RESERVOIR ACRCHITECTURE OF METHANE HYDRATE BEARING TURBIDITE CHANNELS IN THE EASTERN NANKAI TROUGH, JAPAN

Satoshi Noguchi , Toshiko FurukawaJapan Oil, Gas and Metals National Corporation

Than Tin AungSchlumberger Oilfield Services

Nobutaka OikawaJapan Oil, Gas and Metals National Corporation and Japan Petroleum Exploration

ABSTRACTReservoir architecture of methane hydrate (MH) bearing turbidite channels in the eastern Nankai

Trough, offshore Japan is evaluated using a combination of 3-D seismic and well data. On the 3-D seismic section, the MH-bearing turbidite channels correspond to complex patterns of strong seismic reflectors, which exhibit a 3-D internal architecture of the channel complex. Seismic sequence stratigraphic analysis shows that the channel complex comprises various sedimentary conditions (i.e., depositional systems), which primarily controls the reservoir architecture of the turbidite channels. For constructing 3-D facies model, we have interpreted internal sand body and

stacking patterns of the channels to discuss reservoir heterogeneities of MH-bearing formations. Geobody using seismic volume rendering shows that the extracted body is distributed towar d northeast to southwest directions in good agreement with the direction of the channel filled sediments according to the seismic sequence stratigraphic analysis. Stacking patterns of channel facies are identified around the ß1 well. The identified channels exhibit low sinuosity channels consisting of various magnitudes of channel widths ranging from tens to several hundred meters.

Paleo-flow current directions of these channels are typically oriented along north-to-south and north-northeast-to-south-southwest directions. Internal property of the stacking patterns of thechannel sediments is examined using a seismic acoustic inversion analysis. The distribution patterns of the velocity between the BSR and the top of the MH-bearing sediments show that higher velocities are particularly identified along north-northeast direction, which coincides with the sediment supplies of the turbidite channels. Several magnitudes and flow directions of these

velocity distributions suggest that each bottom frame of the channel consists of complex stacking patterns characteristic of several generations of the flow units.

Keywords: methane hydrate, turbidite channel, sequence stratigraphy, reservoir architecture

Satoshi Noguchi: Phone: +81 (43)276 4317 Fax +81 (43)276 4063 E-mail: noguchi-satoshi@jogmec.go.jp

INTRODUCTIONThe eastern Nankai Trough, which is located offshore of central Japan, is considered an attractive potential resource of methane hydrates (MHs) and a candidate for the exploitation and production of MH test fields [1, 2, 3].

Previous studies of the “Nankai Trough”

exploratory test well (which was acquired in 1999)reported that MHs are developed in the pore spaces of the unconsolidated sandy sediment

Proceedings of the 7th International Conference on Gas Hydrates (ICGH 2011),Edinburgh, Scotland, United Kingdom, July 17-21, 2011.

Figure 1 Location map of the 2-D/3-D seismic surveys and the METI “Tokai-oki to Kumano-nada” exploratory test wells.

within the deep submarine-fan turbidite system [4,

5, 6, 7]. Based on these exploratory results, the Japanese Government conducted MH research consisting of a 2-D seismic survey in 2001 (2,802 km), 3-D seismic surveys in 2002 (the total area of three survey fields was 1,960 km2), and a multiple well-drilling campaign in 2004 by the Ministry of

Economy, Trade and Industry (METI). Fujii et al. (2008, 2009b) [1, 8] reported that the MHreservoir is comprised of thin alternating sand and mud layers with an average sandy layer thickness of around 20–30 cm (the maximum thickness is greater than 1 m). In some locations in the eastern

Nankai Trough, MHs are selectively accumulatedin the turbidite formations; such zones are called“MH-concentrated zones”. According to the 2-D/3-D seismic surveys, there are sixteen MH-concentrated zones found in the eastern Nankai Trough survey area. The total gas in place (GIP)

for the MH-concentrated zones accounts for half of the total GIP in the eastern Nankai Trough survey area [1, 2]. Furthermore, these MH-concentrated zones are the best targets for recovering MHs for future exploitation and for thecommercialization of MHs. Hence, we focus on

the MH-concentrated zones to characterize the

reservoir architecture of the MH-bearing

sediments.In this paper, we focus on the submarine channel types of the MH-bearing sediments in one of the MH-concentrated zones in the eastern Nankai Trough. We first discuss reservoir heterogeneity within the MH-bearing turbidite channels using

seismic- sequence stratigraphic analysis to understand the relationship between thedepositional environment of the turbidite system and the reservoir potentials of the MH-concentrated zone. Subsequently, we use further detailed analysis to construct a facies model, using

seismic data around ß1 well to take into account an interpretation of stacked-channel patterns and the internal properties of the channel sediments.

BACKGROUND OF GEOLOGYThe Nankai Trough is situated along the

subduction zone between the Shikoku basin and the overriding Eurasian plate. It extends approximately 670 km from northeast of the Suruga Trough to southwest of the Kyushu-Palau Ridge off the shore of southwest Japan (Fig. 1).Our study area is located in the Tokai-Kumano

forearc basins along the Nankai Trough, central Japan (Fig. 1). The tectonic setting of the eastern

Nankai Trough is strongly influenced by a collision between the Izu-Ogasawara and the Honshu arcs. A large number of active faults have been recognized by the KAIKO-Tokai survey [9, 10]. The stratigraphy of the forearc basin of the eastern Nankai Trough is comprised of the

Pleistocene Kakegawa and Ogasa Groups [11, 12, 13, 14]. Takano et al. (2008, 2009) [13, 14] conducted a seismic-sequence stratigraphic analysis using 2-D/3-D seismic surveys and well data (including cores and logs) to reconstruct the depositional processes of submarine-fan turbidites

in the eastern Nankai Trough area. They classified the Kakegawa and Ogasa Groups into seventeen sequence units based on the well data and seismic reflection terminations, such as onlaps and truncation surfaces on the seismic sections. Furthermore, they interpreted the depositional

system of the turbidites using seismic facies via a seismic geomorphological analysis (e.g., [15]). Table 1 summarizes the depositional sequences, the depositional system, and the tectonic events of our study area based on Takano et al. (2009) [14].The depositional systems of the Kakegawa and

Ogasa groups can be divided into four distinctdepositional stages as follows:The intervals between Sequence Kg-a and Sequence Kg-c are categorized as Stage 1 and are part of the early Pleistocene Kakegawa Group. The braided channel-style submarine fans are

dominant due to a large number of coarse clastics supplied by the island of Honshu.Stage 2 is the early Pleistocene stage of the Kakegawa Group and ranges from Sequences Kg-d to Kg-h. Although the depositional system is composed of small, radial fan-type submarine fans,

the compressional deformation related to plate subduction activity occurred at this stage [14]. Due to this tectonic event, the depositional area was restricted, and the deposition of the sediments ceased, beginning in the early to middlePleistocene stage.

Ta ble 1 Summary of the depositional systems of the turbidites and tectonic events of the forearc basin in the eastern Nankai Trough, based on Takano et al. (2009) [14]

Stage 3 is the middle Pleistocene Ogasa Group, which ranges from sequences Og-a to Og-f. The depositional system is composed of the small, radial fan-type submarine fans, with no compressional deformation occurring during this stage. Several confined trough-filled channels are

observed; these troughs were formed due to synclinal depressions during Stage 2.Stage 4 is the latest Pleistocene stage of the Ogasa Group and ranges from sequences Og-h to Og-j. The channel-levee system on the fans is dominant and has an increasing slope gradient from

sequences Og-h to Og-j. During this stage, the forearc basin of the Atsumi knoll was uplifted and deformed due to a seamount subduction event [13, 14].

RESULTS

MH-B EARING TURBIDITE CHANNELSWith regard to the 3-D seismic volume, the seismic reflectors above the BSR show relatively strong seismic reflectors of positive (i.e., the same polarity as that of the seafloor) and negative (i.e.,reverse polarity with regard to the seafloor) peaks

compared with those of surrounding formations(e.g., the seismic section near the ß1 well in Fig. 2). Well-log data indicate that these strong seismic reflectors are correlated with an interval of higher resistivities that indicate MH-bearing turbidite formations [1, 8]. Amplitude values of both

positive and negative seismic reflectors in the MH-bearing sediments are variable even in the adjacent reflectors (Fig. 2).

Figure 2 Figure showing the seismic cross- section around the ß1 well. We can observe complex patterns of strong seismic reflections comprised of patchy shapes of positive (i.e., of the same polarity

as the seafloor) and negative (i.e., of reverse polarity with respect to the seafloor) reflectors above the BSR.

Around the ß1 well, positive seismic reflectors typically show lower amplitude values (e.g., amplitude values from 50 to 150) compared with the amplitude value of the seafloor (e.g.,

400~900), whereas negative seismic reflectors exhibit slightly lower amplitude values (e.g., –50 to –150) compared with amplitude values of the BSR (e.g., –150 to –250). Alternatively, the outside of the MH-bearing sediments (i.e., the surrounding formations) shows lower amplitude

values ranging from –20 to 20. Hence, we can qualitatively identify the distribution of the MH-bearing sediments by examining these relative

differences in amplitude values between the MH-bearing sediments and the formations surrounding them.The shapes of these strong seismic reflectors

typically exhibit patchy distributions of positive and negative reflectors. Their lateral continuity varies significantly, even over short distances (i.e., less than several tens of meters, Figs. 2 and 3).The manual selection of both positive and negative seismic reflectors above the BSR describes the 3-

D internal architecture of the MH-bearing turbidite channels (Fig. 3).

Figure 3 The 3-D internal architecture of the turbidite channel complex. The channel complex is distributed along the northeast-to-southwest direction. The northeastern side of the channel complex, (a) and (b), consists of thin MH-bearing sediments. In the central part of the channel complex (c), distinct channels are

observed on the northwestern and southeastern sides. The southwest region of the channel complex, (d), exhibits older parts of channels than appeared on the northwestern side.

DEPOSITIONAL SYSTEM AND FACIES DISTRIBUTIONS O F TH E TURBIDITE CHANNELSThe MH-bearing turbidite channels display a channel complex that extends in the northeast-southwest direction. The depositional sequence of

the turbidite channels is the middle Pleistocene Ogasa Group ranging from sequence boundaries (SB) Og-b to SB Og-c (Sequence Og-b). The depositional system of the Sequence Og-b is composed of the trough-filled type of turbidites that are represented by channels, or of stratified

turbidites. According to the seismic-facies distribution around the ß1 well, the interval of the MH-bearing sediments is comprised of seismic facies A to B (channel facies) indicating the sand-dominated turbidite system (Fig. 4).

Figure 4Seismic-facies distributions of sequences Og-a and Og-b in our study area.

Resistivity log data clearly show that a higher resistivity zone exists that corresponds to the

strong seismic reflectors above the BSR in the seismic section (Fig. 5). Fujii et al. (2008, 2009a) [1, 2] suggested that, based on the relationship between LWD ring resistivity and the lithologies of the core samples, the cut-off value for MH-bearing sand layers is 3 ohm-m. At the ß1 well,

there is an interval with resistivity values greater than 3 ohm-m that is approximately 44 m thick, ranging from 1295 m to 1339 m (indicated by the red color of the resistivity image in Fig. 5). The lithologies in this interval gradually change from the lower part of the thick sandy sediments to the

shallower part of the silty-to-muddy sediments. These formations probably correspond to a channel-to-levee transition stage, in agreement with the interpretation of the seismic sequence stratigraphic analysis.

Hence, the seismic sequence stratigraphic analysis indicates that the depositional system of the turbidites roughly correlates with the reservoir potential of the MHs. This may imply that the reservoir architecture of the turbidite channels is largely controlled by the sedimentary conditions

and the topographic changes that have occurred in the study area.

Figure 5 Representative LWD data from the ß1well plotted on the seismic inline sections. The first track shows the measured depth (from mean sea level). The second track indicates borehole

images obtained from Resistivity At Bit images (RAB images). Relatively bright colors exhibit higher resistivity values, which correspond to the MH-bearing sand layers. The third track depicts resistivity images obtained from LWD ring resistivity. Resistivity values of more than 3 ohm-

m are colored red.

DISCUSSIONSFACIES DISTRIBUTION O F TH E TURBIDITE CHANNELS AROUND ß1 WELLAlthough the depositional sequences correlate well with reservoir potentials of the MH, the sequence stratigraphic analysis is a qualitative method for examining the internal architecture of the turbidite channels. In fact, actual MH-bearing turbidite reservoirs are much more complicated even within

a single depositional sequence. Here, we examine further details of reservoir heterogeneities in the turbidite channels. We take into account an

extracting the sand body, an interpretation of the stacked channel patterns and the internal properties of the channel sediments by using seismic data from around the ß1 well.

EXTRACTION OF INTERNAL SAND BODYInternal sand bodies and connectivity within the MH-bearing sediments are evaluated by geobody obtained from seismic volume rendering (Fig. 6).Here we examine a seismic sweetness (reflection strength (amplitude envelope)/square root of instantaneous frequency) to identify the channel

body of the MH-bearing sediments. Figure 6 shows an interpretation of geobody using the seismic sweetness. T wo isolated bodies are identified at below/above the BSR by extracting higher sweetness value (white and green bodies shown in Figure 6). Green body corresponds to the

interval of the MH-bearing sediments. The boundary bet ween the MH-bearing sediments and the sediments without the MHs shows higher impedance contrast which generates higher reflection strength (i.e., high sweetness). Distribution of the extracted green body also

coincides with a direction of the channel filled sediments supplied from northeast side according to the seismic facies distribution (Fig. 4).On the other hand, northern side of the white body distributes a boundary of the BSR and/or below the BSR. The BSR shows physical boundary

between the MH-bearing sediments and gas-bearing sediments below the BSR.

Figure 6 Geobody using seismic volume rendering extracts two isolated bodies around ß1

well.

Thus, the higher sweetness of the extracted white body is probably due to both high reflection strength and low instantaneous frequency caused by the gas effect. Western side of the white body

exhibits a boundary of the gas-water contact implying that the extracted white body represents the gas-bearing sediments. Although distributions of the gas in the sediments may be related to the continuity of the lithofacies (i.e., pathway of gas migration), the extracted white body clearly shows

the physical boundary of the gas-bearing sediments, which do not represent sand body of the turbidite channels.

Figure 7 A graph depicting the identified bottom frame of the channels around the ß1 well. Arrows indicate the direction of the sediment supplies.

IDENTIFICATION O F STACKED CHANNELS Interpretation of the several stacking patterns of channel facies is identified within the sequence Og-b around the ß1 well. We first extracted a bottom frame of the stacked channels above the

BSR (i.e., the lower generation of the channels) and classified it into several distinct channels (Fig. 7 (a), (b)). One should note that some of the channels of the lower generation below the BSR are difficult to see in the seismic section because of the gas-bearing sediments that disturbed the

seismic reflectors (e.g., the seismic cross section at the ß1 well shown in Fig. 2). For this reason, we

extracted the bottom frame of the stacked channels in this study at the lower stability limit of MH up to the BSR line. The typical bottom frame of the channels exhibits variable widths (from several hundreds of meters), whereas the depth of the channels varies (ranging

from several tens to hundreds of meters). The identified bottom frame of the channels mainly descends from the north-northeast to the south-southwest direction (Fig. 7 (c)). The paleo-current direction of the channels is identical to the direction of the sediment supplies (e.g., north-

northeast), and this is indicated by the seismic facies distribution of sequence Og-b (Fig. 4).

Figure 8 Acoustic impedance between the BSR and the top of the MH-bearing sediments.

INTERNAL PROPERTIES O F THE MH-BEARING TURBIDITE CHANNELSThe internal properties of the interpreted channels are evaluated using a seismic impedance inversion analysis. Figure 8 shows the result of acoustic impedance between the BSR and the top of the MH-bearing sediments. We used the constant value of the density as 1000 kg/m3 in order to

evaluate velocity distributions of the MH-bearing sediments. The typical velocities of the MH-bearing sediments around the ß1 well range from 1800 m/s to 2200 m/s. These values overlap slightly with the velocities of the surrounding formations (e.g., 1650–1950 m/s). Check-shot data

from the ß1 well show that the interval velocities of the MH-bearing sediments range from 1820 m/s

to 2130 m/s, whereas check-shot data from above the MH-bearing sediments show interval velocities of 1700–1950 m/s, consistent with those of the impedance inversion result .The correlation between the velocity distributions and the bottom frame of the channels shows that

higher velocities (e.g., greater than 2000 m/s) are rarely observed in some locations: at the northern side of the channels above BF01, BF04, BF06, BF09. The distribution patterns of the higher velocities are oriented toward north-northeast direction (indicated by arrows) which coincides

with the direction of sediment supply of the turbidites. Because elastic responses for the MH-bearing sediments show a positive correlation between MH saturation rates and compressional and shear wave velocities[16, 17], reservoir properties of the MH-bearing formations are

closely related to the facies distribution of the turbidite channels.Hence, several magnitudes and directions of these velocity distributions probably correspond to complex stacking patterns characteristic of several generations of the flow units.

SUMMARYWe have discussed the reservoir architectures of the MH-bearing turbidite channels. The MH-bearing turbidite channels show the channel complex which exhibit different stages of

depositional sequences according to a seismic-sequence stratigraphic analysis. The depositional sequence implies that the reservoir architecture of the turbidite channels varies according to the sedimentary conditions and the topographic changes in the study area.

For constructing a facies model, we evaluated further details of reservoir heterogeneities in the turbidite channels by considering internal sand body and stacking patterns of the channels around the ß1 well.The interpretation of geobody shows that the

extracted body extends to northeast-southwest direction which correlates well with the direction of the channel filled sediments according to the seismic facies distribution. The stacking patternsof the channels also show that paleo-flow currents of the identified channels are typically oriented

along north-to-south and north-northeast-to-south-southwest directions.Internal property of the stacking patterns of thechannel sediments is examined using a seismic acoustic inversion analysis. The distribution patterns of the velocity between the BSR and the

top of the MH-bearing sediments show that higher velocities are particularly identified along north-northeast direction in coincident with the sediment supplies of the turbidite channels. Several magnitudes and flow directions of these velocity distributions are observed above the channels. This

fact suggests that each bottom frame of the channel consists of complex stacking patterns characteristic of several generations of the flow units.

ACKNOWLEDGEMENTThe 2-D/3-D seismic data and the well-log data of the exploratory test wells “Tokai-oki to Kumanonada” were acquired by METI. We would like to thank METI, Methane Hydrate 21 Research Consortium (MH21 Consortium), and Japan Oil,

Gas and Metals National Corporation for providing permission to publish this paper.

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