Rock Physics Case Study Results from the Shipwreck Trough, Offshore Otway Basin, SE Australia: A Cautionary Tale of Ratios

摘要

A rock physics 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 were available from a number of wells in addition to core (both full and sidewall) and associated thin section analyses of composition and texture. The core data are critical 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. However, it is in some instances equally plausible that the shear velocity has increased relative to the compressional velocity and a decreased ratio value is not indicative of any pore-fluid change. 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 AVO as an exploration tool.