Lithological features of hydrate-bearing sediments and their relationship with gas hydrate saturation in the eastern Nankai Trough, Japan

Accepted Manuscript Lithological features of hydrate-bearing sediments and their relationship with gas hydrate saturation in the eastern Nankai Trough, Japan Takuma Ito, Yuhei Komatsu, Tetsuya Fujii, Kiyofumi Suzuki, Kosuke Egawa, Yoshihiro Nakatsuka, Yoshihiro Konno, Jun Yoneda, Yusuke Jin, Masato Kida, Jiro Nagao, Hideki Minagawa PII:

S0264-8172(15)00054-9

DOI:

10.1016/j.marpetgeo.2015.02.022

Reference:

JMPG 2154

To appear in:

Marine and Petroleum Geology

Received Date: 22 September 2014 Revised Date:

4 February 2015

Accepted Date: 16 February 2015

Please cite this article as: Ito, T., Komatsu, Y., Fujii, T., Suzuki, K., Egawa, K., Nakatsuka, Y., Konno, Y., Yoneda, J., Jin, Y., Kida, M., Nagao, J., Minagawa, H., Lithological features of hydrate-bearing sediments and their relationship with gas hydrate saturation in the eastern Nankai Trough, Japan, Marine and Petroleum Geology (2015), doi: 10.1016/j.marpetgeo.2015.02.022. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT

Lithological features of hydrate-bearing sediments and their relationship with gas hydrate saturation in the eastern Nankai Trough, Japan

RI PT

Takuma Ito1, *, Yuhei Komatsu2, Tetsuya Fujii2, Kiyofumi Suzuki2, Kosuke Egawa1, **, Yoshihiro Nakatsuka2, Yoshihiro Konno1, Jun Yoneda1, Yusuke Jin1, Masato Kida1, Jiro Nagao1, Hideki Minagawa1

(AIST), Sapporo, Japan

SC

1. Methane Hydrate Research Center, National Institute of Advanced Industrial Science and Technology

2. Methane Hydrate R&D Division, Technology and Research Center, Japan Oil, Gas and Metals

M AN U

National Corporation (JOGMEC), Chiba, Japan

Present address:

* CO2 Storage Research Group, Research Institute of Innovative Technology for the Earth, Kyoto 619-0292, Japan

Corresponding author:

TE D

** Subsurface Evaluation Unit, Technical Division, INPEX Corporation, Tokyo 107-6332, Japan

Takuma Ito ([email protected])

AC C

EP

Telephone number: +81-0774-75-2321; Fax number: +81-0774-75-2316

1

ACCEPTED MANUSCRIPT

Abstract

RI PT

Gas hydrate-bearing sediments from the eastern Nankai Trough, Japan, are characterized in terms of their lithology, interpreted processes and paleoenvironments of deposition, and various geometric parameters of their grain size distribution. These data are used to determine the relative influence of each characteristic on gas hydrate saturation within the sedimentary column. Four lithologies have been

identified in a single turbidite sequence that can be attributed to hyperpycnal flow deposits, Tc or Td

SC

divisions of a turbidite sequence, a Te division of a turbidite sequence, and hemipelagic mud. Facies association indicates that the sediment core can be vertically divided into units that are characteristic of

M AN U

three depositional environments: a lowermost channel-fill turbidite sequence, an intervening sheet-like turbidite sequence, and an uppermost basin floor sequence. The channel-fill turbidite and sheet-like turbidite sequences are the best hydrate reservoirs, as evidenced by the high levels of gas hydrate contained within them. The relationships between gas hydrate saturation and the grain size distribution parameters of median grain size, sand content, and skewness show that the latter can be useful tools with which to assess the quality of the gas hydrate reservoir in the eastern Nankai Trough area. This result

TE D

provides useful criteria for assessing reservoir quality in the eastern Nankai Trough area.

Keywords:

Nankai Trough, gas hydrate saturation, grain size, geometric parameters, lithology, sediment core,

AC C

EP

depositional process.

2

ACCEPTED MANUSCRIPT

1. Introduction

RI PT

Gas hydrates are ice-like crystalline solids in which gas molecules are caged within a solid lattice of water molecules (Kvenvolden, 1988). These clathrate structures are only stable under high-pressure and

low-temperature conditions, such that gas hydrates are restricted to regions of permafrost, high-latitude lakes, and oceanic margins (Ergov et al., 1999). Many recent studies have attempted to characterize the properties of gas hydrate-bearing sediments in marine environments, including those from the Nankai

SC

Trough, offshore Japan (e.g., Uchida et al., 2004a; Fujii et al., 2008, 2009, 2013), the Ulleung Basin, offshore Korea (e.g., Kim et al., 2011; Bahk et al., 2011, 2013; Kwon et al., 2011), the Cascadia margin

M AN U

(e.g., Riedel et al., 2009; Trehu et al., 2004; Torres et al., 2008), the Gulf of Mexico (Winters et al., 2008, Boswell et al., 2012, Collett et al., 2012), the Alaska North Slope (e.g., Rose et al., 2011; Winters et al., 2011), the South China Sea (e.g., Yun et al., 2006; Wang et al., 2011), and offshore of India (e.g., Collett et al., 2014; Kumar et al., 2014; Winters et al., 2014; Rose et al., 2014). Developing safe technologies that will allow the production of natural gas from such gas hydrate-bearing sediments is an important area of

century.

TE D

current scientific research, as gas hydrates are thought to be a potential energy resource for the 21st

The eastern Nankai Trough was selected as the site at which to test for gas hydrate production (Fujii et al., 2008; Yamamoto et al., 2010). Seismic surveys have identified the presence of a bottom-simulating reflector (BSR) that often appears at the base of a gas hydrate reservoir (Ashi et al., 2002; Hayashi et al.,

EP

2010). Furthermore, seismic reflection patterns and their shapes have indicated that the gas hydrate reservoir occurs as several channel deposits in a deep submarine-fan system (Arato and Takano, 1995;

AC C

Takano et al., 2009; Noguchi et al., 2011). Studies of sediment cores have revealed that gas hydrate mostly assumes a pore-filling morphology in unconsolidated sands, although some occurs in a nodular form in the eastern Nankai Trough region (Tsuji et al., 2004; Uchida and Tsuji, 2004; Uchida et al., 2004a, b; Fujii et al., 2009). Previous studies have documented the role of sand content and median grain size in gas hydrate distribution by demonstrating that gas hydrate selectively accumulates in coarse-grained sediments (Ginsburg et al., 2000; Uchida et al. 2004a; Torres et al., 2008; Fujii et al., 2009; Dugan and

Daigle, 2011; Winters et al., 2011; Bahk et al., 2011; Winters et al., 2014). However, those studies focused specifically on the sand content and median grain size of sediments and did not fully consider the 3

ACCEPTED MANUSCRIPT

influence of other geometric parameters of grain size distribution such as skewness, sorting, and kurtosis (Folk and Ward, 1957). The purpose of this study is to identify the lithological features of gas

RI PT

hydrate-bearing sediments that were recovered from the eastern Nankai Trough during a pressure coring campaign run by the Research Consortium for Methane Hydrate Resources in Japan (MH21). We have also performed detailed investigations into the role of different geometric parameters of grain size distribution throughout the sediment core as related to varying gas hydrate content.

SC

2. Geological and geophysical setting

M AN U

The Nankai Trough is an active convergent plate margin where the Philippine Sea Plate is undergoing northwest-directed subduction beneath the Eurasian Plate. Our study considers gas hydrate deposits that have been documented in the Kumano forearc basin region of the eastern Nankai Trough (Fig. 1) (Tsuji et al., 2004; Uchida and Tsuji, 2004; Uchida et al., 2004a, b; Fujii et al., 2009). A zone with concentrated methane hydrate, several tens of meters in thickness, was discovered during the drilling of a

(Uchida et al., 2009b).

TE D

well in 2004 (Fujii et al., 2009). Most of the methane gas recovered by the well is of microbial origin

At the study site, Quaternary sediments overlie the gas hydrate-bearing Plio–Pleistocene Kakegawa and Ogasa Groups (Hiroki et al., 2004; Arai et al., 2006; Takano et al., 2009). Seismic sequence stratigraphy has indicated that the Kakegawa and Ogasa Groups can be vertically divided into 17

EP

depositional sequences, Kg–a to Kg–h and Og–a to Og–i, in ascending order (Noguchi et al., 2011). Moreover, seismic facies features have shown that the depositional environment was that of a deep

AC C

submarine-fan system (Takano et al., 2009). This submarine-fan system is thought to have changed from initially being dominated by braided

rivers to being dominated by a channel-levee system during the Plio–Pleistocene (Takano et al., 2009; Noguchi et al., 2011). In the late Pliocene Kakegawa Group, the sequence intervals between Kg–a and Kg–c can be categorized as braided river submarine fans, and those between Kg–d and Kg–h as small radial fan-type submarine fans. In the Ogasa Group, the early to middle Pleistocene intervals between Og–a and Og–f can be categorized as trough-filled small radial fan-type submarine fans, and the late Pleistocene intervals between Og–h and Og–i can be interpreted as channel-levee dominated submarine 4

ACCEPTED MANUSCRIPT

fans. This study focuses on the depositional sequence between Og–b and the early stage of Og–c (Fig. 1). In the eastern Nankai Trough area, the BSR and overlying high-amplitude seismic reflectors have

RI PT

been reported to produce large acoustic impedance contrasts due to elastic changes between the gas hydrate-bearing sediments and those containing no gas hydrate (Ashi et al., 2002; Hayashi et al., 2010). At

the study site, it is well known that the BSR is indicative of the lower base of the gas-hydrate stability zone, while the high-amplitude reflectors above the BSR indicate the top of the gas-hydrate concentrated zone

(Tsuji et al., 2004; Noguchi et al., 2011). Because both reflectors occur in the coring interval, the

SC

depositional sequence examined in this study corresponds to the zone of gas hydrate concentration.

3.1.

Coring operations and cores

M AN U

3. Materials and methods

About 60 m of gas hydrate-bearing sediment cores was acquired by a Hybrid pressure core sampler (Hybrid PCS) at the AT-1 well during the 2012 JOGEMC/JAPEX Pressure coring operation using D/V

TE D

Chikyu (Yamamoto, 2015; Inada and Yamamoto, 2015). The sediment core was recovered between 260 and 320 meters below sea floor (mbsf). Overall core recovery efficiency was 61%. In the AT-1 well, the coring well (AT1-C well: water depth of 988.7 m) and the monitoring well (AT1-MC well: water depth of 997.7 m), which produced geophysical well logs, are separated by a horizontal distance of ~40 m at the

EP

seafloor (Fujii et al., 2013).

Once the pressure cores were retrieved, they were immediately transferred into a Pressure Core

AC C

Analysis and Transfer System (PCATS) to perform non-destructive tests such as X-ray imaging and P-wave velocity and gamma-ray measurements (Schultheiss et al., 2011). Afterwards, the sediment core

was cut in PCATS transferred under pressure into 0.3-m- or 1.2-m-long storage chambers, and stored at a pressure of ~15 MPa and temperature of ~5°C. Although depressurized cores are unsuitable for measuring the physical properties of hydrate-bearing sediments (e.g., Waite et al., 2008; Santamarina et al., 2012), the sediment characteristics do not change. Hence, a subset of the pressure cores was depressurized

and stored in liquid nitrogen for analysis of various sediment characteristics. A conventional coring tool, the Extended Shoe Coring System, was used alongside the Hybrid PCS 5

Fig. 1

ACCEPTED MANUSCRIPT

coring. Those conventional cores were depressurized onboard. The sediment core was split into halves, the split core was photographed, and its lithology was described. One of the halves was subsampled for

a long plastic case and imaged using X-ray computed tomography.

RI PT

the purposes of sedimentological, geochemical, and microbiological analysis, and the other was placed in

Subsamples of both pressure and conventional cores were used for grain size analysis to characterize

the properties of the gas hydrate-bearing sediment. The horizons observed in the sediment core correspond to the seismic depositional sequence Og–b to early stage Og–c in the early to middle

Grain size measurements

M AN U

3.2.

SC

Pleistocene deposits (Noguchi et al. 2011) (Fig. 1).

Samples weighing ~100 mg were placed in a 50-ml capacity centrifugal tube and treated with 30% hydrogen peroxide (H2O2) for a week at room temperature in order to remove organic matter. Subsequently, 0.4% sodium diphosphate decahydrate solution (Na4P2O7-10·H2O) was added as a dispersing agent. The grain size analysis was performed at average intervals of 17.3 cm using sliced core

TE D

samples of length 5.4–15.5 cm. The residue was then analyzed with a Microtrac MT3300 EX (Nikkiso Co. Ltd.) laser-diffraction particle size analyzer, which can detect sediment grains in the size range 0.02–2000 µm, and results were expressed as volume percentages. The mechanical reproducibility was better than ±1% for the median grain size. The following geometric parameters were calculated in order to

φ84－φ16 4

+

φ95－φ5 6.6

,

AC C

sorting =

EP

investigate grain size distributions (Folk and Ward, 1957):

kurtosis =

φ84－φ16

2.44(φ75－φ25)

skewness =

(1)

,

φ16 + φ84－2φ50 2( φ84－φ16)

+

(2) φ5 + φ95－2φ50 2( φ95－φ5)

,

(3)

where φ is the phi-scale grain size at the relevant cumulative curve and subscripts refer to the appropriate spot on the cumulative curve (Wentworth, 1929). 3.3.

Mineral composition

6

ACCEPTED MANUSCRIPT

Mineralogical analysis was conducted on a random powered X-ray diffraction (XRD) device RAD-3C (Rigaku Co. Ltd.) with a CuKa target. Analyses were performed using a tube voltage of 40 kV

RI PT

and a current of 30 mA. The divergence, scattering, and receiving slits were 1°, 1°, and 0.15 mm, respectively. Dried powdered samples were mounted on glass holders and X-rayed from 3 to 60°2θ. The scanning speed was 2°2θ / min and the data-sampling step was 0.05°2θ. For mineral identification, the

diffractograms were processed using MacDiff software for smoothing and background calculation. The

Calculation of gas hydrate saturation

M AN U

3.4.

SC

mineral composition was measured using core samples from depths of 272, 285, 293, and 315 mbsf.

As the presence of gas hydrate changes the physical properties of its host sediment, geophysical well logs have been used to estimate in situ gas hydrate saturation (e.g., Collett and Ladd, 2000; Collett, 2002; Lee and Collett, 2011; Miyakawa et al., 2014). We estimated the saturation using Archie’s equation (Archie, 1942) under the assumption that resistivity anomalies in the eastern Nankai Trough sediments are

2004; Fujii et al., 2009):

TE D

caused by the presence of gas hydrate and water in pore spaces (Uchida et al., 2004a, b; Uchida and Tsuji,

Sh = 1 － (a・Rw/φm・Rt)1/n,

(4)

where Sh is gas hydrate saturation, Rw is the resistivity of formation water (0.3 ohm-m), φ is density

EP

porosity derived from the bulk density log, Rt is true formation resistivity derived from a deep reading resistivity log, and a and m are Archie constants. Here, the vertical resolution of the resistivity tool is 25.4 cm. In the eastern Nankai Trough sediments, gas hydrate saturation can be estimated using the parameters

AC C

a = 1.0, m = 2.0, and n = 2.0 (Fujii et al., 2013; Fujii et al., 2015), which closely resemble values suggested

for other regions of the Nankai Trough (Miyakawa et al., 2014) and the South China Sea (Wang et al., 2011). As a result, the saturation estimated from analysis of well logs is strongly correlated with the gas hydrate saturation measured based on the gas volume released from the cores following hydrate dissociation (Fujii et al., 2015).

4. Results

7

ACCEPTED MANUSCRIPT

4.1.

Lithological features

RI PT

The sediment core was divided into four different facies (Fig. 2 and 3), each characterized by a particular set of lithologies. These comprised Facies A (massive sandy silt to silty sand), Facies B (parallel- or cross-laminated sandy silt to sand), Facies C (graded sandy silt with abundant weakly parallel

laminations), and Facies D (massive clayey silt with bioturbation), which are described in detail below.

Lithologies were characterized according to their sedimentary structure (determined by soft X-ray

SC

radiography), grain size, sediment composition (observed by smear slide tests), and degree of bioturbation.

M AN U

Facies A contains dark-olive massive sandy silt to silty sand (Fig. 4). These clastic lithologies are generally coarser than those in other facies, and the sequence is poorly sorted. It has a coarsening-upward unit at the base that is capped by a fining-upward unit above it, with this change occurring over a few meters. Organic remains (e.g., leaf and wood debris) are present in the sandy layers, although no bioturbation was documented. The facies was recorded at depths greater than 305.1 mbsf. Facies B is characterized by dark-olive sandy silt to sand (Fig. 4) in which parallel or cross

TE D

laminations and weak or clear erosional basal boundaries were observed on soft X-ray radiograph. This facies is tens of centimeters in thickness and is capped by graded muddy sediments (Facies C) with gradual or sharp boundaries. These lithologies are found at depths above 305.1 mbsf. Most of the sandy layers are mica-rich and lack evidence of bioturbation. After depressurization, Facies B commonly

EP

exhibited a soupy texture (Fig. 2) that cannot be observed in the pressure core using soft X-ray photography. This texture could be the result of the dehydration process that occurs during gas hydrate

AC C

dissociation.

Facies C is characterized by dark-olive colored silty sand to sandy silt (Fig. 4) that commonly

contains weakly parallel laminations and is several centimeters to several meters in thickness. It has normal grading and no evidence of burrow structures. This facies was recorded at depths of 266.1–266.4 mbsf and 274.7–299.3 mbsf. According to soft X-ray photography, this facies is less dense than Facies D, even though both are fine-grained sediments. Smear slide observation shows that this sediment is mainly composed of detrital minerals with fewer calcareous microfossils than are observed in Facies D. Facies D consists of mainly dark-olive massive clayey silt (Fig. 4) that contains rare spot-like 8

Fig. 2

ACCEPTED MANUSCRIPT

burrows, as determined by soft X-ray photography. Smear slide observation indicates that this sediment contains a higher proportion of calcareous (or other) complete or fragmented microfossils than the other

4.2.

RI PT

lithological facies. Facies D was recorded at depths above 305.6 mbsf.

Features in grain-size distribution

Figure 3 shows a surface plot for the statistical description of grain size distributions. Most of the

SC

samples exhibit a unimodal distribution with a fine tail, although muddy sediments tend to have a broader

distribution than those of sandy sediments. The calculated median grain size (i.e., the value that separates

M AN U

the higher half of a dataset from the lower half) for all samples is in the range 4.0–146.4 µm, with an average of 41.4 µm (Fig. 5a). The median grain size for Facies D is <20.8 µm, due to the lack of stratigraphic variation, and that of Facies C is larger than that of Facies D. However, the median grain sizes for Facies A and B show large fluctuations and are greater than those for Facies C and D. Sorting (equation 1) is a measure of the standard deviation of a dataset, which is the spread of the grain size distribution. Folk and Ward (1957) proposed values of <0.71 for well-sorted sediments,

TE D

0.71–1.00 for moderately sorted sediments, and >1.00 for poorly sorted sediments. The calculated sorting values varied from 0.98 to 2.45 with an average of 1.94 (Fig. 5b). Sorting values for Facies A, C, and D vary from 1.5 to 2.8, and so they are all classed as poorly sorted sediments. Facies B exhibits a larger range of values between 0.98 and 2.44, and so is defined as moderately sorted to poorly sorted.

EP

Kurtosis (equation 3) measures the extent to which the central part of a grain size distribution departs from the normal distribution (i.e., whether it is flatter or more peaked than the normal distribution). Folk

AC C

and Ward (1957) defined a kurtosis value of 0.7–0.9 as platykurtic (flatter than the normal distribution), 0.9–1.1 as mesokurtic (near normal grain size distribution), and 1.1–1.5 as leptokurtic (more peaked than the normal distribution). In our work, kurtosis varied from 0.7 to 2.4 with an average value of 1.0 (Fig. 5c). Values for Facies C and D show no significant stratigraphic variation, and are in the range 0.7–1.5 (platykurtic to leptokurtic) with an average of 0.9 (boundary between platykurtic and mesokurtic). By contrast, Facies A and B show values with a notable stratigraphic variation and are in the range 0.7–2.4 (platykurtic to leptokurtic) with an average of 1.1 (mesokurtic to leptokurtic). Although the calculated kurtosis values overlap among all facies, each average value is distinguishable. The grain size distributions 9

Fig. 3

ACCEPTED MANUSCRIPT

of Facies C and D are sharper than those of Facies A and B. Skewness (equation 2) is a measure of asymmetry of a dataset when compared to a normal

RI PT

distribution, and calculated values varied from −0.1 to 0.7 with an average of 0.3 (Fig. 5d). Folk and Ward (1957) defined values between −1.0 and −0.1 as negative skewness (skewed towards a coarser grain size),

those between −0.1 and 0.1 as nearly symmetrical, and those between 0.1 and 0.3 as positive skewness (skewed towards a finer grain size). Facies C and D show wide variations in their calculated skewnesses, which range from 0 (symmetrical) to 0.6 (positive skewness) with an average of 0.2 (positive skewness).

SC

Skewness values for Facies A and B show less variation than those for Facies C and D, and vary from 0

4.3.

Mineralogical features

M AN U

(symmetrical) to 0.7 (positive skewness) with an average of 0.5 (positive skewness).

Fig. 4 Fig. 5

Minerals identified in the different sediments consist of quartz, feldspars, calcite, chlorite, and various clays (namely illite) (Egawa et al., 2015). Figure 6 shows averaged XRD profiles for all four facies, where it can be seen that Facies D exhibits a notably higher peak for calcite (Fig. 6a) than is

TE D

observed in Facies A–C (Fig. 6b, c, and d). This is due to the presence of calcareous nanofossils in its hemipelagic sediments, although there is no significant change in the constituent mineral composition (Fig.

4.4.

Gas hydrate occurrence

EP

6).

AC C

The monitoring well (AT1-MC) is located 40.0 m away from the pressure coring well (AT1-C)

(Fujii et al., 2013). Thus, their respective depth control is essential for reliable core-log integration. Table 1 shows the results of depth control between each well (Suzuki et al., 2015). The core-to-log depth controls are based on the fitting of distinct peaks that represent major lithological boundaries. These are estimated by resistivity log data acquired at the AT1-MC well and P-wave velocity log data obtained by the PCATS

at the AT1-C well, which take into account the strike and dip of the strata (Table 1). Sand contents can also be used for depth control, as Fujii et al. (2009) suggested that sand content and resistivity show strong correlations with lithological variations at this site. With the depth constraints outlined in Table 1, precise 10

Fig. 6

ACCEPTED MANUSCRIPT

depth controls were performed as peak-to-peak fittings using both sand contents of the sediments and resistivity data. This was achieved by using tie points showing distinct peaks in sand contents (Fig. 7).

RI PT

Consequently, the results of core-to-log depth controls are hereafter used when referring to depth in the sediment core. Figure 5e shows the profile of gas hydrate saturation, where it can be seen that two zones of high gas hydrate saturation occur at 275.1–299.1 mbsf and below 304.6 mbsf. These zones contain notably high gas hydrate contents than are recorded above and below them.

Geometric parameters and their implications for depositional processes

M AN U

5.1.

SC

5. Discussion

Dispersion diagrams showing the relationships between median grain size and other geometric parameters of grain size distribution (Fig. 8) were used to characterize their stratigraphic variation in sediment of the Nankai Trough. These data show that sand content and skewness are linearly related to median grain size (Fig. 8a and c), but sorting and kurtosis are non-linearly related (Fig. 8b and d). In the

TE D

latter case, when considering sorting and kurtosis, Facies D is easily distinguished from Facies A–C by having a median grain size smaller than ~20 µm (Fig. 8b and d). This suggests that the median grain size of Facies A–C increases (coarsens), leading to better sorting, but the median grain size of Facies D increases, leading to poorer sorting (Fig. 8b). This relationship present in Facies A–C agrees with previous

EP

work showing that a coarser median grain size denotes better sorting in turbidite sediments of the Nankai Trough (Suzuki et al., 2009), although this is the first documentation of the opposite trend shown in Facies

AC C

D. This characteristic trend appears to be driven by different depositional processes, as discussed below. Sediments in Facies A–C lack the fine grain sizes that are present in Facies D (Fig. 5a), as indicated

by their positive skewness (Fig. 5d), and are better sorted than Facies D (Fig. 5b). These features suggest that Facies A–C have been winnowed by high hydraulic energy flows. Facies A contains coarse, but poorly sorted, lithologies that are characteristic of concentrated gravity flow deposits (e.g., Pierson and Scott, 1985; Mulder and Alexander, 2001; Sohn et al., 2002). Furthermore, Facies A contains coarsening-upward units that are capped by a fining-upward sequence, which can be attributed to hyperpycnal flow deposits (Mulder et al., 2001). Thus, these two units are likely related to periods of 11

Table 1

ACCEPTED MANUSCRIPT

waxing and waning between conditions of instantaneous surges and longer duration surge-like flows (Kneller, 1995; Mulder et al., 2001). The observation that the sandy sediments contain plant debris also

RI PT

supports the interpretation of hyperpycnal flow deposits reworking coastal deposits during floods and storms events (Mulder et al., 2001). Units of Facies B show parallel or cross laminations with basal

erosional contacts, which seem to be comparable with the Tc or Td divisions of a fine- to medium-grained

turbidite (Bouma, 1962). Facies C (beneath Facies B; Fig. 2) lacks evidence of bioturbation, which

indicates rapid deposition of particles from suspension; hence, it can be confidently correlated to the Te

SC

division of a turbidite sequence (Bouma, 1962).

Facies D contains minor evidence of bioturbation, suggesting that it was deposited in a hemipelagic

M AN U

setting, as fine-grained particles will actively flocculate to form aggregates of clay particles in deep sea and coastal environmental settings. Flocculation and aggregation processes enable fine-grained particles to settle down with the coarse fraction. Such clay-sized particles, including clastics and calcareous nanofossil aggregations associated with the coarser silt-sized clastic particles, were observed in scanning electron microscope imagery. The aforementioned trend showing that a coarser median grain size correlates with poorer sorting (Fig. 8b) can be explained by these aggregates. This is likely because grain size

TE D

distributions were measured in this study by laser diffraction after complete dispersal of these aggregated materials; hence, the generation of additional fine-grained particles would make it appear that the sediment under analysis had poorer sorting than is actually the case. In sediments transported by high hydraulic energy flows such as turbidity currents, such aggregations will disaggregate and be transported

EP

as discrete grains due to internal shear stresses. This mechanism, which primarily affects clay-sized

AC C

particles, appears to influence the sorting between Facies A–C and Facies D.

5.2. Depositional environment interpretation inferred from facies association

The depositional environment of the Pleistocene Ogasa Group has been interpreted from its sequence

stratigraphy as having been a deep submarine-fan system (Takano et al., 2009; Noguchi et al., 2011). The Ogasa Group Og–b and Og–c sequences are most likely trough-filled small radial fan-type submarine fan deposits (Takano et al., 2009; Noguchi et al., 2011). Accordingly, the sediment core can be categorized into three units: a channel-fill turbidite sequence (below 304.3 mbsf), a sheet-like turbidite sequence 12

ACCEPTED MANUSCRIPT

(304.3–273.3 mbsf), and a basin floor sequence (above 273.3 mbsf). The channel-fill turbidite and sheet-like turbidite sequences contain more gas hydrate than the basin floor sequence, which suggests that

RI PT

these turbidite sequences are good reservoirs and that the uppermost basin floor sequence is the cap deposit.

In the sediment core analyzed in this study (Fig. 5), the channel-fill turbidite sequence consists of Facies A (hyperpycnal flow deposits) and the overlying Facies C (turbidite mud). Turbidite mud

overlying hyperpycnal flow deposits indicates that the former was derived from the tail of the post- and

SC

diluted-hyperpycnal flows. Sedimentological features representative of a hyperpycnal sequence in the

sandy sediments imply deposition in a submarine channel, as hyperpycnal flows are generated from a

M AN U

river mouth connecting to a submarine channel during floods or storms (Mulder et al., 2001). Based on geophysical well log data measured near to the study site (Fujii et al., 2009), the coring interval can be attributed to the high resistivity intervals, which are indicators of gas hydrate occurrence. The coring interval is correlated both with the gas hydrate stability zone above the BSR and an upper part of the Og–b sequence where the stacked channels were observed (Noguchi et al., 2011). Stacked channels often form because of channel shifts in middle or lower submarine fan domains. This sequence appears to be

TE D

correlated with seismic Facies A and B reported by Takano et al. (2009) and Noguchi et al. (2010), and to Facies A interpreted by Fujii et al. (2009) as channel-fill deposits based on core obtained from nearby wells.

The sheet-like turbidite sequence consists of Facies B (Tc or Td turbidite divisions) and C (turbidite

EP

mud). Only Tc and Te divisions were observed without a Ta division in this sequence, implying that they were deposited by the tail of a single-surge turbidity flow in the distal part from the channel. The Te

AC C

division was occasionally observed to overlie the Tc or Td turbidite divisions. A thin-bedded sheet-like

morphology identified by seismic profiles (Takano et al., 2009) can be interpreted as deposits in the distal part of channel, such as channel overspill that occurred during transportation of a large amount of detrital particles. This sequence can be correlated to seismic Facies C and D, which appear as seismic signals of

continuous parallel patterns (Takano et al., 2009; Noguchi et al., 2011), and also can be correlated to Facies B or D of Fujii et al. (2009). The base-absent Tc–e and Td–e sequences tend to show better upright-sorting (Fig. 5b); however, no

clear thickening- or thinning-upwards trends were observed, despite both being characteristic of a 13

ACCEPTED MANUSCRIPT

sheet-like turbidite succession. Such overspill deposit volume is largely controlled by the relationship between overspill flow energy and overspill flow thickness (Kane and Hodgson, 2011), and so variations

RI PT

in stratigraphic thickness and the grain size of the turbidite deposit are explained by changes in either (or both) of these factors. Determining the most influential factor is difficult here, but sedimentological

evidence of a better upright-sorting trend indicates that flow energy increases towards the upper part of the sequence so that fine and coarse particles can segregate during sediment transport (Hübscher et al., 1997; Deptuck et al., 2007).

SC

The basin floor sequence consists of thick hemipelagic mud, often intercalated with Facies B (Te division of turbidite), which is interpreted as a basin floor deposit produced by distal turbidity currents

M AN U

from the submarine fan (Walker, 1978). This sequence seems to be correlated with seismic Facies E, indicated by discontinuous and chaotic seismic signals (Takano et al., 2009; Noguchi et al., 2011), and can be attributed to Facies C or E of Fujii et al. (2009). The sequence appears to have been deposited in a hemipelagic setting with a slow rate of deposition based on the combination of calcareous microfossil-bearing fine-grained sediments with evidence of bioturbation.

TE D

5.3 Gas hydrate occurrence in relation to lithological features in the eastern Nankai Trough area and its significance for reservoir characterization

The profiles for gas hydrate saturation detailed in this study have no depth-dependent distribution

EP

because the two zones of high gas hydrate saturation primarily correlate with sand-bearing intervals in the sediment core (Fig. 5e). Egeberg and Dickens (1999) reported that gas hydrate is more saturated towards

AC C

the base of its stability zone, assuming no other lithological influence; however, the gas hydrate distribution presented in this study does not follow their model. Other distribution trends that do not follow this relatively simple model have been reported by Riedel et al. (2009) and Bahk et al. (2011). Possible explanations for this difference are offered by host lithologies that have different capillary entry pressures (Nimblett and Ruppel, 2003; Behseresht and Bryant, 2012; Chatterjee et al., 2014) and the effect of dissolved hydrocarbons in migrating pore fluids (Ginsburg et al., 2000; Weinberger et al., 2005; Torres et al., 2008). It is known that concentrations of gas hydrate can only form when the hydrocarbon content of a fluid exceeds its saturation limit (Xu and Ruppel, 1999), so the large variety of lithologies can instead 14

Fig. 8

ACCEPTED MANUSCRIPT

play an important role in the heterogeneous distribution of gas hydrate ( Behseresht and Bryant, 2012; Chatterjee et al., 2014).

RI PT

Gas hydrate is known to preferentially accumulate in coarse-grained sands (Uchida et al., 2004a, b; Uchida and Tsuji, 2004; Fujii et al., 2004; Winters et al., 2014), which is supported by experimental studies using various quartz sands with different median grain sizes (Liu et al., 2011). It is known that

permeability is significant for hydrate saturation in sediments. Winters et al. (2011) demonstrated that a

strong correlation between median grain size and the measured permeability of a plugged core suggests

SC

that median grain size is responsible for much of the difference in permeability at the Mount Elbert Gas Hydrate Stratigraphic Test Well, Alaska North Slope. This finding suggests that grain size is associated

M AN U

with permeability, and is therefore the factor actually controlling gas hydrate saturation. Moreover, recent studies have shown that the effect of sediments containing a large amount of diatoms or nanofossils (e.g., ooze sediments) and volcanic ash is important (Kraemer et al., 2000; Bahk et al., 2011, 2013; Rose et al., 2014), given that such oozes or volcanic ash have low capillary entry pressures that provide conditions suitable for the formation of gas hydrates. However, no significant changes in sediment composition occur in our core samples from the eastern Nankai Trough, except due to a small amount of nanofossils in

TE D

the hemipelagic mud. Consequently, grain size appears to be one of the important factors that restrict gas hydrate occurrences, and we find that lithological changes in the study area are caused by varying depositional processes, not changes in sediment composition. Figure 9 shows relationships between gas hydrate saturation and geometric parameters for each

EP

lithology, including sand contents. In previous studies, sand content and median grain size were primarily discussed with regard to the lithological control of gas hydrate saturation (Torres et al., 2008; Winters et

AC C

al., 2011; Bahk et al., 2011, 2013; Rose et al., 2014). This relationship can be found in the eastern Nankai Trough because gas hydrate saturation has significant positive correlations to sand content and median grain size (Fig. 9a and b). Gas hydrate saturation is also significantly positively correlated with geometric parameters of skewness (Fig. 9 c), indicating that the profile of gas hydrate saturation correlates with not

only sand content and median grain size but also skewness as geometric parameters. On the other hand, gas hydrate saturation is weakly associated with sorting and kurtosis (Fig. 9d and e). This suggests that these geometric parameters of sand content, median grain size, and skewness are likely to be the most useful measures for reservoir characterization in the eastern Nankai Trough area, where no significant 15

Fig. 9

ACCEPTED MANUSCRIPT

change occurs in sedimentary composition. In addition, sediments derived from hyperpycnal flow deposits (Facies A) and a turbidite sequence containing the Tc or Td turbidite division (Facies B) and Te

RI PT

turbidite mud division (Facies C) have higher gas hydrate saturation, whereas that of hemiplegic mud (Facies D) is lower (Fig. 9). This indicates that the depositional processes of the strata are essential criteria for assessing reservoir quality and production potential in the eastern Nankai Trough area. Because depositional process of the strata led to changes in the lithological features, the analysis of the lithology in

combination with geometric parameters such as sand content, median grain size, and skewness can

SC

elucidate the process of gas hydrate accumulation in the eastern Nankai Trough area.

M AN U

Conclusions

In this paper, we have characterized gas hydrate-bearing sediment core recovered from just above the BSR in the eastern Nankai Trough in terms of its lithology and interpreted process of deposition. Furthermore, we have investigated various sedimentological controls on gas hydrate saturation. Lithologies are divided into four sedimentary facies, which can be attributed to hyperpycnal flow deposits,

TE D

Tc or Td divisions of a turbidite sequence, a Te division of a turbidite, and hemipelagic mud. Facies association indicates that the sediment core can be vertically divided into three units: a lowermost channel-fill turbidite sequence, an intervening sheet-like turbidite sequence, and an uppermost basin floor sequence. The channel-fill turbidite and sheet-like turbidite sequences are the best gas hydrate reservoirs.

EP

Relationships between gas hydrate saturation and the grain size distribution parameters of median grain size, sand content, and skewness shows that the latter are likely to be the dominant controls on the degree

AC C

of gas hydrate saturation in a sedimentary column. These results deliver insight into the process of gas hydrate accumulation in the eastern Nankai Trough area, and provide criteria that can be used to assess reservoir quality in the eastern Nankai Trough area or other areas with similar geological settings or lithostratigraphic sequences.

Acknowledgements

We are grateful to two anonymous reviewers who read this article and provided many valuable 16

ACCEPTED MANUSCRIPT

comments. We also thank METI and the MH 21 Research Consortium for their financial support. We are grateful to the staffs and crews of the 2012 JOGEMC/JAPEX pressure coring operation aboard the D/V

RI PT

Chikyu for acquiring the samples.

References

Archie, G.E., 1942. The electrical resistivity log as an aid in determining some reservoir characteristics.

SC

Transactions of the American Institute of Mining, Metallurgical, and Petroleum Engineers, 146, 54–62.

M AN U

Arai, K., Okamura, Y., Ikehara, K., Ashi, J., Soh, W., Kinoshita, M., 2006. Active faults and tectonics on the upper forearc slope off Hamamatsu City, central Japan. Journal of Geological Society of Japan, 112, 749–759 (in Japanese with English abstract).

Arato, H., Takano, O., 1995. Significance of sequence stratigraphy in petroleum exploration. The Memoirs of the Geological Society of Japan, 45, 43–60 (in Japanese with English abstract). Ashi, J., Tokuyama, H., Taira, A., 2002. Distribution of methane hydrate BSRs and its implication for the

TE D

prism growth in the Nankai Trough. Marine Geology, 187, 177–191. Bahk, J.J., Um, I.K., Holland, M., 2011. Core lithologies and their constraints on gas-hydrate occurrence in the East Sea, offshore Korea: Results from the site UBGH1-9. Marine and Petroleum Geology, 28, 1943–1952.

EP

Bahk, J.J., Kim, D.H., Chun, J.H., Son, B.K., Kim, J.H., Ryu, B.J., Torres, M.E., Riedel, M., Schultheiss, P., 2013. Gas hydrate occurrences and their relation to host sediment properties: Results from Second

AC C

Ulleung Basin Gas Hydrate Drilling Expedition, East Sea. Marine and Petroleum Geology, 47, 21–29.

Behseresht, J., Bryant, S.L., 2012. Sedimentological control on saturation distribution in Arctic gas-hydrate-bearing sands. Earth and Planetary Science Letters, 341–344, 114–127.

Boswell, R., Collett, T.S., Frye, M., Shedd, W., McConnell, D.R., Shelander, D., 2012. Subsurface gas hydrates in the northern Gulf of Mexico. Marine and Petroleum Geology, 34, 4–30. Bouma, A.H., 1962. Sedimentology of some Flysch deposits. Elsevier, Amsterdam. Chatterjee, S., Bhatnagar, G., Dugan, B., Dickens, G.R., Chapman, W.G., Hirasaki, G.J., 2014. The 17

ACCEPTED MANUSCRIPT

impact of lithologic heterogeneity and focused fluid flow upon gas hydrate distribution in marine sediments. Journal of Geophysical Research: Solid Earth, 119, 6705–6732.

RI PT

Collett, T.S., Ladd, J., 2000. Detection of gas hydrate with downhole logs and assessment of gas hydrate concentrations (saturations) and gas volumes on the Blake Ridge with electrical resistivity data. In:

Paull, C.K., Matsumoto, R., Wallace, P.J., Dillon, W.P. (Eds.), Proceedings of the Ocean Drilling Program, Scientific Results, 164, 179–191.

Collett, T.S., 2002. Energy Resource potential of natural gas hydrates. AAPG Bulletin, 86, 1971–1992.

SC

Collett, T.S., Lee, M.W., Zynianova, M.V., Mrozewski, S.A., Guerin, G., Cook, A.E., Goldberg, D.S.,

2012. Gulf of Mexico Gas Hydrate Joint Industry Project Leg II logging-while-drilling data

M AN U

acquisition and analysis. Marine and Petroleum Geology, 34, 41–61.

Collett, T.S., Boswell, R., Cochran, J.R., Kumar, P., Lall, M., Mazumdar, A., Ramana, M.V., Ramprasad, T., Riedel, M., Sain, K., Sathe, A.V., Vishwanath, K., NGHP Expedition 01 Scientific Party, 2014. Geologic implications of gas hydrates in the offshore of India: Results of the National Gas Hydrate Program Expedition 01. Marine and Petroleum Geology, 58, 3–28.

Deptuck, M.E., Sylvester, Z., Pirmez, C., O’Byrne, C., 2007. Migration-aggradation history and 3-D

TE D

seismic geomorphology of submarine channels in the Pleistocene Benin-major Canyon, western Niger Delta slope. Marine and Petroleum Geology, 24, 406–433. Dugan, B., Daigle, H., 2011. Data report: permeability, compressibility, stress state, and grain size of shallow sediments from Sites C0004, C0006, C0007, and C0008 of the Nankai accretionary

EP

complex. In: Kinoshita, M., Tobin, H., Ashi, J., Kimura, G., Lallemant, S., Screaton, E.J., Curewitz, D., Masago, H., Moe, K.T. and the Expedition 314/215/316 Scientists (Eds.), Proceedings of the

AC C

Integrated Ocean Drilling Program, 314/315/316.

Egawa, K., Nishimura, O., Izumi, S., Fukami, E., Jin, Y., Kida, M., Konno, Y., Yoneda, J., Ito T, Suzuki, K., Nakatsuka, Y. and Nagao, J. 2015. Bulk sediment mineralogy of gas hydrate reservoir at the East Nankai offshore production test site, Marine and Petroleum Geology, in this issue.

Egeberg, P.K., Dickens, G.R., 1999. Thermodynamic and pore water halogen constraints on gas hydrate distribution at ODP Site 997 (Blake Ridge). Chemical Geology, 153, 53–79. Ergov, A.V., Crane, K., Vogt, P.R., Rozhkov, A.N., 1999. Gas hydrates that outcrop on the sea floor: stability models. Geo-Marine Letters, 19, 68–75 18

ACCEPTED MANUSCRIPT

Folk, R.L., Ward, W.C., 1957. Brazos river bar: a study in the significance of grain size parameters. Journal of Sedimentary Petrology, 27, 3–26.

RI PT

Fujii, T., Saeki, T., Kobayashi, T., Inamori, T., Hayashi, M., Takano, O., Takayama, O., Kawasaki, T., Nagakubo, S., Nakamizu, M., Yokoi, K., 2008. Resource assessment of methane hydrate in the eastern Nankai Trough, Japan. Proceedings of 2008 Offshore Technology Conference, Houston, Texas, U.S.A., 2008, OTC19310.

Fujii, T., Nakamizu, M., Tsuji, Y., Namikawa, T., Okui, T., Kawasaki, M., Ochiai, K., Nishimura, M.,

SC

Takano, O., 2009. Methane-hydrate occurrence and saturation confirmed form core samples, eastern

Nankai Trough, Japan. In: Collett, T., Knapp, C., Boswell, R. (Eds.), Natural gas hydrate―Energy

Geologists Memoir, 89, 385–400.

M AN U

resource potential and associated geologic hazards. The American Association of Petroleum

Fujii, T., Noguchi, S., Takayama, T., Suzuki, K., Yamamoto, K., Saeki, T., 2013. Site selection and formation evaluation at the 1st offshore methane hydrate production test site in the eastern Nankai Trough, Japan, 75th EAGE conference and Exhibition incorporating SPE EUROPEC 2013. Fujii, T., Suzuki, K., Takayama, T., Tamaki, M., Komatsu, Y., Konno, Y., Yoneda, J., Yamamoto, K. and

TE D

Nagao, J. (2015): Geological settings and characterization of methane hydrate reservoir distributed on the Daini-Atsumi knoll in the Eastern Nankai Trough, Japan., Marine and Petroleum Geology, in this issue.

Ginsburg, G., Soloviev, V., Matveeva, T., Andreeva, I., 2000. Sediment grain-size control on gas hydrate presence, sites 994, 995, and 997. In: Paull, C.K., Matsumoto, R., Dillon, W.P. (Eds.), Proceedings

EP

of the Ocean Drilling Program, Scientific Results, 164, 237–245. Hayashi, M., Inamori, T., Saeki, T., Noguchi, S., 2010. The distribution of BSRs related to methane

AC C

hydrates, offshore Japan. Journal of the Japanese Association for Petroleum Technology, 75, 42–53 (in Japanese with English abstract).

Hiroki, Y., Watanabe, K., Matsumoto, R., 2004. Lithology, Biostratigraphy, and Magnetostratigraphy of gas hydrate-bearing sediments in the eastern Nankai Trough. Resource Geology, 54, 25–34.

Hübscher, C., Spieb, V., Breitzke, M., Weber, M.E., 1997. The youngest channel levee system of the Bengal fan: results from digital sediment echosounder data. Marine Geology, 141, 125–145. Inada, N. and Yamamoto, K. (2015): Data Report: Pressure coring and shipboard core handling in the Nankai Project, Marine and Petroleum Geology, in this issue.

19

ACCEPTED MANUSCRIPT

Kane, I.A., Hodgson, D.M., 2011. Sedimentological criteria to differentiate submarine channel levee subenvironments: Exhumed examples from the Rosario Fm. (Upper Cretaceous) of Baja California,

RI PT

Mexico, and Fort Brown Fm. (Permian), Karoo Basin, S. Africa. Marine and Petroleum Geology, 28, 807–823.

Kim, G.Y., Yi, B.Y., Yoo, D.G., Ryu, B.J., Riedel, M., 2011. Evidence of gas hydrate from downhole logging data in the Ulleung Basin, East Sea. Marine and Petroleum Geology, 28, 1979–1985.

Kneller, B.C., 1995. Beyond the turbidite paradigm: physical models for deposition of turbidite and their

SC

implications for reservoir prediction. In: Hartley, A.J., Prosser, D.J. (Eds.), Characterization of deep marine clastic systems. Geological Society of London Special Publication, 94, 31–49.

M AN U

Kraemer, L.M., Owen, R.M., Dickens, G.R., 2000. Lithology of the upper gas hydrate zone, Blake Outer Ridge: A link between diatoms, porosity, and gas hydrate. In: Paull, C.K., Matsumoto, R., Wallace, P.J., Dillon, W.P. (Eds.), Proceedings of the Ocean Drilling Program, Scientific Results, 164, 229–236.

Kumar, P., Collett, T.S., Boswell, R., Cochran, J.R., Lall, M., Mazumdar, A., Ramana, M.V., Ramprasad, T., Riedel, M., Sain, K., Sathe, A.V., Vishwanath, K., Yadav, U.S., NGHP Expedition 01 Scientific

TE D

Party, 2014. Geologic implications of gas hydrates in the offshore of India: Krishna–Godavari Basin, Mahanadi Basin, Andaman Sea, Kerala–Konkan Basin. Marine and Petroleum Geology, 58, 29–98. Kwon, T.H., Lee, K.R., Cho, G.C., Lee, J.Y., 2011. Geotechnical properties of deep oceanic sediments recovered from the hydrate occurrence regions in the Ulleung Basin, East Sea, offshore Korea.

EP

Marine and Petroleum Geology, 28, 1870–1883. Kvenvolden, L.M., Owen, R.M., Dickens, G.R., 2000. The global occurrence of natural gas hydrate, in

AC C

Natural Gas Hydrates: Occurrence, Distribution, and Detection, Geophys. Monogr. Ser., vol. 124, edited by Paull CK, Dillon WP, 3–18, AGU, Washington DC.

Lee, M.W., Collett, T.S., 2011. In-situ gas hydrate saturation estimated from various well logs at the Mount Elbert Gas Hydrate Stratigraphic Test Well, Alaska North Slope. Marine and Petroleum Geology, 28, 439–449.

Liu, H., Kawasaki, T., Ukita, T., Moudrakovski, I., Fujii, T., Noguchi, S., Shimada, T., Nakamizu, M., Ripmeester, J., Ratcliffe, C., 2011. Particle size effect on the saturation of methane hydrate in sediments – Constrained from experimental results. Marine and Petroleum Geology, 28, 1801–1805. 20

ACCEPTED MANUSCRIPT

Milkov, A.V., Claypool, G.E., Lee, Y., Sassen, R., 2005. Gas hydrate systems at Hydrate Ridge offshore Oregon inferred from molecular and isotopic properties of hydrate-bound and void gases. Geochim.

RI PT

Cosmochim. Acta, 69, 1007–1026. Miyakawa, A., Saito, S., Yamada, Y., Tomaru, H., Kinoshita, M., Tsuji, T., 2014. Gas hydrate saturation

at Site C0002, IODP Expeditions 314 and 315, in the Kumano Basin, Nankai trough. Island Arc, 23, 142–156.

Mulder, T., Alexander, J., 2001. The physical character of subaqueous sedimentary density flows and

SC

their deposits. Sedimentology, 48, 269–299.

Nimblett, J., Ruppel, C.D., 2003. Permeability evolution during the formation of gas hydrates in marine

M AN U

sediments. Journal of Geophysical Research, 108, 2420, doi: 10.1029/2001JB001650. Noguchi, S., Shimoda, N., Takano, O., Oikawa, N., Inamori, T., Saeki, T., Fujii, T., 2011. 3-D internal architecture of methane hydrate-bearing turbidite channels in the eastern Nankai Trough, Japan. Marine and Petroleum Geology, 28, 1817–1828.

Pierson, T.C., Scott, K.M., 1985. Downstream dilution of a lahar: transition from debris flow to hyperconcentrated streamflow. Water Resour. Res., 21, 1511–1524.

TE D

Rose, K., Boswell, R., Collett, T., 2011. Mount Elbert Gas Hydrate Stratigraphic Test Well, Alaska North Slope: Coring operations, core sedimentology, and lithostratigraphy. Marine and Petroleum Geology, 28, 311–331.

Rose, K.K., Johnson, J.E., Torres, M.E., Hong, W.L., Giosan, L., Solomon, E.A., Kastner, M., Cawthern,

EP

T., Long, P.E., Schaef, H.T., 2014. Anomalous porosity preservation and preferential accumulation of gas hydrate in the Andaman accretionary wedge, NGHP-01 site 17A. Marine and Petroleum

AC C

Geology, 58, 99–116.

Riedel, M., Collett, T., Malone, M.J., IODP Expedition 311 scientist, 2009. Gas hydrate drilling transect across northern Cascadia margin – IODP Expedition 311. In: Long, D., Lovell, M.A., Rees, J.G., Rochelle, C.A. (Eds.), Sediment-hosted Gas Hydrates: New Insights on Natural and Synthetic Systems. The Geological Society, London, Special Publications, 319, 11–19.

Santamarina, J.C., Dai, S., Jang, J., Terzariol, M., 2012. Pressure core characterization tools for hydrate-bearing sediments. Scientific Drilling, 14, 44–48. Schultheiss, P., Holland, M., Roberts, J., Huggett, Q., Druce, M., Fox, P., 2011. PCATS: Pressure core 21

ACCEPTED MANUSCRIPT

analysis and transfer system. Proceedings of the 7th International Conference on Gas Hydrates (ICGH 2011), Edinburgh, Scotland, United Kingdom.

RI PT

Shepard, F.P., 1954. Nomenclature based on sand-silt-clay ratios. Journal of Sedimentary Petrology, 24, 151–158.

Sohn, Y.K., Choe, M.Y., Jo, H.R., 2002. Transition from debris flow to hyperconcentrated flow in a submarine channel (the Cretaceous Cerro Toro Formation, southern Chile). Terra Nova, 14, 405–415.

SC

Suzuki, K., Ebinuma, T., Narita, H., 2009. Features of Methane hydrate-bearing sandy-sediments of the Forearc Basin along the Nankai Trough: Effect on Methane hydrate-accumulating mechanism in

M AN U

turbidite. Journal of Geography, 118, 899–912.

Suzuki, K., Takayama, T. and Fujii, T. (2015-a): The density structure report by Logging-while-Drilling data and core data at the first offshore gas production test site on Daini-Atsumi Knoll around eastern Nankai Trough, Marine and Petroleum Geology, in this issue.

Takano, O., Nishimura, M., Fujii, T., Saeki, T., 2009. Sequence stratigraphic distribution analysis of methane-hydrate-bearing submarine-fan turbidite sandstones in the Eastern Nankai Trough area:

TE D

relationship between turbidite facies distributions and BSR occurrence. Journal of Geography, 118, 814–834 (in Japanese with English abstract).

Torres, M.E., Trehu, A.M., Cespedes, N., Kastner, M., Wortmann, U.G., Kim, J.H., Long, P., Malinverno, A., Pohlman, J.W., Riedel, M., Collett, T., 2008. Methane hydrate formation in turbidite sediments of

EP

northern Cascadia, IODP Expedition 311. Earth and Planetary Science Letters, 271, 170–180. Trehu, A.M., Long, P.E., Torres, M.E., Bohmann, G., Rack, F.R., Collett, T.S., Goldberg, D.S., Milkov,

AC C

A.V., Riedel, M., Schultheiss, P., Bangs, N.L., Barr, S.R., Borowski, W.S., Claypool, G.E., Delwiche, M.E., Dickens, G.R., Gracia, E., Guerin, G., Holand, M., Johnson, J.E., Lee, Y.J., Liu, C.S., Su, X., Teichert, B., Tomaru, H., Vanneste, M., Watanabe, M., Weinberger, J.L., 2004. Three-dimensional distribution of gas hydrate beneath southern Hydrate Ridge; constraints from ODP Leg 204. Earth and Planetary. Science Letters, 222, 845–862.

Tsuji, Y., Ishida, H., Nakamizu, M., Matsumoto, R., Shimizu, S., 2004. Overview of the METI Nankai trough wells: a Milestone in the Evaluation of methane hydrate resources. Resource Geology 54, 3–10. 22

ACCEPTED MANUSCRIPT

Uchida, T., Tsuji, Y., 2004a. Petrophysical properties of natural gas hydrates-bearing sands and their sedimentology in the Nankai Trough. Resource Geology, 54, 79–87.

RI PT

Uchida, T., Lu, H., Tomaru, H., MITI Nankai Trough Shipboard Scientists, 2004b. Subsurface occurrence of natural gas hydrate in the Nankai Trough Area: Implication for gas hydrate concentration. Resource Geology, 54, 35–44.Wentworth, C.K., 1929. Method of computing mechanical composition of sediments. Geological Society of America Bulletin 40, 771–790.

Waite, W.F., Kneafsey, T.J., Winters, W.J., Mason, D.H., 2008. Physical property changes in

SC

hydrate-bearing sediment due to depressurization and subsequent repressurization. Journal of Geophysical Research: Solid Earth, 113, doi: 10.1029/2007JB005351.

M AN U

Wang, X., Hutchinson, D.R., Wu, S., Yang, S., Guo, Y., 2011. Elevated gas hydrate saturation within silt and silty clay sediments in the Shenhu area, South China Sea. Journal of Geophysical Research, 116, B05102, doi:10.1029/2010JB007944.

Weinberger, J.L., Brown, K.M., Long, P.E., 2005. Painting a picture of gas hydrate distribution with thermal images. Geophysical Research Letters, 32, 1–4.

Winters, W.J., Dugan, B., Collett, T.S., 2008, Physical properties of sediments from Keathley Canyon and

25, 896-905.

TE D

Atwater Valley, JIP Gulf of Mexico gas hydrate drilling program, Marine and Petroleum Geology,

Winters, W.J., Walker, M., Hunter, R., Collett, T., Boswell, R., Rose, K., Waite, W., Torres, M., Patil, S., Dandekar, A., 2011. Physical properties of sediment from the Mount Elbert Gas Hydrate

EP

Stratigraphic Test Well, Alaska North Slope. Maine and Petroleum Geology, 28, 361–380. Winters, W.J., Wilcox-Cline, R.W., Long, P., Dewri, S.K., Kumar, P., Stern, L., Kerr, L., 2014.

AC C

Comparison of the physical and geotechnical properties of gas-hydrate-bearing sediments from offshore India and other gas-hydrate-reservoir systems. Marine and Petroleum Geology, 58, 139–167.

Wright, F.M., Nixon, F.M., Uchida, T., 1999. Physical properties of sediments from the JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well. In: Dallimore, S.R., et al. (Eds.), Scientific Results from JAPEX/JNOC/GSC Mallik 2L-38 Gas Hydrate Research Well, Mackenzie Delta, Northwest Territories, Canada. Bull. Geol. Surv. Can., 544, 95–100. Xu, W.-Y., Ruppel, C., 1999, Predicting the occurrence, distribution, and evolution of methane gas hydrate in 23

ACCEPTED MANUSCRIPT

porous marine sediments from analytical models, Journal of Geophysical Research, 104, ,5081-5096.

Yamamoto, K., Noguchi, S., Terao, Y., Matsuzawa, K., Kurihara, M., Saeki, T., Shimada, T., 2010. The

RI PT

concept and technical issues of the planned offshore methane hydrate production test in the Eastern Nankai Trough. Proceedings of 2010 Offshore Technology Conference, Houston, Texas, U.S.A. (OTC20740).

Yamamoto, K., 2015, Pressure core-sampling and analyses in the 2012–2013 MH21 offshore test of gas production from methane hydrates in the eastern Nankai Trough, Marine and Petroleum Geology, in this issue.

SC

Yun, T.S., Narsilio, G.A., Santamarina, J.C., 2006. Physical characterization of core samples recovered

AC C

EP

TE D

M AN U

from Gulf of Mexico. Marine and Petroleum Geology, 23, 893–900.

24

ACCEPTED MANUSCRIPT

RI PT

Figure and table captions

Fig. 1. Location of the eastern Nankai Trough and seismic profile of the depositional sequence with the coring interval discussed in this study marked by a black vertical line.

Fig. 2. X-radiographs, photographs of typical sediment lithology, and median grain size profiles

SC

determined for selected core sections. Bold black lines indicate interpreted erosional surfaces. X-radiographs and photographs were taken before and after gas hydrate dissociation.

M AN U

Fig. 3. Gas hydrate reservoir stratigraphic column and surface plot of grain size distribution of the sediment core at the eastern Nankai Trough area. Note that grain size distribution is given on a logarithmic scale.

Fig. 4. Ternary diagram showing lithological classification of sediment core obtained from the gas hydrate

TE D

reservoir. See text for discussion.

Fig. 5. Depth profiles of (a) median grain size, (b) sorting, (c) kurtosis, (d), skewness, and (e) gas hydrate saturation by well log data (Fujii et al., this volume). The gas hydrate saturation profile shows two

EP

zones of relatively high concentration at depths of 275.1–299.1 mbsf and below 304.6 mbsf.

Fig. 6. Averaged X-ray diffraction profiles for sediment core Facies A–D. Profiles represent core samples

AC C

from depths of (a) 272 mbsf (Facies D), (b), 285 mbsf (Facies C), (c) 293 mbsf (Facies B), and (d) 315 mbsf (Facies A).

Fig. 7. The result of core-to-log depth controls by peak-to-peak fitting using sediment core sand contents and resistivity of the log data under the stratigraphic constraints outlined in Table 1.

Fig. 8. Dispersion diagrams showing the relationships between median grain size and the parameters (a) sand content, (b) sorting, (c) skewness, and (d) kurtosis. The relationships between median grain size and these variables are either linear (sand content and skewness) or non-linear (sorting and kurtosis).

25

ACCEPTED MANUSCRIPT

Fig. 9. Gas hydrate saturation determined from resistivity log versus parameters such as (a) sand content, (b) median grain size, (c) skewness, (d) sorting, and (e) kurtosis.

RI PT

Table 1. Depth controls between sediment core and log depths in coring well (AT1-C: water depth of 998.7 m) and monitoring well (AT1-MC: water depth of 997.7 m). This control is based on peak

fitting from the profile of P-wave velocity log by the PCATS at AT1-C well and that of resistivity

AC C

EP

TE D

M AN U

SC

log at the AT1-MC well (Suzuki et al., 2015).

26

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT

Highlights

We document gas hydrate-bearing sediment core from the eastern Nankai Trough, Japan

•

Core contains channel-fill turbidite, sheet-like turbidite, and basin floor units

•

Relationships between gas hydrate content, median grain size, sand content, and skewness are

RI PT

•

AC C

EP

TE D

M AN U

SC

established

1