Subsurface analysis of fluvial systems of Epsilon and Toolachee formations, Cooper Basin, Australia

Abstract

Fluvial channel sandstones form major productive reservoirs in many basins around the world. In these basins, exploration and production activities are increasingly concerned with predicting the origin and distribution of fluvial sandstone bodies, which include a wide variety of stacking patterns, ranging from narrow and single-story sandstone bodies to wide and amalgamated multi-story bodies. Determining the geometry of sandstone bodies is critical to building and conditioning 3D reservoir models which help to determine the most efficient well spacing and thus improve reservoir management. The most critical dimension to be determined is the width of the sandstone bodies, which can be estimated from five methods: (1) well correlations, (2) outcrop analogues, (3) modern analogue, (4) empirical equations, and (5) 3D seismic geomorphology. This study was initiated to determine the geometry of subsurface fluvial sandstone bodies deposited within the Epsilon and Toolachee formations, two of the most productive formations within the Cooper Basin, South Australia. The intracratonic, Late Carboniferous–Middle Triassic Cooper Basin is characterized by three major elongate, synclinal depocenters which are separated by two anticlinal, intrabasin highs; these structures were active during deposition of the units studied here. On a regional scale, the Epsilon and Toolachee formations consist of fluvial, deltaic, and shoreface deposits. The fluvial sandstone reservoirs of interest within the study area of these formations were examined by using 3D seismic and well log data. This work indicates that most of the sandstone bodies within the Epsilon and Toolachee formations cannot be resolved on seismic data because the widths and thicknesses of these sandstone bodies are not great enough to be seismically visible, especially at deep burial depths. Two correlation panels were constructed using the widespread coal bed within the Toolachee formation as a datum to determine the width of the sandstone bodies. These correlation panels with the calculated sandstone portions suggested that sandstone bodies are amalgamated within the Toolachee formation and are isolated within the Epsilon formation; however, some of these sandstone bodies cannot be correlated between adjacent wells.