PROPOSED SITESWest Tasmania: WT-1A and WT-2A
The proposed site is at the crossing of seismic lines Tasmante 125-52 and Sonne SO36B-47 in a present water depth of 2475 m. The evidence from DSDP Site 282 suggests that the Paleogene sequence is mud dominated, and the seismic profiles suggest a deltaic origin in both the Paleogene and the Cretaceous, with the possibility of some porous beds. The Eocene sequence of shallow marine mudstone (200 m thick) at Site 282 consisted of dark organic-rich uppermost middle to upper Eocene nannofossil-bearing mudstone that is believed to have been deposited in a deep basin with restricted circulation. Thus, there is a chance that anoxic sediments will have accumulated in a silled basin on the margins of the opening Southern Ocean. Total nearby sediment thickness is 3500 m.
At WT-1A, the section to 0.8 s (680 m) is moderately transparent on most seismic profiles and unconformably overlies a sequence that is finely cross-bedded on high-resolution profiles. Two cores indicate that the cross-bedded section consists of shallow marine Eocene mudstone. An inferred Oligocene unconformity appears to form the base of the overlying transparent sequence. Within the Neogene sequence, there is some downlap onto a reflector at 0.45 s. We propose to drill through 0.17 s (200 m) of cross-bedded Eocene sequence. The approximate total depth is 880 m and may require reentry. The site lacks any structure and is believed to be safe with respect to potential petroleum entrapment.
Alternate Site WT-2A is designed to penetrate 855 m into Eocene sediments. This site is in an area with little structure, where all sequences dip toward the ocean, so that any hydrocarbons could migrate freely out of the area. The seismic profiles again suggest a deltaic origin, with the possibility of some porous beds, in the Paleogene. Total sediment thickness nearby exceeds 3000 m.
West South Tasman Rise: WSTR-1B
This shallow-penetration (250 m) site is close to the western margin of the STR and is designed to retrieve a thick young Neogene section. The younger part of the Neogene section is seismically transparent and has accumulated in the lee of the Tasman Fracture Zone, which forms a ridge to the west.
Site WSTR-1B is in a gentle structural low with thick, almost flat-lying, sediments. The older Neogene section is well bedded but hummocky in character and probably consists of chalks with some hiatuses. The seismically similar underlying Eocene sequence is probably largely mudstone. The lowest part of the Neogene section appears to be absent at this site.
The proposed site is at the intersection of multichannel seismic profiles Tasmante 125-4 and 9 in a present-day water depth of 3580 m. The site is designed to penetrate 0.23 s (195 m) of Pliocene to Holocene transparent ooze that onlaps 0.22 s (185 m) of upper Oligocene to Miocene ooze and chalk above the Oligocene unconformity. Below the Oligocene unconformity is 0.1 s (110 m) of upper Eocene to lower Oligocene marine mudstone, marl, and chalk, above 0.08 s (90 m) of middle Eocene mudstone. The site is in a local low in the Cretaceous surface and is believed to be safe as regards possible hydrocarbon entrapment.
West South Tasman Rise: WSTR-2A
Proposed Site WSTR-2A is about 60 km east of Site WSTR-1B on the western slope of the culmination of the STR and is designed to penetrate a thicker (and older) lower Neogene sequence than Site WSTR-1B (470 m compared to 185 m). The young transparent ooze sequence of Site WSTR-1B is absent, but the underlying well-bedded sequence is well developed.
Site WSTR-2A consists largely of a sequence of Neogene ooze and is thus unlikely to contain any potential reservoir rocks. The site is located in an area of flat-lying Cenozoic sediments, overlying a structural low in the Cretaceous sequence. The Eocene sequence does contain some bedding, but little evidence of progradation. Although total sediment thickness nearby may reach 2000 m, the site is believed to be without hydrocarbons.
This site is located at the intersection of multichannel seismic profiles SO36B-58 and Tasmante 125-14. The site is designed to penetrate 0.55 s (470 m) of upper Oligocene to Miocene ooze and chalk above the Oligocene unconformity. Below the unconformity, we intend to drill 0.3 s (310 m) of upper Eocene to lower Oligocene sediments. Seismic profiles show that the calcareous sediments below the Oligocene unconformity are well bedded.
South Tasman Rise: STR-2A, STR-1A, SET-1A
Proposed Site STR-2A is designed to penetrate 940 m, well down into the Eocene, in an area where maximum sediment thickness is 3000 m. It may possibly enter the uppermost Paleocene. The site should penetrate 0.30 s (255 m) of Neogene ooze and chalk above the Oligocene unconformity and 0.65 s (685 m) of Eocene mudstone. The assumed top of the Cretaceous is at 1.2 s (~1250 m) below seafloor, and another prominent Cretaceous reflector is at 1.5 s (~1600 m). The site is on the intersection of multichannel seismic profiles AGSO 202-05 and -06. A 20-m core taken by the Marion Dufresne provides a continuous section back to ~900 ka. Sonne coring, Rig Seismic dredging, and data from DSDP Site 281 suggest that the facies is greatly different to that on the Tasmanian margin (DSDP Site 282, Sonne cores, and WT-2A). On the STR, the facies is olive to gray upper Eocene shelf to upper slope mudstone with abundant microfossils, both siliceous and calcareous in places. The site is in a depression from which any hydrocarbons generated in the Cretaceous sequence (about half the section) should migrate updip to the east.
The Neogene section, ~255 m thick, is transparent and virtually flat lying. The assumed Oligocene sequence, about 100 m thick, is strongly bedded and rather hummocky. The underlying well bedded and virtually flat-lying sequence, about 200 m thick, is not very reflective. Beneath this is a poorly reflective and somewhat disturbed section, about 300 m thick, that onlaps a gently dipping, poorly reflective sequence. The site is designed to terminate in the upper part of the dipping sequence.
Alternate proposed Site STR-1A is on the southern slope of the STR and lies about 20 km north northeast of DSDP Site 281, which was drilled on a basement high and is tied through seismic profile SO36B-51 to Site STR-1A. Basement is at about 1.3 s (1500 m), and there is a wedge of Cenozoic and possibly older sediments, reaching a maximum of a little less than 2000 m to the northeast, against a northwest trending fault. Site STR-1A would penetrate ~200 m of Neogene carbonates and 400 m of middle Eocene to lower Oligocene shallow-marine mudstone.
The maximum sediment thickness is about 2500 m. The Cenozoic sediments dip south and there is little chance of up-dip pinchouts. The DSDP evidence shows that the Eocene sequence is mud dominated, but the seismic profiles suggest a deltaic origin, with some possibility of porous beds. Whether the total sequence is Eocene or includes the Paleocene is unknown. The site is in a structural low over shallow basement, and there was no problem with hydrocarbons at DSDP Site 281 on a nearby structural high, so there is little danger of hydrocarbon entrapment.
The site is at the intersection of multichannel seismic profiles Tasmante 125-31 and SO36B-51. The site is designed to penetrate 0.21 s (180 m) of Neogene ooze and chalk above the Oligocene unconformity, 0.32 s (350 m) of upper Eocene to lower Oligocene marls below the Oligocene unconformity, and 0.07 s (70 m) of older Eocene mudstone. The presumed Cretaceous/Tertiary boundary lies 1.06 s (1110 m) below the seafloor.
Alternate Site SET-1A lies in 4060 m of water in the L'Atalante depression on Tasmante profile 125-4, between the STR and ETP. The site is designed to penetrate 400 m of Neogene drift sediments and 100 m of Paleogene clays. Total sediment thickness is about 1500 m.
East Tasman Plateau: ETP-2A and ETP-1A
Proposed Site ETP-2A is designed primarily to penetrate high-resolution carbonate-rich Neogene and Quaternary sequences in an area where the East Australian Current will have had variable influence through time, and it is located at a latitude comparable to the sites west of Tasmania. Secondly, it should penetrate a lower Oligocene and upper Eocene marine sequence whose character is poorly known. The sediments rest on continental or volcanic basement at 1.4 s (about 1400 m). There is little angular break apparent at the Oligocene unconformity.
The present water depth is 2630 m. The site should penetrate 640 m through Cenozoic sediments in an area of almost flat-lying sediments about 1400 m thick. Basement is believed to be Eocene volcanics above cratonic basement. The seismic profiles suggest that there is a Neogene pelagic carbonate sequence overlying a tight Paleogene sequence of volcaniclastic mudstone or muddy sandstone. Given the sediment thickness, there is almost no chance of hydrocarbon generation, and there are no conceivable traps.
The proposed site is on multichannel seismic profiles Tasmante 125-4 and AGSO 202-01 and 13.
The site is designed to penetrate 0.58 s (495 m) of Neogene-Quaternary ooze and chalk above the
Oligocene unconformity, 0.22 s (240 m) of thickly bedded upper Eocene to lower Oligocene chalk
and mudstone below the unconformity, and finish well above volcanic basement (probably Eocene
sills or flows).
This site will reveal much of the postbreakup history of the enigmatic ETP. The site is in a depression in the basement and should be safe from possible hydrocarbon entrapment.
Alternate Site ETP-1A is very like ETP-2A. It is on Tasmante profile 125-3 and is designed to penetrate 615 m through Cenozoic sediments in an area of almost flat-lying sediments about 1150 m thick. Basement is again believed to be Eocene volcanics above cratonic basement.
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