Coal resources: Waikato

Location

The Waikato Coal Region comprises 13 coalfields extending from Drury (30 km south of Auckland) to Mangapehi (20 km south of Te Kuiti in the western central North Island). The main coalfields are Maramarua, Waikare, Huntly and Rotowaro in the north of the region, and Kawhia, Tihiroa and Mangapehi in the south.

Geology

Coal seams are within the Eocene Waikato Coal Measures, which unconformably overlie an erosional surface cut on Mesozoic basement rocks. The Waikato Coal Measures are the basal formation in the predominantly transgressive marine Te Kuiti Group, which may in turn be overlain by younger marine and fluvial sediments. The Kupakupa seam is restricted to the northern part of the region, but is the most widespread and commonly mined seam. The Kupakupa seam is typically 3-10 m thick, but reaches 20 m in parts of Huntly and Waikare coalfields. There are several other named seams, which are less widely distributed, thinner, and commonly more split. Some have been mined. The coal measures were deposited in regional depressions over an undulating basement surface, with minor contemporaneous faulting but widespread compactional effects influencing the lowest seam in particular. Subsequent block faulting and erosion has affected the present distribution, depths from surface, and structure of coal seams that tend to have lenticular geometry.

Coal quality

Waikato coal covers the full sub-bituminous rank range, from sub-bituminous C in the Tihiroa, Kawhia, Waikare and Maramarua fields, to sub-bituminous A in Glen Massey, Rotowaro, Huntly and Pukekawa fields. Seams in the north of the region generally have low to medium ash and low sulphur contents, whereas seams in the south have medium to high ash and sulphur contents.

Resources

Coal-in-ground resources for the region are 2078 Mt, of which 714 Mt are estimated to be recoverable. Nearly half of the recoverable resource is in the Huntly coalfield, from which virtually all opencastable reserves have been mined. The region is essentially fully explored apart from some areas where coal measures are known to exceed 1000 m depth.

Production

Coal has been mined in the Waikato since the late 1840s. By the 1950s, production was over 1 Mtpa, with recent production averaging about 1.4 Mtpa and accounting for 40% of New Zealand's total annual production.

Utilisation

The BHP New Zealand Steel Ltd mill at Glenbrook is currently the largest single market for Waikato coal, using about 800,000 tpa for direct reduction iron making. About 500,000 tpa is supplied to the 1000 MW thermal power station at Huntly, owned by Genesis Power, one of three State Owned Enterprises split from the old Electricity Corporation of New Zealand. Industrial steam raising accounts for most of the balance of current production.

Exploration status

Exploration of the Waikato Coal Region was a major component of the NZCRS programme and was carried out continuously between 1975 and 1989, managed by NZ Geological Survey with operational support from State Coal Mines. Over 1000 holes were drilled, and up to 15 drilling rigs operated at times. Drilling was carried out in Maramarua, Waikare, Huntly, Rotowaro, Glen Massey, Whatawhata, Kawhia, Te Kuiti and Mangapehi coalfields. Scout drilling explored the limits of these coalfields and other areas where there was any geological possibility of unknown coal deposits down to depths exceeding 500 m. As a result, new, entirely subsurface coalfields were discovered at Pukekawa, Whangamarino and Tihiroa. Mining feasibility studies were carried out in Maramarua and Kawhia coalfields.

Maramarua Coalfield

NZCRS exploration of the Maramarua Coalfield began in 1976 with a preliminary drilling programme to investigate reserves of opencast coal and was later extended to include investigations of deeper coal. A total of 122 holes had been drilled at completion of the programme in 1980.

In 1982, a preliminary mining and environmental constraints assessment of Maramarua Coalfield was commissioned. This was the first of a series of formal mining studies of coalfields explored by the NZCRS, and indicated that very low recovery percentages (14-28%) of in situ coal resources could be expected by underground mining in the Waikato.

Waikare Coalfield

Between 1975 and 1982, 403 holes were drilled in the completely concealed Waikare Coalfield, including from a barge-mounted rig which drilled 61 holes on Lake Waikare. Resources discovered were estimated at 219 Mt of coal-in-ground, including 30 Mt of potentially opencastable measured coal in what became known as the Ohinewai Opencast Prospect, the largest block of virgin opencast coal left in the North Island. Evaluation of this resource was taken over by State Coal Mines in 1977.

Huntly Coalfield

Huntly Coalfield had been extensively explored prior to the commencement of the NZCRS, and new exploration between 1975 and 1983 was largely confined to the concealed northern portion of the field beyond the boundary of the Huntly West mine. Exploration was confined to drilling, and 185 holes were drilled, including 13 drilled on Lake Rotongaro.

A total of 630 Mt of coal-in-ground was discovered in seams up to 23 m thick, although the majority of the resource is at depths between 300 and 500 m. The exploration of coal resources at greater depths than 300 m was not regarded as high priority at any stage of the NZCRS but there were concerns about energy resources arising from the 'Energy Crisis' of the time and a desire to undertake limited deep drilling to complete an overall evaluation of coal resources in the Waikato. A number of geological constraints to underground mining at these depths were identified, drawing substantially on experience in the nearby Huntly West mine.

Kawhia Coalfield

The NZCRS programme was in several stages, involving both the NZ Geological Survey and DJ Phelps & Co Ltd. Exploration was partly driven by planning at the time for another thermal power station and it was thought that the relatively high-sulphur Kawhia coals would be suitable. A total of 112 holes were drilled between 1979 and 1986, which together with extensive outcrop mapping and sampling, resulted in the discovery of significant resources adjoining those previously known.

Coal seams were initially mapped and sampled in the southern portion of the coalfield. Scout drilling explored the concealed parts of the coalfield to the north, including what was later determined to be a separate coalfield in the east. DJ Phelps managed a second phase drilling programme in west Kawhia where coal had been intersected in the scout programme. Exploration in east Kawhia was carried out by NZ Geological Survey which also supervised a shallow drilling programme in the south east of the field in the hope of discovering opencastable resources to complement production from underground mining.

Sub-bituminous coal-in-ground resources of 182 Mt were discovered, of which 103 Mt is in one sector in west Kawhia, mainly at 100 - 300 m depth. Of the 60 Mt of in-ground coal in east Kawhia, only 14 Mt is less than 300 m deep.

A detailed preliminary mining assessment of the west Kawhia Coalfield was completed in 1988. Underground mining was considered feasible, although there are considerable mining constraints.

Tihiroa Coalfield

Scout drilling to the east of the Kawhia Coalfield discovered coal in an area that was subsequently found to be separated from the Kawhia Coalfield by a barren zone and was renamed the Tihiroa Coalfield. Between 1979 and 1984, 69 holes were drilled which discovered an estimated resource of 180 Mt of coal-in-ground, mainly between 200 and 400 m deep.

Other Waikato coalfields

Two holes were drilled in Drury Coalfield, and it was concluded that the coalfield had no significant remaining coal resources.

The Pukekawa Coalfield was defined on the basis of 21 holes drilled between 1976 and 1983. A total of 40 Mt was discovered at depths of 400-900 m over most of the coalfield.

A large area underlying the Whangamarino swamp was explored by 34 drillholes which discovered an estimated 29 Mt of coal-in-ground. While coal in the east of the new Whangamarino Coalfield is 100-300 m deep, seam depths elsewhere are up 800 m or more.

Scout drilling in the Rotowaro Coalfield as part of the NZCRS programme discovered coal to the north of the known limits of the coalfield. Subsequent drilling of 28 holes confirmed that in-ground resources of 8 Mt coal did not extend into nearby Huntly Coalfield. NZCRS drilling discovered only limited coal in the northwest of the Glen Massey Coalfield, and the remaining resource potential is low. Scout drilling and mapping in the Whatawhata Coalfield did not locate any coal of significance.

Twenty nine holes were drilled in the Te Kuiti Coalfield in 1983-84 primarily to locate shallow coal. Less than 7 Mt of potentially opencastable coal were identified, and a further 34 Mt of potentially underground mineable coal-in-ground was inferred.

Comments

Expansion of Waikato coal production is being driven by demand to fuel Genesis Power’s 1000 MW Huntly station as gas supplies tighten. Solid Energy’s Waikato production will rise from current levels of 380,000 tonnes a year to about 1.7 million tonnes a year by 2006/7. Genesis Power has also contracted significant production (about 500,000 tpa) from the three minor producers remaining in the Waikato. Because coal mines cannot expand sufficiently quickly to meet dramatic demand swings, Genesis began importing coal in 2003.

Waikato coal, especially from the northern part of the region, is a high quality thermal coal. The large resources in the region include extensive deep coal and the largest virgin block of opencastable coal left in the North Island, at Ohinewai in the Waikare Coalfield. Development of underground mines is hampered by difficult mining conditions, which contributed to the demise of the big Huntly West mine in 1992. Since then, radical reform of the electricity sector in New Zealand saw the closure of Solid Energy's Kopako opencast mine after mothballing of the coal-fired Meremere power station. Coal demand has now reversed as the Maui gas field declines.

Key references

General Waikato references

Kirk PA 1988 Waikato Coal Region coalfields and resources NZ Geological Survey miscellaneous series map 17.

Kirk PA, Sherwood AM, Edbrooke SW 1988. Waikato Coal Region: a summary of geology and coal resources. NZ Geological Survey record 34.

Edbrooke SW and others 1994. Geology of the Waikato Coal Measures, New Zealand. Institute of Geological and Nuclear Sciences monograph 6.

Maramarua

Anon 1983. Preliminary mining and environmental constraints assessment, Pilot Study, Maramarua Coalfield. Unpublished report for Mines Division.

Edbrooke, SW 1981. Maramarua Coalfield: assessment of exploration 1977-1980. NZ Geological Survey report M85. Prepared for Resource Management and Mining, Ministry of Energy.

Lavill, DR 1983. Coal properties and utilisation of the Maramarua Coalfield, New Zealand. Resource Management and Mining coal geology technical report 5.

Waikare

Fowke NC 1987. Waikare Coalfield: assessment of exploration 1975-1982. NZ Geological Survey report M157. Prepared for Resource Management and Mining, Ministry of Energy.

Mitchell, KD 1989. Geology and coal resources of the Waikare Coalfield, New Zealand. Coal geology report 16. Market Information and Analysis, Ministry of Energy.

Huntly

Edbrooke SW 1987. New Zealand Coal Resources Survey. Northern Huntly Coalfield assessment of exploration 1975-1983. NZ Geological Survey report M158. Prepared for Resource Management and Mining, Ministry of Energy.

Mitchell KD, Edbrooke SW 1988. Geology and coal resources of the Northern Huntly Coalfield, New Zealand. Coal geology report 10. Market Information and Analysis, Ministry of Energy.

Kawahia

Anon. 1988. New Zealand Coal Resources Survey, preliminary mining assessment of the West Kawhia Coalfield (two volumes). Unpublished Applied Geology Associates Ltd report for Resource Management and Mining, Ministry of Energy.

Kirk PA 1986. New Zealand Coal Resources Survey, East Kawhia Coalfield: geology and resources. New Zealand Geological Survey report M148. Prepared for Resource Management and Mining, Ministry of Energy.

Mitchell KD, Kirk PA, Phelps DJ 1989. Geology and coal resources of the Kawhia Coalfield, New Zealand. Coal geology report 15. Market Information and Analysis, Ministry of Energy.

Newman J 1989. Coal geology the West Kawhia Coalfield, New Zealand. Coal geology report 17. Market Information and Analysis, Ministry of Energy.

Phelps DJ 1985. New Zealand Coal Resources Survey, West Kawhia Coalfield: geological assessment of coal resources. Unpublished report by D J Phelps & Co Ltd for Resource Management and Mining, Ministry of Energy.

Phelps DJ 1986. New Zealand Coal Resources Survey, South Kawhia Coalfield: geological assessment of coal resources. Unpublished report by D J Phelps & Co Ltd for Resource Management and Mining, Ministry of Energy.

Phelps DJ 1987. New Zealand Coal Resources Survey, West Kawhia Coalfield: update on the geological assessment of coal resources. Unpublished report by D J Phelps & Co Ltd for Resource Management and Mining, Ministry of Energy.

Tihiroa

Kirk PA 1985. Tihiroa Coalfield: an assessment of coal resources survey exploration, 1979-1984. New Zealand Geological Survey report M146. Prepared for Resource Management and Mining, Ministry of Energy.

Other Waikato coalfields

Edbrooke SW 1982. Coal exploration drilling in Hunua mining sector, Drury Coalfield, December 1981. New Zealand Geological Survey Report M120. Prepared for Resource Management and Mining, Ministry of Energy.

Edbrooke SW 1984. Pukekawa Coalfield: assessment of coal resources survey exploration. New Zealand Geological Survey Report M139. Prepared for Resource Management and Mining, Ministry of Energy.

Fowke NC, MacDonell BJ 1986. Te Kuiti Coalfield: geological assessment of shallow drilling programme, August 1983-November 1985. New Zealand Geological Survey Report M150.

Kirk PA 1986. New Zealand Coal Resources Survey: geology and coal reserves of Ruawaro Sector, Rotowaro Coalfield. New Zealand Geological Survey Report M152.

Phelps DJ 1985. New Zealand Coal Resources Survey. Whatawhata Coalfield. Unpublished report by D J Phelps & Co Ltd for Resource Management and Mining, Ministry of Energy.

Unpublished NZ Geological Survey reports of NZCRS exploration, 1977-1983.