Raukumara Basin, North Island, New Zealand: Seismic Stratigraphy, Depositional History and Petroleum System Implications

Raukumara Basin lies beneath Raukumara Plain and Raukumara Peninsula, north-eastern North Island, New Zealand. Published work has established a megasequence seismic-stratigraphic framework for the basin, which contains an estimated sediment thickness of >13 km. Seismic surveys in 2007 and 2011 acquired 4,500 line km of high-quality seismic data. This study uses both the 2007 and 2011 survey and is the first study to map the four megasequences (RAU-MS-W, RAU-MS-X, RAU-MS-Y, and RAU-MS-Z) across the basin using the combined dataset.

The basin evolved through four tectonic phases, with each megasequence relating to a phase: Gondwana margin subduction (RAU-MS-W; Cretaceous); passive margin burial (RAU-MS-X; Cretaceous-Oligocene); allochthon emplacement during inception of the Hikurangi subduction margin (RAU-MS-Y; Early Miocene); and subduction margin underplating, deformation and volcanism (RAU-MS-Z; Miocene-Recent).

Petroleum source rocks are inferred to be correlatives of the Whangai and Waipawa Formations and possibly the Karekare Formation, mapped as sequences within RAU-MS-X. Reservoir rocks are Eocene, Oligocene and Miocene Sandstone and landslide facies (upper RAU-MS-X, RAU-MS-Y, and RAU-MS-Z). Seal units are mudstones that are inferred to be widespread in RAU-MS-X and RAU-MS-Z, and the basal detachment surface or internal faults within RAU-MS-Y.

The critical moment for the petroleum system is within the Early Miocene, after emplacement of RAU-MS-Y and during initial deposition of RAU-MS-Z. The inception of tectonic activity during the Eocene–Early Miocene created sedimentary source areas for reservoir sandstones as topography and structures formed.

Four play concepts were identified. An anticlinal play in RAU-MS-X has an inferred Eocene sandstone reservoir and a mudstone seal. An unconformity play, with truncation of RAU-MS-X against the base of RAU-MS-Y that juxtaposes a potential Oligocene sandstone reservoir against an Early Miocene fault seal. An allochthon play within RAU-MS-Y that has possible sandstone reservoir units of Cretaceous–Eocene age enhanced by fracture permeability, and sealed by an overlying marine mudstone of RAU-MS-Z. A basal sandstone play with Early Miocene sandstone or conglomerate reservoir at the base of RAU-MS-Z, channelled into and contained by troughs on the top surface of RAU-MS-Y, and sealed by mudstone.

I suggest that Raukumara Basin is likely to contain all the elements of a petroleum system, but the qualities of reservoir and source rocks are identified as significant risks. The basin is unusual for New Zealand because the basin is history is inferred to be entirely marine, which may mean that source rocks could be oil-prone (type 2) and that oil: gas ratios may be higher than in other New Zealand petroleum provinces. Onshore oil seeps are consistent with this conclusion and suggest that the offshore region could be prospective. The basin is relatively slightly deformed, as compared to farther south and onshore, and may hence host large accumulations of petroleum.