Advanced Seismic Techniques For Delineation of Petroleum Reservoirs
Authors: Lindseth Roy O. 
Published: March 2010

Introduction

Seismic inversion yields synthetic sonic logs from seismic reflection data recorded at ground surface. Synthetic sonic logs are similar to, and have most of the properties of, long source-receiver borehole sonic logs. They provide a new area/or application of seismic digital process technology.

The synthetics have been found to be particularly useful for the delineation of petroleum reservoirs, including the extent, thickness and porosity of the reservoir rocks. In a typical survey, field data, obtained from seismic line profiles across reservoirs, yield a sequence of synthetic sonic logs. Continuous vertical cross sections which display colour-coded transit time units can be interpreted in terms of lithology and porosity within the limits of interpretation of the single sonic curve and the resolution of the method. Sonic velocity changes are related to porosity and, by establishing the velocity of the matrix rock, the porosity within it can be estimated from synthetic sonic logs.

Applications include optimizing development drilling patterns to obtain maximum production, the design of secondary recovery programs, the positioning of injection wells and the location of offshore drilling platforms to ensure maximum recovery.

Introduction

Delineation and definition of a known new petroleum reservoir is generally the responsibility of the production engineer or geologist, who must rely on a few well logs to make his determination. The advantages of the superb vertical resolution of the logs are offset by poor horizontal sampling; well logs can be obtained only by first drilling expensive boreholes, which are therefore usually spaced at rather wide intervals.

The field was probably discovered with the reflection seismograph, which provides excellent horizontal sampling, but poor vertical resolution. Also, the conventional seismograph merely maps stratigraphic boundaries and pro- vides little or no confirmation about the actual rock units. Hence, it is of little help in determining reservoir properties. As a result, the development, geologist prefers to rely on samples and well logs, as they yield the desired degree of resolution and type of information.

At the borehole location, both well logs and seismic traces sample the same geological column. In fact, a seismic reflection trace is merely a transform of the sonic and density information on borehole logs. For years, geophysicists have computed synthetic seismograms from sonic and density logs. A well constructed synthetic seismogram will match a field-recorded seismogram very closely, and aids in correlating seismograph information to borehole geology at the tie point. The reverse process, of computing a synthetic sonic log from a seismic trace, is also theoretically possible, but it has not worked well in the past, because of a difference "in quality of source signals.

A sonic log is a relatively pure signal, recorded directly from the rock at the level of investigation. A seismic reflection signal, generated at the ground surface, must travel down through the· earth to the level of investigation and back again. In transit, it is often contaminated by external noise, distorted by wave pro" pagation phenomena, and part of it may be lost or attenuated in the transmission.