Gas seepage on the sea floor of Okinawa Trough Miyako Section

ARTICLESchinese science Bulletin 2005 Vol. 50 No 13 1358"-1365rine sediments. Firstly, they are potential energy resourcesGas seepage on the sea floor of and albacora oil and gas fields in the region of a gas seepOkinawa Trough miyakoin the Campos Basin indicated a potential relationshipbetween gas seepage and oil and gas fields"5.WorldwideSectionnge has highlightthe possibility that gas venting from the marine gas seepsLUAN Xiwu& QIN Yunshanmay be making a significant but not well-known contribution to atmospheric methane and cO, concentrations! 3, 41Institute of Oceanography, Chinese Academy of Sciences, Qingdao Gas seeps have also been related to gas hydrate 9-12266071,ChnaThe geophysical properties of marine sediments, suchCorrespondence should be addressed to Luan Xiwu (emaias velocity, density, and acoustic impendence, will be al-tered due to the presence of gas in the sediments. BothAbstract A marine geophysical survey was carried out, P-wave velocity and S-wave velocity of a sediment layeron the R/V Science 1 of the Institute of Oceanography, Chi- containing gasll5will change significantly. Acoustic technese Academy of Sciences (IoCAS), in 2000, at the Miyakoection of Okinawa Trough. Here we present seismic andniques may be used to characterize the P-wave and theacoustic evidence of a gas seep on the sea floor on the west-S-wave velocities and the attenuation characteristics of aern part of the Okinawa Trough, near the lower slope of the sediment layer. The presence of free gas can usually beEast China Sea Slope and discuss the possibility of related seen on seismic records, echo-sounder records, side-scanformation of gas hydrate. a gas column reflection was ob- sonar records, and multi-beam bathymetry. Acoustic turserved in echo-sounder data above a section where the sea bidity, acoustic blanking, strong multiples, bright spotsfloor reflector was missing, on both the echo-sounder and the and phase reversals have combined to indicate the presseismic data for line H14. The seismic data also show an ence of an area of gas-bearing sediments. Further, acousticacoustic curtain reflection and a turbidity reflection at this blankets[2. 16,17). acoustic curtains[16-18)acoustic columns[ 2as seep, which occupies an area 2.2 km in diameter. Based and acoustic turbidity 7, 19 on seismic profiles are used toon the acoustic curtain on line hl4. we believe that the assess the type of gas accumulationamount of gas contained in the sediments below the gas seepSediment compaction and tectonic compression are theis larger than 1% by volume of sediment. Tectonically, the two main factors that lead to the formation of gas seeps ingas seep developed in a small basin controlled by basementarine sediments 20. Diagenesis of sediments, and catag-uplift in the north, south and east. The thickness of the sedi- enesis of organic rich layers produce multi-phase materialment layer can be greater than 3.5 km. A mud diapir struc- at depth. Gravitational loading and tectonic forcing canture was found in layer d beneath the gas seep. make the multi-phase material overpressured 21. 221 Faults,Over-pressure may occur due to the large sediment thickness mud diapirs and the mud volcanoes can be efficient conand also the tectonic basement uplift in the north, south, and duits for multi-phase matter to migrate to the surface andeast. The mud diapir could be the preferential pathway for form the gas seepsmethane- rich fluids. The acoustic curtain may indicate thatfree gas related to the gas seep can be formed on the sea floor.In this study, we present acoustic evidence for a giantWe also note that the layer above the acoustic curtain on gas seep on the sea bottom of the okinawa Trough nearprofile H14 may contain gas hydrate.the lower part of the East China Sea SlopeKeywords: Okinawa Trough, gas seepage, evidences, formation, gas 1 Geological settinThe Okinawa Trough is a young back arc basin formedDOI:10.1360/04w0257by the subduction of Philippine Sea Plate underneath theEast-Asia continental margin. It extends for about 1200The presence of gas has been recognized as a feature of km from southwest Japan to northeast Taiwan, and can bemarine sediments for a long time. Emery and Hoggan first divided into three parts: the North, Middle and Southreported gas in sea floor sediments. Since then, different Okinawa Trough, separated by the Tokara Fault and theauthors have reported gases in marine sediments in the Miyako Fault at about 130- and 127-E respectivelycoastal zone and on continental shelves/2, 3) on continental There are many geological differences among the threesloped. 5), on margin 6l and in accretionary prisms. In parts. For example, the water depth of the Okinawarecent years, with the move of the petroleum industry into Trough is 23deeper water, gas seeps have also been found under deep to about 200V凵中国煤化工 ywateCNMHG reaches a maxiThe scientific and economic interest in gas seepage in mum width of 230 km in the north. The striking directionnarine sediments grew enormously in the last few years. of the North Okinawa Trough is NNE, and then it gentlyThere are many reasons for studying the gas seeps in ma- changes to nE in the middle and Nee in the south (Fig 1)13方数据Chinese Science Bulletin Vol 50 No. 13 July 2005ARTICLES32NEast China Sea Shelforth ot30°N28Nako Isl2EL120°E122°E124E126°E128°E130°E132EFig 1. Map of East China Sea. The block area is the Miyako Section of Okinawa Trough. Bathymetry contour interval is 1000 m for thedeeper area. OT, Short for Okinawa Trough; F, short for Fault.Tectonically, Okinawa Trough is a narrow and deep basin Island, between the middle and south sections of the Okibetween the Diaoyu Island Uplift foldbelt in the west and nawa Trough. The strike direction of the Okinawa Troughthe Ryukyu Uplift foldbelt in the east. Within the trough, begins to change from nE to NEE at the Miyako Sectionthere are some second order tectonic structures, such as ( Fig. 1)the Shelf Margin Sag, the Longwang Uplift, and the Cen- 2 Data acquisition and processingtral graben. all of these second order structures areroughly parallel to the strike of the Okinawa Trough, in arA marine geophysical survey was carried out by I0echelon, rather than continuous, structure particularly for CAS using R/ Science I in 2000, in the region of thethe Central Graben( 25 26( Fig. 2)Miyako Section of the Okinawa Trough. Gravity, mag-In addition to an active fault system, frequent earth- netic, bathymetric, and single channel seismic data wererecorded. During the seismic survey, a 4 air-gun array,fresh igneous rock intrusion, remarkable graben structureswith a total volume of 1330 inwas used, with a samplininterval of 2 ms, and a recording time length of 4 s. theand hydrothermal activity have been observed in the oki- distance from the shot point to the receiver is 160 m. Thenawa Troughl27. Recently, a mud volcano/28,US EPC was used to plot the single channel profile onsub-bottom fan29, and sub-bottom old meanders so have board the ship. The echo-sounder was a bi-frequency 6000lso been found in the Okinawa Trough. Here, we give the m helacevidence of a giant gas seep found on the bottom of Oki中国煤化工d150kHz.TheCNMHGprecision of thenawa Trough Miyako Section near lower bottom of East water depth dChina sea slopeFig. 2 shows the two survey lines passing through theThe Miyako Section of Okinawa Trough is between gas seep Line H14 strikes NE-SW, and is located in the25-N--27-N, west of Kiumi Island, north of Miyako Shelf Margin Sag, near both the lower part of the EastChiese seience Bulletin Vol 50 No. 13 July 2005ARTICLES27.5N27.0°NH12elf Margin SagDiaoyu Island Upl26.5°N26.0°N25.5°N250N124.5E125.0°E125.5°E1260E126.5E270EFig. 2. Map of track lines. Star shows the position of gas seepage. Bathymetry contour interval is 100 mChina Slope and the longwang uplift. Line H12 strikes on the East China Sea Slope, 16 km wide and 700 m deepNW-SE, perpendicular to line H14, across the East China Beyond the canyon, the slope drops steeply by 1800 m toSea Shelf, Slope, the Okinawa Trough and the west mar- the bottom of the Trough(Fig 3(b). From this profile wegin of Ryukyu Islandcan see that the bottom of the trough is quite flat. From the3 Resultslower end of the slope, the bottom of the trough is slopesently dein waterpth IsThe data acquired during the Cruise Science I 2000 only 200 m over 60 km of the trough. The central grabengeophysical survey was processed, and an anomalous re- marked on this profile is 6 km in width and 40 m in depthflection was found on line H14. We believed it is a sign of The bottom profile of line H14, however, is not flat at allgas seep. A similar anomalous reflection was found on (Fig 3(b)). Along this profile, the water depth is roughlyline H12, very near to the site on line H14. To determine if increasing from northeast to southwest. In the northeastthe anomalous reflections were indeed due to a gas seep, part of this profile, the water depth is about 1700 m,whileline H12. We found a topographic ofiles for line H14 and in the southwest part, the water depth is around 2050 mthe exact same position as the seismic reflection anomalynorth to souththe middle of the3. 1 Echo-sounder profileprofile, whic中国煤化工hand200mindepth. TheCN MH Ghe basin is aboutThe echo-sounder profiles from Cruise Science 1 2000 2017 m. Along the pronte, in the center of the basin,thereshow the topography of the Okinawa Trough Miyako Sec- is an area where the seafloor reflection is missing(fition(Figs. 2 and 3(a). Profile HI2 shows a large canyon 3(c). In Fig 3(c), at time 4: 20, the recorded water depth139方数据Chinese Science Bulletin Vol 50 No. 13 July 2005ARTICLES25125.15212531254125125612571258125912861261257258259262612626326426526626726826927Line H12200F SwLine H14400 ShelfRyukyu400Gas seepageArc60080010001200Centra1200Intersect1400Intersect graber1400here with H121600l80020 km1800200(b)20 kmFig 3. Water depth along line Hl4 and line H12 and the anomaly section of echo sounder recorder on line H14is 1854 m increasing to the southwest. At 6: 40, the water layers named A, B, C, D, E, and F from the top of thedepth is 2017 m. The sea floor is clear and flat up to 7: 25, sediment downward(Fig. 4). Layer A is well-developedthen it shallows and disappears. At 7: 37 the sea floor re- throughout the study area with a consistent thickness ofappears, and the water depth is still 2017 m. The missing around 0.4 s(two way travel time). Layer A can be dsection is 12 min(about 2.2 km) long. During this time, vided into two conformable contacted layers. The upperthe echo-sounder and other survey equipment onboard part of layer A is strong, continuous and opaque. Theworked well and no anomaly or failures were recorded. As lower part of layer A is weak, continuous and semitrans-rell as the missing section of the sea floor, another anomparent. The appearance of layer A on line H12 is differentaly on the echo-sounder profile was recorded. There is a from that of on line H14. On line H12(Fig. 4(b), layer Agas column reflection recorded above the missilis continuous and flat, while on line H14, layer A isfloor(Fig. 3(c). The gas column reflection happenestrongly bended and folded. In the north and middle partspIof line H14, layer A is not affected by the underlying layer,7: 03--7: 37, near and above the missing sea floor. There but in the south part of line H14, the fold of layer A iswas no similar anomalous gas column reflection recorded obviously affected by the underlying layer. Comparingon any other echo sounder profilewith layer A layers B and C are weak, semitransparent,3.2 Single channel seismic profilescontinuous layers Layer D is an opaque, continuous layer,stronger than中国煤化工 than layer A.The(i) Stratigraphic division. Thefiles ob strong reflectitained during Cruise science 1 2000rocessed.The makes it easyCNMH一 ottom of layer DHhe line. Layer E isstratigraphic divisions were deduced based on the am- weak but continuous, while layer F is stronger, but disconplitude, frequency, and the combination and distribution of tinuous and hard to explain We believe that layer F is althe reflectors on the seismic profiles. We identified six ready strongly deformed by tectonics. Except for layerChiaese seience Bulletin Vol 50 No. 13 July 200517:0018:0019:0020:0021:00220023:000:001:002003:004:005:006:007:008:009:0010:0011:00120013:0014:0015:00TimeLine h14Intersect here with H123.03.410 kmBasement upliftBasement uplift22:0022:2022:4023:0023:2023:400:000:200:401:001:201:402:002:202:403:003:203:404004:20440TimeSELine hi2Acoustic turbidity Intersect with line H145000gomsos