Summary

In the Tapti-Daman subbasin of Western Offshore, India, stacked 3-D seismic amplitude anomalies were interpreted as gas-charged sand channels and a well was drilled for testing of two such stacked channels. Upon drilling, predicted pays along with an additional gas-sand and one water-sand were encountered. Additional gas and water sands did not show amplitude anomaly in the well. However, stratigraphic level equivalent to the additional gas-sand exhibited high amplitude anomaly away from the well.

This study was undertaken to find the lateral extent and continuity of the additional pay sands in absence of anomaly and structure. The gas sands with anomalies and water sands have high net-to-gross, low impedance, and strong impedance contrast with respect to enclosing shales. The additional gas sands have relatively low net-to-gross and insignificant impedance contrast because of intercalated thin shale layers.

To understand the causatives of high amplitude away from the well, the intercalated gas-sands were modelled for thicker and cleaner sandstone layer by editing the logs and synthetics were generated by using dominant frequency wavelet extracted from seismic. It produced high amplitude indicating that high amplitude away from the well may be generated from thicker and cleaner gas-charged channel sands. To get the recognizable signal from the intercalated sand-shale, synthetics were generated from original logs by using 16 to 120 Hz Ricker wavelets. In frequency range 64-112 Hz higher amplitudes were observed with peak response at 96 Hz.

The data was filtered by using 64 Hz low cut and attributes were re-extracted from filtered volume. Horizon slices, generated from reflection strength volume, showed anomaly at well location. Amplitude maps of dominant and high frequencies were summed to get the lateral extent and continuity and depositional geometry of additional gas sands.

Introduction

In the area of study, seismic amplitudes of Upper Oligocene sandstone reservoirs are often unpredictable and gas sands and water sands, both, can produce bright spots (Harilal et al., 2008). Sometimes gas sands do not produce recognizable reflection and remain unidentified. This paper is related to such a case history where multilevel stacked anomalies were interpreted and drilled to test only two such anomalies. After drilling exploratory well-A (Figure 1), the identified anomalous zones and non-anomalous zones, both, were found gas bearing. In addition, a thick water-sand was also encountered. The sands are named as Pay-1, water-sand, Pay-2, Pay-3 and Sand-4 from top to bottom. The Pay-2 is, seismically, un-recognized in the well-A but its equivalent reflector shows large anomalous feature (channel) towards east and west of the well (Figure 1). The Pay-2 entrapment is not explained structurally. For the reserve estimation, the lateral extent of Pay-2 and its relationship with anomalous feature was to be established. The problem was solved by using a workflow consisting of log signature analysis, forward modelling, interpretive processing and seismic attribute analysis.

The objective of this paper is to find the reasons for not getting identifiable reflection signal from particular pay during predrill interpretation and to demonstrate the forward modeling and interpretive processing methods for mapping the lateral extent and continuity of such hidden pay sands.

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