Upper permian coal measures of the Central Sydney Basin, N.S.W. – seismic data analysis and drilling results

Abstract

The results of a preliminary eight hole coal drilling program in the Dural-Spencer-St. Albans region are presented. The data from the South Colah, Frederick and St. Albans holes are discussed with respect to lithologies, stratigraphy, depositional environments, coal intersections and qualities. The holes were drilled beyond 800m. The north western bores intersected coking coal seams, the thicknesses of which varied between 0.69m and 4.85m. A study was made of the regional seismic reflection data available from petroleum prospecting operations. The data was very useful in the drilling programme in that it enabled the recognition of, and estimation of depths to, the coal measures as a gross unit. However it was not possible to determine seam configurations owing to fundamental limitations in the resolving power of the reflection method. The coal measure sediments intersected in the drill holes comprise: (a) A lower unit in which sandstones are mostly thin; shales, dirty coals and thin tuff beds are common. The beds appear to be lenticular. Evidence of salinity (indicated by burrowing) is common; (b) An upper unit in which sandstone and conglomerates are the dominant rock types. The sandstones appear to be sheet-like bodies. The coals are cleaner and burrowing in the finer sediments is rare. Coking quality coals are developed in this unit, they are thickest in the northwest. Assuming a thick tuff bed to be a time plane, the facies change is diachronous becoming younger from windeyer's Hawkesbury Bore southwards. The relationships in the South Colah drill hole may be more complex. Environmentally the lower is almost entirely deltaic ranging from bay to upper delta plain deposits becoming more distal westwards and downwards. One possible point bar sequence is present in the South Colah hole. The upper unit is mainly fluvial, probably deposited by braided streams but some point bar and deltaic deposits are present in South Colah. The sediments of the upper sequence could have been derived from the northeast or the north to northwest. The simplest model is that the upper and lower units are the downstream deposits of the streams that laid down the coal measures in the Newcastle-Lake Macquarie area.