THE GEOLOGY OF THE MOUNT HORNER OILFIELD, PERTH BASIN, WESTERN AUSTRALIA

Abstract

The Mount Horner oilfield is located 30 km east of the town of Dongara, some 360 km north of Perth, Western Australia. It was discovered by WAPET in 1965 but it was not until 1980-81 that a further four appraisal wells were drilled on the field. One additional appraisal well was drilled in 1987.

The structure consists of a tilted fault block downthrown to the east with a roll-over anticline on the downthrown side of the fault. Sandstones within the Lower Triassic Kockatea Shale produced oil in 1965 from Mount Horner No. 1 on the upthrown tilted fault block. This well was eventually plugged and abandoned due to the low productivity of the reservoirs and the then prevailing low oil price. Production also occurred in the Basal Triassic Sandstone on the downthrown side of the fault. Mount Horner Nos 4 and 5 produced from this horizon from 1984 to 1986, when they were shut in due to increasing water cut.

At the top of the Lower to Middle Jurassic Cockleshell Gully Formation, a complex transition zone exists between the fluvial sandstones of the Cattamarra Coal Measures Member and the marine sediments of the Champion Bay Group. Sandstones in this transition zone produce oil from Mount Horner No. 5 which was re-completed in 1986 and from No. 7 which was drilled in 1987.

There are four distinct oil pools on the Mount Horner structure. Geochemical and maturation studies have shown that the bulk of the reserves was generated from the basal part of the Kockatea Shale and has migrated up the main fault and into the Jurassic reservoirs.

Recent workover and appraisal drilling programs on the field have established the first commercial oil production from Jurassic sediments in the Perth Basin. This opens up an exciting new exploration play in the northern Perth Basin.