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Analysis of the Sealing Capacity of the Flaxmans Formation and the Belfast Mudstone in the Vicinity of the Shipwreck Trough, Otway Basin, Victoria

Damte, Shiferaw Weldetsadik

Honours Degree, 2002

University of Adelaide

Abstract

Hydrocarbon sealing rocks are one of the main components of the petroleum system. Evaluation of sealing within the Shipwreck Trough, Otway Basin is essential to predict the degree of hydrocarbon reservoir fill. A study of the hydrocarbon sealing potential of the Flaxman Formation and Belfast Mudstone was performed to assess the top seal potential of these units within the study area. Fault sealing mechanisms and the potential of the faults to seal or leak are also addressed in this study.

Cuttings and cores were collected and analysed by Mercury Injection Capillary Pressure (MICP) analysis in order to evaluate the sealing capacity of the defined lithologies. Fault seal analysis was performed using Shale Gauge Ratio (SGR) method. This method evaluates the juxtaposition relationships between the footwall and the hanging wall of the faults analysed.

MICP analysis showed that the claystone from the Belfast Mudstone is an effective seal that can support a gas column height up to 467m. The Flaxman Formation also exhibits relatively good sealing capacity. Claystones from this Formation can support a gas column height up to 130m.

The SGR fault analysis suggests that the main sealing mechanism of the faults analysed is the juxtaposition relationship between reservoir and non-reservoir lithologies. The analysis also indicates the presence of low SGR (<20%) values for minimum fault throws on some faults. It is likely that cross fault communication is possible under these parameters. Areas with low SGR values (<40%) are interpreted to represent phyllosilicate framework fault rocks. SGR analyses indicate that the Faults analysed are likely to seal except Fault 7 & 8 at Mussel tarp.

In the Shipwreck Trough there are best sealing lithologies and these rocks can act as an effective cap seal over the structural trap that exists in the area.

Australian School of Petroleum
THE UNIVERSITY OF ADELAIDE

SA 5005 AUSTRALIA

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