Seismoacoustic Study of the Shallow Gas Transport and ... - E-LIB

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Chapter 1 Chapter 1. Introduction 1.1 Gas and fluids in marine sediments In present-day oceans, shallow gas commonly accumulates in marine sediments, which has been reported in over 100 locations worldwide (Fleischer et al., 2001). Methane is almost always the dominant component of the natural gas mixtures. It can originate from biogenic or thermogenic processes (e.g. Floodgate and Judd, 1992). Biogenic methane is produced from organic matter by methanogenic bacteria, which generally occur in shallow marine sediments (Claypool and Kaplan, 1974; Sloan, 1990). However, biogenic methane can also be generated in considerable depths (examples from as deep as 3350 m sub-bottom are quoted by Rice and Claypool, 1981). Methane and higher hydrocarbons are generated thermogenically from organic matter by catagenesis and metagenesis at temperatures between 50 °C and 200 °C at greater depth (Fig. 1), which is normally greater than 1000 m (Floodgate and Judd, 1992). Due to the different nature of the original organic material and the different depth of burial, various hydrocarbon compounds may be formed (Judd, 2003). Methane is the most abundant and mobile gas, and it can migrate to the surface to form shallow gas accumulations. Figure 1 Generalized relationship between temperatures, hydrocarbon generation, diagenesis (after Shu, 2012) Fluid flow is common in marine sedimentary basins, which are sediment-filled depressions and preserve sufficient permeability to permit fluid flow. Many basins contain important mineral and energy resources, which have been generated by fluid flow or the interaction between fluid and rocks (Bitzer, 2001). About the origin of the fluid flow, internally derived fluids such as formation waters and hydrocarbons, and externally derived fluids such as meteoric and metamorphic fluids are distinguished (Lawrence and Cornford, 1995). Another main source of internal fluids is clay dehydration which may lead to the overpressure in the sedimentary basins. Overpressure in the sedimentary 5
Chapter 1 basins can be generated by many dynamic mechanisms, including disequilibrium compaction (Bethke 1986; Shi and Wang 1986), tectonic collision (Ge and Garven 1992), aquathermal expansion (Sharp 1983), clay dehydration (Burst 1969), gravity flow (Toth 1962; Lee and Bethke 1994; Wolf et al. 2005), gas capillary seals (Revil et al. 1998; Lee and Deming 2002) and hydrocarbon generation (Luo and Vasseur 1996; Lee and Williams 2000). Overpressure can create and widen faults and fractures, which serve as the main migration pathway for upward-moving fluids. For fluids to migrate upwards, driving forces are needed to overcome gravity. These include pressure gradient, buoyancy and diffusion. Among these forces, buoyancy plays a dominant role in fluid migration in sediments. It can drive the fluids and hydrocarbons not only through available pathways (e.g. faults, cracks) or porous paths, but may also make their own way through rather impermeable sediment intervals (Nunn, 1996; Ding, 2008). Figure 2 Examples of seismic evidence (e.g. Bright Spots, enhanced reflections, gas chimney and acoustic blanking) are shown in the 2D seismic section in Nyegga (after Plaza-Faverola et al., 2012). Methane can dissolve in the fluid or be in a gaseous phase, migrating either in a diffusive or focused way through sediments (Ginsburg and Soloviev, 1997). Before reaching the sea floor, most of the methane is trapped or consumed, such as dissolved in interstitial water, stored as gas accumulations, condensed to gas hydrates or consumed by the anaerobic oxidation of methane (AOM). Although it has global significance, the mechanism of AOM is still uncertain (Boetius et al., 2000). Although it is difficult to detect the gas directly, variations in seismic attributes may indicate the change of physical properties of marine sediments caused by the gas migration and accumulation (Minshull and White, 1989). According to Judd and Hovland (2007), the main types of seismic evidence of gassy sediments include: (1) Acoustic turbidity, acoustic energy is attenuated by gas bubbles in marine sediments; (2) Enhanced 6
Page 1: Seismoacoustic Study of the Shallow
Page 4 and 5: Table of contents Table of contents
Page 6 and 7: Abstract Abstract In recent years,
Page 8 and 9: Outline of the Dissertation Outline
Page 12 and 13: Chapter 1 reflections, interpreted
Page 14 and 15: Chapter 1 Submarine gas seeps, pock
Page 16 and 17: Chapter 1 which have documented act
Page 18 and 19: Chapter 1 fluid migration related t
Page 20 and 21: Chapter 1 Evidence of gas seepage
Page 22 and 23: Chapter 1 The three main different
Page 24 and 25: Chapter 1 from deeper sources in th
Page 26 and 27: Chapter 1 numerous countries such a
Page 28 and 29: Chapter 1 1.5 Geological setting of
Page 30 and 31: Chapter 1 main sequences consist of
Page 32 and 33: Chapter 1 craterlike depressions wi
Page 34 and 35: Chapter 1 Jurassic - Paleogene, Upp
Page 36 and 37: Chapter 1 respectively, and one wat
Page 38 and 39: Chapter 1 During the seismic data p
Page 40 and 41: Chapter 1 1.6.2 The Parasound sedim
Page 42 and 43: Chapter 1 Caress, D.W., and Chayes,
Page 44 and 45: Chapter 1 Hovland, M., Gardner, J.V
Page 46 and 47: Chapter 1 Luo, X.R. and Vasseur, G.
Page 48 and 49: Chapter 1 Schroot, B.M., Klaver, G.
Page 50 and 51: Chapter 2 Chapter 2. Shallow Gas Tr
Page 52 and 53: Chapter 2 times. Seismically imaged
Page 54 and 55: Chapter 2 spike noise were marked a
Page 56 and 57: Chapter 2 The 2D high resolution se
Page 58 and 59: Chapter 2 Figure 4 A close-up of se
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Chapter 2 within the upper sediment
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Chapter 2 volcanoes between 66 and
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Chapter 2 Figure 7 Geo-seismic sect
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Chapter 2 In the study area, many b
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Chapter 2 volcanism could be calcul
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Chapter 2 along the feeder channel
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Chapter 2 Evpatoriya). Soviet Geol
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Chapter 3 Chapter 3. Sedimentary Ev
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Chapter 3 South (Fig. 2). The water
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Chapter 3 Figure 3 (a) Uninterprete
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Chapter 3 Seven seismic facies type
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Chapter 3 Seismic Unit 5 ranges in
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Chapter 3 3.4.3.5. Seismic Unit 2 A
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Chapter 3 Figure 11 (a) Uninterpret
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Chapter 3 Figure 13 (a) Uninterpret
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Chapter 3 boundary between the Plio
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Chapter 3 Seismic Unit 6 belongs to
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Chapter 3 subsidence was coincident
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Chapter 3 Acknowledgements We thank
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Chapter 3 Tugolesov, D.A., Gorshkov
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Chapter 4 study area at ~900 m wate
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Chapter 4 Figure 2 Bathymetry of th
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Chapter 4 University of Bremen). At
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Chapter 4 and Ridge R2 (GeoB07-139)
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Chapter 4 flares location. Anticlin
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Chapter 4 GeoB07-145. Green arrows
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Chapter 4 4.4.3.2.3. Other anomalou
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Chapter 4 Slope fan deposits with f
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Chapter 4 Figure 12 Possible locati
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Chapter 4 bulk hydrate or even high
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Chapter 4 may add to gas accumulati
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Chapter 4 The narrow-beam Parasound
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Chapter 4 River fan deposits. Being
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Chapter 4 Klaucke, I., Sahling, H.,
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Chapter 4 Wagner-Friedrichs, M., 20
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Chapter 5 deposited; during lowstan
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Chapter 5 accumulations. An acousti
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Acknowledgements Acknowledgements I
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