Rock physics and quantitative interpretation using Lambda-Mu-Rho in the Shipwreck Trough, Otway Basin

Abstract

A rock physics study and AVO modelling study has been completed to assist in the interpretation of seismic amplitude and AVO anomalies in the Shipwreck Trough of the offshore Otway Basin of southeastern Australia. Elastic log data, core data (both full and sidewall) and associated thin section analysis of composition and texture were available on a number of wells; and these data are important in calibrating proposed rock physics models that suggest incorporating cement is critical to understanding anomalies in seismic inversion volumes and measured log data. Lithoprobability volumes based on conventional interpretation paradigms, such as low Vp:Vs values indicating gas presence, that do not incorporate an understanding of the rock physics lead to biased interpretations. Ratios in particular can be misleading as there is ambiguity about whether an anomalous ratio is driven by the numerator or denominator. As a classic gas indicator low Vp:Vs values are interpreted to be driven by a decrease in Vp associated with gas replacing brine in a rock. Using Lamé Impedance terms λρ and μρ, however, provides an alternative interpretation template that does not utilise ratios and can improve insight into rock properties. As in this case study, using LMR can be an important tool when shear velocity has increased relative to the compressional velocity irrespective of any pore-fluid change. We propose that due to quartz cement in the reservoir rocks of the Shipwreck Trough both gas and brine sandstones exhibit very low Vp:Vs, creating substantial challenges to the use of a standard rock physics template. In LMR space, however, the low Vp:Vs data points are clearly characterized by a high shear rigidity – an important point to recognize and incorporate into AVO interpretation workflows.