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AAPG GEO 2010 Middle East
Geoscience Conference & Exhibition
Innovative Geoscience Solutions – Meeting Hydrocarbon Demand in Changing Times
March 7-10, 2010 – Manama, Bahrain

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

Jürgen Hoffmann1; Øyvind Engen1; Nils Bang1; Jens-Petter Nørgård1; Andreas Olaus Harstad1

(1) DNO Iraq AS, Oslo, Norway.

The Tawke anticline is situated in the Kurdistan region of Northern Iraq in the folded zone of the Zagros fold and thrust belt. The region is characterized by folded Cenozoic carbonates and foreland basin siliciclastics, with considerable differences in mechanical properties. Rough terrain, narrow structures and strong velocity contrasts pose significant challenges to seismic imaging in the area.

In 2006, DNO completed the first ever 3D seismic survey in Iraq with the objective of determining the subsurface structure of the Tawke oil field discovered in 2005. Since then, the seismic data have been processed 3 times with progressively more advanced techniques to meet geologists’ and reservoir engineers’ demands for resolution and accuracy.

The first processing involved elevation statics and a fast-track time migration. The main purpose of this dataset was to perform early volume calculations and well planning. Despite several successful wells, the data showed clear limitations in imaging of complex thrust structures.

The second processing project was initiated in 2007 using anisotropic prestack time migration based on the AutoImager velocity model building. This semi-automated method used the initially preprocessed data as input and provided significantly improved imaging of steeply dipping interfaces and faults. The key improvement factor was the AutoImager iterative migration velocity analysis producing a consistent seismic velocity model with anisotropy correction.

Seismic modelling and depth conversion tests were conducted in 2008 using 2D image ray tracing techniques. Results showed significant ray path bending because of strong lateral velocity contrasts between Cenozoic carbonates and siliciclastics. This implies that time migration images are inaccurate and conventional depth conversion by vertical stretching is not applicable. To solve these problems, the Tawke 3D data were reprocessed using prestack depth migration (PSDM) which migrates seismic events to their correct positions provided that the velocity model is correct. The PSDM results show better agreement between seismic events, well logs and VSP data and are consistent with the wider and gentler top reservoir geometry as predicted by ray trace modelling. Following the startup of Tawke oil export on 1 June 2009, the PSDM data are used for reservoir management and infill well planning.