An improvement in lithology interpretation from well logs in the Patchawarra Formation, Toolachee Field, Cooper Basin, South Australia

Abstract

Conventional lithology interpretation from well log data becomes unreliable when the target bed is thinner than the vertical resolution limit of the logging tools. The thin bed log analysis method of Bateman (1990), using the concept of binary lithology and a filtering technique applied to conventional logging suites, is able to detect beds as thin as 15 cm. This paper will focus on the use of the Bateman method to improve log-derived lithology estimates in the Permian Patchawarra Formation sandstone gas reservoirs in the Toolachee Field of the southern Cooper Basin in South Australia. In three representative wells, conventional log analysis overlooks most of the thin sandstone beds and underestimates the gross sandstone by up to 36% when compared with core data. In contrast, the thin bed algorithm identifies the thin sandstone beds and gives a gross sandstone thickness which agrees with the core value to within about 5%. It therefore provides a more correct picture of reservoir rock distribution, and reveals more potential pay than conventional log analysis. The method also makes core to log depth matching easier and more accurate. Log character and flow test results imply the presence of relatively permeable gas-bearing zones within some of the thin sandstone beds, suggesting that overlooking thin bed reservoirs may significantly lower reserve estimates. Since the method does not require the use of any high resolution logging device, it is possible to re-evaluate old log data for which conventional log analysis is subject to such under-estimation.