New Zealand's changing configuration in the last 100 million years: plate tectonics, basin development, and depositional setting
New Zealand's sedimentary basins with proven or perceived prospectivity for hydrocarbons have formed over the last 100 m.y. Today, these basins sit astride or adjacent to the Pacific-Australian plate boundary, and represent the southernmost petroleum provinces on the western Pacific rim. This paper documents the changing shape of New Zealand's landmass and offshore areas in response to tectonic influences through time, and discusses the implications for petroleum systems development.The Cretaceous to Recent sedimentary succession in New Zealand corresponds to a 100 m.y. 1(st)-order transgressive-regressive 'megasequence', that evolved within a tectonic continuum from intra-continental rift to convergent margin orogenic belt. Seven tectonically controlled second-order depositional cycles, of 5 to 25 m.y. duration, are correlatable between regions as major, unconformity-bound transgressive or regressive packages.In the Late Cretaceous a lineament of rift sub-basins existed in the west, linked to a Tasman Ridge transform zone. Syn-rift deposits include coarse-grained potential reservoir units and, more importantly, thick coal-rich source rock intervals. Rapid deposition and high heat flow associated with rifting led to early hydrocarbon generation C, locally. To the east of the active rift zone the sea began to transgress low-lying coastal plain areas from c. 80 Ma.By the end of the Paleocene a passive margin had developed around the sub-continent, including broad marine embayments at the landward ends of the Bounty Trough and New Caledonia Basin. Rising base level allowed the accumulation, on broad coastal plains, of thick coal measure sequences that are proven source rocks. Marine rocks with source potential, most notably the Waipawa Formation black shale, were also deposited. Re-worked transgressive shoreline sandstones, derived from a weathered hinterland, constitute the pre-eminent reservoir fairway in several basins. With ongoing regional subsidence, fine-grained seal rock facies were deposited in distal regions, and gradually transgressed across the shoreline sandstone tracts. By Late Oligocene times land areas were minimal. Oblique extension associated with Emerald Basin rift propagation created a complex series of sub-basins as far north as eastern Taranaki. Highly variable basin-fill lithofacies include deep-water turbidites that are proven reservoirs.Southward propagation of the Hikurangi subduction zone and inception of a through-going plate boundary in the earliest Miocene heralded the onset of convergent tectonics, uplift and expansion of land areas, and increased terrigenous sedimentation within most basins. As dextral convergence gradually increased, the proto-New Zealand landmass was 'squeezed' into an elongate belt parallel to the plate boundary. With southward migration of the Australian-Pacific rotation pole, and rotation of the Hikurangi margin, the focus of compression and arc volcanism shifted southwards, and extension began in the north. Large structural anticlines formed as a result of basin inversion, particularly within central-western fold-thrust belt areas. Thin-skinned structures formed within both fore-arc and back-arc settings in the proto-North Island. The onset of source rock maturity was enhanced by locally rapid and thick sedimentation, and by higher heat flows associated with extension or volcanism. Reservoir units of Neogene age mainly comprise regressive deep-water to shelf sandstones of variable quality, derived from a variety of older sedimentary rocks and uplifted basement terranes.