The Ulan Mine Complex is at the western limit of the geological formation known as the Sydney Basin and at the southern end of the Gunnedah Sub-basin. The resource coal targeted for extraction is from the Illawarra Coal Measures, which are of Permian age and overlain by sedimentary formations of varying thickness and types including sandstones, shales and alluvium. Ten coal seams (nine in the west) occur within the coal measures; eight occur above (seven in the west) and one below the Ulan Seam. These seams range in thickness from approximately 0.4 to 10 metres (with the Ulan Seam being thickest). Apart from the Ulan Seam, all seams within the mining leases are considered uneconomic in the current market due to their high ash content.
The main geological features present within the Ulan Mine Complex and in particular within the underground mining area are Triassic Narrabeen Group sandstones overlying the Permian sedimentary rocks of the Illawarra Coal Measures; and some Gulgong granite. More recent Jurassic Pilliga sandstone (over Gomiala shale) caps the Narrabeen layers in the north east and quaternary sands are present along rivers and major creeks.
While they are very different in character, the Narrabeen and Pilliga sandstones tend to be less resistant to erosion than the Hawkesbury sandstone in eastern NSW. Pilliga sandstone is friable and includes seams of conglomerate, siltstone and shale.
Other features include quaternary silts, sands and gravels which form river flats and flank some creeks, notably at confluences. The Illawarra Coal Measures outcrop below the Narrabeen Group, and are mostly poorly consolidated as are some layers within the Narrabeen Group.