Abstract

The Nile Delta area is well known for its geological and geophysical complexity due to:

  1. the presence of gas clouds, chimneys, channels and growth faults in the Pliocene/Late Pliocene series;

  2. the variability in terms of lithology, structure and thickness of the Messinian formations that cause a highly irregular and laterally varying velocity structure;

  3. the low SNR of seismic data in the pre-Messinian series caused by the above described irregularities combined with the contamination of seismic data due to the presence of multiples.

All these characteristics make the seismic investigation of the area very challenging for both depth imaging and velocity model building, especially when the final goal is the exploration of pre-Messinian targets.

Moreover the Nile Delta region is also well known for being affected by overpressures, especially in the pre-Messinian sequences. Recent deep exploration wells, targeting Mio/Oligocene reservoirs, have raised the importance of predicting the overpressure behaviour during the well design phase in order to assess all the exploration risks. Since pore pressure can be estimated from seismic velocities this requirement puts further importance on accurate estimation of seismic velocity.

In this paper a case history from the offshore Nile Delta is presented, showing the application of an integrated procedure for solving these problems based on a rigorous depth imaging approach. In particular the focus will be put on the added value given by the continuous interaction between geological and geophysical contexts during the processing phase in all their aspects, such as well information, regional geological knowledge of the areas and interpretation on one side, and acquisition methodologies, imaging technologies and rock physics properties on the geophysical side.

Introduction

Areas characterized by complex geology, with strong lateral velocity variations and dipping structures are the natural domain of application of Depth Imaging. There the assumptions at the basis of Time Imaging break down, and Depth Imaging is mandatory in order to obtain reliable images of subsurface structures. In order to reach best results, the most sensitive ingredient is the velocity field. No matter which kind of migration is used (either Kirchhoff, Wave Equation, or RTM), if the velocity is not correct the imaging will be distorted.

The velocity field must be estimated from seismic data by using tomography, but nonetheless seismic data alone do not provide enough information to determine the velocity unambiguously. Some a-priori information must be provided, and one of the available sources of such information is the geological knowledge of the area under study. On the other hand, as it will be shown here, velocity in its turn can be used to support the validation of geological hypotheses, so that the integration of geology in the process can be of benefit not only for velocity estimation but also for geology itself.

Moreover, velocity plays a key role not only for imaging purposes but also for pore pressure prediction, providing a further motivation for getting accurate estimates of seismic velocity [1]. The standard approach to pore pressure prediction based on conventional velocity estimation methodologies deriving from standard time processing does not seem effective enough in the Nile Delta area. In particular, here overpressures and gas presence determine low velocity anomalies that are not properly resolved by the focusing velocities provided by time processing. Hence more sophisticated tools are needed, like those used for depth imaging, in order to improve the reliability of the velocity volume and accordingly to derive reliable pore pressure estimates.

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