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.