Abstract

Use of prestack data for quantitative analysis of hydrocarbon reservoirs has gained popularity over the past decades. Most of these quantitative studies rely on the seismic reflection amplitude variation with offset (AVO) and/or elastic impedance (EI) analysis. AVO/EI makes an assumption that reflection amplitudes, recorded on the prestack seismic data, are primary P-wave reflections and there are no contaminations of these amplitudes from wavefield transmission effects and from other wave modes. This, in turn, is equivalent to a convolutional assumption, i.e., the prestack seismic data can be theoretically predicted by convolving a given seismic wavelet with the reflection coefficient time series, computed for different angles of incidence. In this paper, we demonstrate that for realistic geologic models, convolutional assumption is approximately valid only at relatively small offsets/incidence angles. For large offsets/incidence angles, P-wave reflections are severely affected by interference from other wave modes, and therefore, a convolutional model that is typically employed in an AVO/EI analysis is not valid. To correctly account for the wave interference effects, it is necessary to use a wave equation-based methodology such as prestack waveform inversion (PSWI). PSWI is extremely important for Q-acquired high-resolution seismic data that has been gaining popularity in recent years. Higher offset and frequency sampling of such data allows accurate estimation of elastic parameters that can be used in predicting lithology and reservoir fluids. We show that the consequence of a convolutional assumption is more severe for such high-resolution data compared to a conventional one. Using real and synthetic high resolution prestack seismic data, we demonstrate the value of PSWI in delineating reservoir fluid properties. PSWI is the tool that should ideally be applied at every common midpoint (CMP) location in a 3D seismic data volume. We must, however, note that PSWI is computer intensive, because it computes many forward synthetic models to obtain an estimate of the optimum earth model at a given CMP location. This limits application of PSWI only to a selected few control locations, and for 3D applications, we must use PSWI in conjunction with AVO/EI analysis in a hybrid seismic inversion scheme. We present some results of such hybrid seismic inversion using Qacquired high-resolution seismic data from the deep water Gulf of Mexico. As computers are becoming faster every day, it is expected that in the near future, it will be possible to efficiently compute forward prestack seismograms for complex models, and apply PSWI over large data volumes.