Geology and petroleum potential of the Fairway Basin in the Tasman Sea

Abstract

The Fairway Basin is a large, generally north – south-trending, sediment-filled structure in water 1500 – 3000 m deep, on the eastern slope of the Lord Howe Rise in the Tasman Sea, and is partly within Australian jurisdiction. It was poorly known until a few years ago, when seismic profiling and piston coring cruises were carried out. The basin, about 1100 km long and 120 – 200 km wide, can be divided into three segments—north, central and south—that trend northwest, north and north-northwest, respectively. All three segments probably formed by thinning of continental crust during breakup of Lord Howe Rise and surrounding aseismic continental ridges in the Late Cretaceous and Paleocene. Normal faulting, large inputs of terrigenous sediment and subsidence to bathyal marine depths occurred during that time. A period of compression, perhaps related to overthrusting on New Caledonia, occurred in the Eocene, leading to uplift (and in parts, erosion) of northern Lord Howe Rise, and reversal of faulting in the basin. By the Oligocene, the area was again in bathyal depths, and pelagic ooze and some turbidites accumulated. The basinal sequence is generally 2000 – 4000 m thick, with 1200 – 3200 m of Cretaceous to Eocene sediment concentrated in depocentres, capped by 500 – 800 m of Oligocene and younger sediment. In the depocentres, numerous sedimentary diapirs pierce sedimentary sequences. The sedimentary diapirs appear to be fed by Cretaceous muds deposited during rifting. Often, these diapirs are overlain by faults extending to the seafloor, and hummocky bathymetry is possibly caused by fluid escape. The overall geology suggests that the Fairway Basin may be a large frontier hydrocarbon province. Seismic profiles display a bottom-simulating reflector above many depocentres, 500 – 700 m below the seafloor. The bottom-simulating reflector has positive polarity, which could result from a diagenetic phase transformation, a thin gas hydrate layer with a sharp top, or from the sharp base of a gas layer (probably beneath gas hydrates). Standard piston cores taken above diapirs and apparent fluid-escape features have recovered little gas. Other than drilling, the next steps in assessing petroleum potential are to clearly document fluid-escape structures, and to sample any fluids emitted for hydrocarbons.

Key words:

• bottom-simulating reflector
• Cenozoic
• Cretaceous
• diapirs
• Fairway Basin
• Lord Howe Rise
• petroleum potential
• rifting
• Tasman Sea

Acknowledgements

We wish to thank all those who took part in the ZoNéCo 5 survey of L'Atalante in 1999, and the Franklin survey FR9/01 in 2001, both of which provided essential data for this study, and those who analysed the results thereafter. The Franklin survey was largely funded under Australian National Facility arrangements, and was the third survey in the French – Australian Seismic Transects (FAUST) program. Most figures are drawn from Exon et al. (2004); the captions of the other figures acknowledge their source. Barry Willcox and Phil Symonds of Geoscience Australia carried out thorough and thought-provoking reviews of an early version of the paper, and we particularly thank Phil Symonds for some stimulating discussions. Peter Baillie and Patrick Quilty reviewed the paper for this journal and helped substantially improve it. We thank Angie Jaensch at Geoscience Australia for producing final versions of most of the figures. NFE and PJH publish with the permission of the Chief Executive Officer of Geoscience Australia.