Petroleum potential of deepwater basins around Tasmania: insights from ocean drilling program Leg 189  

Authors / CoAuthors

Exon, N.F. | White, T.S. | Malone, M.J. | Kennett, J.P. | Hill, P.J.

Abstract

During ODP Leg 189, the JOIDES Resolution recovered about 4200 m of continuous core from four sites in sedimentary basins on continental crust off Tasmania. These sites, one off west Tasmania, two on the South Tasman Rise (STR), and one on the East Tasman Plateau (ETP) are 760-968 m deep and in water 2100-2700 m deep. No hydrocarbon accumulations were expected at these sites. The pre-Oligocene section is largely shallow-marine organic-rich mudstone, which seismic profiles indicate is the top of a Cretaceous-Eocene largely deltaic sequence thousands of metres thick. This siliciclastic sequence formed as Tasmania rifted from the surrounding parts of Gondwana. Sedimentation rates were relatively high until the late Eocene, when a condensed siltstone sequence formed as the Antarctic Circumpolar Current first swept the shelves of the separating land masses. From the earliest Oligocene, when Australia finally cleared Antarctica, deposition of several hundred metres of carbonate pelagic ooze and chalk predominated. Sedimentation no longer kept up with subsidence, and at most sites unconformities have removed much of the Oligocene. The cores, in conjunction with seismic profiles, provide information on tectonic and sedimentary history and petroleum potential. New data include evidence of high present-day thermal gradients; marginally mature organic matter less than 1000 m below sea bed, biogenic and probably thermogenic gas, and bitumen generation; and overall source rock potential. Tectonic histories vary, but all basins have sufficient sediment and thermal gradient to generate hydrocarbons. However, of the thick sequences interpreted as deltaic, only the upper shelf mudstones were drilled. TOC content decreased with time as the gulfs around Tasmania widened, and also eastward from the more restricted Australo-Antarctic Gulf into the less restricted early Tasman Sea. Although the thermal gradient is highest in the three western sites, the holes do not reach the petroleum window. Methane is biogenic in the younger sediments, but higher hydrocarbon gases at depth suggest a thermogenic component. In addition, Rock-Eval analyses of the oldest sediments suggest bitumen (S2 peak) and organic matter approaching maturity. Little stratigraphic section is missing, so past overburden was unlikely to exceed present. Whether there are reservoir rocks and suitable hydrocarbon traps remains unknown. On the existing evidence, west Tasmania and STR appear to be reasonably prospective for petroleum, and more prospective than ETP, and we present a speculative play concept for them.

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nonGeographicDataset

eCat Id

36873

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Keywords

• External PublicationConference Paper

( Theme )

• petroleum exploration

( Theme )

• organic geochemistry

( Theme )

• marine survey

( Theme )

• marine

• AU-TAS

Australian and New Zealand Standard Research Classification (ANZSRC)

• Earth Sciences

• Published_Internal

Publication Date

2001-01-01T00:00:00

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Australian Government Security ClassificationSystem    

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Creative Commons Attribution 4.0 International Licence    

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unknown

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geoscientificInformation

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[-50.0, -41.0, 141.0, 152.0]

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