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Opportunities for underground geological storage of CO2 in New Zealand; Report CCS-08/2, The onshore Waikato region



Opportunities for underground geological storage of CO2 in New Zealand; Report CCS-08/2, The onshore Waikato region



GNS Science Report: 72



Carbon capture and storage (CCS) is recognised as a key mitigation measure in the reduction of greenhouse gas emissions. Several major point sources of CO2 are located within the Waikato region and we present an initial assessment of storage potential in the region's onshore sedimentary basins and in southeast Waikato ignimbrite deposits. Other reports in this series assess the potential for CO2 storage in deep unmineable coal seams and in the offshore Waikato area. The most efficient storage of CO2 is achieved when it is in the supercritical fluid state; geothermal and hydrostatic gradients in the region indicate that this will occur at depths greater than about 800 m. Existing data is used to identify areas with CO2 storage potential in saline formations at depths greater than about 800 m. Most of the Waikato region has only a relatively thin (<800 m) sedimentary cover above basement rocks, but locally thicker sequences are preserved in structurally controlled depressions. Regional gravity maps show three prominent negative anomalies and a fourth that is less pronounced. These correlate with thick sedimentary sequences above basement rocks in the South Manukau, Hauraki Plains, Hamilton and Whangamarino areas. There is little information available on the deep sedimentary rocks in these areas and they are generally not well understood. As a result there is significant uncertainty about the storage potential in these areas. The South Manukau area is strategically located close to a major CO2 point source at Glenbrook and is relatively close to Huntly power station, and other major sources in South Auckland. Basement beneath the South Manukau area is as deep as 2700 m in the south, adjacent to Waikato Fault, and rises east and north to about 1000 m near Waiuku. Waitemata and Te Kuiti Group rocks with reservoir potential are present in the area, although their overall distribution and thickness is poorly known. The best storage prospects are in the south, where the basin is deepest, mainly beneath the southern part of Awhitu Peninsula and possibly south of Pukekohe. However, the extent of potential reservoir formations onshore will be limited by the presence of an apparent basement high, of unknown extent, in the Otaua-Aka Aka area. There is also potential for connection with offshore areas that may have structural and stratigraphic traps suitable for large-scale CO2 storage. Further investigation of the South Manukau area is warranted because of the apparent onshore potential and a possible link with thick offshore sequences that may have storage potential. Too little is known of the subsurface geology of the Hauraki Plains area to properly assess its storage potential. Theoretically, there is a large volume of sediment deeper than 800 m that may contain reservoir formations, both beneath the Hauraki Plains and the Firth of Thames. However, the deep sediments are likely to be only weakly consolidated and lack the thick, laterally-extensive mudstone lithofacies necessary to provide a secure seal. Detailed gravity and seismic investigations, as well as deep drilling will be needed before a useful assessment of the storage potential of the Hauraki Plains can be made, but further investigations should not proceed until the risks to long term storage posed by the active tectonic setting of the area are reviewed. The very limited data available suggest that the Whangamarino area may contain deep reservoir formations in the Waitemata and Te Kuiti groups, but most are likely to be too shallow to offer effective CO2 storage. The area is likely to have very limited storage capacity at depths greater than 800 m. The presence of a nationally significant wetland over much of the Whangamarino area, together with its apparently limited storage potential, suggests that the area does not warrant further consideration. A thick Waitemata Group sequence in the Hamilton Basin contains at least four reservoir formations that are up to 136 m thick and separated by mudstone aquitards. The reservoir formations are deepest on the western basin margin and rise eastward in concert with the greywacke basement surface that underlies the basin. Seismic data show the succession is cut by numerous high-angle normal faults, which tend to segment the reservoir formations, and separating mudstones, in some cases isolating them in terms of hydraulic connectivity. The combination of westward regional dip and faulting has elevated some reservoir formations above the 800 m minimum depth required for efficient CO2 storage. Reservoir formations that are truncated by the sub-Tauranga Group unconformity are also at risk of significant leakage into the Tauranga Group aquifer complex. The negative structural aspects of the basin are likely to limit storage capacity to relatively modest levels. Reprocessing the existing seismic data and running some new seismic lines would allow a better assessment of storage potential, and further test drilling is needed to obtain detailed information on reservoir characteristics. Significant urban and industrial development north of Hamilton is likely to hamper new field investigations and any future implementation. A preliminary assessment of the capacity for ignimbrite deposits to store CO2 raises serious doubts about the suitability of these deposits for long term storage. The characteristics of the deposits suggest that they offer little chance of secure containment. This review of the onshore Waikato region has not produced an obvious site with clear potential for large-scale CO2 storage in deep saline formations. The region's deep basins are either too poorly known or lack suitably extensive reservoir formations within the required depth range. Each of the four basins identified, and the southeast Waikato ignimbrite deposits, have factors of varying significance limiting their potential for large-scale CO2 st

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