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Occurrence and seismic characteristics of gas hydrate in the Ulleung Basin, East Sea
Marine and Petroleum Geology 47 (2013) 236e247
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Marine and Petroleum Geology journal homepage: www.elsevier.com/locate/marpetgeo
Occurrence and seismic characteristics of gas hydrate in the Ulleung Basin, East Sea Dong G. Yoo a, *, Nyeon K. Kang a, Bo Y. Yi a, Gil Y. Kim a, Byong J. Ryu a, Keumsuk Lee b, Gwang H. Lee c, M. Riedel d a
Petroleum & Marine Research Division, Korea Institute of Geoscience and Mineral Resources (KIGAM), 124 Gwahang-no, Yuseong-gu, Daejeon 305-350, Republic of Korea Korea National Oil Corporation (KNOC), Anyang 431-711, Republic of Korea c Department of Energy Resources Engineering, Pukyoung National University, Busan 608-737, Republic of Korea d National Resources Canada, Geological Survey of Canada, 9860 West Saanich Road 16, Sidney, BC V8L 4B2, Canada b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 February 2013 Received in revised form 1 July 2013 Accepted 2 July 2013 Available online 12 July 2013
Multi-channel seismic reflection and well-log data from the Ulleung Basin, East Sea reveal several seismic signatures indicative of gas-hydrate occurrence in the Ulleung Basin that are associated with vertically and/or laterally stacked mass-transport complexes. The seismic indicators include (a) a bottom simulating reflector (BSR), (b) a seismic chimney, (c) high amplitude reflections within the gas hydrate stability zone (GHSZ), (d) acoustic blanking, (e) enhanced reflections below the BSR, and (f) seafloor gas-escape features. The BSR, associated with enhanced reflections below, is most commonly found over much of the basin indicating a physiochemical boundary of gas hydrates overlying free gas. Seismic chimneys are characterized by velocity pull-up and reduced reflectivity on the seismic sections, which appear to be caused by active migration of fluid gas vertically into the GHSZ. The logging data retrieved from the seismic chimneys showed elevated electrical resistivity (>80 Ohm-m) and P-wave velocity (>2000 m/s), indicating the presence of gas hydrate. Another seismic characteristic observed in gas hydrate bearing sediments is the strong amplitude reflections, defined by the relatively high reflectivity within the GHSZ. Acoustic blanking is likely to be the result of hydrate accumulation in the sediments causing a significant reduction of acoustic impedance contrast between sedimentary layers. Where the upward migrating gas seeps into the deep water column, seafloor pockmarks and mud mounds may be formed. Gas hydrate was recovered from the Ulleung Basin, East Sea in 2010 during the Second Ulleung Basin Gas Hydrate Drilling Expedition (UBGH2) under the Korean National Gas Hydrate Program. Based on the results, gas-fluids migrate into the GHSZ through two distinct pathways: (1) structural conduits which include fault and fracture systems associated with seismic chimneys and (2) stratigraphic conduits associated with inclined turbidite/hemipelagic layers. Two types of gas-hydrate occurrence were identified in the basin: (1) a stratally-bound type (pore filling) within turbidite sand layers and (2) a locally concentrated type (massive, nodule or fracture filling) within upward-growing chimneys associated with near vertical faults. Relatively high concentrations of gas hydrate, however, tend to occur in localized seismic chimneys, rather than in the strata-related features. The successful recovery indicates that the Ulleung Basin provides favorable conditions for gas-hydrate formation. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Seismic indicators of gas hydrate Occurrence types Multi-channel seismic data Ulleung Basin
1. Introduction Occurrences of marine gas hydrates are well known from numerous geophysical and geological studies (e.g. Hyndman and Spence, 1992; Kvenvolden, 1993; Lee et al., 1993; Xu and Ruppel, 1999). In seismic sections, the presence of gas hydrate can be * Corresponding author. Tel.: þ82 42 86 3324; fax: þ82 42 868 3417. E-mail address: [email protected] (D.G. Yoo). 0264-8172/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.marpetgeo.2013.07.001
identified by the existence of Bottom Simulating Reflectors (BSRs), which have high amplitude and reverse polarity and tend to be parallel to the seafloor topographic surface (Shipley et al., 1979). Therefore, a BSR remains perhaps the most common indicator of gas hydrates, although it may not be essential for gas hydrate recovery (Kvenvolden and Barnard, 1983; Shipley et al., 1979; Hyndman and Spence, 1992; Holbrook et al., 1996). The existence of gas hydrate within sediment pore space may reduce the acoustic impedance contrasts of sedimentary layers, leading to a decrease in
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seismic reflectivity (i.e., acoustic blanking) above a BSR (Shipley et al., 1979; Hovland et al., 1997; Wood and Ruppel, 2000; Diaconescu et al., 2001). Seismic chimneys or near vertical wipeouts associated with seismic blanking are also seen in seismic data as a gas hydrate indicator, together with pockmarks or mounds on the seafloor in many areas (Ginsburg, 1998; Hyndman et al., 2001; Riedel et al., 2006). The presence of free gas below the BSR also may be inferred from the observation of enhanced reflections, which show high-acoustic impedance contrast in seismic profiles (Holbrook et al., 1996; Wood and Ruppel, 2000; Bünz et al., 2003; Hustoft et al., 2007). The probable existence of gas hydrates in the Ulleung Basin has been suggested by several seismic indicators including BSR, the acoustic blanking zone, and seismic chimneys (Gardner et al., 1998; Lee et al., 2005; KIGAM, 2007; Ryu et al., 2009; Bahk et al., 2013a). Horozal et al. (2009) identified various seismic indicators of gas hydrate and free gas in the Ulleung Basin. Haacke et al. (2009) attempted to relate the seismic images of chimney structures in the deep-water basin to the concentration of gas hydrates. Mass-transport deposits associated with gas hydrate occurrence in the basin were documented by Riedel et al. (2012) and Scholz et al. (2012). Although the recent recovery of gas hydrate by drilling and piston coring has confirmed the gas hydrate accumulations in the central basin, the regionally distributed characteristics of the seismic indicators of gas and gas hydrates for the entire basin have been only rarely documented. In the present study we interpreted multi-channel seismic reflection and well-log data from the East Sea off Korea to identify and document the seismic characteristics related to gas hydrate and gas. We also present the occurrence types of gas hydrate associated with migration pathways for fluid-gas upwelling into the GHSZ. 2. Geologic setting The East Sea (Fig. 1A) is a back-arc basin bounded by the eastern Asian continent in the west and the Japan islands both in the east and the south. The semi-enclosed basin consists of three deep-water basins (Japan, Yamato, and Ulleung Basins), separated by submerged continental remnants (i.e., the Korea Plateau, Oki Bank, Yamato Ridge, and Kita-Yamato Ridge). The Ulleung Basin, a bowl-shaped basin, is located in the southwestern East Sea adjacent to the eastern coast of Korea and is bounded by the steep continental slope of the Korean Peninsula to the west and by the rugged Korea Plateau to the north (Fig. 1B). The gentle slopes of the Oki Bank and the Japanese Islands form the eastern and southeastern margins of the basin, respectively. The seafloor is generally smooth and deepens progressively to the northeast; water depths range from less than 1500 m in the south to over 2300 m in the northeast where the basin joins the deeper Japan Basin through a long and narrow interplain gap between the two islands of Ulleungdo and Dokdo (Fig. 1B). The Ulleung Basin was formed during the Late Oligocene to Early Miocene by crustal extension associated with the southward drift of the Japan Islands (Tamaki et al., 1992; Jolivet et al., 1995; Chough and Barg, 1987). At the end of the Middle Miocene, the tectonic stress regime changed from tensional to compressional (Yoon and Chough, 1995), leading to thrust faultings and foldings in the southern and western margin of the basin and to the sediment compression and consolidation that probably has been responsible for the upward flow of gas-rich fluids and hydrate formation (Ryu et al., 2009). Since this time of tectonic movement, the basin has progressively subsided until the present and has been fed with a significant quantity of siliciclastic sediments (e.g., debris-flow deposits), filling the larger part of the basin. In the Ulleung Basin, the Neogene sediments are characterized by vertically/laterally widespread deposition of mass-transport complexes, caused by margin-wide slope failures in response to back-arc closure (Lee and
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Suk, 1998). Mass transport deposits in the basin show a zoned distribution pattern that approximately follows the bathymetric contours: slide/slump deposits on the upper slope, debris-flow deposits on the lower slope or base-of-slope, and turbidites on the northern basin (Chough et al., 1997). 3. Data The total of 6600 L-km of multi-channel seismic reflection data used in this study were collected by the Korea Institute of Geoscience and Mineral Resources (KIGAM) using the research vessel Tamhae II in 2005 for the exploration of gas hydrate resources in the Ulleung Basin, East Sea offshore of Korea (Fig. 1B). The seismic data acquisition included a 240-channel and 3-km long streamer with a 1035 in3 air-gun array that provides a wide range of frequency up to 250 Hz (KIGAM, 2007). The receiver-group interval, and the near and far offsets are 12.5 m, 162 m, and 3149 m, respectively, yielding 60-fold coverage. The distance between shots is 25 m, resulting in a CDP spacing of 6.25 m. The data were digitally recorded with a sample rate of 1 ms and a maximum recording length of 7 s. The seismic data were processed at KIGAM (KIGAM, 2007). True amplitude was preserved during data processing, which is important scheme to clearly interpret amplitude blanking and seismic chimneys. Data processing included resampling, anti-aliasing filtering, bandpass filtering, true amplitude recovery (spherical divergence correlation), velocity analysis, normal moveout correlation, and stacking. The interpretation of the seismic data was performed using Geographix manufactured by Landmark. The Second Ulleung Basin Gas Hydrate Expedition (UBGH2) in 2010 conducted LWD and coring at 13 sites to identify the overall distribution of gas hydrate in the Ulleung Basin (Fig. 1B; Ryu et al., 2012; Bahk et al., 2013b). The LWD phase was conducted using the Schlumberger’s logging tools of the GeoVision, TeleScope, EcoScope and SonicVision. LWD log data include natural gamma, resistivity, velocity, porosity, and density. 4. Seismic characteristics of gas hydrate and associated gas On the basis of the multi-channel seismic data, we identified six seismic signatures indicative of gas-hydrate existence in the deepwater basin filled with vertically and/or laterally stacked masstransport complexes (Figs. 3e8). The seismic indicators include BSRs, seismic chimneys, high reflection amplitude within the GHSZ, acoustic blankings, enhanced reflection below the BSR, and seafloor expressions that are illustrated in the following. Figure 2 is a distribution map of these seismic indicators in this area. 4.1. The bottom simulating reflector (BSR) In the Ulleung Basin, the BSR is commonly observed over much of the basin (Fig. 8) and is well imaged in the multi-channel seismic sections (Figs. 3 and 4). The BSR runs almost parallel to the seafloor and frequently cross-cuts reflectors. It also has high reflection amplitude and reversed polarity with respect to the seafloor reflection (Fig. 3C). Based on the reflection amplitude strength and continuity, we grouped the BSR into two types such as high amplitude with good continuity and low amplitude with poor continuity. The seismic sections from the southern continental slope associated with mass-transport deposits show continuous to discrete BSRs, following the topography of the corresponding seafloor. An example of a continuous and strong BSR is shown in Figure 3. However, as the water depth deepens to the north, the BSR becomes relatively weaker and less continuous than those in the southern slope (e.g., Fig. 4). The strong BSR is also overlain by localized significantly enhanced reflections (Fig. 5A and B); thus the two acoustic
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Figure 1. (A) Physiography of the East Sea. Insert box is the Ulleung Basin shown in B. (B) Detailed bathymetry and track lines of multi-channel seismic data. Heavy lines denote the selected seismic sections shown in Figures 3e8.
anomalies improve the visibility of the BSR on the seismic profile. There is another well-defined example of a crosscutting-BSR observed in association with a gentle anticline structure in the northwestern basin plain (Fig. 5C), but the reflectivity underneath the flat BSR is not strong as that on the southern slope. 4.2. Seismic chimneys associated with reduced reflectivity The deep-water seismic profiles show a large number of seismic chimneys associated with acoustic blanking. These seismic
indicators are characterized either by relatively narrow vertical zones of significant amplitude reduction, or by disturbed seismic reflection patterns (Figs. 4, 6A, and 7A). Most of the seismic chimneys occur in the northern basin plain where the turbidite/hemipelagic sediments are dominant, but the number of chimney structures is substantially decreased in the southern continental slope of the area where mass-transport deposits form thick sedimentary wedges (Fig. 2). In most cases, the seismic chimneys do not fade out upward but terminate rather abruptly before reaching the seafloor (Figs. 4 and 6A).
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Figure 2. Map showing the distribution of seismic indicators of gas hydrate and gas. The seismic indicators in the Ulleung Basin include a bottom simulating reflector (BSR), a seismic chimney, high reflection amplitude within the gas hydrate stability zone (GHSZ), acoustic blanking, enhanced reflections below the BSR, and seafloor gas-escape features. MTD, mass-transport deposits; BSR, bottom simulating reflector.
The vertical extension of the chimneys varies from less than 10 m to over 100 m but there are only a few chimneys with seafloor expressions (i.e., pockmarks or mounds) (Figs. 6A and 7A). The chimney itself at this site is located in an area of strong sediment deformation (folding) associated with several faults. The chimney structures are noticeable not only because of their low amplitudes but because of their upward-bending internal reflectors (Figs. 4C and 7A). Inside any one seismic chimney, velocities are significantly increased to over 1800 m/s relative to the sediments outside of the chimney structure, which generally show values around 1700e1850 m/s (Ryu et al., 2009; Kim et al., 2013). The seismic chimneys in the basin range from less than 100 m to over 1000 m in width. Their horizontal crosssections are not well determined by 2D seismic data, but their geometries, showing roughly circular shape, are well defined by 3D seismic cube, in which the seismic chimneys show circular features delineated by an amplitude contrast between acoustic blanking and the neighboring reflectors (Kang et al., 2011). Some of these chimneys are associated with vertical faults or fractures. Based on log data at site UBGH2-7 recovered from seismic chimney, resistivity and P-wave velocity within seismic chimney are overall much higher (Fig. 6B). Especially, the resistivity log within chimney structure is characterized by very high values exceeding 100 Ohm-m. The P-wave velocity also reveals relatively high values exceeding 2500 m/s. The gas hydrate saturations at this interval show high values, ranging from 30% to 95% (Fig. 6B). 4.3. High amplitude reflections within the GHSZ Another peculiar phenomenon observed in hydrate bearing sediments is strong amplitude reflections, defined by relatively high reflectivity within the GHSZ (Fig. 8A). Anomalous high amplitude seismic reflections, characterized by well-stratified and seafloor-parallel seismic facies, were observed in the northwestern margin of the basin (Fig. 2). High amplitude reflectors terminate
against a normal fault just above the BSR. A small scale anticline structure is also cut by a strong BSR, which appears locally beneath this anticline. Based on log data at site UBGH2-6, the high amplitude interval (Fig. 8A) is well correlated with high resistivity, velocity, and saturation (Fig. 8B). At Site UBGH2-6, the upper part above w3.125 s is characterized by consistently low resistivity and velocity with little variation, whereas the lower part exhibits high excursions in the both logs (Fig. 8B). The resistivity log reveals a series of 1e2 m thick high resistivity sections (with values as high as 60 Ohm-m) within the interval just above and below the strong amplitude zone, which may indicate gas hydrate-bearing sediments (Fig. 8B). At the interval, the P-wave velocity also shows high values between 1600 m/ s and 1850 m/s. Core sediments within this strong amplitude zone consist mainly of mud interbeded with thin-to-thick sand beds (Ryu et al., 2012; Kim et al., 2013). The thickness and frequency of sand beds are variable and the maximum thickness reaches up to about 50 cm. In the core recovered from UBGH2-6, gas-hydrate-bearing sand (>30 cm thick) layers were found within the interval, corresponding to high amplitude reflection zone (Ryu et al., 2012). The gas hydrate saturations calculated from Archie’s equation at the interval show high values, ranging from 20% to 90% (Fig. 8B). 4.4. Acoustic blanking Acoustic blanking is a common feature in the Ulleung Basin (e.g., those shown in Fig. 4A). As previously mentioned, true amplitude was preserved during data processing, which is important scheme to clearly interpret amplitude blanking. The low amplitude zone occurs widely in the northern basin of turbidite/ hemipelagic sediments, whereas only a few are observed in the southern part of the area (Fig. 2). One typical example of a wide blanking zone up to 20 km wide in well-layered turbidite/hemipelagic sediments is shown in Figure 4. Acoustic blanking along
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Figure 3. Seismic profile from the southern slope area showing BSR and gas seepages (for location, see Fig. 1B). (A) The BSR associated with mass-transport deposits on the southern slope is characterized by relatively high amplitude and high continuity. (B) The BSR closely follows the topography of the corresponding seafloor and (C) its polarity is opposite to the seafloor reflection, as expected for high-velocity hydrate over low-velocity gas. MTD, mass-transport deposits; BSR, bottom simulating reflector.
vertical faults extends to near the seafloor. In some areas, a laterally distributed acoustic blanking zone is observed just above the BSR (Fig. 5A). The acoustic blankings are often seen together with seismic chimneys whose reflectors are upward bending (Fig. 4C), suggesting the possibility of high-velocity hydrate concentrations (Haacke et al., 2009). 4.5. Enhanced reflections below the BSR Enhanced reflections below the BSR are characterized by strong amplitudes suggesting gas charge within a series of dipping strata below a clear BSR (Fig. 5A). In terms of distribution, enhanced reflections occur in a NeS direction, nearly parallel to the bathymetry between 1500 m and 2000 m (Fig. 2). These zones show a strong and well developed BSR that crosscuts the beddings of the deepwater fine-grained sediments dipping to the east, forming a relatively steep continental slope (Fig. 5A). A series of high amplitude reflectors below the BSR appears to be correlated with a low
velocity zone which is a typical seismic response of free gas (KIGAM, 2007). 4.6. Seafloor gas features and seafloor expressions Most seafloor gas-escape features associated with pockmarks or mounds are mainly found in the southern continental slope area, which contains mass-transport deposits (Figs. 2 and 3). Locally disrupted seafloor features associated with vertical faults, characterized by small hyperbolic reflections, are also observed in the southern slope area (Bahk et al., 2013b). Pockmarks associated with seismic chimney are observed in the northern or northeastern area where seismic chimneys are seen (Fig. 6). High-resolution chirp profiles collected from the southern slope reveal that the associated seafloor mounds are high up to 20 m with the maximum slope of 3 to 4 (Bahk et al., 2013b). Bahk et al. (2013b) also suggest that the seafloor mounds are well correlated with the abnormally high multibeam backscatter intensity.
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Figure 4. Seismic profile from the basin plain showing seismic indicators such as BSR, acoustic blanking, and seismic chimneys (for location, see Fig. 1B). (A) and (B) BSR become relatively weaker and less continuous than those in the southern slope (e.g., Fig. 2) and it is hard to be identifiable when running parallel to the near- or flat reflections associated with well-layered turbidite/hemipelagic sediments. An example of a wide blanking zone up to 20 km wide is also seen within well-layered turbidite/hemipelagic sediments. (C) Seismic chimneys show upward-bending internal reflectors result from the velocity pull-up due to the hydrate-bearing sediments. MTD, mass-transport deposits; THS, turbidite-hemipelagic sediments; BSR, bottom simulating reflector.
5. Discussion 5.1. Geophysical evidence for the occurrence of gas and gas hydrate The present study has demonstrated that the Ulleung Basin contains several kinds of seismic indicators related to gas and gas hydrate, including BSRs associated with enhanced reflections, seismic chimneys, acoustic blanking of reduced reflectivity, high amplitude reflectors, and seafloor expressions. The BSR, interpreted to be a physiochemical boundary between hydrates above and gas below, is a major indication of gas hydrate occurrence (Kvenvolden and Barnard, 1983; Shipley et al., 1979; Hyndman and Spence, 1992; Holbrook et al., 1996). In the Ulleung Basin, the BSR is most commonly found in the basin (Fig. 2), and is defined by its reflection polarity opposite to the seafloor, its seafloor-parallel reflection pattern, and its occurrence depth corresponding to the expected base of the GHSZ (Fig. 3C; Lee et al., 2005; KIGAM, 2007). Recently, Ryu et al. (2009) reported that the seafloor reflection coefficient is about 0.10, whereas the calculated BSR reflection coefficient is about 0.04. As a comparison, BSR reflection coefficients calculated in the Ulleung Basin are much lower than those observed in a
number of other areas, e.g., the Cascadia margin offshore Vancouver Island (e.g. Yuan et al., 1999; Chapman et al., 2002), and the Beaufort Sea margin (Andreassen et al., 1995), the Nankai Trough (e.g., Foucher et al., 2002) and Lake Baikal (Vanneste et al., 2001). In general, the BSR reflection coefficients suggest a velocity decrease of about 100 m/s assuming a constant density across the interface. This reflection coefficient suggests either low amounts of high-velocity hydrate above the BSR or very small amounts of low-velocity free gas below the BSR. If the reflection is solely due to overlying highvelocity hydrate, the concentration could be about 10% saturation, using the simple porosity reduction model (e.g., Hyndman and Spence, 1992). However, various researchers have reported that reflection presence and properties appears to be mainly due to underlying concentrations of free gas. These results indicate that the BSR in this area originates at the interface and is characterized by a strong decrease in acoustic impedance due to the presence of free gas below the BSR. In the core recovered from site UBGH1-14 located in the BSR area, the gas hydrate was not nearly founded within the BSR zone (KIGAM, 2007). Thus we suggest that the concentrations of gas hydrate were generally low in the BSR zone, although the occurrence of BSR does imply that some gas hydrate is present.
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Figure 5. Seismic profiles showing the BSR, strong reflections, and enhanced reflections below the BSR (for location, see Fig. 1B). (A) The strong and well developed BSR crosscuts the dipping strata. Additionally, enhanced reflections below the BSR are also seen that may indicate the presence of some free gas. (B) Seismic profile shows strong reflections just above the BSR and enhanced reflections below the BSR. (C) The flat BSR crosscuts gentle anticline structure. MTD, mass-transport deposits; THS, turbidite-hemipelagic sediments; BSR, bottom simulating reflector, ER, enhanced reflection.
The seismic data show numerous localized zones of seismic chimneys associated with reduced reflectivity in sediments (Figs. 4 and 6). In most cases, internal reflectors within seismic chimneys are characterized by pull-up or upward-bending shapes in time sections; these shapes can be interpreted as the result of the presence of high-velocity hydrate in the sediments (Fig. 6B, Haacke et al., 2009). Detailed velocity analysis showed strongly increased interval velocities inside seismic chimneys defined by pull-up structures. Previous studies (Lee et al., 2005; KIGAM, 2007; Ryu et al., 2009) also showed increased interval velocities within seismic chimneys; these increased velocities are associated with pull-up structures due to hydrate saturation. High seismic velocities compared to those of normal sediments are the most important indicator of gas hydrate (Lee et al., 1993; Singh et al., 1993;
Diaconescu et al., 2001). Seismic methods of gas hydrate quantification are based on the assumption that, compared to the normal increase of velocity with depth due to sediment compaction, the Pwave (Vp) of sediments containing hydrates increases with hydrate saturation above the BSR, and the velocity decrease below the BSR is due to the presence of gas (Lee et al., 1993). Seismically similar features have been investigated in a variety of geographic areas, including the Niger Delta (Hovland et al., 1997), the Norwegian margin (Hovland and Svensen, 2006), and the Cascadia margin (Riedel et al., 2006). According to these studies, features such as those in the study area have been shown to contain gas hydrate. This interpretation is applicable to our examples from the Ulleung Basin, where stacking-velocity analysis shows that the chimney structures correspond to positive velocity anomalies likely due to
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Figure 6. (A) Seismic profile across the site UBGH2-7 located in the northwestern basin plain showing seismic chimney associated with pockmark and mound structure (for location, see Fig. 1B). (B) Diagrams showing P-wave velocity, resistivity, and gas-hydrate saturation at site UBGH2-7 (for location, see Fig. 1B). Just beneath the site UBGH2-7 the diapir-like structure associated with possible fluid or gas migration can be seen. MTD, mass-transport deposits; THS, turbidite-hemipelagic sediments; BSR, bottom simulating reflector.
high-velocity gas hydrate (Park et al., 2008; Haacke et al., 2009; Ryu et al., 2009). Recent near-seafloor gas hydrate discoveries from some seismic chimneys associated with velocity pull-up structures support our interpretation (Ryu et al., 2012). Similar chimney structures were surveyed on the northern Cascadia Margin and a massive reservoir of gas hydrate was also drilled and sampled by the IODP Expedition 311 (Riedel et al., 2006). Along with seismic chimneys, strong reflection amplitude, defined by relatively high reflectivity, within the GHSZ is a major indication of gas hydrate occurrence in the Ulleung Basin. Based on well-log data, high amplitude interval is well correlated with high resistivity, velocity, and saturation (Fig. 8B). The resistivity log reveals a series of 1e2 m thick high resistivity sections just above
and below the strong amplitude zone, which may indicate gas hydrate-bearing sand beds (Fig. 8B; Ryu et al., 2012). At the interval, the P-wave velocity also shows high values between 1600 m/s and 1850 m/s. In the core recovered from site UBGH2-6, gas-hydratebearing sand (>30 cm thick) layers were founded within the interval, corresponding to high amplitude reflection zone (Ryu et al., 2012). The gas hydrate saturations at the interval show high values, ranging from 29% to 90% (Fig. 8B). Also, the gas hydrate saturations estimated from chlorinity excursions in the gas-hydrate-bearing zone range from 12% to 80%, averaging about 52% (Ryu et al., 2012). Thus, the strong reflection amplitude in the GHSZ should be expected within the gas hydrate layer and is often used as an indicator of gas hydrate.
Figure 7. (A) Seismic profile from the southern slope area shows seismic chimneys with mound structure (for location, see Fig. 1B). (B) Diagrams showing P-wave velocity, resistivity, and gas-hydrate saturation at site UBGH2-3 (for location, see Fig. 1B). MTD, mass-transport deposits; THS, turbidite-hemipelagic sediments; BSR, bottom simulating reflector.
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Figure 8. (A) Seismic profile across the site UBGH2-6 located in the northwestern part of the area showing the strong amplitude reflections just above the BSR (for location, see Fig. 1B). (B) Well-log data showing P-wave velocity, resistivity, and gas-hydrate saturation at site UBGH2-6 (for location, see Fig. 1B). Note that the high amplitude interval is well correlated with high velocity, resistivity, and gas-hydrate saturations. Just south of the site UBGH2-6 normal faults can be seen. MTD, mass-transport deposits; THS, turbiditehemipelagic sediments; BSR, bottom simulating reflector.
Acoustic blanking can help to discriminate gas hydrate bearing sediments (Dillon et al., 1994). Gas hydrate layers commonly display acoustic blanking effects, i.e. reduced acoustic impedance contrasts within the hydrated sediments, presumably due to the cementation of the stratal interfaces by the gas hydrate molecules (Shipley et al., 1979; Judd and Hovland, 1992; Hovland et al., 1997; Wood and Ruppel, 2000; Diaconescu et al., 2001; Vanneste et al., 2001). Diaconescu et al. (2001) also reported a blanking zone associated with a high velocity anomaly (w2100 m/s) in the Caspian Basin. Thus a reduction of the seismic amplitudes is often used as an indicator of gas hydrate. However, as the results of UBGH2, the acoustic blanking indicated minimal gas hydrate in fine-grained sediments (Ryu et al., 2012). Consequently, the acoustic blanking within the GHSZ could be either the result of physical properties of sediment itself, or a gas hydrate induced reflection anomaly. Enhanced reflections below the BSR are defined by a high acoustic impedance contrast on seismic profiles. The presence of gas will be shown in seismic data as a high amplitude anomaly (Holbrook et al., 1996; Wood and Ruppel, 2000). The gasbearing sediments can generate anomalously high amplitude reflections because even small amounts of gas (less than 2%) may cause a marked P-wave velocity drop (Domenico, 1976). In the Ulleung Basin, enhanced reflections are recognized just below the BSR, suggesting the presence of free gas in turbidite/ hemipelagic sediments (Fig. 5A). Based on the drilling results of UBGH2 (Ryu et al., 2012), however, the occurrence of enhanced reflections was not directly correlated with the existence of free gas below the GHSZ. Thus we think that the enhanced reflection attribute could be either the result of characteristics of sediment itself, or a free gas induced reflection anomaly, although the existence of enhanced reflections does imply that some free gas is present.
5.2. Gas migration into the GHSZ Gas fluids in deep marine sediments may migrate into the GHSZ through favored conduits (e.g., faults, fractures, diapirs and other permeable pathways; Nimblett and Ruppel, 2003; Hornbach et al., 2004; Trehu et al., 2004; Cook et al., 2008). Figure 9 summarizes two styles of migration pathways for fluid-gas upwelling into the GHSZ: (1) structural conduits including fault and fracture systems associated with seismic chimneys and (2) stratigraphic conduits mainly consisting of inclined turbidite/hemipelagic layers. The deepwater seismic profiles show a large number of vertical faults, which probably focus fluid flux (Fig. 6). Previous researchers (Lee et al., 2005; KIGAM, 2007; Horozal et al., 2009; Ryu et al., 2009) have suggested that, as a migration pathway of fluid-gas into the GHSZ, the fault or fracture systems in this area are very important for the formation of gas hydrate. Similar features are commonly recognized in seismic sections as migration pathways of gas or fluid in other regions (Heggland, 1997; Dillon et al., 1998; Judd and Hovland, 1992; Riedel et al., 2002, 2006; Cook et al., 2008). Weinberger and Brown (2006) also reported that complicated fracture networks are the primary method of fluid flow for free gas at the Hydrate Ridge, offshore Oregon. More recently, Barnes et al. (2009) reported that fault or fracture systems are commonly interpreted as important conduits for the upward flow of fluid or gas. We thus conclude that vertical faults or fractures associated with seismic chimneys in the basin have played an important role in gas-hydrate accumulations as structural conduits of fluid or gas that originates from below the GHSZ (Fig. 9; KIGAM, 2007; Ryu et al., 2012). The western margin of the Ulleung Basin shows an example of stratigraphic conduits related to inclined turbidite/hemipelagic layers. The seismic section shown in Figure 5A indicates that the BSR crosscuts the dipping strata, which probably consist of alternating beds of sand and mud. Hustoft et al. (2007) suggested that enhanced
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Figure 9. A schematic diagram showing migration pathways of gas-fluids and occurrence types of gas hydrate as well as various seismic signatures of gas hydrate and gas identified in the Ulleung Basin. Note that gasefluids migrate into the GHSZ through two distinct pathways: (1) structural conduits including fault and fracture systems associated with seismic chimneys (blue arrow) and (2) stratigraphic conduits mainly consisting of inclined, permeable turbidite/hemipelagic layers (red arrow). Two modes of gas hydrate accumulation were identified in the deep sea basin: (1) a stratally-bound type (pore filling) within turbidite sand beds and (2) a locally concentrated type (massive, nodule, and fracture filling) within upward-growing chimneys associated with near vertical faults. MTD, mass-transport deposits; THS, turbidite-hemipelagic sediments; BSR, bottom simulating reflector.
reflections below the BSR caused by free gas in the Norwegian margin are correctly correlated with permeable, coarse-grained layers; the free gas migrates into the GHSZ through the permeable layers. Baba and Yamada (2004) also reported that gas-bearing sediments consisting of alternating beds of sand and mud developed in the Nankai Trough and that gas-fluids migrate into the GHSZ through permeable sand layers. Similar acoustic features were also reported in the Storegga area by Bouriak et al. (2000), who believed that these anomalies are caused by fluid or gas expulsion though the dipping strata. In the Ulleung Basin, we also suggest that the inclined, permeable layers characterized by enhanced reflections have played an important role as stratigraphic conduits for the upward migration of fluid or gas that originates from below the GHSZ (Fig. 9).
5.3. Gas hydrate occurrence in the Ulleung Basin From ten drill sites during the UBGH2, various types of gas hydrate were recovered (Ryu et al., 2012). Based on onboard and post-cruise analysis, gas hydrate recovered from the Ulleung Basin can be grouped into two types: (1) a stratally-bound (pore filling) hydrate within turbidite sand layers and (2) a locally concentrated hydrate within upward growing chimneys associated with near vertical faults, such as massive, nodule, and fracture filling types (Fig. 10). Figure 10B shows an example of pore-filling type hydrate in turbidite sand beds. Based on seismic and well-log data, high amplitude reflection zone was observed within the GHSZ and this interval is well correlated with significant high values of resistivity,
Figure 10. Two types of gas hydrate recovered from the Ulleung Basin, East Sea in 2010 during the UBGH2. (A) local concentrated type within vertical chimneys such as fracture filling, massive, and nodule type hydrate recovered from the UBGH2-7, (B) statally-bound (pore filling) type hydrate in sand layer observed by IR image. The sample was recovered from the UBGH2-2-1 (for location, see Fig. 1B).
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velocity, and hydrate saturations (Fig. 8). Also, in the core recovered from UBGH2-6, gas-hydrate-bearing sand (>30 cm thick) layers were founded within the interval, corresponding to high amplitude reflection zone (Fig. 10B; Ryu et al., 2012). These results support that high amplitude zone associated with well-stratified reflectors are considered to result from gas-bearing sands, as a stratallybound (pore filling) type (Figs. 9 and 10B). In the western margin of the basin, enhanced reflections within alternating strata are abruptly terminated by the strong BSR and are in turn overlain by acoustic blanking of reduced reflectivity (Fig. 5A). The acoustic blanking zone just above the BSR is well correlated with the high velocity zone, strongly suggesting the presence of high-velocity gas hydrate, whereas the area of enhanced reflections below corresponds to a relatively low velocity zone (KIGAM, 2007; Horozal et al., 2009). On the other hand, a wide blanking zone associated with reduced reflectivity in the turbidite/hemipelagic sediments occurs within the GHSZ (Fig. 4). Based on the velocity analysis, the blanking zone was found to correspond to a positive velocity anomaly due to the high-velocity gas hydrate (KIGAM, 2007; Ryu et al., 2009). The blanking zone is also connected with vertical faults or chimneys as migration pathways for fluid-gas upwelling into the GHSZ. We conclude that such an acoustic blanking zone may also indicate a concentrated zone of gas hydrate in the Ulleung Basin, as shown in Figure 9. Figure 10A shows examples of vertically concentrated hydrate such as massive, nodule, and fracture filling types. Chimney structures are commonly seen in deep-water basins, suggesting gas hydrate concentrations (Figs. 2, 4 and 6). Near-surface hydrates have been discovered in piston corings from the basin plain (Bahk et al., 2009). At one site, a 130 m thick layer of hydrate-bearing sediments was recovered in localized chimney structures (Park et al., 2008; Kim et al., 2011; Ryu et al., 2012). One well-studied example associated with vertical chimneys is the Bullseye structure off western Canada (Riedel et al., 2002). In the Bullseye structure, a massive quantity of gas hydrate was piston cored just below the seafloor. Drilling by IODP shows a w50 m thick layer of gas hydrate extending from just below the seafloor (Riedel et al., 2006). Schwalenberg et al. (2005) also reported that a number of chimney structures in that area contain high concentrations of gas hydrate, mostly near the seafloor in areas associated with vertical chimneys. We thus suppose that relatively high concentrations of gas hydrates in the Ulleung Basin tend to occur in localized chimney structures associated with vertical faults, as a locally concentrated mode. 6. Conclusions 1. Based on the multi-channel seismic data, the seismic evidences for the gas hydrate and associated gas in the Ulleung Basin include (a) BSRs, (b) a seismic chimney, (c) high reflection amplitude within the gas hydrate stability zone (GHSZ), (d) acoustic blanking, (e) enhanced reflections below the BSR, and (f) seafloor gas-escape features. 2. The BSR is most commonly found over much of the basin and is defined by its reflection polarity, which is opposite to that of the seafloor, and by its seafloor-parallel reflection. Seismic chimneys associated with velocity pull-up appear to be caused by active migration of fluid gas vertically into the gas hydrate stability zone. Another seismic characteristic observed in gas hydrate bearing sediments is the strong reflection amplitude, defined by the relatively high reflectivity within the GHSZ. Acoustic blanking is likely to be attributed to hydrate cementation in the sediments, which causes a significant reduction of acoustic impedance contrast. Enhanced reflections immediately below the BSR indicate the presence of gas. Sub-circular depressed/domed seafloor features may represent gas
seepages manifested by the occurrence of mud mounds and pockmarks through which gas escapes into the water column. 3. Gas-fluids migrate into the GHSZ through two distinct pathways: (1) structural conduits including fault and fracture systems associated with seismic chimneys and (2) stratigraphic conduits mainly consisting of inclined turbidite/hemipelagic layers. 4. Two types of gas hydrate accumulation were identified in the Ulleung Basin: (1) a stratally-bound type (pore filling) within turbidite sand beds and (2) a locally concentrated type (massive, nodule or fracture filling) within upward-growing chimneys associated with near vertical faults. Relatively high concentrations of gas hydrate, however, tend to occur in localized gas chimneys, rather than in the BSR-related features. Acknowledgments This study is a contribution of the gas hydrate R&D project of Ministry of Trade, Industry and Energy, Korea. We thank the Ministry of Knowledge Economy (MKE) and Gas Hydrate R/D Organization (GHDO) for funding. We thank the crews and scientific parties onboard the R/V Tamhae II during the seismic survey. We also wish to thank those that contributed to the success of the Second Gas Hydrate Drilling Expedition in the Ulleung Basin (UBGH2). References Andreassen, K., Hart, P.E., Grantz, A., 1995. Seismic studies of a bottom simulating reflection related to gas hydrate beneath the continental margin of the Beaufort Sea. J. Geophy. Res. 100, 12659e12673. Baba, K., Yamada, Y., 2004. BSR and associated reflections as an indicator of gas hydrate and free gas accumulation: an example of Accretionary Prism and Forearc Basin system along the Nankai Trough, off Central Japan. Resource Geol. 54, 11e24. Bahk, J.J., Kim, J.H., Kong, G.S., Park, Y.S., Lee, H., Park, Y., Park, K.P., 2009. Occurrence of near-seafloor gas hydrates and associated cold vents in the Ulleung Basin, East Sea. Geosci. J. 13, 371e385. Bahk, J.J., Kim, G.Y., Chun, J.H., Kim, J.H., Lee, J.Y., Ryu, B.J., Lee, J.H., Son, B.K., Collett, C., Riedel, M., 2013a. Characterization of gas hydrate reservoirs by integration of core and log data in the Ulleung Basin, East Sea. J. Marine Pet. Geol. 47, 30e42. Bahk, J.J., Yoo, D.G., Um, I.K., Lee, S.H., Woo, K.S., 2013b. Sedimentation and seafloor mound formation in the southern slope of the Ulleing Basin, East Sea, Korea, since the Last Glacial Maximum. Geosci. J. 17, 151e161. Barnes, P.M., Lamarche, G., Bialas, J., Henrys, S., Pecher, I., Netzeband, G.L., Greinert, J., Mountjoy, J.J., Pedley, K., Crutchley, G., 2009. Tectonic and geological framework for gas hydrates and cold seeps on the Hikurangi subduction margin, New Zealand. Mar. Geol.. http://dx.doi.org/10.1016/j.margeo.2009.03.012. Bouriak, S., Vanneste, M., Saoutkine, A., 2000. Inferred gas hydrates and clay diapirs near the Storegga Slide on the southern edge of the Voring Plateau, offshore Norway. Mar. Geol. 163, 125e148. Bünz, S., Mienert, J., Berndt, C., 2003. Geological controls on the Storegga gas-hydrate system of the mid-Norwegian continental margin. Earth Planet. Sci. Lett. 209, 291e307. Chapman, N.R., Gettrust, J., Walia, R., Hannay, D., Spence, G.D., Wood, W., Hyndman, R.D., 2002. High resolution deep-towed multichannel seismic survey of deep sea gas hydrates off western Canada. Geophysics 67, 1038e1047. Chough, S.K., Barg, E., 1987. Tectonic history of the Ulleung Basin margin, East Sea (Sea of Japan). Geology 15, 45e48. Chough, S.K., Lee, S.H., Kim, J.W., Park, S.C., Yoo, D.G., Han, H.S., Yoon, S.H., Oh, S.B., Kim, Y.B., Back, G.G., 1997. Chirp (2e7 kHz) echo characters in the Ulleung Basin. Geosci. J. 1, 143e153. Cook, A.E., Goldberg, D., Kleinberg, L., 2008. Fracture-controlled gas hydrate systems in the northern Gulf of Mexico. Mar. Pet. Geol. 25, 932e941. Diaconescu, C.C., Kieckhefer, R.M., Knapp, J.H., 2001. Geophysical evidence for gas hydrates in the deep water of the South Caspian Basin, Azerbaijan. Mar. Pet. Geol. 18, 209e221. Dillon, W.P., Lee, M.W., Coleman, D.F., 1994. Identification of marine hydrates in situ and their distribution off the Atlantic Coast of the United States. In: Sloan, Tr., Happep, J., Hnatow, M.A. (Eds.), Internal Conference on Natural Gas Hydrates, Annals of the New York Academy of Science 715, pp. 364e380. Dillon, W.P., Danforth, W.W., Hutchinson, D.R., Drury, R.M., Taylor, M.H., Booth, J.S., 1998. Evidence for faulting related to dissociation of gas hydrate and release of methane off the southeastern United States. In: Henriet, J.P., Mienert, J. (Eds.), Gas Hydrates-relevance to World Margin Stability and Climate Change, Geol. Soc. Special Publ. 137, pp. 293e302. Domenico, S.N., 1976. Effect of brine-gas mixture on velocity in an unconsolidated sand reservoir. Geophys. 42, 882e895.
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Occurrence and seismic characteristics of gas hydrate in the Ulleung Basin, East Sea Dong G. Yoo a, *, Nyeon K. Kang a, Bo Y. Yi a, Gil Y. Kim a, Byong J. Ryu a, Keumsuk Lee b, Gwang H. Lee c, M. Riedel d a
Petroleum & Marine Research Division, Korea Institute of Geoscience and Mineral Resources (KIGAM), 124 Gwahang-no, Yuseong-gu, Daejeon 305-350, Republic of Korea Korea National Oil Corporation (KNOC), Anyang 431-711, Republic of Korea c Department of Energy Resources Engineering, Pukyoung National University, Busan 608-737, Republic of Korea d National Resources Canada, Geological Survey of Canada, 9860 West Saanich Road 16, Sidney, BC V8L 4B2, Canada b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 February 2013 Received in revised form 1 July 2013 Accepted 2 July 2013 Available online 12 July 2013
Multi-channel seismic reflection and well-log data from the Ulleung Basin, East Sea reveal several seismic signatures indicative of gas-hydrate occurrence in the Ulleung Basin that are associated with vertically and/or laterally stacked mass-transport complexes. The seismic indicators include (a) a bottom simulating reflector (BSR), (b) a seismic chimney, (c) high amplitude reflections within the gas hydrate stability zone (GHSZ), (d) acoustic blanking, (e) enhanced reflections below the BSR, and (f) seafloor gas-escape features. The BSR, associated with enhanced reflections below, is most commonly found over much of the basin indicating a physiochemical boundary of gas hydrates overlying free gas. Seismic chimneys are characterized by velocity pull-up and reduced reflectivity on the seismic sections, which appear to be caused by active migration of fluid gas vertically into the GHSZ. The logging data retrieved from the seismic chimneys showed elevated electrical resistivity (>80 Ohm-m) and P-wave velocity (>2000 m/s), indicating the presence of gas hydrate. Another seismic characteristic observed in gas hydrate bearing sediments is the strong amplitude reflections, defined by the relatively high reflectivity within the GHSZ. Acoustic blanking is likely to be the result of hydrate accumulation in the sediments causing a significant reduction of acoustic impedance contrast between sedimentary layers. Where the upward migrating gas seeps into the deep water column, seafloor pockmarks and mud mounds may be formed. Gas hydrate was recovered from the Ulleung Basin, East Sea in 2010 during the Second Ulleung Basin Gas Hydrate Drilling Expedition (UBGH2) under the Korean National Gas Hydrate Program. Based on the results, gas-fluids migrate into the GHSZ through two distinct pathways: (1) structural conduits which include fault and fracture systems associated with seismic chimneys and (2) stratigraphic conduits associated with inclined turbidite/hemipelagic layers. Two types of gas-hydrate occurrence were identified in the basin: (1) a stratally-bound type (pore filling) within turbidite sand layers and (2) a locally concentrated type (massive, nodule or fracture filling) within upward-growing chimneys associated with near vertical faults. Relatively high concentrations of gas hydrate, however, tend to occur in localized seismic chimneys, rather than in the strata-related features. The successful recovery indicates that the Ulleung Basin provides favorable conditions for gas-hydrate formation. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Seismic indicators of gas hydrate Occurrence types Multi-channel seismic data Ulleung Basin
1. Introduction Occurrences of marine gas hydrates are well known from numerous geophysical and geological studies (e.g. Hyndman and Spence, 1992; Kvenvolden, 1993; Lee et al., 1993; Xu and Ruppel, 1999). In seismic sections, the presence of gas hydrate can be * Corresponding author. Tel.: þ82 42 86 3324; fax: þ82 42 868 3417. E-mail address: [email protected] (D.G. Yoo). 0264-8172/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.marpetgeo.2013.07.001
identified by the existence of Bottom Simulating Reflectors (BSRs), which have high amplitude and reverse polarity and tend to be parallel to the seafloor topographic surface (Shipley et al., 1979). Therefore, a BSR remains perhaps the most common indicator of gas hydrates, although it may not be essential for gas hydrate recovery (Kvenvolden and Barnard, 1983; Shipley et al., 1979; Hyndman and Spence, 1992; Holbrook et al., 1996). The existence of gas hydrate within sediment pore space may reduce the acoustic impedance contrasts of sedimentary layers, leading to a decrease in
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seismic reflectivity (i.e., acoustic blanking) above a BSR (Shipley et al., 1979; Hovland et al., 1997; Wood and Ruppel, 2000; Diaconescu et al., 2001). Seismic chimneys or near vertical wipeouts associated with seismic blanking are also seen in seismic data as a gas hydrate indicator, together with pockmarks or mounds on the seafloor in many areas (Ginsburg, 1998; Hyndman et al., 2001; Riedel et al., 2006). The presence of free gas below the BSR also may be inferred from the observation of enhanced reflections, which show high-acoustic impedance contrast in seismic profiles (Holbrook et al., 1996; Wood and Ruppel, 2000; Bünz et al., 2003; Hustoft et al., 2007). The probable existence of gas hydrates in the Ulleung Basin has been suggested by several seismic indicators including BSR, the acoustic blanking zone, and seismic chimneys (Gardner et al., 1998; Lee et al., 2005; KIGAM, 2007; Ryu et al., 2009; Bahk et al., 2013a). Horozal et al. (2009) identified various seismic indicators of gas hydrate and free gas in the Ulleung Basin. Haacke et al. (2009) attempted to relate the seismic images of chimney structures in the deep-water basin to the concentration of gas hydrates. Mass-transport deposits associated with gas hydrate occurrence in the basin were documented by Riedel et al. (2012) and Scholz et al. (2012). Although the recent recovery of gas hydrate by drilling and piston coring has confirmed the gas hydrate accumulations in the central basin, the regionally distributed characteristics of the seismic indicators of gas and gas hydrates for the entire basin have been only rarely documented. In the present study we interpreted multi-channel seismic reflection and well-log data from the East Sea off Korea to identify and document the seismic characteristics related to gas hydrate and gas. We also present the occurrence types of gas hydrate associated with migration pathways for fluid-gas upwelling into the GHSZ. 2. Geologic setting The East Sea (Fig. 1A) is a back-arc basin bounded by the eastern Asian continent in the west and the Japan islands both in the east and the south. The semi-enclosed basin consists of three deep-water basins (Japan, Yamato, and Ulleung Basins), separated by submerged continental remnants (i.e., the Korea Plateau, Oki Bank, Yamato Ridge, and Kita-Yamato Ridge). The Ulleung Basin, a bowl-shaped basin, is located in the southwestern East Sea adjacent to the eastern coast of Korea and is bounded by the steep continental slope of the Korean Peninsula to the west and by the rugged Korea Plateau to the north (Fig. 1B). The gentle slopes of the Oki Bank and the Japanese Islands form the eastern and southeastern margins of the basin, respectively. The seafloor is generally smooth and deepens progressively to the northeast; water depths range from less than 1500 m in the south to over 2300 m in the northeast where the basin joins the deeper Japan Basin through a long and narrow interplain gap between the two islands of Ulleungdo and Dokdo (Fig. 1B). The Ulleung Basin was formed during the Late Oligocene to Early Miocene by crustal extension associated with the southward drift of the Japan Islands (Tamaki et al., 1992; Jolivet et al., 1995; Chough and Barg, 1987). At the end of the Middle Miocene, the tectonic stress regime changed from tensional to compressional (Yoon and Chough, 1995), leading to thrust faultings and foldings in the southern and western margin of the basin and to the sediment compression and consolidation that probably has been responsible for the upward flow of gas-rich fluids and hydrate formation (Ryu et al., 2009). Since this time of tectonic movement, the basin has progressively subsided until the present and has been fed with a significant quantity of siliciclastic sediments (e.g., debris-flow deposits), filling the larger part of the basin. In the Ulleung Basin, the Neogene sediments are characterized by vertically/laterally widespread deposition of mass-transport complexes, caused by margin-wide slope failures in response to back-arc closure (Lee and
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Suk, 1998). Mass transport deposits in the basin show a zoned distribution pattern that approximately follows the bathymetric contours: slide/slump deposits on the upper slope, debris-flow deposits on the lower slope or base-of-slope, and turbidites on the northern basin (Chough et al., 1997). 3. Data The total of 6600 L-km of multi-channel seismic reflection data used in this study were collected by the Korea Institute of Geoscience and Mineral Resources (KIGAM) using the research vessel Tamhae II in 2005 for the exploration of gas hydrate resources in the Ulleung Basin, East Sea offshore of Korea (Fig. 1B). The seismic data acquisition included a 240-channel and 3-km long streamer with a 1035 in3 air-gun array that provides a wide range of frequency up to 250 Hz (KIGAM, 2007). The receiver-group interval, and the near and far offsets are 12.5 m, 162 m, and 3149 m, respectively, yielding 60-fold coverage. The distance between shots is 25 m, resulting in a CDP spacing of 6.25 m. The data were digitally recorded with a sample rate of 1 ms and a maximum recording length of 7 s. The seismic data were processed at KIGAM (KIGAM, 2007). True amplitude was preserved during data processing, which is important scheme to clearly interpret amplitude blanking and seismic chimneys. Data processing included resampling, anti-aliasing filtering, bandpass filtering, true amplitude recovery (spherical divergence correlation), velocity analysis, normal moveout correlation, and stacking. The interpretation of the seismic data was performed using Geographix manufactured by Landmark. The Second Ulleung Basin Gas Hydrate Expedition (UBGH2) in 2010 conducted LWD and coring at 13 sites to identify the overall distribution of gas hydrate in the Ulleung Basin (Fig. 1B; Ryu et al., 2012; Bahk et al., 2013b). The LWD phase was conducted using the Schlumberger’s logging tools of the GeoVision, TeleScope, EcoScope and SonicVision. LWD log data include natural gamma, resistivity, velocity, porosity, and density. 4. Seismic characteristics of gas hydrate and associated gas On the basis of the multi-channel seismic data, we identified six seismic signatures indicative of gas-hydrate existence in the deepwater basin filled with vertically and/or laterally stacked masstransport complexes (Figs. 3e8). The seismic indicators include BSRs, seismic chimneys, high reflection amplitude within the GHSZ, acoustic blankings, enhanced reflection below the BSR, and seafloor expressions that are illustrated in the following. Figure 2 is a distribution map of these seismic indicators in this area. 4.1. The bottom simulating reflector (BSR) In the Ulleung Basin, the BSR is commonly observed over much of the basin (Fig. 8) and is well imaged in the multi-channel seismic sections (Figs. 3 and 4). The BSR runs almost parallel to the seafloor and frequently cross-cuts reflectors. It also has high reflection amplitude and reversed polarity with respect to the seafloor reflection (Fig. 3C). Based on the reflection amplitude strength and continuity, we grouped the BSR into two types such as high amplitude with good continuity and low amplitude with poor continuity. The seismic sections from the southern continental slope associated with mass-transport deposits show continuous to discrete BSRs, following the topography of the corresponding seafloor. An example of a continuous and strong BSR is shown in Figure 3. However, as the water depth deepens to the north, the BSR becomes relatively weaker and less continuous than those in the southern slope (e.g., Fig. 4). The strong BSR is also overlain by localized significantly enhanced reflections (Fig. 5A and B); thus the two acoustic
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Figure 1. (A) Physiography of the East Sea. Insert box is the Ulleung Basin shown in B. (B) Detailed bathymetry and track lines of multi-channel seismic data. Heavy lines denote the selected seismic sections shown in Figures 3e8.
anomalies improve the visibility of the BSR on the seismic profile. There is another well-defined example of a crosscutting-BSR observed in association with a gentle anticline structure in the northwestern basin plain (Fig. 5C), but the reflectivity underneath the flat BSR is not strong as that on the southern slope. 4.2. Seismic chimneys associated with reduced reflectivity The deep-water seismic profiles show a large number of seismic chimneys associated with acoustic blanking. These seismic
indicators are characterized either by relatively narrow vertical zones of significant amplitude reduction, or by disturbed seismic reflection patterns (Figs. 4, 6A, and 7A). Most of the seismic chimneys occur in the northern basin plain where the turbidite/hemipelagic sediments are dominant, but the number of chimney structures is substantially decreased in the southern continental slope of the area where mass-transport deposits form thick sedimentary wedges (Fig. 2). In most cases, the seismic chimneys do not fade out upward but terminate rather abruptly before reaching the seafloor (Figs. 4 and 6A).
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Figure 2. Map showing the distribution of seismic indicators of gas hydrate and gas. The seismic indicators in the Ulleung Basin include a bottom simulating reflector (BSR), a seismic chimney, high reflection amplitude within the gas hydrate stability zone (GHSZ), acoustic blanking, enhanced reflections below the BSR, and seafloor gas-escape features. MTD, mass-transport deposits; BSR, bottom simulating reflector.
The vertical extension of the chimneys varies from less than 10 m to over 100 m but there are only a few chimneys with seafloor expressions (i.e., pockmarks or mounds) (Figs. 6A and 7A). The chimney itself at this site is located in an area of strong sediment deformation (folding) associated with several faults. The chimney structures are noticeable not only because of their low amplitudes but because of their upward-bending internal reflectors (Figs. 4C and 7A). Inside any one seismic chimney, velocities are significantly increased to over 1800 m/s relative to the sediments outside of the chimney structure, which generally show values around 1700e1850 m/s (Ryu et al., 2009; Kim et al., 2013). The seismic chimneys in the basin range from less than 100 m to over 1000 m in width. Their horizontal crosssections are not well determined by 2D seismic data, but their geometries, showing roughly circular shape, are well defined by 3D seismic cube, in which the seismic chimneys show circular features delineated by an amplitude contrast between acoustic blanking and the neighboring reflectors (Kang et al., 2011). Some of these chimneys are associated with vertical faults or fractures. Based on log data at site UBGH2-7 recovered from seismic chimney, resistivity and P-wave velocity within seismic chimney are overall much higher (Fig. 6B). Especially, the resistivity log within chimney structure is characterized by very high values exceeding 100 Ohm-m. The P-wave velocity also reveals relatively high values exceeding 2500 m/s. The gas hydrate saturations at this interval show high values, ranging from 30% to 95% (Fig. 6B). 4.3. High amplitude reflections within the GHSZ Another peculiar phenomenon observed in hydrate bearing sediments is strong amplitude reflections, defined by relatively high reflectivity within the GHSZ (Fig. 8A). Anomalous high amplitude seismic reflections, characterized by well-stratified and seafloor-parallel seismic facies, were observed in the northwestern margin of the basin (Fig. 2). High amplitude reflectors terminate
against a normal fault just above the BSR. A small scale anticline structure is also cut by a strong BSR, which appears locally beneath this anticline. Based on log data at site UBGH2-6, the high amplitude interval (Fig. 8A) is well correlated with high resistivity, velocity, and saturation (Fig. 8B). At Site UBGH2-6, the upper part above w3.125 s is characterized by consistently low resistivity and velocity with little variation, whereas the lower part exhibits high excursions in the both logs (Fig. 8B). The resistivity log reveals a series of 1e2 m thick high resistivity sections (with values as high as 60 Ohm-m) within the interval just above and below the strong amplitude zone, which may indicate gas hydrate-bearing sediments (Fig. 8B). At the interval, the P-wave velocity also shows high values between 1600 m/ s and 1850 m/s. Core sediments within this strong amplitude zone consist mainly of mud interbeded with thin-to-thick sand beds (Ryu et al., 2012; Kim et al., 2013). The thickness and frequency of sand beds are variable and the maximum thickness reaches up to about 50 cm. In the core recovered from UBGH2-6, gas-hydrate-bearing sand (>30 cm thick) layers were found within the interval, corresponding to high amplitude reflection zone (Ryu et al., 2012). The gas hydrate saturations calculated from Archie’s equation at the interval show high values, ranging from 20% to 90% (Fig. 8B). 4.4. Acoustic blanking Acoustic blanking is a common feature in the Ulleung Basin (e.g., those shown in Fig. 4A). As previously mentioned, true amplitude was preserved during data processing, which is important scheme to clearly interpret amplitude blanking. The low amplitude zone occurs widely in the northern basin of turbidite/ hemipelagic sediments, whereas only a few are observed in the southern part of the area (Fig. 2). One typical example of a wide blanking zone up to 20 km wide in well-layered turbidite/hemipelagic sediments is shown in Figure 4. Acoustic blanking along
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Figure 3. Seismic profile from the southern slope area showing BSR and gas seepages (for location, see Fig. 1B). (A) The BSR associated with mass-transport deposits on the southern slope is characterized by relatively high amplitude and high continuity. (B) The BSR closely follows the topography of the corresponding seafloor and (C) its polarity is opposite to the seafloor reflection, as expected for high-velocity hydrate over low-velocity gas. MTD, mass-transport deposits; BSR, bottom simulating reflector.
vertical faults extends to near the seafloor. In some areas, a laterally distributed acoustic blanking zone is observed just above the BSR (Fig. 5A). The acoustic blankings are often seen together with seismic chimneys whose reflectors are upward bending (Fig. 4C), suggesting the possibility of high-velocity hydrate concentrations (Haacke et al., 2009). 4.5. Enhanced reflections below the BSR Enhanced reflections below the BSR are characterized by strong amplitudes suggesting gas charge within a series of dipping strata below a clear BSR (Fig. 5A). In terms of distribution, enhanced reflections occur in a NeS direction, nearly parallel to the bathymetry between 1500 m and 2000 m (Fig. 2). These zones show a strong and well developed BSR that crosscuts the beddings of the deepwater fine-grained sediments dipping to the east, forming a relatively steep continental slope (Fig. 5A). A series of high amplitude reflectors below the BSR appears to be correlated with a low
velocity zone which is a typical seismic response of free gas (KIGAM, 2007). 4.6. Seafloor gas features and seafloor expressions Most seafloor gas-escape features associated with pockmarks or mounds are mainly found in the southern continental slope area, which contains mass-transport deposits (Figs. 2 and 3). Locally disrupted seafloor features associated with vertical faults, characterized by small hyperbolic reflections, are also observed in the southern slope area (Bahk et al., 2013b). Pockmarks associated with seismic chimney are observed in the northern or northeastern area where seismic chimneys are seen (Fig. 6). High-resolution chirp profiles collected from the southern slope reveal that the associated seafloor mounds are high up to 20 m with the maximum slope of 3 to 4 (Bahk et al., 2013b). Bahk et al. (2013b) also suggest that the seafloor mounds are well correlated with the abnormally high multibeam backscatter intensity.
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Figure 4. Seismic profile from the basin plain showing seismic indicators such as BSR, acoustic blanking, and seismic chimneys (for location, see Fig. 1B). (A) and (B) BSR become relatively weaker and less continuous than those in the southern slope (e.g., Fig. 2) and it is hard to be identifiable when running parallel to the near- or flat reflections associated with well-layered turbidite/hemipelagic sediments. An example of a wide blanking zone up to 20 km wide is also seen within well-layered turbidite/hemipelagic sediments. (C) Seismic chimneys show upward-bending internal reflectors result from the velocity pull-up due to the hydrate-bearing sediments. MTD, mass-transport deposits; THS, turbidite-hemipelagic sediments; BSR, bottom simulating reflector.
5. Discussion 5.1. Geophysical evidence for the occurrence of gas and gas hydrate The present study has demonstrated that the Ulleung Basin contains several kinds of seismic indicators related to gas and gas hydrate, including BSRs associated with enhanced reflections, seismic chimneys, acoustic blanking of reduced reflectivity, high amplitude reflectors, and seafloor expressions. The BSR, interpreted to be a physiochemical boundary between hydrates above and gas below, is a major indication of gas hydrate occurrence (Kvenvolden and Barnard, 1983; Shipley et al., 1979; Hyndman and Spence, 1992; Holbrook et al., 1996). In the Ulleung Basin, the BSR is most commonly found in the basin (Fig. 2), and is defined by its reflection polarity opposite to the seafloor, its seafloor-parallel reflection pattern, and its occurrence depth corresponding to the expected base of the GHSZ (Fig. 3C; Lee et al., 2005; KIGAM, 2007). Recently, Ryu et al. (2009) reported that the seafloor reflection coefficient is about 0.10, whereas the calculated BSR reflection coefficient is about 0.04. As a comparison, BSR reflection coefficients calculated in the Ulleung Basin are much lower than those observed in a
number of other areas, e.g., the Cascadia margin offshore Vancouver Island (e.g. Yuan et al., 1999; Chapman et al., 2002), and the Beaufort Sea margin (Andreassen et al., 1995), the Nankai Trough (e.g., Foucher et al., 2002) and Lake Baikal (Vanneste et al., 2001). In general, the BSR reflection coefficients suggest a velocity decrease of about 100 m/s assuming a constant density across the interface. This reflection coefficient suggests either low amounts of high-velocity hydrate above the BSR or very small amounts of low-velocity free gas below the BSR. If the reflection is solely due to overlying highvelocity hydrate, the concentration could be about 10% saturation, using the simple porosity reduction model (e.g., Hyndman and Spence, 1992). However, various researchers have reported that reflection presence and properties appears to be mainly due to underlying concentrations of free gas. These results indicate that the BSR in this area originates at the interface and is characterized by a strong decrease in acoustic impedance due to the presence of free gas below the BSR. In the core recovered from site UBGH1-14 located in the BSR area, the gas hydrate was not nearly founded within the BSR zone (KIGAM, 2007). Thus we suggest that the concentrations of gas hydrate were generally low in the BSR zone, although the occurrence of BSR does imply that some gas hydrate is present.
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Figure 5. Seismic profiles showing the BSR, strong reflections, and enhanced reflections below the BSR (for location, see Fig. 1B). (A) The strong and well developed BSR crosscuts the dipping strata. Additionally, enhanced reflections below the BSR are also seen that may indicate the presence of some free gas. (B) Seismic profile shows strong reflections just above the BSR and enhanced reflections below the BSR. (C) The flat BSR crosscuts gentle anticline structure. MTD, mass-transport deposits; THS, turbidite-hemipelagic sediments; BSR, bottom simulating reflector, ER, enhanced reflection.
The seismic data show numerous localized zones of seismic chimneys associated with reduced reflectivity in sediments (Figs. 4 and 6). In most cases, internal reflectors within seismic chimneys are characterized by pull-up or upward-bending shapes in time sections; these shapes can be interpreted as the result of the presence of high-velocity hydrate in the sediments (Fig. 6B, Haacke et al., 2009). Detailed velocity analysis showed strongly increased interval velocities inside seismic chimneys defined by pull-up structures. Previous studies (Lee et al., 2005; KIGAM, 2007; Ryu et al., 2009) also showed increased interval velocities within seismic chimneys; these increased velocities are associated with pull-up structures due to hydrate saturation. High seismic velocities compared to those of normal sediments are the most important indicator of gas hydrate (Lee et al., 1993; Singh et al., 1993;
Diaconescu et al., 2001). Seismic methods of gas hydrate quantification are based on the assumption that, compared to the normal increase of velocity with depth due to sediment compaction, the Pwave (Vp) of sediments containing hydrates increases with hydrate saturation above the BSR, and the velocity decrease below the BSR is due to the presence of gas (Lee et al., 1993). Seismically similar features have been investigated in a variety of geographic areas, including the Niger Delta (Hovland et al., 1997), the Norwegian margin (Hovland and Svensen, 2006), and the Cascadia margin (Riedel et al., 2006). According to these studies, features such as those in the study area have been shown to contain gas hydrate. This interpretation is applicable to our examples from the Ulleung Basin, where stacking-velocity analysis shows that the chimney structures correspond to positive velocity anomalies likely due to
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Figure 6. (A) Seismic profile across the site UBGH2-7 located in the northwestern basin plain showing seismic chimney associated with pockmark and mound structure (for location, see Fig. 1B). (B) Diagrams showing P-wave velocity, resistivity, and gas-hydrate saturation at site UBGH2-7 (for location, see Fig. 1B). Just beneath the site UBGH2-7 the diapir-like structure associated with possible fluid or gas migration can be seen. MTD, mass-transport deposits; THS, turbidite-hemipelagic sediments; BSR, bottom simulating reflector.
high-velocity gas hydrate (Park et al., 2008; Haacke et al., 2009; Ryu et al., 2009). Recent near-seafloor gas hydrate discoveries from some seismic chimneys associated with velocity pull-up structures support our interpretation (Ryu et al., 2012). Similar chimney structures were surveyed on the northern Cascadia Margin and a massive reservoir of gas hydrate was also drilled and sampled by the IODP Expedition 311 (Riedel et al., 2006). Along with seismic chimneys, strong reflection amplitude, defined by relatively high reflectivity, within the GHSZ is a major indication of gas hydrate occurrence in the Ulleung Basin. Based on well-log data, high amplitude interval is well correlated with high resistivity, velocity, and saturation (Fig. 8B). The resistivity log reveals a series of 1e2 m thick high resistivity sections just above
and below the strong amplitude zone, which may indicate gas hydrate-bearing sand beds (Fig. 8B; Ryu et al., 2012). At the interval, the P-wave velocity also shows high values between 1600 m/s and 1850 m/s. In the core recovered from site UBGH2-6, gas-hydratebearing sand (>30 cm thick) layers were founded within the interval, corresponding to high amplitude reflection zone (Ryu et al., 2012). The gas hydrate saturations at the interval show high values, ranging from 29% to 90% (Fig. 8B). Also, the gas hydrate saturations estimated from chlorinity excursions in the gas-hydrate-bearing zone range from 12% to 80%, averaging about 52% (Ryu et al., 2012). Thus, the strong reflection amplitude in the GHSZ should be expected within the gas hydrate layer and is often used as an indicator of gas hydrate.
Figure 7. (A) Seismic profile from the southern slope area shows seismic chimneys with mound structure (for location, see Fig. 1B). (B) Diagrams showing P-wave velocity, resistivity, and gas-hydrate saturation at site UBGH2-3 (for location, see Fig. 1B). MTD, mass-transport deposits; THS, turbidite-hemipelagic sediments; BSR, bottom simulating reflector.
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Figure 8. (A) Seismic profile across the site UBGH2-6 located in the northwestern part of the area showing the strong amplitude reflections just above the BSR (for location, see Fig. 1B). (B) Well-log data showing P-wave velocity, resistivity, and gas-hydrate saturation at site UBGH2-6 (for location, see Fig. 1B). Note that the high amplitude interval is well correlated with high velocity, resistivity, and gas-hydrate saturations. Just south of the site UBGH2-6 normal faults can be seen. MTD, mass-transport deposits; THS, turbiditehemipelagic sediments; BSR, bottom simulating reflector.
Acoustic blanking can help to discriminate gas hydrate bearing sediments (Dillon et al., 1994). Gas hydrate layers commonly display acoustic blanking effects, i.e. reduced acoustic impedance contrasts within the hydrated sediments, presumably due to the cementation of the stratal interfaces by the gas hydrate molecules (Shipley et al., 1979; Judd and Hovland, 1992; Hovland et al., 1997; Wood and Ruppel, 2000; Diaconescu et al., 2001; Vanneste et al., 2001). Diaconescu et al. (2001) also reported a blanking zone associated with a high velocity anomaly (w2100 m/s) in the Caspian Basin. Thus a reduction of the seismic amplitudes is often used as an indicator of gas hydrate. However, as the results of UBGH2, the acoustic blanking indicated minimal gas hydrate in fine-grained sediments (Ryu et al., 2012). Consequently, the acoustic blanking within the GHSZ could be either the result of physical properties of sediment itself, or a gas hydrate induced reflection anomaly. Enhanced reflections below the BSR are defined by a high acoustic impedance contrast on seismic profiles. The presence of gas will be shown in seismic data as a high amplitude anomaly (Holbrook et al., 1996; Wood and Ruppel, 2000). The gasbearing sediments can generate anomalously high amplitude reflections because even small amounts of gas (less than 2%) may cause a marked P-wave velocity drop (Domenico, 1976). In the Ulleung Basin, enhanced reflections are recognized just below the BSR, suggesting the presence of free gas in turbidite/ hemipelagic sediments (Fig. 5A). Based on the drilling results of UBGH2 (Ryu et al., 2012), however, the occurrence of enhanced reflections was not directly correlated with the existence of free gas below the GHSZ. Thus we think that the enhanced reflection attribute could be either the result of characteristics of sediment itself, or a free gas induced reflection anomaly, although the existence of enhanced reflections does imply that some free gas is present.
5.2. Gas migration into the GHSZ Gas fluids in deep marine sediments may migrate into the GHSZ through favored conduits (e.g., faults, fractures, diapirs and other permeable pathways; Nimblett and Ruppel, 2003; Hornbach et al., 2004; Trehu et al., 2004; Cook et al., 2008). Figure 9 summarizes two styles of migration pathways for fluid-gas upwelling into the GHSZ: (1) structural conduits including fault and fracture systems associated with seismic chimneys and (2) stratigraphic conduits mainly consisting of inclined turbidite/hemipelagic layers. The deepwater seismic profiles show a large number of vertical faults, which probably focus fluid flux (Fig. 6). Previous researchers (Lee et al., 2005; KIGAM, 2007; Horozal et al., 2009; Ryu et al., 2009) have suggested that, as a migration pathway of fluid-gas into the GHSZ, the fault or fracture systems in this area are very important for the formation of gas hydrate. Similar features are commonly recognized in seismic sections as migration pathways of gas or fluid in other regions (Heggland, 1997; Dillon et al., 1998; Judd and Hovland, 1992; Riedel et al., 2002, 2006; Cook et al., 2008). Weinberger and Brown (2006) also reported that complicated fracture networks are the primary method of fluid flow for free gas at the Hydrate Ridge, offshore Oregon. More recently, Barnes et al. (2009) reported that fault or fracture systems are commonly interpreted as important conduits for the upward flow of fluid or gas. We thus conclude that vertical faults or fractures associated with seismic chimneys in the basin have played an important role in gas-hydrate accumulations as structural conduits of fluid or gas that originates from below the GHSZ (Fig. 9; KIGAM, 2007; Ryu et al., 2012). The western margin of the Ulleung Basin shows an example of stratigraphic conduits related to inclined turbidite/hemipelagic layers. The seismic section shown in Figure 5A indicates that the BSR crosscuts the dipping strata, which probably consist of alternating beds of sand and mud. Hustoft et al. (2007) suggested that enhanced
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Figure 9. A schematic diagram showing migration pathways of gas-fluids and occurrence types of gas hydrate as well as various seismic signatures of gas hydrate and gas identified in the Ulleung Basin. Note that gasefluids migrate into the GHSZ through two distinct pathways: (1) structural conduits including fault and fracture systems associated with seismic chimneys (blue arrow) and (2) stratigraphic conduits mainly consisting of inclined, permeable turbidite/hemipelagic layers (red arrow). Two modes of gas hydrate accumulation were identified in the deep sea basin: (1) a stratally-bound type (pore filling) within turbidite sand beds and (2) a locally concentrated type (massive, nodule, and fracture filling) within upward-growing chimneys associated with near vertical faults. MTD, mass-transport deposits; THS, turbidite-hemipelagic sediments; BSR, bottom simulating reflector.
reflections below the BSR caused by free gas in the Norwegian margin are correctly correlated with permeable, coarse-grained layers; the free gas migrates into the GHSZ through the permeable layers. Baba and Yamada (2004) also reported that gas-bearing sediments consisting of alternating beds of sand and mud developed in the Nankai Trough and that gas-fluids migrate into the GHSZ through permeable sand layers. Similar acoustic features were also reported in the Storegga area by Bouriak et al. (2000), who believed that these anomalies are caused by fluid or gas expulsion though the dipping strata. In the Ulleung Basin, we also suggest that the inclined, permeable layers characterized by enhanced reflections have played an important role as stratigraphic conduits for the upward migration of fluid or gas that originates from below the GHSZ (Fig. 9).
5.3. Gas hydrate occurrence in the Ulleung Basin From ten drill sites during the UBGH2, various types of gas hydrate were recovered (Ryu et al., 2012). Based on onboard and post-cruise analysis, gas hydrate recovered from the Ulleung Basin can be grouped into two types: (1) a stratally-bound (pore filling) hydrate within turbidite sand layers and (2) a locally concentrated hydrate within upward growing chimneys associated with near vertical faults, such as massive, nodule, and fracture filling types (Fig. 10). Figure 10B shows an example of pore-filling type hydrate in turbidite sand beds. Based on seismic and well-log data, high amplitude reflection zone was observed within the GHSZ and this interval is well correlated with significant high values of resistivity,
Figure 10. Two types of gas hydrate recovered from the Ulleung Basin, East Sea in 2010 during the UBGH2. (A) local concentrated type within vertical chimneys such as fracture filling, massive, and nodule type hydrate recovered from the UBGH2-7, (B) statally-bound (pore filling) type hydrate in sand layer observed by IR image. The sample was recovered from the UBGH2-2-1 (for location, see Fig. 1B).
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velocity, and hydrate saturations (Fig. 8). Also, in the core recovered from UBGH2-6, gas-hydrate-bearing sand (>30 cm thick) layers were founded within the interval, corresponding to high amplitude reflection zone (Fig. 10B; Ryu et al., 2012). These results support that high amplitude zone associated with well-stratified reflectors are considered to result from gas-bearing sands, as a stratallybound (pore filling) type (Figs. 9 and 10B). In the western margin of the basin, enhanced reflections within alternating strata are abruptly terminated by the strong BSR and are in turn overlain by acoustic blanking of reduced reflectivity (Fig. 5A). The acoustic blanking zone just above the BSR is well correlated with the high velocity zone, strongly suggesting the presence of high-velocity gas hydrate, whereas the area of enhanced reflections below corresponds to a relatively low velocity zone (KIGAM, 2007; Horozal et al., 2009). On the other hand, a wide blanking zone associated with reduced reflectivity in the turbidite/hemipelagic sediments occurs within the GHSZ (Fig. 4). Based on the velocity analysis, the blanking zone was found to correspond to a positive velocity anomaly due to the high-velocity gas hydrate (KIGAM, 2007; Ryu et al., 2009). The blanking zone is also connected with vertical faults or chimneys as migration pathways for fluid-gas upwelling into the GHSZ. We conclude that such an acoustic blanking zone may also indicate a concentrated zone of gas hydrate in the Ulleung Basin, as shown in Figure 9. Figure 10A shows examples of vertically concentrated hydrate such as massive, nodule, and fracture filling types. Chimney structures are commonly seen in deep-water basins, suggesting gas hydrate concentrations (Figs. 2, 4 and 6). Near-surface hydrates have been discovered in piston corings from the basin plain (Bahk et al., 2009). At one site, a 130 m thick layer of hydrate-bearing sediments was recovered in localized chimney structures (Park et al., 2008; Kim et al., 2011; Ryu et al., 2012). One well-studied example associated with vertical chimneys is the Bullseye structure off western Canada (Riedel et al., 2002). In the Bullseye structure, a massive quantity of gas hydrate was piston cored just below the seafloor. Drilling by IODP shows a w50 m thick layer of gas hydrate extending from just below the seafloor (Riedel et al., 2006). Schwalenberg et al. (2005) also reported that a number of chimney structures in that area contain high concentrations of gas hydrate, mostly near the seafloor in areas associated with vertical chimneys. We thus suppose that relatively high concentrations of gas hydrates in the Ulleung Basin tend to occur in localized chimney structures associated with vertical faults, as a locally concentrated mode. 6. Conclusions 1. Based on the multi-channel seismic data, the seismic evidences for the gas hydrate and associated gas in the Ulleung Basin include (a) BSRs, (b) a seismic chimney, (c) high reflection amplitude within the gas hydrate stability zone (GHSZ), (d) acoustic blanking, (e) enhanced reflections below the BSR, and (f) seafloor gas-escape features. 2. The BSR is most commonly found over much of the basin and is defined by its reflection polarity, which is opposite to that of the seafloor, and by its seafloor-parallel reflection. Seismic chimneys associated with velocity pull-up appear to be caused by active migration of fluid gas vertically into the gas hydrate stability zone. Another seismic characteristic observed in gas hydrate bearing sediments is the strong reflection amplitude, defined by the relatively high reflectivity within the GHSZ. Acoustic blanking is likely to be attributed to hydrate cementation in the sediments, which causes a significant reduction of acoustic impedance contrast. Enhanced reflections immediately below the BSR indicate the presence of gas. Sub-circular depressed/domed seafloor features may represent gas
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