Reassessing the Petroleum Prospectivity of the Offshore Northern Perth Basin, Western Australia

Jones, Andrew¹; Kennard, John¹; Nicholson, Chris¹; Rollet, Nadege¹; Mantle, Daniel¹; Grosjean, Emmanuelle¹; Boreham, Chris¹; Jorgensen, Diane¹; Robertson, Danielle¹; Bernardel, George¹; Greinert, Jens²; Kempton, Richard³; Langhi, Laurent³; Zhang, Yanhua³; Hall, Lisa¹; Hackney, Ron¹; Johnston, Stephen¹; Petkovic, Peter¹; Bernecker, Tom¹; Bradshaw, Marita¹
¹Energy Division, Geoscience Australia, Canberra, ACT, Australia.
²Marine Geology, Royal Netherlands Institute for Sea Research, Texel, Netherlands.
³Petroleum, CSIRO, Perth, WA, Australia.

A geological assessment of the offshore northern Perth Basin, Western Australia, has been completed as part of the Australian Government's Offshore Energy Security Program. The study provides new insights into the petroleum prospectivity of the basin and integrates results of a newly-developed tectonostratigraphic framework, regional trap integrity analysis and a hydrocarbon seepage survey. The results enhance the potential for new oil and gas discoveries and reduce exploration risk.

Most petroleum accumulations in the northern Perth Basin are associated with Permian and Triassic reservoir and source intervals, and are found onshore and nearshore. New sequence stratigraphic and geochemical studies of key offshore wells, in conjunction with revised biostratigraphy, have shown that the Late Permian-Early Triassic Hovea Member (Kockatea Shale) source interval responsible for these accumulations is regionally extensive offshore and has good to excellent source-rock potential for generating oil. This is supported by fluid inclusion data which identified palaeo-oil columns in most wells analysed in Permian shallow marine-fluvial reservoirs below the regional Kockatea Shale seal. In addition, a palaeo-oil column in Houtman-1 demonstrates an effective oil-charge system in Jurassic strata for the Houtman Sub-basin.

Loss of petroleum accumulations because of trap breach is considered a major exploration risk. A trap integrity study evaluated the potential for fault reactivation associated with renewed mid Jurassic extension and Valanginian breakup. 3D geomechanical models indicate that fault strike (NNW-SSE to ESE-WNW) and fault planes intersections represent a first order risk factor.

Results of a recent seepage survey provide additional support for the presence of an active petroleum system on this part of the continental margin. Recent faulting and amplitude anomalies in the shallow strata correlate with raised, high-backscatter regions and pockmarks on the seabed. A series of hydroacoustic flares identified with sidescan sonar may represent bubbles rising through the water column. ROV underwater video footage shows a dark-coloured fluid which may be oil that seeped from the seabed. This immiscible fluid was observed at a depth of about 500 m in the vicinity of the sidescan flares. The integration of the above mentioned datasets builds a case that is indicative of natural oil seepage.

AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012