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Abstract

The Reinga Basin occupies a northwest-southeast bathymetric d epression between the West Norfolk and Reinga ridges and has an area of about 100,000 sq. km. Rock samples have been dredged from surrounding ridges, but no boreholes have been drilled. We present a seismic stratigraphy developed using 5,135 line km of new 2D seismic-reflection data and 20,000 line km of older data, and we tie this stratigraphy to boreholes in the nearby Northland and Taranaki basins. We identify six phases of basin evolution. The first phase involved extension across northwest-trending normal faults. The region subsided passively during phase 2, and we infer from regional considerations that this phase lasted from Late Cretaceous until middle Eocene time. Phase 3 was late Eocene compression, which we interpret to be related to the initiation of the Tonga-Kermadec subduction. This led to uplift and erosion of the West Norfolk and Reinga ridges and deposition of detrital material at the center of the Reinga basin. Oligocene to early Miocene regional subsidence (phase 4) resulted in flooding of structures created during phase 3. Uplift of the Wanganella Ridge, in the northwest part of the Reinga Basin, occurred at the end of the early Miocene (phase 5). The last phase is tectonically passive, but with ongoing sedimentation up until the present day (phase 6).

Upper Cretaceous units in the nearby Taranaki Basin contain coaly source rocks, and coal has been dredged from the ridge on the southwest margin of the Reinga Basin. Maturation models of three sites in the Reinga Basin predict that Cretaceous type III coaly source rocks within basal strata would begin to generate and expel petroleum in early Cenozoic time and expulsion would continue to the present day. The top of the oil expulsion window is modeled at 4.0 +/- 0.5 km below the sea bed, implying a potential kitchen area of approximately 15,000 sq km for Cretaceous source rocks, or a broader area if Jurassic source rocks are present. Most oil and gas expulsion is predicted to be later than the Eocene to Miocene folding and reverse faulting events that created structural traps. It is outside the scope of our study to develop play concepts or analyze direct hydrocarbon indicators, but our regional stratigraphic and tectonic study, combined with a consideration of petroleum system components that may be present, indicates that the Reinga Basin is prospective for oil and gas.

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