Gas Hydrate BSR and Possible Fluid Migration in the Nankai Accretionary Prism off Muroto

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1 Gas Hydrate BSR and Possible Fluid Migration in the Nankai Accretionary Prism off Muroto Sumito Morita 1), Yasuyuki Nakamura 2), Shin ichi Kuramoto 3), Nathan Bangs 4) and Asahiko Taira 3) 1) Geological Survey of Japan, AIST-GREEN, Tsukuba, Ibaraki , Japan. 2) Ocean Research Institute, University of Tokyo, Tokyo , Japan. 3) Center for Deep Earth Exploration, Japan Marine Science and Technology Center, Yokosuka, Kanagawa , Japan 4) Institute for Geophysics, The University of Texas at Austin, Austin, TX , USA Abstract: Three dimensional seismic survey carried out in the western Nankai Accretionary Prism off Muroto succeeded in imaging of clear geologic structure and detailed distribution of BSR. Topography and geologic structure influence the distribution of BSR in the survey area. Especially, depths of BSRs obviously change along out-of-sequence thrusts. In large thrust zone, BSR in the hanging wall of a large thrust reaches down to the thrust, while the footwall does not have clear BSR. Heat flow values calculated from depth of BSR indicate that more intensive fluid convection occurs toward the toe of the Nankai prism. Heat flow increases seaward in each faults unit bounded by out-of-sequence thrusts, and the heat flow drops down at the out-of-sequence thrusts. The same seaward increase and the drop are duplicated at each fault block cut by minor scale thrust. These features give rise to more speculation related to an interaction between geologic structure, fluid migration, and development of BSR and gas hydrate deposits. 1. Introduction A bottom simulating reflector (BSR) is considered to be the bottom of the gas hydrate stability field, commonly thought to have a free gas layer below. It has been known from the previous studies that BSR occurs widely within the Nankai Accretionary Prism off southwest Japan. At Site 808, Ocean Drilling Program Leg 131, gas hydrate was actually recovered in association with some plant debris at the toe of the Nankai prism off Cape Muroto in the western Nankai Trough (Taira, Hill, Firth, et al., 1991). In 2000, gas hydrate-bearing sandstone was obtained at the Ministry of Economy, Trade and Industry s test well off Tokai area in the eastern Nankai Trough (Takahashi et al., 2001). However, the development of gas hydrate deposits and the spatial distribution of gas hydrate are not understood very well. In this study, an extensive three dimensional (3D) seismic survey was performed in the western Nankai Trough, to investigate detailed distribution of BSR, which gives information on the spatial distribution of gas hydrate paired with free gas below. The BSR can be also used to calculate heat flow values from its depth. To support the heat flow data, heat flow values measured on NGH99 cruise in September, 1999 were used. This new BSR data enable us to

2 discuss a correlation between geologic structure and the formation of gas hydrate deposits. 2. Data Acquisition Japan-US cooperative 3D seismic investigation with R/V Maurice Ewing (EW9907/08 cruise) was carried out in the western Nankai Accretionary Prism from June to August in The survey area is located south off Cape Muroto, Shikoku Island. The boat tracked km lines with 100 m separation between lines. The survey covered an area 80 km by 8 km from the trench to the older accretionary prism (Figure 1). Figure 1: The three dimensional seismic box in the western Nankai Trough. 81 km long survey lines with 100 m intervals cover 80 km x 8 km area off Cape Muroto. Tuned array air-guns (4,276 cubic inch in total) and a 6,000 m long 240 channel streamer cable were deployed (Table 1). A differential global positioning system was adopted with receivers on the boat and tail buoy. This powerful and precisely positioned seismic system made it possible to see a clear and high-resolution image of the Nankai prism.

3 Performance : June August, 1999 Name of boat : R/V Maurice Ewing (LDEO, Univ. Columbia) Total Length : 73 m Weight : 1,978 t Survey area : 640 km 2 (80 km x 81 2d-lines) Positioning : Differential global positioning system Source : Tuned-array Air-guns Number of guns : 14 Total capacity : 4,276 cu.in. (70 l) Air pressure : 2,000 psi Shot interval : 50 m Towing depth : 10.5 m Receiver : Single digital streamer Length : 6,000 m Number of channels 240 ch. Interval of channel : 25 m Towing depth : 10 m Sampling rate : 2 msec Recording length : 12 sec. CDP interval : 12.5 m Mean stacking number : 60 Table 1: Specifications of the three dimensional seismic survey. 3. Distribution of BSR The bottom of Figure 2 shows the topography and an interpretation of geologic structure and BSR on an inline cross-section. Geologic structure in the survey area is basically composed of imbricate stack of seaward-vergent thrust sheet, which is generally present in a primary accretionary prism. There are some step-ups in topography toward the land to the northwest. At each foot of these step slopes, there are out-of-sequence thrusts, larger than the general imbricated thrusts. In the frontal thrust zone, which is the zone from the deformation front at the trough floor to the first step of the out-of sequence thrust, the accreted sediments is less than 1.3 km thick and its geologic structure has been well investigated (Moore et al., 2001). BSR in this zone is continuous. The zone in the middle part of the survey area is called the large thrust zone, where the present exploration revealed very deep structure of the accretionary prism for the first time (Figure 3). Geologic structure 3 km deep or more was imaged clearly on the processed data. In this zone, BSR shows a clear relation with geologic structure. BSR is typically discontinuous, and generally in the hanging wall of a large thrust reaches down to the thrust, while a footwall does not have clear BSR. In the northmost zone of the survey area, very continuous and high amplitude BSR occurs in the older accretionary prism. The general distribution of BSR was mapped on the top of Fig. 2. Tone in gray scale indicates depth of BSR, where lighter is shallower and darker the deeper. The distribution of BSR is very well related to the geometry of the Nankai Accretionary Prism. BSR extends widely in the frontal thrust zone and in the northmost zone in the survey area. However, in the large thrust zone,

4 where BSR is discontinuous on cross-section, fragmentary BSRs extend laterally and form bands along contour lines. Figure 2: Distribution of BSR in the three dimensional seismic box. Figure 3: Occurrence of BSR in the large thrust zone. BSR in a hanging wall of a large thrust reaches down to the thrust, while a footwall does not have clear BSR.

5 The depth of BSR shows rapid changes relative to big topographic changes, mainly at the out-of-sequence thrusts. The depth of BSR increases seaward from 0.4 or 0.5 sec at the northmost zone, and down to 0.6 or 0.7 sec approaching the foot of the slope of the large thrust zone at 40 km landward from the trough axis. On the other hand, the depth of BSR rapidly switches to decreasing seaward from 0.4 to 0.2 sec at the toe of the accretionary prism. This feature must indicate higher fluid circulation around the frontal thrust zone and the toe of the Nankai Prism. 4. Heat Flow Estimations To investigate the variation of the fluid circulation, heat flow was estimated from depth of BSR. For this estimation, we used porosity data from Site 808 of the Ocean Drilling Program to obtain thermal conductivity (Taira, Hill, Firth, et al., 1991). Figure 4 shows the variation of the heat flow values. For comparison, data from the NGH99 cruise is also plotted. The data of NGH99 is measured with a needle probe on the seafloor along the same survey area as the 3D s. The data estimated from BSR correlates very well with that measured on the seafloor, indicating the accuracy of estimates. Figure 4: Heat flow estimated from BSR and its implication with geologic structure. The heat flow value increases exponentially toward the trough axis, and reaches the maximum values of over 150 mw/m 2 at the toe of the prism and over 200 mw/m 2 at the trough floor. Variation of the heat flow increases seaward in each faults unit bounded by out-of-sequence thrusts, and drops at the out-of-sequence thrusts. Furthermore, this same seaward rise and subsequent drop of the heat flow is duplicated at each fault block cut by the thrust. Thus, the heat flow values in the survey area generally show saw-teethed variation, where the heat flow repeats its seaward rise in fault blocks in each scale, but drops sharply crossing the thrusts, out-of-sequence thrusts and imbricated thrusts.

6 5. Fluid Migration As mentioned before, in the large thrust zone, BSR is discontinuous and shows a clear difference between hanging wall and foot wall occurrences. The heat flow rises seaward in each fragmentary BSR and drops down between the chopped BSRs. The different occurrences of BSR indicate that there are both gas hydrate and free gas deposits in a hanging wall, being more matured than that in a foot wall. The reason why the heat flow increases seaward in a fault block is possibly because a higher volume of fluid is supplied to the forward portion of the fault block from the deeper portion of the structure. On the basis of these considerations, the following two cases of fluid migration may be inferred. In one case, fluid generally migrates upward along porous beds in the strata, and it is likely that major volume of fluid would be supplied to the forward thrust in a fault block (Figure 5). Because the geologic structure in the survey area is composed of folds and thrusts as the basic structure of an accretionary prism, a thrust dips steeper than strata in each fault block. In the other case, a large amount of fluid comes up through thrust planes, and upward fluid diffusion may occur from the thrust planes in the upper part of the thrusts due to lower overburden pressure near the seafloor (Fig. 5). Considering conventional geological concepts, it may be likely a composite fluid migration system, consisting of the above two systems, may contribute to the typical occurrence of BSR in the large thrust zone of the Nankai Accretionary Prism. Thus, the occurrence of BSR may vary according to the fluid migration system in an accretionary prism. So, it can be considered that the formation of gas hydrate deposits depends on critical variations in geological structure. Figure 5: Two basic models of fluid migration system in the large thrust zone.

7 6. Conclusions The observations and the heat flow estimations using the data from the 3D seismic survey in the western Nankai Trough made it possible to discuss the relationship of geologic structure, occurrence of BSR, fluid migration, and the development of gas hydrate deposits. The results lead to the following conclusions: 1) In the survey area, topography and geologic structure markedly influence distribution and depth of BSR. 2) BSR is distributed continuously in the frontal thrust zone and in the northmost zone in the survey area. However, BSR is discontinuous in the large thrust zone, and varies between hanging wall and footwall occurrences. The BSR in the hanging wall reaches to the forward thrust, while the footwall does not have clear BSR. 3) Heat flow estimated from BSR indicates saw-teethed variations along an inline of the survey area, where the heat flow values a show seaward increase on three different scales: across each thrust, across each out-of-sequence thrust unit and across the survey area, with a rapid drop in each thrust. 4) It is likely that the discriminated occurrence of BSR in the large thrust zone is caused by a composite fluid migration system, comprising of source volume differences and diffusion from thrust planes. 5) The occurrence of BSR and the formation of gas hydrate deposits may be related to critical variations in geological structure. Acknowledgements We would like to thank Dr. Juichiro Ashi, Assistant Professor at Ocean Research Institute, University of Tokyo, who provided practical suggestions on estimation of heat flow from depth of BSR. We also thank all scientists and the crew of EW9907/08 and NGH99 cruises for their support in the success of the cruises. References Moore, G. F., Taira, A., Klaus, A., Becker, L., Boeckel, B., Cragg, B. A., Dean, A., Fergusson, C. L., Henry, P., Hirano, S., Hisamitsu, T., Hunze, S., Kastner, M., Maltman, A. J., Morgan, J. K., Murakami, Y., Saffer, D. M., Sanchez-Gomez, M., Screaton, E. J., Smith, D. C., Spivack, A. J., Steurer, J., Tobin, H. J., Ujiie, K., Underwood, M. B. & Wilson, M. (2001). New insights into deformation and fluid flow processes in the Nankai Trough accretionary prism: Results of Ocean Drilling Program Leg 190. Geochemistry, Geophysics, Geosystems, 2, /2001GC Sloan Jr., E. D. (1998). Clathrate Hydrates of Natural Gases, 2nd ed., Marcel Dekker, Inc., New York. Taira, A., Hill, I., Firth, J. V., et al. (1991). Initial Report of Ocean Drilling Program, 131, Texas A & M, ODP.

8 Takahashi, H., Yonezawa, T. & Takedomi, Y. (2001) Exploration for natural hydrate in Nankai-Trough wells offshore Japan. Proceedings of Offshore Technology Conference 2001, OTC13040.