PESA SA/NT: March Technical Luncheon with Peter McCabe, ASPER

Photo: Neck of a sand volcano in the Triassic of the Sydney Basin.

Abstract:

Injectites – an important element of many petroleum systems.  Lessons from the Sydney Basin

Sandstone injectites were first recognized in outcrops almost 200 years ago and are common in many sedimentary successions. However, they were not seen as significant aspects of any petroleum systems until the 1990s. Injectites are now important plays in several mature petroleum provinces. The sandstones can be both major reservoirs and important conduits for fluid migration. In outcrops, recognition of sandstone dikes, can be easy but other types of injectite features are more subtle and are easily misinterpreted.  This is especially the case for larger features. In the subsurface, it can be particularly difficult to identify injectites whether from core, well log or seismic data and, as a result, a lot of injectites are probably unrecognized, especially in frontier basins.

The Triassic strata of the Sydney Basin contain many large injectite features and they can constitute a significant part of the strata: forexample, injectite sandstones comprise ~20% of one 200m thick section.  Associated with the injectites are a few ejectite structures.  The neck of the largest observed sand volcano is 27m in diameter – a scale equivalent to the largest sedimentary volcanos observed today. The compaction of thick coals in the underlying Permian strata and associated biogenic methane production may have provided an abundant supply of water and gas for fluidization of overlying sands. The triggering mechanism for the upward movement of fluids and mobilization of sands in the subsurface was probably seismic activity associated with thrusting in the nearby New England Fold Belt, although volcanic activity within the basin may also have been a factor. There is abundant evidence of both earthquakes and volcanic eruptions coeval with deposition of the Triassic sediments.

The lecture will emphasize criteria for recognition of injectites in the subsurface based on observations from outcrops and cores in the Sydney Basin.