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

Mustafa Toker,Hüseyin Tur

doi:10.1007/s12665-021-10039-4

Mustafa Toker, Hüseyin Tur

Open Access

https://doi.org/10.1007/s12665-021-10039-4

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Abstract

The high-resolution analysis of single-channel, seismic reflection data from Lake Erçek (Eastern Anatolia) revealed a wide range of shallow gas anomalies consisting of enhanced reflections, seismic chimneys, acoustic blanking/acoustic turbidity, strong reflectors, and pockmarks, including both surface and buried pockmarks. The enhanced reflections are represented by the higher amplitude reflection 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 reflection 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 reflector was identified in the W-section of the lake, simulating the lake floor. This reflector is due to extended enhanced reflections, suggesting shallow free gas. Pockmarks observed in the lake are structurally classified 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 faulting-driven depositional control and earthquakes, that provide the seismicity-driven overpressure control. The shallow gas is then vertically–horizontally distributed and shaped by asymmetric depositional–stratigraphic factors. This study of Lake Erçek presents complementary information about a possible tectono-thermal origin of observed shallow gas.

Highlights

Methane gas emission, migration and concentration, one of the most potent threats to the Earth’s atmosphere (Jaśniewicz et al, 2019), is a strong mechanism for the production of greenhouse gases (Judd et al 2002)
The gas-induced acoustic impedance variations result in high amplitude reflections, representing the enhanced reflections, which are most densely observed in the NW-corner of the lake
Some of the seismic chimneys are closely associated with pockmarks, and are characteristic of active vent structures

Summary

Introduction

Migration and concentration, one of the most potent threats to the Earth’s atmosphere (Jaśniewicz et al, 2019), is a strong mechanism for the production of greenhouse gases (Judd et al 2002). The global warming potential of methane is more than 20 times higher than CO2 (Hovland and Judd 1988, Hovland et al 1993, Judd et al 2002, Lowe and Walker 1997; Vardar and Alpar, 2016) These mechanisms include methane fluxes in the seabed and into the air due to the exploitation of fossil fuels leading to potential instability in gas reserves in shelves and slopes (Çifçi et al, 2003) and free gas accumulation in sediment pore spaces (e.g., Prior and Coleman 1984; Hovland and Judd 1988; Yun et al 1999; Çifçi et al, 2003). Previous studies report that shallow water gas emission contributes as much as 20% of total methane emission to the air (Hovland et al 1993; Fleischer et al 2001; Jaśniewicz et al, 2019)

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