Thermal
Feature Articles
Hunter Valley Coalfield Seams—Geology and Quality
September 2021
Hunter Valley Coalfield is located within the Sydney-Gunnedah Basin. Almost all major coal resources in NSW are located in the Sydney-Gunnedah Basin, which extends from the south of Wollongong to the north of Newcastle and is 500km and 150km wide. Coal measures of this basin are bituminous in rank and Permian in age. These vast measures contain a wide variety of coal, from high quality thermal coal to low volatile semi-hard coking coals. There are five major coalfields within the basin: Hunter, Newcastle, Southern, Western and Gunnedah.

The Hunter Valley Coalfield mines currently produce ~102Mtpa thermal coal, most of it for the export market. Over 25 seams are mined in the coalfield, and they set a benchmark for their high quality and favourable specifications.

 

Who Mines What

Almost 90% of the thermal coal produced at the 16 operating mines in the Hunter Valley Coalfield is from the Wittingham Coal Measures. The remainder is from the underlying Greta Coal Measures and is extracted from the Mangoola and Muswellbrook mines. The table (below) lists the Hunter Valley coalfield mines from south to north going down the table.

 

 

Hunter Valley Geology

The Hunter Valley Coalfield is the largest coal producing region in NSW. Sixty identified coal seams are located within three distinct sequences: the Greta Coal Measures, the Wittingham Coal Measures and the Newcastle Coal Measures (previously known as the Wollombi Coal Measures). A large proportion of coal within the Hunter Valley Coalfield is at relatively shallow depths, making it accessible to large-scale, multi-seam open-cut mining operations. The coalfield contains significant reserves of export quality low-ash, high calorific value thermal coals and low-ash semi-soft coking coals.

Over 28 seams in the Hunter Valley coalfield are mineable (at least 20 in the Wittingham Coal Measures and 8 in the Greta Coal Measures), with some seams having multiple plies. The Liddell Seam comprises the Upper, Middle and Lower Liddell seams and the Barrett and Hebden each have upper and lower plies.

The Hunter-Mooki Thrust fault, bounding the northeast of the coalfield, and other large-scale thrust faults associated with it, influence the burial depth and temperature, and therefore rank, of the coal seams. A series of gentle folds across the Hunter Valley Coalfield further affects the mine geology. The Muswellbrook Anticline is a major structural feature in the coalfield.

Coal seams in mines south of the anticline (shown above the dividing line in the table) are higher in rank due to their increased depth and greater structuring, and produce semi-soft coking coal as well as thermal coal. Those north of the anticline (below the dividing line) are far less affected by faulting and structuring and produce thermal coal only from their lower rank seams.

In the central part of the coalfield, the lower seams—particularly the Liddell Seam, but also the Pike Gully, Arties, Barrett and Hebden Seams—yield semi-soft coking coal as well as thermal coal. The seams at Rix’s Creek South mine yield the highest proportion of semi-soft coking coal in the coalfield due to the structuring associated with the nearby Hunter Thrust. Likewise, in the southern area of the coalfield, the Mt Thorley thrust influences coal rank. Mt Thorley mine, lying close to the thrust fault, has higher rank seams and, like Rix’s Creek South, produces ~40% semi-soft coking coal.

The Greta Coal Measures are the oldest of the economic coal measures in the Sydney Basin. They reach shallow depths in the north of the Hunter Valley coalfield. The coals are generally high to very high in volatile matter. While parts of some Greta coal seams commonly have a higher sulphur content than other Australian coals, the Greta coals near Muswellbrook have only slightly higher sulphur content than the Wittingham coals.

 

Hunter Valley Thermal Coal Qualities

Premium export thermal coal from the Hunter Valley mines typically has medium ash, low-to-medium sulphur and high volatile matter content. Hunter Valley coal has a medium Hardgrove Grindability Index, indicating that it pulverises fairly readily in the mills at power stations. The ash fusion temperatures are higher than average, indicating a lower propensity to form slag in the boiler. Slagging will occur if furnace exit gas temperatures exceed the fusion temperature of ash from the coal being fired. Poor fuel balance resulting from poor fuel fineness also increases slagging.

Hunter Valley mines may also produce a higher ash, 5,500kcal/kg product for the export market. These coals have slightly lower volatile matter contents compared to the premium products. The ash fusion temperatures of the high ash export coals are not generally available, but are expected to be similar to those of the premium export thermal coal.