Abstract:
During ODP Leg 189, the JOIDES Resolution recovered about 4,200 m of continuous core from deep sites in sedimentary basins on continental crust off Tasmania. These four sites, one off west Tasmania, two on the South Tasman Rise (STR), and one on the East Tasman Plateau (ETP) are in water 2,100-2,700 m deep and contain sections 760-968 m thick. No hydrocarbon accumulations were expected. The pre-Oligocene section consists largely of shallow-marine organic-rich mudstone, which seismic profiles indicate is the top of a largely deltaic Cretaceous-Eocene 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 separated completely from Antarctica, there was deposition of <500 m of pelagic carbonate ooze. Sedimentation no longer kept up with subsidence and, at most sites, unconformities have removed some of the Oligocene. The cores, in conjunction with seismic profiles, provide information on the tectonic and sedimentary history and the petroleum potential. New data include evidence of high present-day thermal gradients; marginally mature organic matter less than 1,000 m below sea floor, biogenic and probably thermogenic gas, and possible bitumen generation; as well as overall source rock potential. Tectonic histories vary, but all basins have sufficient sediment to generate hydrocarbons. TOC content decreased with time as the gulfs around Tasmania widened, and also decreased eastward from the more restricted Australo-Antarctic Gulf into the less restricted early Tasman Sea. The thermal gradient is highest in the three western sites, and limited vitrinite reflectance data suggest some basal sediments may reach the "petroleum window". Biogenic methane occurs in the younger sediments, but higher hydrocarbon gases at depth suggest a thermogenic component. In addition, Rock-Eval pyrolysis of the oldest sediments suggests bitumen (double 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 moderately prospective for petroleum, more prospective than ETP, and we present a speculative play concept for them.