Summary:
The Wilpinjong Mine is located in the Permian Illawarra Coal Measures on the northwest margin of the Sydney Basin. This coal measure is around 115m thick in the area, where the dominant lithologies are mudstone, siltstone, sandstone, coal, carbonaceous mudstone and tuffaceous claystone. The coal seams of interest at Wilpinjong include the Moolarben seam and the Ulan seam.
The Wilpinjong mine accesses the Ulan Seam, a 15m-thick seam that is hosted at the base of the late Permian aged Illawarra coal measures. The paleoenvironment is a protected swamp environment on the stable Carboniferous granite basement. The Ulan seam consists of ten plies, including plies of good quality coal, high ash coal, stony coal and partings of claystone, carbonaceous claystone, tuffs and other non-coal lithologies. The working plies at the Ulan seam are inter-bedded with clay stone and siltstone horizons. The seam is shallow and sub crops in the deposit area.
The Coal Reserves reported are high volatile bituminous in rank. The various coal products making it marketable for thermal use in domestic electricity generation and export.
Peabody classifies the Wilpinjong property as a coal deposit with low geological complexity based on the following factors:
• The Ulan seam is laterally continuous and can be correlated using geophysical logs across large distances with high confidence;
• The seam is relatively flat lying (1 to 3 degrees), gently dipping towards the north-northeast;
• There are no major faults in the area;
• The Ulan seam is currently mined across the area at two other mining operations.
The Moolarben seam consists of three plies, of which the lower half of the basal ~0.50m thick ply (M4) is currently being mined. The other plies – M1, M2 and M3 all exceed 40% in ash and have yields of less than 50% and are not considered mining targets. The Moolarben seam has not been mined in the local area in the past.
The Ulan seam ranges between 11 and 22.5 m in total thickness, however the mineable coal plies have a combined thickness of 5.7 to 9.0 m. The seam consists of a number of coal and stone plies that are correlated across the Wilpinjong resource area and into adjacent mines and projects. From these plies, Wilpinjong Coal Mine is currently using selective mining and washing, with some bypass to produce both domestic and export thermal coal products.
The overall confidence in the geological interpretation of the deposit is high. This is due to low variability (both structural and coal quality) as evidenced by the laterally consistent seam dip, lack of structure and relatively homogeneous coal quality (ply by ply basis).
Two areas of relatively high variability are around intrusions (dykes, sills and plugs) and palaeochannels (adjacent to Triassic age ridges and hilltops). In these areas a multi-faceted exploration approach has been utilized to increase confidence in the geological interpretation; including ground mapping, geophysical surveys and associated validation drilling.
The surface geology of the Wilpinjong Mine is dominated by subcrops of the Illawarra Coal Measures. This unit overlies the Shoalhaven Group, which crop out immediately south of Wilpinjong. The coal measures are overlain by the Narrabeen Group, which forms the cliff-lines and plateaus to the north and south, the ridges that protrude out from these plateaus and outliers such as those adjacent to the Slate Gully area. In places, the Illawarra Coal Measures are concealed beneath younger alluvial deposits, particularly those that occupy abandoned channel-fill, referred to on site, as “palaeo-channels”. Quaternary alluvial deposits also flank Wilpinjong and Cumbo Creeks.
In the Wilpinjong Coal Mine area, the shallow nature of the coal seam, combined with flat topography has resulted in extensive interaction between the base of weathering and the coal seam, to the extent that in some areas the seam has been completely oxidised. In addition, several extensive paleochannels have been identified adjacent to ridgelines that have deeply incised the coal seam. Locally, dips are relatively flat (1 to 3 degrees), with strata dipping to the north-northeast.
The Moolarben seam consists of three plies, of which the lower half of the basal ~0.50m (1.6ft) thick ply (M4) is currently being mined. The other plies – M1, M2 and M3 all generally exceed 40% in ash and have yields of less than 50% and are generally not considered mining targets. The Moolarben seam has not been mined in the local area in the past.
The Ulan seam ranges between 11 and 22.5m (36 to 74ft) in total thickness, however the mineable coal plies have a combined thickness of 5.7 to 9.0m (18.7 to 30.5ft). The seam consists of a number of coal and stone plies that are correlated across the Wilpinjong resource area and into adjacent mines and projects. The Ulan seam has minimal stone partings in the north west of the project with midburden partings opening up to the east.
The Ulan Seam is broken up into the A, B, C, D, E, F and G plies. These sections are selectively mined and campaign washed or bypassed at the CHPP to produce product coal at a specific ash point for both domestic and export thermal coal products. Some coal plies are mined across the whole site including A12, B1, B23, D1, D2, E1 and G. Other coal plies are mined only in certain pits (eg. C1 and D0 taken in Pit 6). The plies of the D seam (D0, D1, D2, DD2) are mined selectively or combined depending on the coal quality of the mining block. Several smaller splits occur either approximately on an north-south orientation (such as the G floor coal ply) or an east-west trend (such as B1 splitting away from B23). Generalised coal quality trends area also present across Wilpinjong for different coal plies in similar north-south and east-west orientations that the structure of the seams follow.
The rank of the coal seams is high volatile bituminous (ASTM D388 ‘Standard classification of Coal by Rank’), based on the volatile matter (daf) content of the coal plies, which is generally in the range 35 to 44%. The average volatile matter content of the Ulan seam plies ranges from 11 to 35%. For the plies that are less than 40% ash ad, the volatile matter content is between 20 and 35%. The low volatile matter content of parts of the Ulan seam is largely a function of the high inertinite content of the dull coal plies. The basal ply of the Moolarben seam is 32% ad. The air-dried moisture content of the Ulan seam averages 2.9% ad and is around 2% for high ash plies, and ranges from 2.5 to 3.5% ad for the coal sections. The basal section of the Moolarben seam has an average air-dried moisture content of 3.8% ad. The total sulphur content of the majority of coal plies is <0.5% ad. Certain plies are known to be locally higher in sulphur content (e.g., E and G plies) with values generally in the range 0.5 to 1.2% ad; and isolated analyses over 2.0%. The calorific value of the raw coal closely follows that of the ash content. On an air dried basis, coal that is less than 28% (ad) ash yields greater than 24 MJ/kg (ad) (5730 kcal/kg).
The surrounding ridges of resistant Triassic strata have combined with the thick seam and shallow dips resulting in an extensive area of shallow coal that is amenable to open cut mining. These ridges are generally within National Park and are excluded from mining.
No major faults have been identified within the Wilpinjong Coal Mine area, however, some minor faulting (<5m vertical throw) produces normal faults with a few meters throw or small thrust faults producing localised seam rolls that have limited impact on the current mine’s coal recovery.