EAGE/SEG Workshop - Depth Imaging of Reservoir Attributes

Abstract was made available at the workshop

3-D ray + Born and 3-D ray + Kirchhoff migration/inversion schemes are considered as the most accurate tools for quantitative depth imaging in 3-D heterogeneous media. The both formulations are presented here with a brief theoretical review and some comparisons on a 3-D real data set. Important theoretical and practical points (such as acquisition footprints, velocity model quality, image post-processing) are underlined with the help of Common Offset gathers in order to anderstand the method capacities for subsequent reservoir attribute studies.

The limitations of classical ray theory, in case of triplicated ray fields, are well known singularities at caustics, shadow zones, ... Lots of theoretical works have been done for avoiding these artefacts.

Even when subsurface velocities vary only moderately, accurate imaging of reservoir attributes in depth remains a challenge. Two crucial components of the solution are the determination of an accurate velocity model and the use of an appropriate imaging algorithm. We examine the depth accuracy and qualitative AVO response of images generated using different velocity models and different imaging methods. We conclude that velocities determined from offset checkshot data in conjunction with a 3-D Kirchhoff imaging algorithm yield prestack images whose depths and amplitudes are in better agreement with well data than 3-D prestack migration implemented as a multi-step process (MZO cascaded with zero offset migration).

Here we present a technique for estimating the velocity associated with a final preSDM image in great detail. Essentially we employ techniques of continuons velocity estimation used for iterative model update, but apply them in a different context. Rather that using the velocity estimator strictly for velocity model building (on a set of sparse lines), we employ the estimation techniques to the ARP gathers resulting from the final migration for all CRPs in the 3D survey. This achieves two things: firstly it estimates a residual velocity correction for stacking the final CRP gathers to obtain an enhanced image, and secondly, it delivers a dense estimate of velocity for the entire 3D volume in a geologically coherent manner. It is this second feature which is the subject of this paper.

In the Vienna Basin are located important hydrocarbon reservoirs, characterised by fractured rocks in a complex fault system. The pre-stack depth migration is a basic tool for identifying the major geological formations. This method is particularly effective when the necessary macro-model for the velocity field and layer interfaces is provided by seismic tomography. There is a trade-off, However, from the practical point of view: seismic tomography is more necessary for relevant complexities but, in such cases, the picking of traveltimes becomes extremely difficult, if not unfeasible. So a goal of our work was to verify the effectiveness of the tomographic imaging in this difficult area.

We present the new data of the delineation of reservoirs and connected fault zones based on the simultaneous registration of the readings of a specially selected group of high precision gravimeters and measurement of natural microseisms.

Operto et al. (1997) presented an application of 3D ray+Born inversion to an onshore-type sub-dataset for imaging a dip section of the SEG/EAGE Overthrust model.

An erroneous amplitude picking can severely degrade AVO (Amplitude Versus Offset) analysis results and may lead to incorrect drilling decisions. A major cause of errors in AVO picking is the presence of coherent and incoherent noise that interfere with the target reflection. AVO picking is therefore a crucial step in AVO analysis and care must be taken in the rejection of coherent noise and in the exploitation of the available range of incidente angles. The joint estimation of AVO and kinematic parameters (JAK: Joint Amplitude and Kinematic) (Grion, Mazzotti, Spagnolini, 1998) is an AVO picking technique that meets these requirements.