Deposits of the Hanifa Supersequence represent the filling of the restricted-marine, intra-shelf Hanifa Basin during Jurassic time. The Hanifa Supersequence is a distinctive source rock-bearing section bounded by prominent regional unconformities that includes the Hanifa Formation along with the Tuwaiq Mountain Formation. This Hanifa/Tuwaiq Mountain section can be subdivided into four sequences (Figure 1).In ascending stratigraphic order, these sequences are the 1) Tuwaiq Mountain Sequence, 2) Hadriya Sequence, 3) Lower Hanifa sequence, and 4) Upper Hanifa Sequence. Each of the sequences has a similar distribution of depositional thickness. Thickest deposits are located in the western part of the study area in Saudi Arabia and thin to the east in the central part of the study area in Qatar. Although not shown in Figure 1, the deposits thicken again in the eastern part of the study area in Abu Dhabi. An important exception to this general trend in the distribution of the deposits is truncation of section due to erosion at the unconformity that caps the supersequence. Significant amounts of the youngest two sequences are removed by this truncation over much of the study area. The youngest sequence, the Upper Hanifa Sequence is particularly impacted by this truncation and is only preserved in the northeast part of the study area.
Each of the four sequences represents a discrete episode of carbonate platform development and progradation into the Hanifa Basin. Each sequence consists of thick platform deposits of skeletal/oolitic grainstone, packstone and wackestone that grade basinward into organic-rich mudstone (Figure 2).The thick accumulation of grainstone, packstone and wackestone represent stacked carbonate shoal complexes that individually grade off the platform into organic-rich mudstone which characterizes deposition in the restricted basin adjacent to the platform. The sequences can be further subdivided into a lower Transgressive Systems Tract and an upper Highstand Systems Tract with the boundary between the two marking the maximum flooding surface of a sequence. The Transgressive Systems Tract section is relatively thin forming a gentle ramp from platform to basin.
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.
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