Use of spectral decomposition in identifying stratigraphic baffles below seismic resolution; A case study of UTB field, offshore Niger Delta

Abstract

During a reservoir characterization study of the UTB field, Niger Delta, a 20-psi pressure difference was observed between two wells, X6 and X8, drilled into the F1 reservoir. Initial seismic interpretation results showed no flow barriers between the wells that would have caused this difference. In order to solve this problem, we attempted using conventional attribute analysis. However, this approach was ineffective as it was unable to highlight any flow barrier in the form of baffles. As a way forward, we deploy spectral decomposition, an advanced attribute analysis method, capable of imaging these subtle features. This study therefore demonstrates application of advanced spectral decomposition technique in resolving the stratigraphic baffles in the F1 reservoir. Two algorithms, including Short Time Fourier Transform (STFT) and Continuous Wavelet Transform (CWT), were adapted for identifying and delineating the hydraulic flow barriers. Analysis of frequency map revealed potential faults, thickness, lateral lithology variations, as well as hydraulic flow barriers within the reservoir. The modular formation dynamic tester (MDT) pressures, together with an understanding of the reservoir's geology, were integrated to validate and interpret the results of the spectral decomposition. The frequency maps between 10 Hz −26 Hz show the pay zone of the F1 reservoir as a continuously connected hydrocarbon pool. We interpreted them as shale baffles, forming the flow barriers that resulted in the pressure differential between the two wells. Thus, the spectral decomposition tool as applied in this study is a robust and cost-effective way to identify intra-reservoir and sub-seismic compartmentalization without drilling additional wells.