Report on coal exploration in portion of Coal Licence 172 Port Keats area, Northern Territory

Abstract

During the period October - December 1966, the coal potential of Coal Licence 172, in the Port Keats area Northern Territory was investigated on behalf of Thiess Bros Pty Ltd by Geotechnics (Aust) Pty Ltd, Consulting Geologists. Port Keats is situated 155 miles south-easterly of Darwin. The sediments in the area are in a geosynclinal trough on the eastern edge of the main Bonaparte Gulf Basin. More than 14,000 feet of Palaeozoic sediments were encountered in Kulshill 1 bore hole. Outcrops are few, the rocks of the area being largely covered by deep red soil and the alluvial deposits of extensive inland plains. The major topographic features are laterite - capped mesas which are remnants of a highly dissected old peneplain. Permian sediments of the Port Keats Group, in which two coal bearing horizons occur, have a total thickness of over 5,000 feet. The Group consists mainly of Lower Permian sandstones, siltstones and shales. A thick glacial tillite has been deposited during the Lower Permian times. The subsurface geology of the Port Keats Group is known from the 1905 - 1909 coal bores and the 1965 -1966 oil bores. Four formations, designated Formation I-IV were recognised in the Port Keats Coal Bores Nos 1-4 which reached a maximum depth of 1,505 feet. Formation I is Triassic. The formations are mutually conformable. A fifth formation, named Formation V, recognised in coal bores at Anson Bay and Cliff Head, was originally correlated with all or part of Formations I-IV. Formation V probably correlates with the Sugarloaf Formation and the Kulshill Formation defined from the oil bores. Three oil bores, Kulshill 1, Kulshill 2 and Moyle 1 were drilled at Port Keats by Australian Aquitaine Petroleum Pty Ltd, during 1956 - 1966. Kulshill 1 started in sediments stratigraphically equivalent to part of Formation IV of the old coal bores. The sediments of the Port Keats Group intersected in the oil bores were subdivided into the Sugarloaf Formation and the Kulshill Formation. The Sugarloaf Formation consists of the Shale Member and the Sandstone Member. The Kulshill Formation consists of the Greywacke Member, the Microconglomeratic Shale Member and the Basal Silicified Sandstone Member. The eastern edge of the Basin is bordered by a northerly trending fault complex. East of the fault complex a thin veneer of upthrown Permian Sediments overlie Proterozoic basement rocks. Oil bore Moyle 1 drilled on the eastern side of the fault encountered basement rock at 1,695 feet. Two coal measures, referred to herein as the Upper and Lower Coal Measures, about 1,000 feet apart, are known in the Port Keats Group. The Upper Coal Measures are in Formation III and were intersected only by Port Keats Coal Bores Nos 3 and 4. Coal streaks and thin lenses of coal are interbedded with mainly non-marine shales and sandstones. The Lower Coal Measures are in the Sandstone Member of the Sugarloaf Formation and contain bright, bituminous coal ranging from thin streaks to 10 feet thick seams. The Sandstone Member has been deposited mainly in a restricted, shallow water, near-shore marine environment in reducing conditions. Thick coal beds may be found near the surface south-east of Kulshill 1 on the eastern limb of a shallow syncline. Near-surface coal beds may also be found on the western (and possibly eastern) side of the major fault complex where the sediments are thought to have been dragged up. The Upper Coal Measures have been correlated with the Permian Liveringa Formation of the Fitzroy Basin. The Lower Coal Measures possibly correlate with the lower beds of the Permian Nookanbah Formation, or the upper beds of the Poole Formation of the Fitzroy Basin. The extensive alluvial plains and deep soil cover in the area probably hide any coal beds which might otherwise outcrop.