Stagpoole, V.M.; Bushe, H.; Milner, M. 2009 Opportunities for underground geological storage of CO2 in New Zealand : report CCS-08/4, offshore Waikato region. Lower Hutt, N.Z.: GNS Science. GNS Science report 2009/57 43 p.
Abstract: Interpretation of seismic data in the offshore Waikato region between Manukau Harbour and Kawhia Harbour, has identified potential sedimentary reservoir formations at suitable depth for CO2 storage. In addition, the proximity of the region to major point sources of CO2 production, primarily the Huntly coal and gas fired power station and Glenbrook Steel Mill at Waiuku, near Port Waikato, support the need for an assessment of CO2 storage options in this area. Interpretation of the structure and stratigraphy in the offshore Waikato region, based on seismic reflection data acquired for petroleum exploration during the 1980s and 90s, suggests that there are three possible CO2 storage reservoir intervals: Early Pliocene Mangaa Formation sandstone. This is a proven reservoir formation that occurs at 800 to 2200 m depth and has over 100 m of net sandstone in Taranaki Basin wells. Porosities average 25% and permeabilities range from 100 to 250 mD. The main CO2 storage challenge with this formation is distance from the coast (> 35 km, typically 50 km offshore) and water depths (up to 800 m). The Late Miocene succession in piggy-back basins east of the Taranaki Fault is potentially an ideal target for CO2 storage, although it is unproven. There is a large thickness (> 1500 m) in shallow water (< 100 m) less than 30 km from the coast. The seismic character and depositional setting suggest the presence of sands at appropriate depths for CO2 storage, and fault bounded structures are interpreted in the offshore region between the entrance to Manukau Harbour and Port Waikato. However, the lack of any onshore outcrop or well data, and the poor seismic coverage in the near-shore region are significant challenges for evaluation of CO2 storage at this level. Further west, in the Northern Graben of the Taranaki Basin, Late Miocene Mangaa Formation has good reservoir potential but is remote from land. The Early Miocene succession, although poorly imaged on seismic reflection data, is interpreted to underlie most of the offshore region and is at suitable depths for CO2 storage in a region between the entrance to Manukau Harbour and Port Waikato that is less than 30 km from the coast. The Early Miocene Waitemata Group sandstone crop out widely in the onshore with measured average porosity of 19%. The Waitemata Group is not proven offshore; there are no well penetrations and the lack of seismic data coverage inhibits the search for large structures in the near-shore region. Middle Miocene basaltic and andesitic lavas and volcaniclastics, and sporadic younger volcanic centres in the region are not considered for CO2 storage because of the limited information available on the reactivity of these volcanics with CO2 and concerns with top-seal integrity. However, it is recommended that preliminary studies be undertaken on samples to assess the potential of these as a CO2 storage option. Strata older than Early Miocene are considered too deep or too far from the coast to be viable reservoirs for CO2 storage. The existing seismic data set is of insufficient quality to define and model suitable locations for CO2 storage. Recommendations in this report centre on the acquisition of new outcrop, seismic and well data to better define the reservoir potential of Miocene offshore strata and to clearly map structures in the region between the entrance to Manukau Harbour and Port Waikato where water depths are less than about 100 m. (auth/DG)