Petroleum geology of North Wanganui Basin, North Island, New Zealand

Abstract

North Wanganui Basin extends from Kawhia Harbour in the north to Stratford in the south. It covers an area of about 12,500 km2 and is seperated from the Taranaki peninsula by the Taranaki Fault. The Rangitoto-Hauhungaroa-Kaimanawa Ranges seperate the basin from the Taupo Volcanic Zone and the hypothetical Pipiriki High forms a southern boundary. The region has undergone a limited amount of petroleum exploration with 16 wells being drilled. Although several small oil seepages have been reported from the region, no commercial accumulations have been discovered as yet. Most sediments in the basin are refered to the Mahoenui and Te Kuiti Groups. The Mahoenui Group consists of mudstones, sandstones, siltstones and limestones. Mahoenui deposits vary in thickness from 320m near Taumaranui to 1384m in the south. The Te Kuiti Group is a predominantly carbonate interval with sandstones and mudstones also present. Te Kuiti deposits vary in thickness from 20m in the north to 275m in the southeast. Well cuttings and geophysical logs from Ararimu-1, Waikaka-1, Koporongo-1, Tupapakurua-1 and Puniwhakau-1 boreholes have been studied. Quartz is the predominant mineral in the Mahoenui and Te Kuiti mudstones and sandstones. Feldspar and clay minerals, especially kaolinite and illite, are also abundant. Widespread diagenetic effects include quartz precipitation as syntaxial overgrowths, precipitation of authigenic clay minerals in pore spaces, carbonate cementation and replacement of quartz by calcite. In the Te Kuiti Group a large quantity of calcite is present. Aragonite, pyrite, quartz, feldspars and clay minerals are also present. The probable provenance for the Mahoenui Group was a plutonic-igneous source. No granite outcrops in the North Island, so the source is unknown; however, it may have been basement rock currently buried or perhaps now seperated by transcurrent faulting. Some possible hydrocarbon source rocks are present within the Te Kuiti and Mokau Groups, but they usually have low organic carbon contents and are mostly thermally immature. Reservoir rocks are only poorly developed. Furthermore, diagenetic reduction of porosity and permeability has taken place by mechanical compaction, silica cementation as syntaxial overgrowths on quartz grains, calcite cementation as pore-lining micrite and pore-filling blocky calcite, pore-filling kaolinite and pore-lining illite. A commercial hydrocarbon discovery in North Wanganui Basin depends on finding source beds that are more thermally mature in conjunction with better developed reservoir facies. At present the prospects must be rated poor.