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The Deepwater Sedimentology of the Outer Browse Basin Shelf, Slope and Canyons - an Integration of Shallow Piston Cores, Swath Bathymetry and Shallow Seismic Data

Deller, Kerrie E.

Honours Degree, 2003

University of Adelaide

Abstract

The shelfal and upper slope margins of the Browse Basin are a significant Australian hydrocarbon province, which contain major undeveloped gas and condensate fields.   Known reserves of 3.02 mbbl of oil, 593.19 mbbl of condensate and 30.33 TCF of gas exist in the region, the largest being the Brecknock and Scott Reef fields.  In contrast, the deepwater parts of the outer Browse Basin, especially the canyons and deepwater stratigraphic and topographic highs, have been largely unstudied, yet they contain potentially prospective exploration targets. 
Shallow seismic data interpretation reveals that the continental shelf migrated northwest as a product of slow progradation during the Paleogene, before major progradation occurred during the Miocene.  A transgressive sea dominated the Pliocene and Quaternary sedimentation which was the result of processes associated with plate collision such as sediment loading, regional subsidence and the formation of the Timor Trough.  This subsidence, combined with warm waters supplied by the Indonesian Throughflow, produced an ideal environment for carbonate reef formations and essential deepwater carbonate sedimentation, to become established in the Pliocene.

Multibeam bathymetry over selected areas of Scott Plateau and Rowley Terrace show that slope geometries and submarine canyons vary in magnitude from the gentle, gullied slopes of Rowley Terrace, to the steep fore-reef slope west of Scott Reef.  Large canyon systems exist along the Leveque lineament and along the Rowley Terrace, which transport sediment from the shelf margin.  These canyons have gradients varying from 0.01 to 0.08 degrees, with widths varying from 3.4 to 21 kilometres, and lengths ranging from 13 to 172 km.  They are located in water depths ranging from 800 to 3800 metres.

This study provides the first detailed examination of recent deepwater sedimentation of the outer Browse Basin by using a suite of shallow seismic data, swath multibeam bathymetry and shallow sea floor sediment sampling.  A total of 40 sea floor piston cores were subjected to X-ray diffraction, laser particle size analysis, scanning electron microscope and carbonate acid digestion techniques to determine the sediment mineralogy, grainsize distribution, palaeontology and quantitative carbonate concentrations.

The sediment samples analysed during this study contained between 83% and 46% total carbonate mixed with terrigenous clays and biogenic silica.  Low magnesium calcite and aragonite, which dominated the carbonate component, varied in concentration with increasing distance from the Scott and Seringapatam reefs.  The decline of aragonite with increasing water depth resulted in the determination of the regional aragonite compensation depth of approximately 2600 to 2900 metres, a value which may be influenced by the convective nature of the Indonesian Throughflow.  The grainsize distributions were sporadic, influenced by large sand-sized pelagic foraminiferal ooze on topographic and structural highs.  The overall grainsize of the samples was silt to clay size (less than 63 microns).  The classification of eight facies groups ranged from the pure fine-grained carbonate muds of Rowley Terrace to the proximal reef debris on the fore-reef slopes west of the reef complexes.

Potential plays of the outer Browse Basin include canyon spurs and structural highs, where contourite deposits contain high concentrations of sand-sized planktonic foraminifera.  These contemporary foraminiferal sands contain significant porosity.  If equivalents exist in the underlying Tertiary carbonates, they could potentially represent an excellent reservoir rock.  Sealing potential of the sediments varies according to the mineralogy and grainsize distribution.  The fine-grained carbonate and terrigenous muds containing no foraminifera or aragonitic components represent the best sealing facies of the region. 
The sedimentary processes documented in this study may provide an analogue, both for the underlying Tertiary sequences of the outer Browse basin, and also for other deepwater regions of the North West Shelf, and even South East Asia.

Australian School of Petroleum
THE UNIVERSITY OF ADELAIDE

SA 5005 AUSTRALIA

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