Tectonic Oil Potential Relationships, Marmara Sea, NW Turkey

Abstract

Gas discovery in the K. Marmara field in northern Marmara Sea proved the generation, migration, and accumulation of hydrocarbons in the offshore area in NW Turkey. Two principal tectonic events determined the petroleum potential of this region: subduction systems during Late Cretaceous time due to the obduction of the Istranca-Rhodope Massif southward onto the Karakaya Zone, which started rifting since Late Triassic-Early Jurassic, and the emplacement of the Ergene Wedge, distinguishable on the Thrace Basin's gravimetric map to the Northwest of the Marmara Sea in Miocene. Latest Miocene tectonic activities, which generated the North Anatolian Fault (NAF) and its splays in the Thrace Basin and in the Marmara Sea, improved the reservoir characteristics with the appearance of fractures and structures in the reservoir units. Evaluations and interpretations of seismic and aeromagnetic data indicate E–W oriented subduction zones at the central part of the Marmara Sea. The westward onshore extension of these zones continues toward the Sarkoy-Gelibolu-Saros Gulf area. While the prospective Eocene Sogucak reefoidal carbonates were deposited on the Late Cretaceous ophiolitic paleohighs, tested in Sarkoy-1, Tepekoy-1, Murefte-1, and Eriklice-1 onshore and Doluca-1 and Isiklar-1 offshore oil exploration wells, the Oligocene Osmancik sandstones, other prospective units, were sedimented in a deltaic environment. On the other hand, an important tectonic and structural feature, the Ergene Wedge, which is reflected by gravimetric map, occurs to the northwest of the study area during Miocene time. It widens toward the Greek boundary but narrows eastward. Due to tectonic activities, high temperatures, pressures, and strains should be accumulated in this zone, which generated necessary conditions for oil generation and migration in and around the Marmara Sea. The Eocene Sogucak (Mecidiye) and Oligocene Osmancik (Gazhanedere) formations constitute the main reservoir units, while the Eocene Hamitabat and Eocene-Oligocene Mezardere shales are accepted as the principal oil source rocks in the Thrace Basin. Geological and geophysical interpretations reveal that the Sogucak and Osmancik formation display interesting prospective trends in the Marmara Sea.