oa Silvereye 1 Case Study – the False Positive

The objective of the recent Silvereye 1 exploration well in Bass Basin was to test Paleocene and Cretaceous sandstone reservoirs of the Eastern View Coal Measures (EVCM). The well was designed to test a faulted four-way dip closure and stratigraphic trap interpreted as a sand filled channel. Pre-drill analysis relied on a recent 3D marine seismic survey with well control provided by 2D seismic ties to wells located in adjacent exploration permits.

The pre-drill interpretation of a gas-bearing sandstone-filled channel was based on a brightening of seismic amplitude (a predominantly class 3 AVO anomaly) associated with the channel feature. The AVO behaviour was consistent with the response at the same gas-bearing stratigraphic level in the nearby White Ibis 1 well, hence the presence of hydrocarbons (gas) was considered likely.

The well intersected the predicted stratigraphy but failed to encounter hydrocarbons at any of the reservoir intervals. A channel sand was intersected within a thick claystone interval at the pre-drill proposed stratigraphic trap. The claystone is characterised by low-velocity and high density, while the sandstone has slightly higher porosity than sand typically encountered in this section. It was the contrasting characteristics of these lithologies that determined the class 3 AVO response and the mis-interpretation of the anomaly. We show that a close examination of the rock physics trends of reservoir and non-reservoir rocks in surrounding wells could have allowed this scenario to be recognised pre-drill.

Failure to recognise all possible lithological characteristics in the pre-drill AVO model meant that not all outcomes were analysed, and hence were not included in the risking of the prospect . The post drill evaluation has recognised that rock physics studies are an important tool for recognising all possible scenarios to aid in prospect evaluation.

Had a more comprehensive rock physics evaluation been conducted, the actual outcome would likely have been recognised as one of several possibilities, but would not necessarily have prevented drilling of the well.