Geological controls on the gas hydrate system of Formosa Ridge, South China Sea

C. Berndt, G. Crutchley1, I. Klaucke, M. Jegen, E. Lebas, S. Muff, K. Lieser, T. Roth

Marine Geodynamics GEOMAR Helmholtz Centre for Ocean Research Kiel

Kiel, Germany 1Now at: GNS Science, Lower Hutt, New Zealand

W.-C. Chi Institute of Earth Sciences

Academia Sinica Taipei, Taiwan

T. Feseker Fachbereich Geowissenschaften

University of Bremen, Bremen, Germany

S. Lin, C.-S. Liu, L. Chen, H.-H. Hsu Institute of Oceanography

National Taiwan University Taipei, Taiwan

Abstract—Formosa Ridge is one of many topographic ridges created by canyon incision into the eastern South China Sea margin. The northwestern termination of the ridge is caused by beheading of the ridge due to a westward shift of the canyon that originally formed to the eastern flank of Formosa Ridge. Below Formosa Ridge a bottom simulating reflector (BSR) exists. Its depth below sea floor coincides with the theoretical base of the gas hydrate stability zone and the reflection has reverse polarity suggesting that it is caused by free gas below gas hydrate accumulations. The BSR is ubiquitous but shows significant variations in depth below sea floor ranging from 150 ms TWT (or approximately 180 m) underneath the incised canyon in the north to up to 500 ms (or approximately 460 m) underneath the crest of Formosa Ridge. Predominantly this depth variation is the result of topography on subsurface temperature, but comparison with the average BSR depth underneath the surrounding canyons suggests that recent canyon incision in the north has perturbed the thermal state of the sediments. Formosa Ridge consists of a northern half that is dominated by refilled older canyons and a southern half that consists mainly of contourite deposits. However, judging by the reflection seismic data this difference in origin seems to have little effect on the distribution of gas hydrate.

Keywords—gas hydrate; sedimentary systems; heat flow

I. INTRODUCTION Gas hydrate is an ice-like substance stable at high pressure

and low temperature [1]. Gas hydrate forms from free gas that combines with water molecules resulting in a substance called clathrate. As the gas stored in gas hydrate is primarily methane, gas hydrate has been identified as a potential future energy source. In particular countries that do not have a large amount of other fossil fuels are keen to develop gas hydrate as an alternative means to satisfy their energy demand. Therefore, there is a lot of interest in establishing how much gas hydrate is

present in continental margins, how it is distributed and how quickly it forms and distintegrates as pressure and temperature conditions change. Gas hydrate has also been identified as a potential agent in slope destabilization, and as a threat to global warming. For all these reasons we conducted a cruise on the german research vessel Sonne in 2013 to understand the underlying geological processes that control the distribution and dynamics of gas hydrates off Taiwan. The seas SW off Taiwan are an excellent study site for gas hydrate investigations as a lot of background information is available and the area allows to compare the effect of structural deformation on gas hydrate processes [2].

Here, we report on the results of the 3D-seismic investigation of one of the study sites – Formosa Ridge (Fig. 1). The objective is to determine the influence of geological controls on the distribution of free gas and gas hydate within Formosa Ridge.

II. P-CABLE 3D SEISMIC DATA The P-Cable system delivers high-resolution 3D data at a

fraction of the cost and operating requirements of full-scale industry 3D acquisition, making it a feasible and attractive option for academic surveys. The system used in this study consisted of 12 streamers attached to the cross cable that is towed perpendicular to the ship’s steaming direction (Figure 2A). The cube covers a surface area of 30 km2 (12 km long in-lines by 2.5 km long cross-lines). The source consisted of one 210 in3 GI gun towed at a depth of 4 m 15 m behind the vessel. It was fired at 5 s intervals. Data were sampled at 1 ms and revealed a dominant frequency of ~100 Hz. Processing of the 3D seismic data included navigation corrections using direct wave travel times, frequency filtering, and 3D time migration with water velocity.

The German ministry of Education and Research (BMBF) and the Taiwanese National Science Council (NSC) sponsored this work.

978-1-4799-3646-5/14/$31.00 ©2014 IEEE978-1-4799-3646-5/14/$31.00 ©2014 IEEE This is a DRAFT. As such it may not be cited in other works. The citable Proceedings of the Conference will be published in

IEEE Xplore shortly after the conclusion of the conference.

III. RESULTS Formosa Ridge is approximately 30 km

wide (Fig. 1). In the West, East and North the by up to 700 m deep canyons. Its flanks are angles exceeding 30 degrees. The study area northern 8 km of the ridge. In this area two sumare approximately at 1050 m water depth. Ththe study area is approximately 2600 m deep. show an almost continuous BSR underlying(Fig. 2). The BSR varies in sub-bottom depbelow the incised canyons to 500 ms two-(TWT) below the crest of the ridge. Thinterrupted below the southern summit wamplitude anomaly reaches from the sea floBSR. On both sides of this amplitude anomalupward (Fig. 2c). East of the southern summit BSR below the regional BSR (Fig. 3).

The 3D seismic data reveal the internal Formosa Ridge. Within the northern half of theare at least seven nested unconformities cuttin(Fig. 2b). These unconformities form elongthat strike in N-S direction. Frequently, thewith high amplitude anomalies in the deeseismic facies within the depressions funconformities is horizontally layered with som

The southern part of the study arunconformities. Instead the sedimentary reflecthickness variations on a wave length of m

Fig. 1. Location of the study area at the northern eto 2800 m (green).

long and 5 km ridge is bounded steep with slope is located in the

mmits exist. Both he deepest part of

The seismic data g Formosa Ridge pth from 150 ms -way travel time his BSR is only where a seismic oor down to the ly the BSR bends there is a second

structure within e study area there ng into eachother gated depressions ey are associated epest parts. The formed by the me undulations.

rea lacks such ctors show lateral more than 1 km

comprising most of the ridge withizone

TreveasimuwhicridgeamplpolarcausspacBSRevidewithiThe the gas aridgebelowbelowand of thnorthshoubecamorpsedimsidesa do

hydrate stability zone. However, theof the BSR below the canyon in th200 m below the sea floor may be eof the canyon and a gas hydrate sysstate conditions (Fig. 2a). Other evidthe gas hydrate stability conditiondouble BSR below the central easterThe occurrence of double BSR has ochanges in the temperature and presthe depth of the gas hydrate stabilitin a critical state below the gas hydrhydrates may remain meta-stable time.

The 3D seismic data reveal the fthat fuels the chemosynthetic ecsummit of Formosa Ridge [4]. Thegas rise to the surface at least from stability zone. Seismic attribute aseismic reflections show that the fvertically in a seismic pipe structuanomalies between the BSR and thare accumulations of free gas insidzone below the seep. As gas shouldthese pressure and temperature condis a dynamic three-phase equilibriuof free gas, water, and gas hydrate w

The 3D seismic data also prsediment facies that constitute Fo

nd of Formosa Ridge. Water depth ranges from 200 m (red)

in the gas hydrate stability e (Fig. 2d).

IV. DISCUSSION The new 3D seismic data al a prominent bottom-

ulating reflector (BSR) ch is underlying most of the e (Fig. 2). The high litudes and its reversed rity show that the BSR is ed by free gas in the pore e and the observation of the

R is direct geophysical ence for gas hydrates in the overlying sediments. wide-spread occurrence of BSR implies an efficient advection system within the e. The depth of the BSR w the sea floor is greatest w the summit of the ridge shallowest below the floor

he submarine canyon in the h of the ridge. This pattern uld be expected simply ause of the sea floor phology which allows the ments to cool from both s within the ridge leading to ownward shift of the gas e particularly shallow depth he North of approximately evidence for recent incision stem that is outside steady-dence for a dynamic past of ns is the observation of a rn part of the ridge (Fig. 3). often been attributed to past ssure conditions that control ty zone leaving gas hydrate ate stability zone [3]. These for a significant length of

fluid conduit to the seep site cosystem at the southern e data show that fluids and the base of the gas hydrate anomalies and interrupted

fluids are ascending almost ure. Several high-amplitude he sea floor show that there de the gas hydrate stability d normally form hydrate at ditions it is likely that there

um that allows co-existence within the sediments.

rovide information on the ormosa Ridge. Undulating

seismic reflections and laterally variable tseismic units are clear indications that bottocontrolled sedimentation along this part of theof the time during which the ridge has deveThus it is likely that the bulk of the ridge grained sediments. However, the 3D seismicprominent unconformities that can be traced conclusive evidence for repeated episodes ofand infill (Fig. 2b). They suggest that tsedimentation regime, which forms the marHigh-amplitude seismic reflection strength adeepest points of the unconformities maydeposits within the thalwegs of former canyooccur within the gas hydrate stability zone, i.esuch sand deposits may be promising targetsexploration as they would have much higher pthe surrounding fine grained sediments.

The fact that there is no significant character from the area dominated by the re

Fig. 2. 3D seismic cross-section along the Formosa Rthermal control on the gas hydrate system. Fig. section through the pipe structure underneath thethe seismic character of the sediment waves in examples in Fig 2b-d. The location of the line is s

thickness of the om currents have e margin for most eloped (Fig. 2d). consists of fine-

c data also show laterally and are

f canyon incision the present day rgin is not new. anomalies at the

y represent sand ons. Where these e. above the BSR, s for gas hydrate permeability than

change of BSR efilled submarine

canyons as compared to the area thawaves suggest that the style ofinfluence significantly the formationPossibly the sediments are not undulations of the reflections in thethat they were also influenced by bcontourites in the southern part ofmost of Formosa Ridge consists except for the thalweg of some of thconsist of sand as indicated by the h

V. CONCLU

Geomorphologically Formosa structure as opposed to the compremargin farther to the southeast. Tsediment wave deposits in the soutand by infilled canyons in its norhalves of the ridge are of differpossibly of different age.

Ridge axis. Fig.2a shows the overall sub-bottom depth variation of the 2b shows one of the unconformities that are caused by previous canyo

e southern summit of Formosa Ridge with the site of chemosynthetic ethe southern part of the study area. The black boxes in Fig. 2a indi

shown in Fig. 1.

at is dominated by sediment f sedimentation does not n of hydrate at a later stage. too different. The slight

e canyon infill may suggest bottom currents just as the f the study area. Probably of fine grained sediments

he older canyons which may igher seismic amplitudes.

USIONS Ridge is an erosional

ssional ridges of the active The ridge is made up of thern half of the study area rthern half. Thus, the two rent geological origin and

BSR which indicates the strong on incisions. Fig. 2c. is a cross-cosystems above. Fig. 2d shows cate the position of the seismic

Fig. 3. 3D seismic cross-section across the Formosa

characteristics of a weak double BSR east of the sFormosa Ridge. The location of the line is shown

There is ample evidence for gas as a hydrate-related BSR is presenthe significantly different depth of tprimarily to topographic effects oridge. However, there are indicaincision has perturbed the thermaleading to even shallower BSR canyon.

The different geological originridge does not seem to affect the very much as the BSR exists eveamplitude.

REFERENC

[1] E. D. Sloan Jr, “Gas hydrates: Review

Energy and Fuels, vol. 12, no. 2, pp. 19[2] C.-S. Liu, P. Schnurle, Y. Wang, S.-H

Hsiuan, “Distribution and charactersouthwestern Taiwan,” Terr. Atmos. 615–644, 2006.

[3] J. P. Foucher, H. Nouzé, and P. Heinterpretation of a double BSR on thevol. 3109, pp. 1–15, 2002.

[4] C.-W. LIN, S. TSUCHIDA, S. Lin“Munidopsis lauensis Baba & de SAnomura, Munidopsidae), a newly reseep in Taiwan,” Zootaxa, vol. 3737, n

Ridge showing the southern summit of in Fig. 1.

hydrates in the entire ridge nt everywhere. We attribute the BSR below the seafloor n the thermal state of the ations that recent canyon al state of the sediments depth below the northern

n of different parts of the distribution of gas hydrate

erywhere and is of similar

ES

w of physical/chemical properties,” 91–196, 1998. H. Chung, S.-C. Chen, and T.-H. rs of gas hydrate offshore of Ocean. Sci., vol. 17, no. 4, pp.

enry, “Observation and tentative Nankai slope,” Marine Geology,

, C. Berndt, and T.-Y. CHAN, Saint Laurent, 1992 (Decapoda, corded squat lobster from a cold o. 1, p. 92, Nov. 2013.