Shallow seismic characteristics and distribution of gas in lacustrine sediments at Lake Erçek, Eastern Anatolia, Turkey, from high-resolution seismic data

Abstract

The high-resolution analysis of single-channel, seismic re ection data from Lake Erçek (Eastern Anatolia) revealed a wide range of shallow gas anomalies consisting of enhanced re ections, seismic chimneys, acoustic blanking/acoustic turbidity, strong re ectors, and pockmarks, including both surface and buried pockmarks. The enhanced re ections are represented by the higher-amplitude re ection patterns resulting from high acoustic impedance variations. They are mostly clustered in the NW-corner of the lake. Seismic chimneys are represented by vertical and thinned columnar disturbances of amplitude blanking and mostly occurred in deep basinal and faulted sections in the West and East of the lake. Some seismic chimneys, occurring together with pockmarks, represent vertical vent activations. Acoustic gas-masking was represented by chaotic and diffuse seismic re ection patterns, including acoustic blanking and acoustic turbidity. As diffuse acoustic turbidity indicates gas-charged sediments, columnar disturbances showing acoustic blanking indicate degassing of the sediments. These features extend from SE to NW, coinciding with the deep basin morphology of the lake. A very local strong re ector was identi ed in the western section of the lake, simulating the lake oor. This re ector is due to extended enhanced re ections, suggesting shallow free gas. Pockmarks observed in the lake are structurally classi ed into the two distinct types; surface (active) pockmarks found in the SE-part of the lake and buried (passive) pockmarks found in the NW. The former enlarge through deeper gas reservoir feedback, as the layering is impermeable, while the latter have resulted from a cessation of the reservoir feedback mechanism and/or permeable layering. In the lake, shallow gas distribution is controlled by faults, that provide the faultingdriven depositional control and earthquakes, that provide the seismicity-driven overpressure control. The shallow gas is then both vertically and horizontally distributed and, nally, distribution is shaped by asymmetric depositional and stratigraphic factors. This study of Lake Erçek presents complementary information about a possible origin of observed shallow gas and is important in identifying the pockmark types to be targeted for oil/gas exploration and valuable geochemical proxies.

Controls on shallow gas distribution, migration, and associated geohazards in the Yangtze subaqueous delta and the Hangzhou Bay

“…Formation of the gas chimney is generally thought to be associated with gas migration upwards (Hustoft et al, 2010;Cukur et al, 2013). The gas chimney is characterized as a narrow area of vertical disturbances, where the reflection phase is undular and distorted (main performance is acoustic turbidity) compared with the surrounding horizontal and vertical gas-free sediments (Ye et al, 2003;Cukur et al, 2013;Toker and Tur, 2021) (Figures 2, 4). They serve as gas migration pathways from deep gas-charged sediments upwards.…”

“…Seismic and acoustic explorations have been widely used for investigating gas-related anomalous reflections and mapping gas distribution in sub-bottom sediments (Ye et al, 2003;Hu et al, 2012;Cukur et al, 2013;Schneider von Deimling et al, 2013: Hu et al, 2016Yang et al, 2019). Because gas-charged sediments can effectively absorb and scatter the energy of sound waves, which rapidly attenuate inner gas-charged sediments along the vertical direction (Hovland and Judd, 1988;Ye et al, 2003;Coughlan et al, 2021;Toker and Tur, 2021;Yang et al, 2022b), stratal structures usually display acoustic anomalous reflections including acoustic turbidity, acoustic blanking, enhanced reflection, bright spot, gas chimney, and pockmark (Ye et al, 2003;Visnovitz et al, 2015;Coughlan et al, 2021;Toker and Tur, 2021). Shallow gas has been surveyed to be widely distributed in coastal zones, such as the Gulf of Mexico, the Baltic Sea, and the East China Sea (Hovland and Judd, 1988;Zhang et al, 2004;Zhang et al, 2008;Lin et al, 2015;Liu et al, 2019;Chen et al, 2020).…”

Research on the characteristics of fluid escape near the seabed in Bohai Bay

“…The highresolution geophysical data obtained by sub-bottom profiler, single beam echo sounder and multi beam echo sounder all have a good response to shallow gas. Toker and Tur (2021) used high-resolution single-channel seismic data to identify and analyse the features of shallow gas accumulation in Erçek Lake in eastern Turkey, including seismic chimneys, enhanced reflections and acoustic blanking/acoustic blurring. There is also a combination of geophysical data collected in different frequency ranges in previous research data (Duarte et al, 2007;Naudts et al, 2009;Tóth et al, 2014).…”

Geophysical and Geochemical Exploration of the Pockmark Field in the Gulf of Patras: New Insights on Formation, Growth and Activity

“…They often occur along continental margins commonly linked to subsurface structural and/or depositional features, including faults and salt diapirism in several settings [15][16][17], which reinforce fluid escape. Pockmarks have also been identified in shallow waters, including semi-enclosed shallow embayments and even in lakes [18,19] and lagoons, as well as in deep water environments [20,21].…”