High-Resolution Seismic Characterization of Shallow Gas Accumulations in the Southern Shelf of Marmara Sea, Turkey

Vardar, Denizhan; Alpar, Bedri

doi:10.1515/acgeo-2015-0059

Cited by 14 publications

(19 citation statements)

References 55 publications

(69 reference statements)

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“…After the destructive 1999 Kocaeli Earthquake by fracturing of the North Anatolian Fault (NAF) in the east of the Marmara region, a number of scientific cruises (listed in Appendix) have been carried out in the Sea of Marmara, providing evidence of fluids and free gas emissions along or near the NAF-N (Alpar, 1999;Armijo et al, 2005;Bayrakci et al, 2014;Crémière et al, 2012;Embriaco et al, 2014;Geli et al, 2008;Halbach et al, 2004;Kuşçu et al, 2008;Tryon et al, 2012). Additionally, a number of pockmarks, gas accumulations and seepages have been observed in the Southern Shelf of the Marmara Sea along the central branch of the NAF (Okay and Aydemir, 2016;Vardar and Alpar, 2016). Tectonic control of the NAF-N seems as the most significant factor for the occurrence of fluid flows (gas, oil, groundwater, brine, etc.)…”

Section: Introductionmentioning

confidence: 99%

Gas occurrence and shallow conduit systems in the Western Sea of Marmara: a review and new acoustic evidence

et al. 2018

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Based on 3D and 2D high-resolution multichannel seismic reflection data in the Western High-Sea of Marmara, this study reviews shallow gas occurrence and related structures and classifies gas conduit systems within the upper, few hundred meter-thick sediment layers below the seafloor. Acoustic anomalies including high amplitude-reverse polarity reflections (bright spots), low amplitude transparent zones, chaotic or discontinuous reflections, pull-down effects, and plumes in the water column are interpreted in terms of natural gas occurrence and fluid flow structures (e.g., mud volcanoes, pockmarks). The gas occurrence is thought to be mostly of thermogenic origin. Mud volcanoes are one of the primary gas conduits forming craters on the seabed due to overpressure of fluidized gassy sediment flows. Following the reach of the Northern Branch of the North Anatolian Fault (NAF-N) to the Western High, the thermogenic fluids are believed to migrate vertically and horizontally to shallow depths mainly through the faults. Natural gas most probably originates from the Thrace Basin Eocene source rock or the Eocene-Oligocene reservoir rock, which extends below the Western High. Shallow gas is distributed by minor faults and gas pipes. Gas, to some extent, emanates from the seafloor via pockmarks and mud volcanoes or is trapped by the crests of the anticlines coinciding with erosional surfaces, impermeable sediments, and gas hydrate-bearing layers. Shallow traps below the tectonized "Western High" structure are likely located in thin layers of sands imbedded with impermeable silty clay layers. However, there is no shallow reservoir in the usual sense within the upper layers imaged by the 3D seismic data (< 300 ms two-way travel time). The existence of gas is an indicator of hydrocarbonrich layers at depth and of active tectonics, and it also impacts the global climate and marine life conditions.

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Section: Introductionmentioning

confidence: 99%

Gas occurrence and shallow conduit systems in the Western Sea of Marmara: a review and new acoustic evidence

et al. 2018

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“…Gas bearing sediments are a globally phenomenon observed from coastal areas to the deep ocean but are quite common in shallow environments with a depth of less than 50 m (Judd, 2004, Vardar andAlpar, 2016). The shallow gas accumulation in the first kilometer of sediment may have biogenic or thermogenic origin (Iglesias and García-Gil, 2007;Wessels, et al, 2010).…”

Section: Introductionmentioning

confidence: 99%

“…The gassy sediments can be easily identified by highresolution reflection seismic data (Vardar and Alpar, 2016). Depending on their seismic character, size and geometry, various descriptive features have been used for gas charged sediments, such as acoustic blanket, black shadow, plumes, curtain, and acoustic turbidity (Taylor, 1992;Yun et al, 1999;Garcia-Gil et al, 2002;Baltzer et al, 2005).…”

Section: Introductionmentioning

confidence: 99%

High-Resolution Acoustic Mapping of Gas Charged Sediments and Living Benthic Foraminifera Assemblages From the Ne Region of the Guanabara Bay (Rj, Brazil)

et al. 2016

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. High-resolution acoustic mapping of gas charged sediments and living benthic foraminifera assemblages from the NE region of the Guanabara Bay (RJ, Brazil). Journal of Sedimentary Environments, 1 (3): 360-384. AbstractThis work was performed in the NE region of the Guanabara Bay, a highly impacted Brazilian coastal system, located in Rio de Janeiro State. It aimed to: i) identify and map the areas with occurrence of gas in the sediment, as well as its acoustic signature; ii) characterize the physical properties of the sediments and; iii) document the response of microbenthic organisms (living benthic foraminifera) to changes in quantity and quality of organic matter. Seismic surveys at the frequency of 12 kHz identified a large area with about 50% gas charged sediments in the study area.The main acoustic signatures of the shallow gas were black shadow and gas blanket. In addition, features related to gas seepages to the water column (acoustic plumes and pockmarks) and gas percolation within the sediments (intrasedimentary plumes, turbidity pinnacles) were also identified. The gas has a biogenic origin and results from the high acumulation rate between 0.03 to 0.9 cm.year -1 and from the decomposition of large amount of organic matter (10-20%). Vertical distribution of gas ranges from few centimeters to 9 m below the water-sediments interface. These occurrences are related to both gas migration from lower sedimentary layers to Holocene muds above, and to recent generation in near-surface sediments as the area display favorable conditions for gas production. Cores ranging from 150-240 cm in length have predominantly muddy sediments and variations in the P-wave velocity followed the changes in sediment density, controlled mainly by the presence of gas in sediments, bioclasts accumulation, textural variation and percentage of organic matter. Journal of Sedimentary Environments

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“…Each author has conducted different background studies researching gas concentration in near-surface sediments. Broadly, there are four kinds of background study, as follows: • Geological and geophysical analysis about how the gas is formed, how the acoustic anomalies emerge, and how its morphological features such as free gas-pockmark (Webb et al, 2009;Canet et al, 2010;Zsuzsanna, 2013;Vardar & Alpar, 2016). Gas bearing sediments form transparent zones and mask deeper reflections on chirp seismic sections.…”

Section: Introductionmentioning

confidence: 99%

“…Other clues supporting the evidence of free gas in sediment are the pockmarks. The appearance of shallow sediments characteristic depend on tectonic setting and depositional systems (Vardar & Alpar, 2016). • Hydrocarbon exploration (Hakan et al, 2018;Canales, 2001).…”

Section: Introductionmentioning

confidence: 99%

Acoustic And Geochemical Evidences Of Shallow Gas Distribution Offshore Waropen Basin, Papua, Indonesia

Firdaus¹,

Albab²,

Subarsyah³

et al. 2021

BGSM

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Sub-Bottom Profiling (SBP) records and results of geochemical analysis of 12 surficial sediment cores from various water depths collected from the offshore Waropen Basin-Papua are presented. Presence of gas is clearly observed on sub-bottom profiler records. Shallow gas was identified through acoustic response due to gas accumulation and gas escape on sub-bottom profiles. Acoustic evidences of gas accumulations within near surface geology consist of high amplitude reflections and associated acoustic blanking, gas plumes and morphological features like pockmarks. Total organic carbon analysis of 12 surface sediment cores varies between 0.5% to 1.3% which indicate that the sediments have an abundance of organic matters. Gas chromatographic analysis of hydrocarbon composition detected only methane, a biogenic origin of shallow gas. Acoustic and geochemical evidence in the Waropen Basin indicates extensive shallow gas accumulations in the Late Quaternary sediments, some trapped within these deposits and some escape from seabed into the water column which then created a high distribution of pockmarks.

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High-Resolution Seismic Characterization of Shallow Gas Accumulations in the Southern Shelf of Marmara Sea, Turkey

Cited by 14 publications

References 55 publications

Gas occurrence and shallow conduit systems in the Western Sea of Marmara: a review and new acoustic evidence

Gas occurrence and shallow conduit systems in the Western Sea of Marmara: a review and new acoustic evidence

High-Resolution Acoustic Mapping of Gas Charged Sediments and Living Benthic Foraminifera Assemblages From the Ne Region of the Guanabara Bay (Rj, Brazil)

Acoustic And Geochemical Evidences Of Shallow Gas Distribution Offshore Waropen Basin, Papua, Indonesia

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