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

The high-resolution analysis of single-channel, seismic reflection data from Lake Ercek (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 Ercek presents complementary information about a possible tectono-thermal origin of observed shallow gas. Yuzuncu Yil University, Scientific Research Projects-Coordination Unit (SRP-CU), Van, Turkey; Yuzuncu Yil UniversityYuzuncu Yil University [2015-MIM-B119]; University of Oulu (Oulu, Finland) The authors would like to thank Yuzuncu Yil University, Scientific Research Projects-Coordination Unit (SRP-CU), Van, Turkey for supporting the Lake Ercek Seismic Survey project, 2015 (Toker, 2015) and the research vessel scientific crew during the survey. This research was undertaken as part of a multidisciplinary LESS-2015 project of Istanbul University Cerrahpasa (IU), Department of Geophysical Engineering, Istanbul and Yuzuncu Yil University (YYU), Division of Earth Physics, Van (Turkey). The LESS-2015 project was supported by Research Fund of the Yuzuncu Yil University (under Scientific Research Project Number: 2015-MIM-B119), and was partly supported by the University of Oulu (Oulu, Finland) post-doctoral research grant. The authors offer their greatest thanks to the editors and the two anonymous reviewers for their constructive comments and suggestions, which helped to improve the manuscript. The maps in this paper were generated using public domain generics.