RTM Guided Migration Velocity Analysis: A West Africa Case History

Andreolli, Marco ¹; Andreoletti, Clara ¹; Ciaccio, Tania ¹; Brajucha, Riccardo ¹; Bienati, Nicola ¹
(1) eni e&p, San Donato, Italy.

The goal of paper is to present a PSDM case history based on the application of Reverse Time Migration (RTM) not only for the final imaging stage but also in the salt model estimation loop. In particular we will show the improvement obtained in the definition of salt geometries and thus in the global accuracy of velocity model.

During salt flooding it is common to use One Way wave equation migration (WEM), but with WEM the salt base can be imaged only by waves that transmitted through top of salt and therefore suffered a strong attenuation. WEM may therefore result in a low quality image of the salt base. Using RTM the base and the adjacent sediments are likely to be imaged from different directions, for example by overturning waves that travelled only through sediments and therefore carry more energy, resulting in a more reliable imaging of salt bottom.

We applied RTM guided velocity model building to an offshore West Africa dataset. In this area the Miocene deepwater turbidite channel systems represent a classic exploration play thanks to their optimal petrophysical characteristics. The traps are always a combination of stratigraphic and structural components, very often related to the typical salt induced tectonics of the Lower Congo Basin. The salt tectonics deforms the channel complex and makes the interpretation below the salt very hard.

One of the biggest problems is the salt shape definition. We started the project defining the salt geometry with WEM. Then we switched to RTM to improve this initial model. In order to reduce RTM run-time we limited the maximum frequency to 30Hz. Actually this choice allowed to refine the salt bottom interpretation and improve imaging. In particular with RTM the thickness of the salt drops was strongly reduced thanks to the fact that after RTM some sediments below salt started to come out. After reducing salt thickness, reflection tomography was able to update velocity close and below the salt drops. The increased accuracy of the updated velocity field allowed us to make the most of RTM up to 60Hz. The imaging improvements below the salt were huge as a lot of channels and structures became interpretable.

In conclusion, this project demonstrated the effectiveness of RTM in the definition of the velocity model. In particular we saw that for RTM up to 30Hz is adequate for the salt geometry definition, while for an accurate geology interpretation below the salt it was necessary to use higher frequency (up to 60Hz).

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.