The Greenshank anomaly – An AVO case history

The Greenshank anomaly straddles the border between WA–246–P and WA–l–P, in the Dampier Sub–basin on the North West Shelf of Western Australia. It is located approximately 70 km northwest of Dampier and 25 km northwest of the Stag Oilfield.

Seismic data over the Greenshank anomaly displays a series of high amplitude events, dipping basinward near the centre of a northeasterly trending fault block. These anomalous amplitudes were correlated from regional seismic control to the lower M. australis Sandstone within the lower Cretaceous. Pre–drill seismic interpretation, AVO and acoustic impedance studies over the anomaly suggested the presence of a stratigraphic trap that comprised either a high porosity brine filled sand, or a hydrocarbon saturated low acoustic impedance sand. However, the drilling of Greenshank–1 in June 1996 encountered a low acoustic impedance shale with minor gas shows beneath a higher velocity cemented claystone at the level of the seismic anomaly. The higher acoustic impedance lower M. australis Sandstone intersected below the shale was very poor quality and water wet.

A post–drill AVO study was conducted to explain the difference between the observed AVO behaviour and the well results. Synthetic forward AVO modelling was performed using the Greenshank–1 measured sonic, density and synthetic shear sonic data. This study showed that the fluid factor anomaly is caused by the low acoustic impedance shale above the reservoir and not the sand as interpreted pre–drill. The acoustic impedance and Poisson’s ratio in the shale decreases relative to the overlying high acoustic impedance claystone unit, resulting in a positive AVO anomaly.