Story: Coal and coal mining
Page 2 – Coal resources
A comprehensive coal survey made between 1976 and 1989 established that New Zealand had an estimated 15 billion tonnes of in-ground coal. The amount that could be recovered depends on geological, engineering, economic and environmental constraints. The largest resource, the lignites of Southland and Otago, has technically recoverable reserves of about 75,000 petajoules (PJ) of energy – by comparison, the huge Māui gas field originally contained 4,500 PJ.
North Island resources
North Island coalfields are located in Northland, the Waikato and northern Taranaki. Most known areas of sub-bituminous coal in Northland have been worked out, and the last mine closed in 1955.
Thirteen coalfields, extending from Drury (30 kilometres south of Auckland) to Mangapēhi (20 kilometres south of Te Kūiti), are geologically grouped as Waikato coalfields. In these, there are 2,078 million tonnes of in-ground coal. However, much of this is in deep seams, which will be very difficult to mine by conventional means. Techniques such as underground coal gasification or coal seam methane production may be needed to extract the energy.
Waikato coals are all sub-bituminous, laid down in the Eocene and Oligocene periods. Seams in the north of the region generally have low to medium ash and low sulfur contents, whereas seams in the south have medium to high ash and sulfur contents. The main seam is typically 3–10 metres thick, but reaches 20 metres or more in parts of the Huntly and Waikare coalfields. Some of the thinner seams have been worked out.
Coal measures is a name for coal-bearing sedimentary rocks, which include sandstone, mudstone and conglomerates as well as coal seams. Coal measures mined in New Zealand are generally a few tens to several hundreds of metres thick, and they are between 10 and 75 million years old.
There are five coalfields in northern Taranaki, of which the Mōkau coalfield near the west coast is the largest. Previous mining has been on a small scale, and there has been no production since the last mine, in the Waitewhena coalfield, closed in 1990. In-ground resources amount to 380 million tonnes of sub-bituminous coal.
The South Island’s West Coast region contains New Zealand’s only bituminous coals. Some have unusual properties that are in demand in international coal markets. There are 13 coalfields of various sizes between Greymouth and Seddonville (40 kilometres north of Westport). The Buller, Greymouth, Pike River and Reefton coalfields are the most economically important.
There are 983 million tonnes of in-ground coal in the region. Over three-quarters of recoverable reserves are in the Greymouth coalfield (mostly underground) and Buller coalfield (mostly opencast). Pike River is a coalfield under development in the Paparoa Range, north-east of Greymouth.
Coal from the top of a mountain
One of the unusual features of the West Coast coalfields is that bituminous coal is found at high altitudes. In the 1940s, geologist Harold Wellman recognised that this was due to a complex two-stage geological history. Maximum burial occurred about 35 million years ago, then the deeply buried basins were lifted, so that they are now preserved in the mountains – a process later named inversion tectonics.
There are two main types of coal measures (coal-bearing sedimentary rocks): late Cretaceous to earliest Tertiary Paparoa coal measures, and Eocene Brunner coal measures. Both Paparoa and Brunner coalfields contain seams up to 20 metres thick. Paparoa coals are characterised by low ash and sulfur contents, and Brunner coals have lower ash and varying sulfur content.
West Coast coals are almost all bituminous, with a small deposit of anthracite at Fox River. Their high quality and low level of mineral contaminants enable them to command premium prices on world markets.
Otago and Southland
The bulk of New Zealand’s coal resources are in the south of the South Island. Over 7 billion tonnes of mineable lignite resources have been proven in Otago and Southland, enough to provide a significant proportion of New Zealand’s energy needs for a long period.
Otago coalfields include very large lignite deposits in Central Otago, the Kaitangata coalfield south of Dunedin, and several smaller coalfields. Central Otago lignites are in seams up to 90 metres thick and typically have 40%–50% in-ground moisture and low to medium ash and low sulfur contents.
The eastern Southland lignite fields are comparable to other large deposits in the world and are, by far, New Zealand’s biggest fossil fuel energy resource. They form extensive, multiple seams up to 18 metres thick, and typically have 40%–65% in-ground moisture and low to medium ash and low sulfur contents.
The Ōhai coalfield in central Southland has seams up to 23 metres thick containing sub-bituminous and bituminous coals, generally with low ash and sulfur contents.