Geology and Exploration of Three Greater Bass Strait Basins, Australia: ABSTRACT

Abstract

Three major Mesozoic-Tertiary basins lie in succession along the eastern one-third of the south coast of Australia within a distance of about 700 miles. The total area embraced is approximately 100,000 square miles, and it includes parts of three of Australia's six states. Fully three-fourths of the area is classified as an 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 have northeasterly or northwesterly trends, suggesting that rotational or transcurrent stresses and subsidence were involved in the breakup. Sedimentation began at least as early as Late Jurassic. The succeeding Mesozoic development lacks uniformity over the area, but the Tertiary is more uniformly developed throughout. Several unconformities are recognized. Though not all sediments carry marine fossils, the contained waters are saline beyond the limits of the fresh water flushing onshore. The Gippsland or eastern basin covers about 22,000 square miles. More than 12,000 feet of rapidly deposited Jurassic-Cretaceous clastic rocks fills a downfaulted central trough and overlaps the basin shelves on the north and south. About 10,000 feet of more widely extending Tertiary sandstone, shale, marl, limestone, and some coal, completes the basin fill. The deeply silled Bass basin, which separates the island State of Tasmania from the mainland, covers about 35,000 square miles. The section is composed of 12,000 feet or more of sandstone, shale, marl, limestone, and some coal. Deposition began in the central part of the basin, probably as early as Late Cretaceous time, and continued through the Tertiary, progressively overlapping radially in all directions. The western or Otway basin covers more than 40,000 square miles. The Mesozoic consists of sandstone, shale, siltstone, and mudstone. Deposition began during Late Jurassic time and continued, with laterally differing breaks in deposition, into the Paleocene; a maximum thickness of more than 15,000 feet was deposited. Approximately 8,000 feet of overlapping Tertiary sandstone, shale, marl, and limestone completes the basin fill. Potential traps for petroleum accumulation of the following types occur: tectonic folds; fault or fault-block structures; massive, elongate sandstone bodies associated with pronounced transgressive overlap and compaction drape; porosity abutment both above and below extensive low-angle unconformities; unconformable overlap of basin-sink sediments over broad bottom highs and against and over major fault scarps; structural noses; extensive progressive flank overlap around a deeply silled basin by a section composed of sandstone, shale, marl, and carbonate rocks; and porosity pinchouts. Approximately 30 exploratory wells drilled onshore in the extensively fresh-water-flushed basin flank, found numerous non-commercial oil and gas shows. The first offshore well drilled in the Gippsland basin 20 miles from the coast (the first offshore well in Australia) resulted, early in 1965, in a major wet gas discovery in thick, very porous Tertiary sandstone. End_of_Article - Last_Page 639------------