Multi-resolution seismic analysis of a cold seep from İzmir Gulf, Aegean Sea using seismic attributes

Abstract

This study primarily aims to characterize both the shallow and deep structures of shallow gas accumulations and gas seepage by utilizing multi-resolution seismic data. Additionally, it seeks to establish a link between shallow and deep stratigraphy for these gas accumulations and seepage in the outer İzmir Gulf by employing various seismic attributes, such as average energy, relative acoustic impedance, apparent polarity, sweetness, pseudo-relief as well as complex trace attributes. Multi-resolution seismic datsets have been collectively interpreted to obtain deep and shallow architecture of gassy structures in the outer İzmir Gulf. Chirp and sparker seismic data provided information about the shallow gas accumulations as well as seep structures while multi-channel seismic data reveals deep structural and stratigraphic formation and their possible connection with the shallow fluid-flow structures.

Analyses of seismic data and calculated multi-attribute sections indicate that both biogenic and thermogenic gases co-exist in the outer İzmir Gulf. It is concluded that Uzunada Fault Zone acts as a conduit for fluid escape, facilitating gas seeps in the water column above the fault plane. Specifically, the Chirp datasets acquired in 2008 and 2023 indicate that the gas seep along the Uzunada Fault Zone has been active over a period of 15 years along this fault plane.

Depending on the analyses of seismic datasets, observed gas is classified into two groups in terms of its origin: (1) Biogenic gas generated in-situ within the highstand sediments of the Gediz Delta, and (2) thermogenic gas from deeper reservoirs within the Upper Miocene sediments. It is concluded that the gas from the deltaic sediments in the eastern part of the gulf is biogenic in origin formed as a result of the biogenic degradation of organic matter in terrigenous sediments transported by the Gediz River. The possible thermogenic gas accumulations, on the other hand, is originated from the presumed sandy layers interbedded with shale laminations in the Upper Miocene sediments, and migrate along the fault planes through the Plio-Quaternary sediments and accumulate in the shallow sediments.