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Three major Mesozoic-Tertiary basins adjoin in east-west alignment along the eastern third of the south coast of Australia, for a distance of about 700 miles. The total area embraced by the three basins is about 100,000 square miles, and it includes part of three of Australia's six states. Three-fourths of the area is offshore shelf.
The general east-west alignment of the basins resulted from sharp taphrogenic breakdown across the generally north-south Paleozoic orogenic trend of eastern Australia and Tasmania. The main faults and many of the basin features tend to have northeasterly or northwesterly trends, suggesting that rotational or transcurrent stresses were involved in the breakup and subsidence.
Sedimentation began at least as early as the Late Jurassic. The Mesozoic is an estuarine to marine, poor- to well-sorted terrigenous clastic series. The Tertiary is largely marine and is more uniformly developed throughout. The lower third of the Tertiary contains extensive, generally highly porous and permeable coastal sandstone beds together with some shale and varying amounts of coal and carbonaceous beds. The middle beds consist mainly of shale. The upper third or more has a considerable amount of limestone and marl. Several unconformities are recognized. Though not all sediments bear marine fossils, the contained waters are saline beyond the limits of the rather extensive fresh-water flushing onshore.
The Gippsland or eastern basin includes approximately 20,000 square miles. At least 10,000 feet of rapidly deposited Upper Jurassic-Cretaceous terrigenous clastics fills a down-faulted central trough and overlaps the basin shelves on the north and south; and about 9,000 feet of more widely extending Tertiary sandstone, shale, marl, and limestone comprises the remainder of the basin fill.
The deeply silled Bass basin separates the island State of Tasmania from the mainland. It occupies an area of 35,000 square miles. The section consists of 12,000 feet or more of sandstone, shale, marl, and limestone, together with some coal, of Eocene and earlier ages. Deposition began in the central part of the basin, probably at least as early as Late Cretaceous time, and it continued throughout the Tertiary. The deposition progressively overlapped toward all flanks.
The western, or Otway, basin includes an area of about 45,000 square miles. In this basin, the Mesozoic consists of sandstone, shale, siltstone, and mudstone. Deposition began during the latter part of Jurassic time and it continued, except for recognizable intervals of non-deposition, into the Paleocene. The maximum thickness exceeds 15,000 feet. A maximum of about 8,000 feet of overlapping Tertiary sandstone, shale, marl, and limestone completes the basin fill.
Potential petroleum traps of the following types occur: tectonic folds; fault or fault-block structures; massive, elongate sandstone bodies associated with pronounced transgressive overlap, shale interfingering, and compaction drape; porosity abutment both above and below extensive low-angle unconformities; unconformable overlap by basin-sink sediments across broad-bottom highs and against and over major fault scarps; structural noses; extensive progressive flank overlap around a deeply silled basin by a sandstone, shale, marl, and limestone section; and porosity pinchouts.
Since the mid-1920s, 130 exploratory borings have been drilled onshore in the extensively freshwater-flushed basin flanks. Numerous non-commercial oil and gas shows were logged. In 1965 and 1966, five exploratory wells were drilled offshore, up to the April, 1966, date of this paper. Three of these were located on well-defined closure in the Gippsland basin and resulted in major wet gas and oil discoveries in Eocene and Upper Cretaceous sandstone reservoirs.
Prior to farmout of each of the three basins, successively from east to west, 18,000 miles of aerial magnetic and 5,320 miles of conventional seismic surveys were carried out under the direction and assistance of the writers. Comprehensive basin studies and compilations completed the preliminary investigations. Not included in the foregoing are various geophysical surveys on a much smaller scale by other companies and government agencies. Also not included are many hundreds of miles of additional shooting by the farmee preparatory to drilling.
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