ILLITE/SMECTITE CLAYS PRESERVING POROSITY AT DEPTH IN LOWER PERMIAN RESERVOIRS, NORTHERN PERTH BASIN

Abstract

The appraisal well Hovea–2, drilled in July 2002, was the first well in the onshore Perth Basin to discover commercial volumes of gas in sandstone reservoirs of the Artinskian (Early Permian) High Cliff Sandstone sealed beneath basal shales and siltstones of the Irwin River Coal Measures. A drill stem test in this formation, in the interval 2,370–419 mMDRT (measured depth below rotary table), flowed gas to surface at 16.5 MMcfd. Thereafter, the High Cliff Sandstone became an important play for exploration in the basin; however, no additional discoveries have been made in this reservoir, even though it has been tested in another seven wells (Jingemia–1, Eremia–1, Kunzia–1, Corybas–1, Yardarino–6, Hakia–2 and Bunjong–1).

In Corybas–1, drilled in 2005, this reservoir was found to be water bearing, but an 18 m core was taken within the topmost High Cliff Sandstone. This core provided important insights into the fluvial depositional environment of these sands. Importantly, porosity and permeability are highly variable within the reservoir, with some coarser grained beds retaining anomalously high porosity and permeability due to the coating of constituent quartz grains by illite/smectite that inhibited late stage quartz overgrowth. Study of this core along with core from overlying Irwin River Coal Measures indicates that there may be some facies control on the presence of the grain-coating clays in the High Cliff Sandstone, and that the fluvial to tidal facies are more likely to be an effective petroleum reservoir than the overlying fluvial/alluvial facies. This reservoir is the primary objective in the upcoming exploration wells Beharra Springs Deep–1 and Drakea–1, to be drilled in 2007.