SEISMIC EVENTS IN THE UPPER MIOCENE – PLIOCENE SEDIMENTARY SUCCESSION IN THE GULF OF İZMİR (WESTERN ANATOLIA): IMPLICATIONS FOR HYDROCARBON PROSPECTIVITY

Z. Altan^1*, N. Ocakoğlu¹, G. Böhm² and K. Tuncer Sarıkavak³

¹ Istanbul Technical University (ITU), Faculty of Mines, Department of Geophysical Engineering, Turkey.

² Istituto Nazionale di Oceanografia e Geofisica Sperimentale (OGS), Italy.

³ Mineral Research and Exploration General Directorate (MTA), Turkey.

^* corresponding author: zaltan@itu.edu.tr

Key words: Direct hydrocarbon indicator, low velocity zone, reflection tomography, pre-stack depth migration, AVO analysis, Gulf of Izmir, Western Anatolia, Turkey.

An analysis of multi-channel seismic reflection data integrating reflection tomography, pre-stack depth migration, AVO analysis, seismic modelling and seismic attribute analysis was used to investigate the Miocene – Quaternary stratigraphy of the Gulf of İzmir, western Anatolia. In this area, the east-west oriented Gediz graben intersects with the NE-SW oriented Bakırçay Graben. A velocity-depth model together with pre-stack depth migration allowed two seismic stratigraphic units (SSU1 and SSU2) to be distinguished. These units can be correlated with the stratigraphic succession at the offshore Foça-1 well and correspond to the Upper Miocene to Recent Bozköy, Ularca and Bayramiç Formations with a combined thickness of 1.75 km. The units rest on acoustic basement (SSU3) which has a basin-and-ridge morphology, and which corresponds to the Lower-Middle Miocene Yuntdağ Volcanics. A number of lateral velocity variations were identified. In particular, a ~90 m wide and ~500 m long lenticular-shaped low-velocity zone with an interval velocity of 1.68 km/s was identified in the Quaternary Bayramiç Formation. The structure is bounded by negative reflections whose amplitude increases with offset at the top and by strong positive reflections whose amplitude increases with offset at the base, interpreted as possible bright and flat spots respectively. These amplitude events point to the presence of gas-saturated sediments within the study area. The lenticular structure is bounded by strike-slip faults on either side, and by a Miocene – Pliocene unconformity surface below and by shales of the Bayramiç Formations above. It is therefore interpreted as a possible structural – stratigraphic trap. The strike-slip faults may allow the migration of hydrocarbons from source rocks located at greater depths. The presence of a low-velocity zone above the lenticular structure reaching up to seafloor may indicate the upward leakage of hydrocarbons from the trap. These observations will contribute to future hydrocarbon exploration activities in the study area.

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