This Feature Class contains contours that depict the interpreted vertical thickness (isochore) in metres between the P50 and N10 seismic horizons mapped within the Northwest Province of the Atlas of Petroleum Prospectivity (namely the Taranaki, Deepwater Taranaki, and Reinga-Northland basins). This isochore interval encompasses rocks of c. 35-21 Ma age (Oligocene-earliest Miocene). For more information about the seismic horizon naming scheme used in this project please refer to: Strogen, D.P.; King, P.R. 2014. A new Zealandia-wide seismic horizon naming scheme. Lower Hutt, NZ: GNS Science. GNS Science report 2014/34. 20 p. The depth and isochore grids, from which these contours were derived, were generated in Paradigm 2015 SeisEarth software using an adaptive fitting algorithm and a grid resolution of 1 km x 1 km. Faults have not been incorporated into the regional-scale gridding of the TWT or depth maps in this project. The TWT and depth grids were then exported from SeisEarth (Zmap format), checked for internal consistency in Zetaware (Trinity 4.61), edited as needed to account for gridding errors and then exported as ArcView Ascii grids, imported and contoured with Esri's ArcGIS software. Contour intervals are 250 m. Given the considerable variability in the spacing and density of 2D seismic data over the entire NWP area the "smoothness" of the depth maps varies accordingly. In the central Taranaki Basin, where seismic line spacing may be 5 km or less, the 1 km x 1 km grid resolution captures the geological complexity well. However, in areas of the NWP such as the Deepwater Taranaki Basin to the west and Reinga Basin in the north, seismic line spacing may be in the order of 15 to 30 km. In such areas the underlying geological complexity is only captured where seismic interpretation exists along the available 2D seismic lines. For further information on these data, please refer to: Arnot, M.J. and Bland, K.J. et al. (Compilers), 2016. Atlas of Petroleum Prospectivity, Northwest Province: ArcGIS geodatabase and technical report. GNS Science Data Series 23b.