Abstract

During recent years there has been a significant downsizing in the industry. This environment of limited resources can be assisted in technical areas by partnering with contractors. This paper demonstrates a new geophysical technique as well as describing the process by which it was developed.

In the recent past there have been both successful and non-successful case studies of the use of Amplitude Vs. Offset for reservoir prediction. The failures in using AVO usually derive from the uncertainties in picking the parameters utilized in the AVO modelling. If there was a way to reduce these uncertainties, then the reservoir prediction would be more reliable. To improve this reliability a technique utilizing AVO Walkaway VSP's to calibrate the surface seismic gathers over prospect areas has been devised.

The technique involves utilizing Schlumberger's ASI (Acoustic Seismic Imager) Tool. The AVO Walkaway VSP itself is recorded by clamping an array of triaxial geophones immediately above a target interface and deploying a moving seismic source along a line with even spaced offset shooting. As the downgoing energy and reflected energy can be separated and measured for each shot, the reflection coefficient of the interface can be measured as a function of angle or offset.

After the AVOWSP is shot and processed, the interface of study is then interpreted both on the AVOWSP and the surface seismic gather over the borehole. The amplitude with offset for both seismic representations are displayed and compared. With this information, an Offset Dependent Gain Function (ODGF) is derived which is utilized in calibrating the surface seismic gather so that the AVO at the interface matches that of the AVOWSP.

Over the prospect area, all specially processed surface gathers are then picked at the interface of study. The ODGF is then applied to each retrieved amplitude from the gather interpretation. Following this, a Gradient Stack Trace calculation then takes place for all surface seismic offset amplitudes and these values are then gridded and mapped over the area.

Anomalies in the maps which may indicate prospective areas are then identified and a tight parameter constricted inversion takes place utilizing the ODGF'd gather over the identified anomaly. This then allows for superior rock property prediction.

To test the validity of the technique, inversions were carried out over previously drilled well locations in the Gas Area within the Maxus/SES License area in the Sunda Basin. The 3D area within this study area will be the case study presented in this paper.