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The Petrology And Petrophysics Of The Pretty Hill Formation In The Penola Trough, Otway Basin.

Little, Bridget

Degree of Master of Science, 1996

University of South Australia


The Pretty Hill Formation is an Early Cretaceous sandstone found in the Penola Trough, in the onshore sector of the Otway Basin, South Australia. The lithology ranges from a sub-litharenite to a feldspathic litharenite. The diagenetic assemblage includes chlorite, laumontite, carbonate and kaolinite. Commercially viable gas reserves have been discovered in the Katnook, Ladbroke Grove and Haselgrove fields.

A characteristic of the resistivity logs in the Pretty Hill Formation is the lack of contrast between the gas bearing and water bearing zones. This problem makes it difficult to quantify gas reserves. It was believed that the complex mineralogy was the cause of the anomalous electrical behaviour. Clays and zeolites are known to be highly conductive and theoretically should have a significant effect on the logs. Chlorite is the most prolific clay but it was found to he relatively non-conductive as well as being present in the gas and water zones thus making it unlikely to be the cause. Laumontite was found to be electrically resistive hence does not cause the lack of contrast in the resistivity logs.

The presence of laumontite was found to the most accurate indicator of the gas water contact. It has been preserved in the water leg only. of the gas producing wells. There are trace concentrations of laumontite in low permeability zones in the gas zone suggesting strongly that there has been dissolution. In non-producing wells such as Laira #1 and Zema #1 laumontite does not appear at the top of the Pretty Hill Formation. This is most likely due to a palaeo gas column having dissolved the laumontite. The onset of laumontite cementation is the palaeo gas-water contact.

A method to measure the electrical properties of the Pretty Hill Formation independently from the resistivity logs was investigated. This involved calculating the Cation Exchange Capacity (CEC) from core plugs using a non-destructive method. This method was found to be inappropriate and was abandoned.

Australian School of Petroleum



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