oa Improved Structural Understanding of a Complex Anticline through Advanced Seismic Processing: A Case Study from Northern Iraq

The Tawke anticline is situated in the Kurdistan region of Northern Iraq in the folded zone of the<br>Zagros fold and thrust belt. The region is characterized by folded Cenozoic carbonates and foreland<br>basin siliciclastics, with considerable differences in mechanical properties. Rough terrain, narrow<br>structures and strong velocity contrasts pose significant challenges to seismic imaging in the area.<br>In 2006, DNO completed the first ever 3D seismic survey in Iraq with the objective of determining the<br>subsurface structure of the Tawke oil field discovered in 2005. Since then, the seismic data have been<br>processed 3 times with progressively more advanced techniques to meet geologists’ and reservoir<br>engineers’ demands for resolution and accuracy.<br>The first processing involved elevation statics and a fast-track time migration. The main purpose of this<br>dataset was to perform early volume calculations and well planning. Despite several successful wells,<br>the data showed clear limitations in imaging of complex thrust structures.<br>The second processing project was initiated in 2007 using anisotropic prestack time migration based on<br>the AutoImager velocity model building. This semi-automated method used the initially preprocessed<br>data as input and provided significantly improved imaging of steeply dipping interfaces and faults. The<br>key improvement factor was the AutoImager iterative migration velocity analysis producing a<br>consistent seismic velocity model with anisotropy correction.<br>Seismic modelling and depth conversion tests were conducted in 2008 using 2D image ray tracing<br>techniques. Results showed significant ray path bending because of strong lateral velocity contrasts<br>between Cenozoic carbonates and siliciclastics. This implies that time migration images are inaccurate<br>and conventional depth conversion by vertical stretching is not applicable. To solve these problems, the<br>Tawke 3D data were reprocessed using prestack depth migration (PSDM) which migrates seismic<br>events to their correct positions provided that the velocity model is correct. The PSDM results show<br>better agreement between seismic events, well logs and VSP data and are consistent with the wider<br>and gentler top reservoir geometry as predicted by ray trace modelling. Following the startup of Tawke<br>oil export on 1 June 2009, the PSDM data are used for reservoir management and infill well planning.