Abstract - The Southern Taranaki Basin, located between Taranaki Peninsula and northern South Island, contains sedimentary rocks up to 8km thick that record multiple phases of faulting and folding since ca 80 Ma. The deformation history provides constraints on New Zealand plate boundary development together with the timing and locations of potential hydrocarbon traps. Deformation has been examined using 2D and 3D seismic reflection lines to generate fault displacement-time curves, isopach maps, and structure contour maps. Up to 16 seismic reflectors tied to 35 wells have been mapped within the basin, producing a temporal resolution of 1-5 Ma since 22 Ma and 5-10 Ma prior to the Miocene. More accurate temporal resolution on the timing of faulting and development of associated trap structures is achieved here utilising all of the available seismic and well data in a detailed basin-wide interpretation. Three main phases of tectonic activity have been recognised; Late Cretaceous and Early Palaeogene extension (ca 80-55 Ma), Oligocene and younger contraction, and Plio-Pleistocene (ca 4-0 Ma) extension. Most of the largest faults (e.g., Cape Egmont fault) accrued displacement during the Late Cretaceous and were reactivated one or more times during subsequent episodes of deformation. The oldest phase of extension occurred during Gondwana breakup, was ubiquitous throughout the basin and controls the distribution of Late Cretaceous-Plaeocene source rocks. Contraction commenced on the largest faults along the eastern margin of the basin (i.e. Taranaki and Manaia faults) no later than Early Oligocene and is inferred to have been induced by the onset of subduction along the Hikurangi margin. Contraction migrated westward episodically throughout the Miocene, with reverse faults and folds in the westernmost parts of the basin not initiating until the latest Miocene (ca 8-5 Ma). Fault growth analysis and hydrocarbon modelling suggest that hydrocarbon charge of these anticlines mainly occurred post 10 Ma. Shortening was followed by Plio-Pleistocene extension which migrated from the northern Taranaki Basin and was accompanied by southward migration of contraction which is now mainly confined to the northern South Island. Highs in the footwalls of late-stage normal faults produce potential traps, however, these faults compromise the regional stratigraphic seal, while fault-bound traps may also be prone to leakage. Channelised up-dip flow in the last 3-4 Myr is an important charge mechanism for Miocene sandstones.
Abstract of talk given to:
Advantage Geotechnical Petroleum Forum, Wellington, New Zealand, April 2014.