlated with log-resistivity.The minor hydrate accumulation at ~170 mbsf was notdepicted by the resistivity log (Fig. 10A). These dis-crepancies are probably because of higher clay contentsin these horizons.

The sonic log of PSW-1, 10 m northeast of the mainhole, exhibited two conspicuous high velocity anomalyzones (~2.50 km/s) at ~238 mbsf and 260-265 mbsf(Inamori and Hato, 2004), which correspond to lowchloride anomaly zones (Zones B and C) (Fig. 13).PSW-1 also revealed weak but significant low velocityanomalies at 265-273 and 287-295 mbsf, suggesting apossible existence of free gases in sediments (Inamoriand Hato, 2004). The impedance gap between the highvelocity zone (260-265 mbsf) and low velocity zone(265-273 mbsf) likely to corresponded to the upperBSRs (BSR-1) at ~263 mbsf, whereas the low velocityzone at 287-295 mbsf may underlie the lower BSRs(BSR-2) (Fig. 13).

2.5. Thermal gradients

The TCPT probe yielded a thermal gradient of4.3°C/100 m, whereas the TEMP probe yielded a ther-mal gradient of 4.0°C/100 m (Fig. 11). The regressioncoefficient is much better for TCPT rather than TEMP(Fig. 11). The diagram also shows the phase boundaryof methane hydrate and free methane+water in seawa-ter. Extrapolated regression line of TCPT intersects the


0 100 200 300 400 500 600

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Fig. 10 Resistivity profile and core analysis data of the main hole (A). About 3 m offset is recognized between core data andlog data (B).

phase boundary curve at around 230mbsf, implying that the base of gashydrate (methane hydrate) occurred at230 mbsf. This is shallower than thebase of the gas hydrate-bearing sand-stones (265 mbsf), the base of highresistivity zones (267 mbsf), and BSR-1 (~263 mbsf).

2.6. Core temperature anomalies

Cores were grouped into the shal-low warm core group (14 to 20°C) anddeeper cool/cold core group (-4 to15°C), and the two groups were sepa-rated by a formation temperature curveas shown in Figure 12. Warm coresrepresent warming-up during and aftercore retrieval, whereas the cool/coldtemperatures of deeper cores areascribed to the endothermic dissocia-tion of gas hydrates and/or degassingand expansion of dissolved methane.Anomalously low temperatures (-2 to -4°C) at around 210 mbsf, 240 mbsf,and 260 mbsf well corresponded to gashydrate-bearing sands. The sharp tem-perature drop at ~230 mbsf coincidedwith the top of the gas hydrate-bearingsands. The moderately low tempera-

ture anomaly at 285 to 300 mbsf was correlat-ed to the low velocity, and perhaps the free-gas-containing zone.

In summary, anomalies of the chloride con-centrations of the interstitial waters were con-sistent with the formation micro images (FMI)and log data, except for a few meters differ-ence with sonic log (Fig. 13). This is becausethe sonic log was obtained in the hole ~10 maway from the main hole. Two BSRs could becorrelated to base of gas hydrate zone C andtop of the low velocity zone, respectively.The current BGHS determined by the presenttemperatures appeared at depths much shal-lower than the gas hydrate zones and BSRs.

3. Discussion

Our integrated geochemical and geophysicalexploration of subsurface gas hydrates of theNankai Trough has delineated the occurrenceand amount of gas hydrates, but also raisedpuzzling issues that bear on the stability of gashydrates in active margins. One issue relates tothe discrepancy between the BGHS and


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Fig. 11 Formation temperatures and inferred geothermal gradients. TCPTprobe seems to show reliable conditions. Temperature measurementsindicate that the base of gas hydrate stability should be around 230 mbsf.

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Thermal gradient change in gas hydrate zones?

5

Fig. 12 Core temperatures as measured after recovery of the cores tothe deck of the drill rig. Note that the temperatures of almost all coresbelow 200 mbsf are colder than the in situ formation temperatures.

observed base of gas hydrate zone, and the other relatesto the regional distribution of a double BSR. These issuesare closely related to each other and provoke the follow-ing questions: Are the subsurface gas hydrates in theNankai Trough stable or unstable under the present P-T-C conditions? Are the gas hydrates in the Nankai Troughbeing dissociated or being precipitated in pore spaces?

3.1. Discrepancy between BGHS and observed base ofgas hydrate zones

The discrepancy between the theoretical BGHS and theobserved base of gas hydrate-bearing sediments was firstdocumented in the Blake Ridge sediments in the westernAtlantic during ODP Leg 164 (Paull and Matsumoto,2000), however the theoretical BGHS on the Blake Ridgeis deeper than the observed base of gas hydrate stability.The discrepancy has not been fully explained as yet. Thediscrepancy in the eastern Nankai Trough is differentfrom that of the Blake Ridge, posing another problemrelated to the stability of gas hydrates. The possible mech-anisms of the discrepancy are discussed below.

3.1.1. Effect of the interstitial water chemistry: Thephase boundary curve for seawater may not be applica-ble to subsurface gas hydrates in fine-grained marinesediments where the interstitial water composition is

different from seawater. Assuming that the BGHS cor-responds to the base of gas hydrate-bearing sands at 265mbsf, the phase boundary curve should shift in thedirection of higher temperatures by ~1.5°C (Fig. 11). Lu(2003) documented an increase of the equilibrium tem-perature in sulfate-free waters, but the effect is notenough to fully explain the observed discrepancy.

3.1.2. Capillary effect: Handa and Stupin (1992)defined the effect of small pore size (i.e., capillaryeffect) in the formation of gas hydrates in porous mediafor pore sizes as small as 70 nm. As described by Handaand Stupin, the capillary effect requires lower tempera-ture-higher pressure for the formation of gas hydrates,thus shifting the phase boundary curve to the left inFigure 11 and shoaling the BGHS. Thus the effect can-not explain the observed discrepancy.

3.1.3. Gas composition: If gas hydrates contain even asmall amount of guest gases such as propane, ethane orCO2, the phase boundary would move to the highertemperature side in Figure 11. This would cause a com-mensurate downward shift in the BGHS. However, thegas hydrates of the Nankai Trough consist of nearlypure methane hydrate (Waseda et al., 2004).

3.1.4. Uplift of BGHS: This model assumes that the


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Fig. 13 Correlation of temperature, velocity, resistivity, FMI (Formation Micro-Scanner), gas hydrate (Sh%) and chloride logsin the exploratory wells in the eastern Nankai Trough. Solid horizontal lines indicate the predicted base of gas hydrate sta-bility (BGHS-0), shallow BSRs (BSR-1) and deeper BSRs (BSR-2).

active BGHS is at ~230 mbsf, just above the gashydrate-bearing sands (zone B), providing that the ther-mal gradient is correct and phase boundary curve isapplicable in this case. The fundamental problem withthis model is that most of the gas hydrate-bearing sandsexist outside the gas hydrate stability field. Accordingto this model almost all of the gas hydrates are belowBGHS and unstable under present conditions.

Despite the fact that it describes a situation wheremost of the gas hydrates are presently unstable, thismodel cannot be ruled out from consideration, particu-larly given the existence of double BSRs. The distancebetween BSR-1 and BSR-2 (30 to 40 m) implies that anupward or downward migration of the BGHS occurredrather rapidly. Foucher et al. (2002) explained that thedouble BSRs of the Dai-ichi-Tenryu knoll most likelyresulted from a rapid upward migration of BGHS due torapid uplift of the knoll.

In the study area, the Nankai Trough has three‘BGHSs’. BGHS-0 exists at ~230 mbsf and is defined bytemperature measurements. BGHS-1 exists at 265 mbsfand corresponds to the base of gas hydrate-bearing sandsand BSR-1. BGHS-2 exists at 289~295 mbsf and corre-sponds to BSR-2. BGHS-2 is now underlain by a smallamount of free gas. BGHS-1 appears to have free gasbelow and solid gas hydrates above. BGHS-0 is neither atthe base of gas hydrate nor at the top of free gas zone.BGHS-0 is “active” and exists in equilibrium with thebase of gas hydrate stability, however, the build-up of afree gas zone is not significant and gas hydrates have notyet been effectively accumulated in this location.Meanwhile, gas hydrates remain in between BGHS-1 andBGSH-2 as a fossil, relic gas hydrate zone.

3.1.5. Change in thermal gradient: Formation tempera-tures were measured only for the depth interval of 0-170mbsf, and the thermal gradient was extrapolated downto 250 mbsf in order to estimate the formation tempera-tures and the depth of BGHS. The thermal gradient maynot necessarily be linear but may change with depthreflecting variation in the thermal conductivities, possi-ble endo- and exothermic reactions, and fluid flows.Given the large difference in thermal conductivities ofice (~2.2 W/m.K) and water (~0.6 W/m.K), gashydrate-bearing sediments are thought to be more con-ductive than gas hydrate-free sediments. If this is thecase, then the thermal gradient line should becomesteeper (Fig. 11) for the gas hydrate-bearing zone whereformation temperatures were not directly measured.Thus, the BGHS should appear at a deeper level. Wecannot confirm this model as we do not have completeand reliable temperature profiles for the entire depthrange. A direct measurement of the temperatures andthermal conductivity across the BGHS and BSR zones

will help to resolve this issue.

3.2. Eustasy, tectonics, and gas hydrate accumulation

As described above, models 1, 2, and 3 are unlikelyto explain the observed discrepancy between BGHS andBSRs. Model 5 may be the cause of the discrepancy butcannot explain the occurrence of double BSRs. DoubleBSRs strongly suggest that abrupt upward migration ofBGHS occurred just recently (model 4).

Bottom water temperatures did not change greatly inthe last million years as documented by the oxygen iso-topic composition of benthic foraminifera, then the tem-perature change can be ruled out from the principal causeof the migration of BGHS. Shoaling of a BGHS can pre-sumably be caused by rapid sea-level drop. However, inthe last 18 – 10 ky there has been abrupt glacio-eustaticsea-level rise which would cause a deepening, not shoal-ing of the BGHS. If the glacio-eustatic sea-level fall forthe last 120 – 18 ky is responsible for the BGHS migra-tion, this phenomenon should be recognized worldwide.In fact double BSRs are not common features of gashydrate regions and are mainly identified in tectonicallyactive areas such as the Nankai Trough and CascadiaMargin (Suess et al., 2001; Torres et al., 1999, 2004).Eustatic sea-level changes are geologically rapid process-es, but the dissociation and formation of gas hydrates aremore rapid processes, easily obliterating the eustasy-induced relic BGHS. Therefore, we propose that theNankai Trough sediments have undergone very rapid tec-tonic uplift causing a rapid upward-migration of BGHSin gas hydrate-bearing sedimentary sequences.

A 26 m shift from BGHS-2 to BGHS-1, and a 33 mshift from BGHS-1 to BGHS-0 correspond to ~100 muplift, and ~140 m uplift, respectively, if the bottom watertemperatures are constant. Such a large uplift may nothave occurred at once but probably occurred as minorrepeated events within a short period. If these eventsoccurred during the last interglacial, the average uplift rateexceeds 1 cm per 100 years. This is anomalously highrate, but is not unreasonable. The study area is in the mid-dle of an active accretionary prism, only 10 km north ofthe active Tokai Thrust, and ~39 km of the main thrust.Uplift rates as high as ~ 0.5 cm/100 years have beenreported at land stations 60 km north of the study area.

Tectonic uplifting of gas hydrate bearing areas causesnot only upward migration of BGHS but also denseaccumulation of gas hydrates (Fig. 14). As has been dis-cussed elsewhere (Matsumoto, 2002), much of themethane present in these gas hydrates has been derivedfrom dissociation of precursor gas hydrates in strati-graphically lower levels. In accordance with progressivesedimentation and burial, the BGHS would graduallymigrate upward, and the precursor gas hydrates wouldeventually be dissociated. Some of the methane from dis-


sociating hydrates may be released from the ocean floor,but most of the methane would be accumulated as newgas hydrates within the new stability zone (‘Recycling ofHydrate Methane’). Rapid tectonic uplift and shoaling ofthe BGHS would result in an effective recycling anddense accumulation of gas hydrates, perhaps formingeconomically important gas hydrate deposits.

3.3. Future studies

To test the hypothetical models that attempt toexplain the unusual occurrence of gas hydrates in theNankai Trough, we propose to conduct the followingstudies:

1. Obtain direct measurements of the equilibrium for-mation temperatures for the entire depth range down tothe free gas zone below BGHS.

2. Reconstruct the paleo-water depths by means ofintegrated biostratigraphy, magnetostratigraphy, andoxygen isotopic analysis of benthic foraminifera.

3. Conduct high resolution, long-term monitoring ofbathymetric change of the outer ridge and fore-arc basinof the Nankai Trough.

4. Measure and monitor shallow and deep fluidmigration in combination with temperature monitoring.

4. Summary and Conclusions

Geochemical analysis of the interstitial waters of sed-iment cores taken from the site survey hole, BH-1, andthe exploratory hole, “MITI Nankai Trough” drilled inthe eastern Nankai Trough, as well as the integrated logand seismic data obtained from these same locationslead us to the following conclusions:

1. A 300 m thick sedimentary sequence, depositedover approximately 1.8 million years can be separatedinto Unit 1 (0-75 mbsf; monotonous dark green clay andsilt), Unit 2 (70-150 mbsf; dark green mud with occa-sional thin ash beds), Unit 3 (150-250+ mbsf; darkgreen mud with thin ash and sand beds), and Unit 4(270-310 mbsf; dark green mud with sandstone beds ofvariable thickness). Units 2, 3, and 4 are gently dippingto the northwest.

2. SO42- concentration of the interstitial waters

extracted from sediment cores rapidly decreased to zeroat 15.7 mbsf. This steep decline of SO4

2- suggests ahigh methane flux from below.

3. Cl- concentrations in the interstitial waters werearound 545 mM in Units 1 and 2, with spiky low anom-alies in sand beds at around 207 mbsf (zone A), 234-240 mbsf (zone B), and 258-265 mbsf (zone C) in Unit3. The deepest cores in Unit 4 were slightly depleted inCl- (~500 mM).

4. Gas hydrate saturation calculated from the chlorideanomalies reached values as high as 60 % (zone A) to80 % (zones B, C) in sands but only a few % in clay andsilt.

5. Gas hydrate accumulation zones (A, B, and C)were identified by the sonic and resistivity logs, thoughthe amplitude of anomaly was not always consistentwith the amount of gas hydrates.

6. 2D and high-resolution seismic studies revealedtwo BSRs: one at ~263 mbsf and the other at ~289mbsf. The upper, stronger BSR-1 was correlated to thebase of gas hydrate-bearing sands, while the lowerBSR-2 corresponded to the top of the low velocity zonein Unit 4.


gas hydrate zone

BGHS = BSR

oldBGHS = BSR2

newBGHS = BSR1

Accumulation ofrecycling gas hydrate

Mixed gas hydrate& free gas zone( = Disssociation zone)

RAPID UPLIFTsea-levelsea-level

Fig. 14 Schematic diagram showing that a rapid uplift causes shoaling of the BGHS, dissociation of gas hydrate between theold and new BGHS, and re-precipitation of gas hydrates in shallower horizons (recycling process). This process is thoughtto be very important for the accumulation of dense gas hydrate deposits.

7. Uplift of the study area most likely caused theupward migration of BGHS. The upper BSR-2 corre-sponds to the new BGHS, and the lower, to the oldBGHS. Relic gas hydrates may survive in the intervalbetween BSR-1 and BSR-2.

8. Direct measurements of formation temperatureindicated a geothermal gradient of ~4.3°C/100 m. Thetheoretical base of BGHS was estimated to be ~230mbsf from the observed thermal gradient and the exper-imentally determined phase boundary of methanehydrates in seawater. The theoretical base was estimat-ed to be ~30 meters shallower than the observed base ofgas hydrate distribution (=BSR-1).

9. The occurrence of the BGHS at ~230 mbsf, shal-lower than gas hydrate zones, can best be explained by arecent, rapid uplift of the area which raised the BGHS.Accordingly, the gas hydrates and free gas have not beenredistributed or recycled as yet. An alternative explana-tion is that the geothermal gradient at deeper levels isactually much steeper than current estimates placing thetheoretical BGHS at ~265 mbsf, the BSR-1 horizon.

10. The double BSRs (263 and 289 mbsf) and probablenew BGHS (~230 mbsf) in the exploration wells implythat a step-wise and rapid migration of the BGHS by tec-tonic uplift has enhanced dense and massive accumulationof gas hydrate deposits through methane recycling.

11. Direct measurements of the equilibrium forma-tion temperatures for the entire gas hydrate-bearing andfree gas zones, and long-term monitoring of bathymet-ric changes in the study area would help to test thehypothetical models that explain the unusual occurrenceof hydrates in Nankai Trough sediments and to developa geologic model for the accumulation of gas hydratesin an active margin.Acknowledgments: We extend our sincere gratitude tothe participants of the MITI Nankai Trough drilling pro-ject for 1997 and 1999-2000, in particular to Y. Hiroki,T. Uchida, A. Waseda, T. Fujii, and K. Baba who workedon board to take interstitial waters and sediment samples.We thank F. Colwell (INEEL) and anonymous reviewerfor their comments and suggestions. METI (Ministry ofEconomy, Trade and Industry) and JNOC (JapanNational Oil Corporation) allowed us to use the dataobtained during the project. This study was partly sup-ported by the Grant-in-Aid from the Ministry ofEducation and Culture to RM.

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detection and evaluation of gas hydrates in the eastern nankai trough by geochemical and geophysical...

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