Impact of near surface topography on seismic imaging and reservoir characterization of deep targets - a case study from onshore Abu Dhabi

Abstract

Near surface conditions of alternating sabkhas and high sand dunes up to 200 meters of thickness pose challenges in seismic data processing. It adversely affects seismic imaging and characterization for the deep targets. Earlier processing results of surface consistent statics and amplitude compensation were deemed suboptimal and resulted in degrading the effectiveness of noise attenuation, the loss of signal due to destructive interferences, poor imaging of deep targets and the loss of amplitude integrity. Near surface topography change (sand dunes, sabkhas and gravel) causes remarkable effects on amplitude variation of the deep targets and on the wavelet signature underneath. The ringing pattern observed in the data is mainly concentrated beneath the sabkhas while the majority of the random and backscattered noise is located beneath the high sand dunes areas. Earlier seismic data processing did not preserve amplitude integrities that introduced considerable uncertainties in quantitative seismic interpretation for reservoir characterization. Hence reliable porosity prediction by the seismic inversion could not be achieved for development well planning in the field. Consequently, a remedial processing sequence was established to minimize near surface variation effects on the deep targets amplitudes integrity and structural imaging. A state of the art processing sequence was desinged for this data which includes Iterative and inter-related processing steps such as noise attenuation, surface consistent amplitude and statics corrections and deconvolution. The statics solution comprised of five (5) steps; 3D GLI utilizing uphole information, two passes of near offset reflection statics and two passes of residual statics. The new statics solution successfully reduced structural interpretation uncertainties especially in the crest of the subsurface structure. In such a complicated near surface topography condition, combinations of conventional amplitude compensation approaches do not make the grade amplitude variation on subsurface targets. In this case study, a new constrained surface consistent amplitude compensation approach utilizing reflectivity map of a shallow reflector resolved the amplitude lateral variabilities caused by near surface conditions. Amplitude integrity of the new reprocessing at reservoir targets are encouraging so that we performed post stack seismic inversion to validate the results. The inversion produced a reliable porosity prediction across the pilot area which was confirmed by correlation with well porosity logs. This new processing initiative provided a considerable solution for seismic reservoir characterization in this surface-related overburden problematic onshore field.