Summary

It is not sufficient to use seismic amplitude anomaly alone to support the validity of a prospect, especially for stratigraphic traps. An essential part of prospect evaluation is not just the prediction of the presence of reservoirs and their aerial distribution, but also the prediction of possible hydrocarbons within the reservoir. This paper illustrates a method that can be used to evaluate areas of exploration potential in the deepwater Kutei Basin, East Kalimantan. The method applied to a Kutei deep water 3D dataset is designed (”tailored method”) to enhance the amplitudes related to the low frequency spectrum of the final stacked data. Eventually the calibrated AVO response, the DHIs and the seismic amplitudes are analyzed in combination. Well calibration is an essential part of this process. Using this approach, the amplitude anomalies can be more confidently associated to the prediction of the fluid properties in a reservoir, and this in turn should lead to a more accurate prospect risk assessment. A similar approach has been applied by ZapSibNIIGG in Tyumen Russia and the results were encouraging. This method is considered a promising tool in the early stage of application. It is hoped that future drilling campaigns will help confirm its validity.

Introduction

A worldwide review of the success ratio of predictive seismic investigations based on AVO (Amplitude Variation with Offset) and Direct Hydrocarbon Indicators (SEG presentation by Kurt W. Rudolph Exxon Mobil) reveals that the most successful criteria to discriminate between false and valid anomaly are based on the following analyses: 1. AVO, 2.Amplitude Terminations, 3.Amplitude conformance to structure, 4.Amplitude strength, 5.Lateral contrast, 6.Fluids contact reflection. These are all well known and commonly used analyses worldwide nowadays. Recently a new idea was promoted by Dr John P. Castagna about the use of spectral decomposition as a new method to support the prediction of hydrocarbons in new Exploration prospects. The concept is based on the analysis of Amplitude Variation with Frequency (AVF). Geophysical modeling showed that high amplitudes are indicative of different situations depending on the frequency range. In a frequency range lower than 10 Hz amplitudes from hydrocarbon filled sands will show a higher amplitude response than a brine filled interval. In a spectrum range between 10 Hz and 21 Hz and a maximum amplitude value at 16 Hz, the situation is reversed and amplitudes from brine filled sands are higher than those related to the hydrocarbon fill. In the 21 Hz to 47 HZ range with maximum amplitude of 32 Hz the amplitudes related to hydrocarbons are again higher than those related to water wet sands. The ISA workflow is shown in Figure 1. Wavelet analysis is performed on seismic migration stack data in order to perform a conversion to time frequency domain, and compute ISA (Instantaneous Spectral Analysis method uses a wavelet transform technique to produce single-frequency reflection events that are accurately localized temporally; each full-spectrum reflection can be seen at its uncontaminated single-frequency equivalent) frequency sections that will be used for analysis and visualization.

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