ABSTRACT: Offshore Maryborough Basin, southeast Queensland: Extent, Structure and Petroleum Potential from New Geophysical Survey Data

HILL, PETER J., Bureau of Mineral Resources, Canberra, Australia, and LEN C. CRANFIELD, Geological Survey, Queensland Department of Resource Industries, Brisbane, Australia

The Maryborough basin covers an area of about 10,000 sq km onshore and at least 15,000 sq km offshore. The basin is of latest Triassic-Early Cretaceous age and contains a thick sequence of continental to marine clastic sediments and coal measures, and some volcanics. It represents one of the thickest Mesozoic deposits in the Australian region. Deformation in the Late Cretaceous produced folding and high-angle faults.

Petroleum exploration has been directed mainly at the Early Cretaceous Maryborough Formation (shallow marine) and Burrum Coal Measures (lacustrine/deltaic). Limited exploration drilling onshore has resulted in gas shows from various stratigraphic intervals within these units, as well as a gas flow from the Gregory Sandstone Member near the base of the Maryborough Formation. The offshore part of the basin has not been tested by drilling.

In late 1989, the Bureau of Mineral Resources used RV Rig Seismic to collect 2900 km of multichannel seismic reflection and other geophysical data (sonobuoy refraction, gravity, magnetics and bathymetry) off southeast Queensland. One of the main objectives was the investigation of the offshore Maryborough basin. It was the first petroleum-oriented geophysical survey to be conducted in the region since 1974.

In Hervey Bay, the seismic data show the Maryborough basin as a deep northwest-trending synclinal structure, the northeast flank of which subcrops in the middle of Hervey Bay at a prominent erosional unconformity overlain by several hundred meters of late Cainozoic sediments and basalts. The prospective Early Cretaceous Maryborough Formation and Burrum Coal Measures attain a combined thickness of about 5 km. A strong seismic horizon at 4 s (twt)--approximately 8 km depth--probably represents Paleozoic-Triassic basement.

The survey provides the first evidence of a substantial sedimentary beneath the relatively wide continental shelf southeast of Fraser Island. At least 3 km of folded and faulted Maryborough basin sediments are overlain by a wedge of Cainozoic shelf sediments ranging in thickness from about 100 m close to shore to about 1 km at the shelf edge.

AAPG Search and Discovery Article #91015©1992 AAPG International Conference, Sydney, N.S.W., Australia, August 2-5, 1992 (2009)