Reservoir characterisation of compacted sandstones in the Osmancık Formation, Thrace Basin, Türkiye

The Thrace Basin houses the main gas-producing fields of Türkiye. The gas reservoirs in the basin are mostly located in deltaic sediments of the Oligocene Osmancık Formation where the hydrocarbon-bearing sandstone reservoirs generally reflect class 3 type of amplitude-versus-offset (AVO) anomalies. Our results indicate that AVO behaviour changes from Class 3 to Class 2 response due to the ascending compaction trend of the lithology with increasing depth. Acoustic impedances (AI) show an abrupt increase, because of higher compaction of the sandstones of the Osmancık Formation. Bright spots are the distinctive characteristics of gas sands encased in shale lithology in seismic stack sections. However, at higher depths of the Osmancık Formation, sweet spots tend to disappear due to lower porosity and permeability at the producing reservoir intervals. We propose a workflow to integrate pre-stack seismic analysis and lithology prediction of the field to characterise the gas-producing reservoirs. Spatial distribution of gamma ray values obtained with neural network prediction indicates highly lateral variation of shaliness, which is directly related to reservoir quality in the deeper Osmancık Formation. Therefore, diagenesis in sandstones and increase in shaliness play a crucial role in terms of evaluation of petrophysical properties of the reservoir, such as porosity and permeability.