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An Integrated Depth Imaging Workflow to De-risk Pre-Rosetta Prospects in the Nile Delta

Marco Andreolli, Juan Di Croce, and Alessandro Tansini

The explorative targets of the proposed case history are the Cretaceous sequences in a ENE-WSW fold related to the “Syrian Arc” event and the overlying Oligocene to Miocene sequences that pertain to the Nile Delta System. The structural pattern is sealed by the evaporites associated with the Messinian Salinity Crisis.

The evaporites, characterized by high velocity contrasts and variable thickness, generate several geophysical challenges for conventional time imaging such as strong internal multiples, energy absorption and difficult velocity picking. The resulting seismic imaging suffers of deformed geometries in the pre-salt.

We propose an integrated workflow between interpreters and geophysicists: interpreters guide geophysicists by providing plausible geological models to be used as soft constraints in the velocity estimation; geophysicists provide images that, according to the focusing achieved step by step, validate the hypothesis of the interpreters. An integrated working platform is required to speedup the process that requires strong interaction and a certain number of iterations to achieve the desired degree of confidence. For the velocity in the post-Rosetta we used a global grid tomographic approach; for the Rosetta we started with a tentative uniform velocity that resulted in some push-down and pull-up effects. Geophysical and geological evidences provided information on the real geometries of Rosetta, allowing to tune velocity variations within salt. In the deepest part, the velocity analysis was even more complex because of the limited offset/depth (6000/8000m). A modeling exercise gave confidence on the effective illumination of the deeper reflectors and a large scale grid tomography was performed. The final velocities in the pre-Rosetta showed two significant velocity inversions validated by geological models and analogues.

The proposed workflow led to significant improvements of the imaging of pre Rosetta sequences, both in terms of SNR and geological reliability of the prospect structure with a consequent de-risking of the prospects.

AAPG Search and Discovery Article #90161©2013 AAPG European Regional Conference, Barcelona, Spain, 8-10 April 2013