Abstract

The Skua 2 well in the Browse Basin, offshore North-western Australia tested oil from a Cretaceous sand that contained up to 50 percent pyrite. The zone was evaluated by a comprehensive suite of modern logs, cores, RFT pre-tests and samples and production testing.

The presence or pyrite in the reservoir affects log responses in such a manner that lower porosity and high water saturation are calculated if conventional log analysis techniques are used. Log responses from the zone of interest, which can be mistaken for erroneous response attributable to tool failure, are explained in terms of lithology, porosity type and fluid content. A computerised interpretation of the zone of interest is presented. Since both resistivity and porosity logs are adversely affected by the presence of even small amounts of pyrite, it is possible to overlook oil accumulations unless all available information is utilised.

This paper documents a case history of a pyrite-rich reservoir which could have easily been overlooked if only conventional log interpretation techniques were employed.