Rock physics AVO depth trends: Implications for exploration in Saudi Arabia

Abstract

To properly characterize the Z Sandstone reservoir in Saudi Arabia it is imperative to determine how depth trends affect the amplitude variation with offset (AVO) signature. Rock physics using Hertz‐Mindlin theory and the Modified Upper and Lower Hashin‐Shtrikman bounds is used to model the effects of mechanical (uncemented) and chemical (cemented) compaction. Uncemented sands portray a Class IV AVO anomaly while cemented sands exhibit a Class III anomaly. The transition depth from uncemented to cemented sands is controlled by the regional geothermal gradient. The presence of cement stiffens the rock, which reduces the porosity, and therefore increasing the velocity. On the AI‐Vp/Vs ratio crossplot, the onset of cementation substantially diminishes our ability to discriminate between brine and gas saturated Z Sands. This is caused by the shift in the Vp/Vs ratio for brine filled sands to values similar to gas bearing sands. Zones that are affected only by mechanical compaction (uncemented) exhibit a clearer separation in the Vp/Vs ratio for different pore fluids. Due to the complexity and heterogeneity of the Z Sands, these depth dependent AVO trends are used to constrain amplitude variation with angle (AVA) simultaneous inversion, thus reducing the ambiguity in interpreting the inversion results.