Cross-equalization and prestack seismic inversion of time-lapse land seismic data, Central Thrace Basin, Turkey

Abstract

The Thrace Basin of Turkey has sandstone and carbonate reservoirs of Eocene and Oligocene age. Mapping undrained sweet spots from seismic data is currently a challenge, especially because there is no water drive after production. Time-lapse (4D) land seismic data was used to investigate undrained liquid gas pockets and to help with the planning of gas storage into the depleted reservoirs. Two vibroseis 3D baseline and monitor surveys were acquired in 2002 and 2011, respectively. The 4D seismic data has low repeatability and required conditioning to reduce the nonrepeatable 4D noise that is not related to production. Cross-equalization is applied to the base and the monitor data to bring out similarities (noise cancellation by subtracting seismic volumes), and the inverted rock properties primarily show changes due to production. We cross-equalized the corresponding angle-band stack cubes prior to prestack inversion as the available cross-equalization software was implemented for poststack seismic data. The cross-equalized angle-band stacks from the base and the monitor surveys were separately inverted to acoustic impedance (Zp), shear impedance, and density. The differences of inverted rock properties and other pore-fill indicators like Zp, P-wave velocity over S-wave velocity, and BulkRho cubes were created to map possible undrained zones after 10 years of production. A poststudy well that was drilled following the 4D analysis penetrated an undrained gas pocket.