Publication Name: Eastern Australian Basins Symposium 2001
Authors: P.A. Arditto
Date Published: November 2001
Number of Pages: 34
Reference Type: Magazine Article
Abstract:
The Permian Nowra Sandstone, a widespread nearshore marine sandstone unit, was a common target in the deeper hydrocarbon tests of the southern and central Sydney Basin between 1960 and 1970, but its hydrocarbon potential is now largely overlooked. The unit is interpreted to have been deposited as a lowstand wedge related to a significant, but short lived, forced regression as this relatively thin, high energy, estuarine channel to delta front and shoreface package lies between outer shelf mud-rich deposits of the Wandrawandian Siltstone, below, and Berry Siltstone, above. These latter two units have the potential to be both a seal and hydrocarbon source. Isopach and isochron data for the Wandrawandian Siltstone have a NW-SE orientation, thickening NE,whereas a symmetrical NE-SW orientation is indicated for isopachs of the overlying Nowra Sandstone; this latter thickness geometry is interpreted to reflect deposition within a broad, low relief incision into the top of the Wandrawandian Siltstone. The isopach and isochron data for the overlying transgressive Berry Siltstone is oriented essentially N-S, thickening E, reflecting syndepositional down-to-the-east normal faulting. The top of the Nowra Sandstone was abruptly drowned by this transgression and appears to approximate a time-synchronous surface.
More recent drilling results demonstrate a currently active gas-generative petroleum system (both carbon dioxide free and ethane enriched), probably sourced from both the Wandrawandian Siltstone and Berry Siltstone. Although previous tests were not located on valid structural traps, a tighter grid of modern seismic data and more detailed structural mapping may yield valid exploration targets within previously under-explored regions. The part of the Sydney Basin under review is adjacent to the Moomba to Sydney gas pipeline and strategically close to both the Wollongong and Sydney gas markets, implying relatively small gas discoveries have the potential to be viable.