Advanced Geophysical Methods for Imaging Carbonate Reservoirs

We present in this work an exhaustive seismic study which was performed on a subset of a wide azimuth Ocean Bottom Cable seismic acquisition shot over a carbonate field from the U.A.E. Most of these results were published in Guilloux et al. (2012). A first challenge was to reprocess these data ensuring that the azimuthal information is properly preserved all along the sequence. An additional challenge is related to the very shallow water environment which brings complexity like dispersive ground-roll and water layer related guided waves, multiples which can be generated at sea-surface, sea-floor or internally, near-surface heterogeneities. On top of that the high velocities which are encountered in these carbonate formations are detrimental to a high resolution description of reservoirs. Two main issues are presented:  Wide azimuth processing: the whole sequence was designed in order to preserve the azimuthal information. Multiples were attenuated in a cascaded approach including 3D predictive deconvolution and wide azimuth high resolution radon filtering. The Common Offset Vector strategy was followed at the imaging step to build 3D Common Image Gathers versus offset and azimuth, which enables to apply an azimuthally variant residual velocity correction and then stack all azimuthal contributions in an optimal manner.  Heterogeneities and fracture characterization: we tested a newly developed technique for imaging the diffractions. This approach is not limited to description of fractures, it can describe and characterize any kind of heterogeneity, it is then well suited to describe carbonate reservoirs. This pilot processing study enabled to set up an operational wide azimuth processing sequence on this dataset, several processing scenarii could be tested and compared. In particular the sequence for attenuation of multiples was considered efficient, bringing a lot of character to the final result. The implementation of multi-azimuth Diffraction Imaging over this carbonate reservoir is proving very encouraging. Numerous detected features display geologically meaningful characteristics, some of them being not visible on conventional structural attributes. This study would be among the first applications of the methodology with tangible results providing more static and dynamic fracture data become available for calibration.