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Seismic Sequence Stratigraphy Of The Early Carboniferous, Petrel Sub-Basin, Northwestern Australia.

Mackie, Victoria Krystyna

Honours Degree, 1999

University of Adelaide

Abstract

Future petroleum exploration in the Petrel Sub-basin of the Bonaparte Basin is dependent on the establishment of a sequence stratigraphic framework and depositional model to help predict reservoir, seal, and source facies away from well control and in areas of poor seismic data quality.

A study area was selected in the Joseph Bonaparte Gulf, comprising an offshore depocentre approximately 45 by 75 km with several seismic surveys of different vintages and sparse well coverage. In order to conduct a sequence stratigraphic study, seismic sequence stratigraphic principles and concepts were used. The study focuses on the Early Carboniferous succession estimated at over 2500m thick.

The study identified six high order systems tracts which were grouped into two megasequences. Each megasequence is interpreted to represent a discrete phase of basin evolution with a unique balance between subsidence, sediment supply and eustasy. Megasequence A was influenced by high subsidence rates, possibly driven by salt withdrawal, and a diminished eustatic signature producing low frequency systems tracts. Deposition of Megasequence B was controlled by decreasing subsidence and increasing eustatic influence producing higher frequency systems tracts.

Seismic stratigraphic and facies analyses identified potential sand-prone turbidite fans within Megasequence A, at the base of the Early Carboniferous succession. A comparison with known turbidite sandstone reservoirs of the eastern Barrow Sub-basin suggests the fans are of moderate reservoir quality.

Play analysis reveals that the fans comprise a near-complete petroleum system. The mounded nature of the fans provides a potential stratigraphic trap and onlap to the Turtle-Barnett High provides an additional trapping mechanism. Seals are supplied by marine shales that condense basinward. Source rocks from underlying Devonian sediments are more likely to charge the turbidite reservoir than source intervals higher in the succession. Source location, migration pathways, and reservoir quality are considered the highest risks to the identified petroleum system.

An understanding of the depositional history and controlling mechanisms enhances the exploration potential within the Early Carboniferous succession of the Petrel Sub-basin. The lowstand fan petroleum system identified in this study is an excellent exploration target in a section of the stratigraphy with otherwise poor reservoir potential. The sequence stratigraphic framework established by this study could be expanded and applied to other, similar depocentres in the Petrel Sub-basin.

Australian School of Petroleum
THE UNIVERSITY OF ADELAIDE

SA 5005 AUSTRALIA

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