Avoiding ocean bottom complexities by using wave equation redatuming

Abstract

Subsurface structural complexities are the main cause of seismic image distortions – a bad imaging – that can cause mistakes in interpretation, give bad constrains in leads mapping, can cause lost of exploratory opportunities and damage volume HC estimation. A wave front generated by a punctual source in a homogeneous media is perfectly spherical. As the media becomes less homogeneous, the wave front becomes more complex. One way to avoid shallow complexities is redatuming to horizons beneath the structural complexity interface. In this paper we present full wave equation redatuming based on elastic formulation aiming a better image of a gas reservoir located just beneath a great canyon (cause of severe distortions in the target). Introduction Structural geology complexities are the main causes of bad imaging. In a homogeneous isotropic media, a wave front generated by a single point source is a perfect sphere. As the media becomes more and more inhomogeneous, wave front turns farer from the expected spherical shape and also becomes what we will call here a complex wave front. The conventional approach used in common seismic processing is not suitable to deal with structural complexities. The idea of CMP itself is corrupted because a point in CMP gather is severely smeared (Berryhill, 1986). In other words, an event is not sampled in the same CMP point. As a consequence we will have poor coherence or semblance what will corrupt the velocity analysis, resulting in a bad velocity field and finally, a bad imaging cited in the first line of this topic. If our prospect or reservoir were above a structural complexity it would not be a problem. But it is almost never the case. Wang et al (2006) pointed the need of redatuming using wave equation and Kirchhoff techniques in order to obtain better images under salt bodies in GoM. But, as offshore exploration is going to deeper water depths, we migrate from horizontal monotonous continental shelves to steep continental slopes. Most of nowadays offshore targets or producing reservoirs are located under steep continental slopes cut by oceanic canyons. In those unlucky situations, the steep or canyon shaped ocean bottom turns the well behaved spherical wave front into a complex wave front. Just the first interface corrupts all the reflections from beneath. In such case the processing flow would be started only after redatuming procedure over all seismic database. If the objective of processing were to improve a previous study for reservoir and producing tasks the necessity of more confidence in contacts and structures would be much more dramatic as they usually have an economic impact. Redatuming for reservoir purposes is also needed. In this paper we present the use of wave equation based redatuming in order to better imaging a reservoir that is beneath a huge canyon. The geological model over which the synthetic seismic was performed is real case based and contains a clastic reservoir completely saturated with gas what conferred it amplitude anomaly on its top. Despite the objective of this paper is focused on imaging – to reach better geometry of reservoir boundaries – we have applied elastic wave equation algorithm. The Geological Model The model is 20 km in length, it is orthogonal to the major dip of a continental slope and stays in a water depth varying from 400m to 800m. The section cuts a 15 km