Assessment of paleo-depositional environments and reservoir potential of the Late Cretaceous North Cape Formation, Nelson, New Zealand.

The Taranaki Basin contains New Zealand’s only producing hydrocarbon systems with commercial production limited to Paleocene, Eocene, Oligocene Miocene to latest Pliocene successions. Potential hydrocarbon reservoirs have been identified in Late Cretaceous successions within the Pakawau Sub-basin of within the southern region of the Taranaki Basin. The lowermost Rakopi Formation has been identified as both a hydrocarbon source and a potential reservoir, while the less well understood North Cape Formation is thought to represent a more unconventional petroleum play in that it contains both reservoir and a disseminated hydrocarbon source. Despite their economic significance the Late Cretaceous potential sandstone reservoirs are not well defined. In this study, highly detailed stratigraphic sections were constructed from selected outcrops of the North Cape Formation with sedimentary analyses revealing obvious lateral variation in the lithofacies. The North Cape Formation is interpreted to have been deposited in a tidally dominated, sandy estuarine setting. There is a distinct change in average grain size across the field area, with the northeastern region characterised by medium sandstones and conglomerates, while the central, southern and western regions are dominated by fine sandstones. An active bay head fan delta dominated deposition in the northeast of the field area and partially sheltered tidal embayments and local salt marshes separated by smaller scale tidal distributary channels characterise the central regions of the study area. There is a marked reduction in the occurrence of tidal signatures in the uppermost North Cape Formation successions, marking a change from delta front to delta plain dominated deposition. The outcropping North Cape Formation and the framework for the characterisation of paleoenvironments presented in this study can be used as analogues for similar estuarine succession in the geological record. Lithofacies and petrophysical analyses indicate the North Cape Formation contains viable hydrocarbon reservoirs. Porosity and permeability measurements assign the best quality reservoirs to conglomerate, wavy and crossbedded sandstone lithofacies, with moderate quality reservoirs identified in both the heterolithic and planar laminated sandstone lithofacies. A number of organic rich sandstone, siltstone and freshwater and salt water influenced coal lithofacies have also been identified which may represent potential source rock units. Gamma Ray profiles confirm the onshore North Cape Formation is dominated by sandstone, with less interbedded siltstones and coal beds than the underlying Rakopi Formation. This work suggests that future exploration within the North Cape Formation may regard the formation as both a potential reservoir and source rock interval. Acknowledgements I would first like to acknowledge the New Zealand MBIE for the partial funding of this research through the GNS Science research programme on New Zealand petroleum source rocks, fluids, and plumbing systems (contract C05X1507). I would also like to extend thanks to OMV New Zealand Limited, GNS Science and the University of Canterbury Mason Trust for additional financial support for field related expenses over the course of this research. Considerable thanks go to my supervisors Dr Kari Basset and Dr Greg Browne for their incredible, and on-going, support throughout this process. Greg’s bank of knowledge on the field area and what seems a photographic memory of all relevant resources has been invaluable. Kari was an important anchor bringing my often wild thoughts back into context and ensuring I stayed focused on my project aims. Between their technical expertise, publishing experience and most importantly patience I have been well-looked after. It’s important for me to also acknowledge the technical support from the University of Canterbury Geology Department Laboratory Technicians; Rob Spiers, Cathy Higgins and Sarah Pope. They each played an important role in facilitating data acquisition during what was an incredibly stressful time. Field work was possible through access granted by land owners and made considerably more comfortable through the use of the Victoria University of Wellington’s Onekaka field station. Thanks to Sarah and Sophie for their willingness to think tank on our shared field area and especially Sophie for simplifying technical processes for me and paving the way for lab analyses. Finally, thank you to my friends and family for their encouragement and welcomed distractions when the going got tough. It’s been an incredibly rewarding experience, but one I could not have done without those acknowledged here.