Regularly spaced fractures crosscut a mechanical unit boundary
The sandstone layers that comprise this cliff are in the Triassic Narrabeen Group, which outcrops throughout the Blue Mountains northwest of Sydney, Australia. Most famously, they are the same beds that form the Three Sisters rock formation, which tourists make pilgrimage to.
People have built houses near the top of this cliff in order to provide a scale for measuring the height of these vertical fractures. A 2-story house is about 7m high. Focusing just on the two central, thicker sandstone layers, the upper is 85 m thick and the lower is 55 m. Some fractures penetrate the underlying sandstone layer as well (another 15 m). Plus we can’t see into the tree-covered recessive layers above and below the sandstones, hence it is likely that some fractures extend well beyond 150 m in height.
If these strata and their fractures were in a deeply buried hydrocarbon reservoir we would anticipate effective gravity drainage due to the great height of these fractures. If this reservoir were fully oil-saturated and you injected lower-density gas into the fractures, it would move buoyantly to the top of the fractures and then laterally from fractures to matrix. This buoyant gas would displace some oil from the matrix by pushing it downwards, which would seep into the lower part of the gas-filled fractures, making room for more gas to move into the matrix. By continuing such a cycle the oil is efficiently drained from the reservoir. A similar process can function with injected water except the density contrast of the oil/water system is opposite that of the gas/water system, thus the buoyancy-induced drainage process is upside down by comparison.
The prominent bedding layer that divides the upper sandstone layer from the one below it appears to be well-bedded, and the foliage growth suggests it is easier to root here than in the cliff faces. One might expect it would be a mechanical unit boundary that would arrest fracture propagation. And one would be wrong, at least with regards to the tall fractures, most of which appear to cut right across this bedding surface.
In the left half of the scene, as well as the rightmost quarter of the scene, the tall fractures are nearly uniformly spaced. Fracture spacing is about equal to the thickness of the upper thick sandstone. But most of the fractures transect both of the thick sandstone layers as more-or-less continuous tall fractures, thus fracture spacing is about half the height of the fractures.
Fractures always leave us wondering. Just to the right of center, both sandstone layers are crossed by a zone in which fractures are much more closely spaced than the more-typically spaced fractures to either side. As with most fracture clusters, or corridors, seen in previously posted photos this image shows no evidence to suggest why a cluster developed here.