Distribution and complex trace analysis of shallow gas accumulation in the Southern Marmara Sea

The aim of this study is the investigation of shallow gas accumulations along North Anatolian Fault and its branches in Southern Marmara Shelf in Sea of Marmara. For this purpose, 1000 km 2D multi-channel seismic reflection data, 1300 km chirp data and 967 km sparker data have been collected in 2013 and 2014. In collected data set, the units which can be considered as shallow gas accumulation anomalies, such as pockmark, buried pockmark, acoustic blanket, gas chimney, gas seep to water column, bright spot, strong reflection and velocity pull-down have been determined. Multi-channel seismic reflection and sparker data has being subjected to standart data processing and then interpreted. Moreover, Complex Trace Analysis (Attribute) are applied to some of the multi-channel seismic reflection and sparker data. The interpretaitons of envelope, instantaneous frequency, apparent polarity, instantaneous phase, integral and derivative sections, have another evidential value for the gas potential of Southern Marmara Shelf. Shallow gas accumulation anomalies are mapped in the Southern Marmara Shelf. In study area, gas seep and active pockmarks have been discovered. It is interpreted that gas seeps and pockmarks are associated with active tectonics and some of the pockmarks occured along fault. Buried pockmarks are evident below the Holocene deposits. Organic-rich materials from rivers during lowstands caused formation of acoustic blankets and gas columns in front of the Kocasu, Gönen and Biga Rivers and these anomalies are associated with the sediments that are before the last sea level change. These units have been mainly observed together with strong reflection with positive polarity and bright spots with inverse polarity. In chirp data and onboard echosounder data, it has been observed that some of the gas chimneys reached to seafloor and form seeps to the water column. Decrease in velocity because of the gas accumulations caused velocity pull-downs. There are also structures, which are called mud diapirs on sections and these structures are thought to be related with the pockmark formation. It is thought that gas accumulations in the Southern Marmara Shelf are hydrocarbons, which has biogenic origin in front of the river and in delta areas, and at the outer shelf the gas accumulations could be thermogenic origin migrating through deep faults. Keywords: Sea of Marmara, Southern Marmara Shelf, shallow gas accumulations, seismic reflection, complex trace analysis