Acoustic evidence for shallow gas accumulations in the sediments of the Eastern Black Sea*

Transcription

1 Acoustic evidence for shallow gas accumulations in the sediments of the Eastern Black Sea* Mustafa Ergün, Derman Dondurur and Günay Çifçi Dokuz Eylu l University, Department of Geophysics, Tınaztepe Campus, 35160Kaynaklar-Buca, Izmir, Turkey ABSTRACT The Black Sea contains immense gas accumulations. Exploration of gas accumulations is geologically and economically important because migration of methane in sediments may cause massive slope failures and the methane seeps may indicate deeper hydrocarbon reservoirs. Human activity both in and on the seafloor (oil industry) and natural activity (earthquakes, cyclones) trigger mechanisms for seafloor failure and gas release that may have a local and possibly global environmental impact. Recently, sonar and high-resolution seismic surveys were carried out to obtain information about the effects of gas and gas-filled sediments throughout the Turkish margin of the Eastern Black Sea, and shallow gas was detected on the subbottom profiler records. It continues about m below the sea floor and is marked by bright and cloudy spots, sometimes pockmarks and acoustic voids. The lower section of the Turkish shelf is an extensive pockmarked plateau. The pockmarks are seen as circular structures with high backscattering on the sonar records. Terra Nova, 14, , 2002 Introduction The Black Sea is a large marginal sea located within complex folded chains of the Alpine system, represented by the Balkanides Pontides belt to the south, and by the Crimea Mountains and Caucasus to the north and northeast. Although the Black Sea is today a single depocentre, it comprises two major extensional basins, probably of different ages, separated by a complex NW SE-trending continental mid- Black Sea Ridge (Finetti et al., 1988; Robinson et al., 1996). The Western Black Sea Basin (WBSB) was opened by the separation of the Western and Central Pontide continental strip from the Moesian Platform and Odessa Shelf (S engo r and Yılmaz, 1981; Zonenshain and Le Pichon, 1986; Manetti et al., 1988; Kazmin et al., 2000). The Eastern Black Sea Basin (EBSB) was opened between the Shatsky Ridge and the Mid-Black Sea Ridge by rotation about a pole west of Crimea (Finetti et al., 1988; Okay et al., 1994; Robinson et al., 1995, 1996). The WBSB is floored by oceanic crust, whereas the crust of the EBSB is highly thinned, possibly oceanic (Ross, 1977). The mountain-building processes and their subsequent erosion around the basin have contributed to high sediment input. Seismic studies indicate a 15-km-thick blanket of sediments with relatively low seismic velocities ( km s )1 ) (Finetti et al., 1988). The study area, where an extensive deltaic and alluvial plain is present, comprises the Turkish shelf, continental slope and apron areas as well as the Turkish margin in the EBSB, and is under a general compressional tectonic regime (Fig. 1). The geological setting of the study area is marked by the Archangelsky Ridge (southern part of the mid-black Sea Ridge) and the eastern Pontides belt. The Archangelsky Ridge defines the transition from EBSB to the Turkish Margin. The Turkish Continental Slope was developed towards the basin margin of the Archangelsky Ridge. On both sides, the Archangelsky Ridge is surrounded by normal fault systems, which were active mainly in the Pliocene (Rangin et al., 2002). The Eastern Pontides Belt, a complex structure formed by a sequence of orogenic Correspondence: Derman Dondurur, Dokuz Eylu l University, Department of Geophysics, Tınaztepe Campus, Kaynaklar-Buca, Izmir Turkey. derman.dondurur@deu.edu.tr *Paper presented at the XI th Meeting of the European Union of Geoscientists, Strasbourg, France, Fig. 1 The locations of the data sources used in this study (solid circles). The numbers next to the circles represent the corresponding figure numbers. The profile directions are indicated in each figure. Ó 2002 Blackwell Science Ltd 313

2 Acoustic evidence for shallow gas accumulations, Black Sea M. Ergün et al. Terra Nova, Vol 14, No. 5, events during the Mesozoic and Cenozoic, bound the Archangelsky Ridge to the south. The northern zone, forming the Turkish margin of the Black Sea, is formed by Upper Cretaceous and Eocene volcanic rocks, asymmetric overturned folds and southward-dipping thrusts (Finetti et al., 1988). Recent studies in marine geology indicate potential geo-resources in the Black Sea (TPAO BP Eastern Black Sea Project Study Group, 1997). The oil-prone source rock is most probably Late Eocene in age and is currently generating both oil and gas in the post-rift basin (Robinson et al., 1996). Methane seeps are common features around the Black Sea basin. Reeburgh et al. (1991) suggested that shelf and slope sediments of high deposition rate are methane sources in the Black Sea. In the deep basin, mud volcano eruptions were observed in the Black Sea and gas hydrates were sampled (Ivanov et al., 1996). The presence of gaseous hydrocarbons in near-surface sediment has been a source of considerable interest to the petroleum community for two main reasons (Kvenvolden et al., 1981): (1) in some instances gaseous hydrocarbons signal the existence of deeper and more extensive hydrocarbon accumulations in frontier basins, and (2) gaseous hydrocarbons may represent a hazard for drilling operations and offshore construction. The main objective of this study was to demonstrate the existence of shallow gas accumulations in the Eastern Black Sea Turkish margin and nearshore areas, and to examine their effects on the shallow sediments. For this purpose, a set of MAK-1 deep-towed side scan sonar and subbottom profiler data was used. The MAK-1 system has a swath range of 500 m per side in long-range (30 khz) mode and of 200 m per side in highresolution (100 khz) mode. The subbottom profiler system operates at a frequency of 6 khz. High-resolution seismic and sonar investigations in the Eastern Black Sea basin Parallel bedded sedimentation in the Eastern Black Sea abyssal plain The Eastern Black Sea abyssal plain is a flat area at a depth of about 2100 m. The shallow geology within the Abyssal Plain mainly consists of parallel thin-to medium-bedded sediments, often interrupted by gas piercing the sequence. The presence of diffusely distributed shallow gas in sediments may cause an acoustically transparent zone on the subbottom profiler sections, often resulting in masking of the sedimentary sequence. There are two distinctive types of bottom surfaces on the abyssal plain: (1) smooth seabed, and (2) hilly surface areas with hummocks (Fig. 2). There are some irregularities on the seabed due to the presence of disturbed sparse hummocky features that are 2 10 m in length, 2 6 m in width and 1 2 m in height. In Fig. 2, the smooth seabed shows low backscattering on the sonographs, whereas the irregular seabed with hillocks has a relatively high backscattering. Acoustic blanking at the Turkish apron The Turkish apron defines the transition from the abyssal plain to the continental slope between 2140 and 1800 m water depth. This area represents a change from the flat and featureless abyssal plain to the Turkish continental slope and margin. Figure 3 shows this transition zone with a number of gas seepages at the sea bottom. The slope apron is characterized by gentle gradients (2 3 ) and disappearance of the channels and gentle subhorizontal, well-bedded recent sediments (C ifc i et al., 1998). In this region large amounts of methane Fig. 2 Record example of side scan sonar (top) and subbottom profiler (bottom) showing regularly spaced hillocks next to a flat and low-backscattering seabed of the abyssal plain. 314 Ó 2002 Blackwell Science Ltd

3 Terra Nova, Vol 14, No. 5, M. Ergün et al. Acoustic evidence for shallow gas accumulations, Black Sea Fig. 3 Gas seepages on the subbottom profiler record from the transition zone of the continental slope to abyssal plain (Turkish apron). Fig. 4 Subbottom profiler record from the Turkish apron region. Uplifting of shallow gas plumes has disturbed the uppermost sediments. Ó 2002 Blackwell Science Ltd 315

4 Acoustic evidence for shallow gas accumulations, Black Sea M. Ergün et al. Terra Nova, Vol 14, No. 5, accumulations may occur. The depth of the shallow gas within the Turkish apron of the Eastern Black Sea area is generally less than 10 m. An extensive discontinuity is observed within the subbottom sediments of the Turkish apron as a result of acoustic blanking of the shallow gas. Shallow gas accumulations appear to have great influence on all of the processes occurring in this region. Figure 4 shows an area of gas plumes through the sedimentary sequence. Gas uplifting clearly disturbs the upper sedimentary layers. Slumps and slides in regions of more mobile sediments, with free gas filtration in the continental slope areas, are commonly observed (C ifc i et al., 2000). Growth faults and gas-saturated sediments at the continental slope areas Water depths range from approximately 150 to 1800 m on the Turkish continental slope. Parallel-bedded slope sediments with a thickness of 5 40 m overlie an unconformable Fig. 5 Side scan sonar (top) and subbottom profiler (bottom) record of a gas dome in the Turkish continental slope. A gas-escape feature is seen as a circular high backscattering structure. 316 Ó 2002 Blackwell Science Ltd

5 Terra Nova, Vol 14, No. 5, M. Ergün et al. Acoustic evidence for shallow gas accumulations, Black Sea Fig. 6 Subbottom profiler record showing vertical growth faults on the upper side of the continental slope. Fig. 7 Subbottom profiler section of a broad mound at the upper side of the Turkish continental slope in the Yesilırmak fan. This mound is important in transportation of the fan sediments to the continental slope. Ó 2002 Blackwell Science Ltd 317

6 Acoustic evidence for shallow gas accumulations, Black Sea M. Ergün et al. Terra Nova, Vol 14, No. 5, surface, above wavy parallel and faulted strata (Ergu n et al., 2001). This area has an average slope of 8 and contains slide-slump masses (C ifc i et al., 2000). Parallel layers abruptly become gas-filled sediments appearing as gas domes (Fig. 5). Several vertical growth faults were observed along almost the entire upper side of the Turkish continental slope (Fig. 6). The throws of these faults are not more than 10 m. Faults intersecting gas-filled sediments may sometimes act as conduits for gas migration to the surface. Several large submarine slide scars are present along the fault traces. Pockmarks and shallow gas pockets on the Turkish shelf The Turkish Shelf of the Eastern Black Sea has an average slope of only a few degrees, and the shelf edge is located at about 150 m depth. Very soft sediments cover almost the entire area, and consist of different gravely sands and silty clay mud (Ergu n and C ifc i, 1999). The area is characterized by intense gas masking, gas seepage and a mottled surface on the shallower Fig. 8 Subbottom profiler record example from the Turkish shelf showing several gas seepage zones (pockmarks). Fig. 9 Subbottom profiler section from Turkish shelf showing a shallow gas pocket. Note the abrupt cutting of the natural and almost parallel sedimentary sequence. 318 Ó 2002 Blackwell Science Ltd

7 Terra Nova, Vol 14, No. 5, M. Ergün et al. Acoustic evidence for shallow gas accumulations, Black Sea part, while parallel beds with a rippled unconformable surface are found on the deeper part. The Turkish shelf area is affected by the Yesilırmak and Kızılırmak river fans (see Fig. 1). Huge amounts of plant debris and organic material from these rivers cause extensive biochemical gas generation (mostly methane) on the shelf (C ifc i et al., 1998). A broad mound of about 3 km width and 50 m height was determined in the direction of the shelf area in the Yesilırmak fan (Fig. 7). This small ridge could play an important role in large-scale accumulation of sediments rich in organic material that produce biochemical gas preventing the further transportation of the sediments from the Yesilırmak River down to the continental slope. The origin of the pockmarks is probably related to local subsidence and faulting due to the escape of gas. In the Black Sea, pockmarks are formed mainly by escape of both biogenic and hydrothermal gas. Pockmarks as circular depressions mark the shelf area with a diameter up to 50 m and a relative depth of several metres (Fig. 8). They show up as dark circular patches on the side scan sonar records. The depth of the shallow gas remains constant at about 20 m in the Turkish shelf area. Gas uplifting often masks the continuation of the recent sediment bedding on subbottom profiler records. Figure 9 is an example of a subbottom profiler record from the Turkish shelf showing a shallow gas pocket that caused an acoustic void. Conclusions The Eastern Black Sea has extensive methane and methane hydrate accumulations, and structures related to gas plumes, gas-saturated sediments and pockmarks can be observed all around the Eastern Black Sea region. Sediments with a high deposition rate at shelf and slope areas are the most common methane sources. The Eastern Black Sea abyssal plain is defined as a parallel and undisturbed sediment sequence. The surface is composed of small hillocks and hummocks. In places, shallow gas continues under the sea floor at about m depth. Because of gas percolation within them, sediments in areas with steep slopes are more mobile than sediments in the continental slope regions. This mobility could distort the natural bedding and promote slope failure and slumps. Two large rivers, the Yesilırmak and Kızılırmak, transport large amounts of organic matter to the study area resulting in extensive generation of biochemical methane on the Turkish shelf. Gas seepage to the sea floor in this region produces large amounts of pockmarks or circular features in tectonically relaxed zones or in regions containing more porous sediments. These structures are seen on the sonographs as dark, circular-shaped depressions. The depth of the gas front is almost constant at 20 m on the Turkish shelf. Gas uplifting sometimes masks the natural sediment bedding resulting in abrupt changes in the continuation on subbottom profiler records. The threat of environmental disaster looms over the Black Sea because of theposition of this marginal enclosed water body. Therefore, the Black Sea has been included within the Regional Seas Action Plan of the UNDP. If the expanding exploration and production industries of the region are to operate safely, the threat from methane seeps must be properly assessed. Therefore, from a geohazards point of view, for safe installation and operation of offshore constructions, the behaviour of the sea floor where gas seepage occurs and the regions of methane accumulations with and without gas hydrate formations should be properly evaluated. References C ifçi, G., Dondurur, D. and Ergu n, M., Sonar and high resolution seismic studies in the Eastern Black Sea Basin. 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8 Acoustic evidence for shallow gas accumulations, Black Sea M. Ergün et al. Terra Nova, Vol 14, No. 5, Margins (A. E. M. Nairn, W. H. Kanes and F. G. Stehli, eds), pp Plenum Publications, New York. Şengo r, A.M.C. and Yılmaz, Y., Tethyan evolution of Turkey. Tectonophysics, 75, TPAO BP Eastern Black Sea Project Study Group, A promising area in the Eastern Black Sea. Leading Edge, 16, 911. Zonenshain, L.P. and Le Pichon, X., Deep basins of the Black Sea and Caspian Sea as remnants of Mesozoic back-arc basins. Tectonophysics, 123, Received 23 October 2001; revised version accepted 6 June Ó 2002 Blackwell Science Ltd