A Consistent Workflow From Seismic Acquistion to Well Positioning - For Challenging Data in Block 11, Offshore Qatar

Abstract

This paper describes the seismic workflow for exploration in Block 11, offshore Qatar, from the decision to acquire a 3D seismic survey, via processing, interpretation, time/depth conversion and reservoir characterization to positioning of wells. The area is generally very flat in the morphology of the horizons and the targets are relatively shallow, therefore, the decision for acquiring 3D seismic data had been taken rather late. It turned out to be a textbook example why 3D seismic is imperative even in such generally flat areas. The structural history of the area, from the old basement lineaments up to the earliest Cretaceous-Tertiary aged reactivation movements, could be outlined by 3D seismic data and it shows a very complex structural pattern. Introduction Early 2007, Wintershall Holding AG acquired a 3D seismic survey in Block 11, offshore Qatar. The area is generally very flat in the morphology of the horizons and the targets are relatively shallow, therefore, the use of 2D seismic had been considered as appropriate in the past. However, after several wells - including a discovery and another well with poor depth prediction, it had been decided to acquire the before mentioned 3D seismic survey with the following objectives:to get a consistent 3D high quality volume allowing a continuous interpretation of geological horizons and faults,to get a seismic velocity model suited for more accurate time/depth conversion, andto get better understanding of compartments and fault patterns, that will allow a better understanding of hydrocarbon migration path ways and seal failure risk,to get seismic data suited for state of the art technologies, like seismic reservoir characterization,to assess leads and prospects, and to define future drilling locations. Most precise velocities are of particular importance, because the overburden shows significant lateral velocity changes that cause important differences between time and depth maps. Depth structures are not necessarily visible on time maps, and apparent time structures are not necessarily existent in depth.