Rock Physics Guided AVO: A Holistic Approach to Sand Detection
Husam M. AlMustafa
Rock physics plays a critical role in lithology and fluid prediction, and amplitude-versus-offset (AVO) modeling. It is a quantitative tool and a necessary step in understanding and interpreting seismic amplitudes for predicting lithofacies distribution. In this case study, I employed AVO inversion, guided by rock physics, to detect reservoir quality sands in the Unayzah Reservoir in central Saudi Arabia. The inversion scheme is a simultaneous pre-stack inversion algorithm based on Zoeppritz’s equations. The procedure uses near- and far-angle stacks and angle-dependent wavelets to determine the elastic parameters: P-wave im-pedance, S-wave impedance and Vp/Vs ratio. To interpret the pre-stack inversion results the data was constrained by rock physics. Initially rock physics templates (RPT) were built, which are geologically driven, basin-specific, theoretical rock physics models. Geologic constraints on RPT included lithology, mineralogy, porosity, depth (pressure) and tempera-ture. RPT cross-plots were built for acoustic impedance (AI) versus Vp/Vs, which were overlain by trends for lithology and porosity. This allowed performing rock physics analy-sis, not only on well log data, but also on seismic data (pre-stack inversion results). Ini-tially, RPT were validated by well-log data before applying them to seismic data. Input data and model assumptions affect the accuracy of the information obtained from the template. I used RPT to guide the classification of the seismic inversion results. The pre-stack inver-sion of AI and Vp/Vs within the target zone was projected onto the template to generate lithofacies map that help distinguish reservoir sands from silty shales.
AAPG Search and Discovery Article #90077©2008 GEO 2008 Middle East Conference and Exhibition, Manama, Bahrain