Basin Development and Petroleum Potential of Offshore Otway Basin, Australia

P. E. Williamson, G. W. O'Brien, M. G. Swift, A. S. Scherl, M. S. Marlow, N. F. Exon, D. A. Falvey, J. Lock, K. Lockwood

The Bass Strait region in southeastern Australia contains three sedimentary basins, which are, from east to west, the Gippsland, Bass, and Otway basins. The offshore Gippsland basin is Australia's most prolific petroleum-producing province and supplies over 90% of the country's production. In contrast, exploration has been unsuccessful in the offshore portion of the Otway basin; 17 wells have been drilled, and although shows of oil and gas have been common, no commercial discoveries have been made. Many of these wells, drilled in the 1960s and 1970s, were sited using poor-quality seismic data and, as a consequence, were frequently off structure. Seismic data quality has, however, improved significantly in recent years.

The present study by the Australian Bureau of Mineral Resources (BMR) involved the collection, in the offshore Otway basin, of 3,700 km of high-quality, 48-channel seismic reflection data by the BMR research vessel R/V Rig Seismic. These data have been integrated with existing industry seismic data, well data, limited dredged material, and geohistory analyses in a framework study of basin development and hydrocarbon potential in this under-explored area.

The offshore Otway basin extends 500 km along the southern coastline and is typically 50 km wide in water depths of less than 200 m. It contains up to 10 km of predominantly late Mesozoic to early Cenozoic sediments, which are overlain by a thin sequence of middle to late Tertiary shelf carbonates. It has been divided into three main structural elements: the Mussel Platform in the east, the central Voluta Trough, and the Crayfish Platform in the west. The basin was initiated at the end of the Jurassic as part of the Bassian rift. Up to 6 km of Lower Cretaceous sediments were deposited prior to breakup at the end of the Early Cretaceous and the onset of sea-floor spreading between Australia and Antarctica. After that time, subsidence was most rapid in the Voluta Trough, where up to 4 k of Late Cretaceous to Quaternary strata were deposited. The Crayfish Platform, in contrast, has subsided less than 1 km since the end of the Early Cretaceous.

The basal Upper Cretaceous Waarre Sandstone overlies the normal-faulted rift unconformity, and a favorable thermal and maturation history, and source rock association indicate the Waarre reservoir is the most attractive exploration play for the Voluta Trough and Mussel Platform. On the Crayfish Platform, geohistory analysis suggests that the intra-Lower Cretaceous Pretty Hill Sandstone reservoir, which underlies Eumerella Formation source rocks, is more prospective; a similar, untested play may exist on the northern Mussel Platform. Intense reactivation of rift faulting in the shallow section in the western Voluta Trough and along the margins of the Crayfish Platform has produced potential trapping structures and may have allowed migration of hydrocarbons into uppermost Cretaceous Cur ies/Paaratte Formation reservoirs.

The results of this study of stratigraphy, structure, subsidence, and maturation history of the Otway basin gives renewed optimism for future significant hydrocarbon discoveries. Further geological sampling in early 1987 should enable additional control of deep-water parts of the basin.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.