Derisking Prerosetta Prospects In The Nile Delta Through Depth Imaging
Authors: Andreolli M. , Bienati N. , Gentile E. , Manera M. , Tansini A. 
Published: May 2016

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. When facing such complexity the integration between interpreters and geophysicists is the key point in order to obtain a reliable imaging and hence a reliable geological model. Moreover, an integrated working platform is required to speedup the process that requires strong team interaction and a certain number of iterations to achieve the desired degree of confidence. The workflow started from the estimation of the velocity in the post-Rosetta where a global grid tomographic approach was used. Then for the Rosetta we initially assumed a tentative uniform velocity that resulted in some push-down and pull-up effects. Such effects were removed after geophysical and geological evidences provided information about 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 (6000m/8000m) ratio. 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.

Introduction

Evaporites are one of the typical imaging challenge in the Nile Delta. They are characterized by high velocity contrasts with respect to the surrounding sediments and a very irregularly variable thickness. The resulting lateral velocity variability can not be properly handled by standard Pre-Stack Time Migration (PSTM), making depth imaging mandatory in order to obtain a reliable geometry reconstruction in the pre-salt. However the presence of evaporites generates also strong internal multiples, energy absorption and diffractions that significantly increase the complexity of the task faced by depth imaging.

In order to make the Pre-Stack Depth Migration (PSDM) approach more robust with respect to the mentioned issues, in this case history we propose an integrated workflow involving both interpreters and geophysicists.

Method

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 at each step, validate the hypothesis of the interpreters. An integrated working platform is mandatory to speed up the process that requires strong interaction and a certain number of iterations to achieve the desired degree of confidence. Such integrated platform was implemented within the framework of a proprietary depth imaging infrastructure. The software platform used for depth imaging was entirely Eni proprietary software was used to build the velocity field, while Kirchhoff PSDM was used for the final depth migration and during the tomographic updates of the velocity field.