Stratigraphic framework and source-rock potential of Maastrichtian to Paleocene marine shale, East Coast, North Island, New Zealand : hydrocarbon prospects

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Leckie, D.A.; Morgans, H.E.G.; Wilson, G.J.; Uruski, C.I. 1992 Stratigraphic framework and source-rock potential of Maastrichtian to Paleocene marine shale, East Coast, North Island, New Zealand : hydrocarbon prospects. Lower Hutt: Institute of Geological & Nuclear Sciences. Institute of Geological & Nuclear Sciences science report 92/05. 35 p.

Abstract: East Coast Basin, New Zealand, contains thick source rocks, several potential reservoir systems and several large structures but is poorly explored. Gas and oil seeps are common. Late Cretaceous to Paleogene strata, the subject of this field trip, include about 1 km of dominantly marine siltstone with lesser sandstone. Siliceous siltstone of the Whangai Fm has low TOC content and HI except for the Calcareous Member. The Te Uri Member, at the top of Whangai, consists of glauconitic sandstone, mudstone and poorly-sorted conglomerate. The mid to late Paleocene Waipawa Fm, up to 50 m thick, represents a marine condensed section. TOC contents are up to 5.3%, with Type II organic matter, and HI to 550. TOC content and HI increase above a pebbly mudstone interpreted as glacial dropstones representing a correlative conformity. All known outcrop of the Waipawa Fm is just at the top of the oil window, but eastwards, sedimentary cover places the shale in the oil window. The volume of Waipawa Formation source rock present in the offshore may be at least 500 km3. Seismic sections show that a unit inferred to be the Wanstead Formation downlaps onto the Waipawa and represents highstand marine shale with Type III organic matter and low HI. Potential reservoirs include the Tertiary to Quaternary fractured calcareous and siliceous mudrock, turbidites, and shallow-marine sandstone and conglomerate, coquina limestone and fractured limestone. The Late Cretaceous-Paleocene strata were uplifted and exposed by Neogene movement on the Hikurangi subduction margin, active since the late Oligocene/early Miocene. East Coast occupies an accretionary prism on the eastern flank of the Australian Plate above the westward, obliquely-subducting Pacific Plate.