Failure to discover hydrocarbons within Late Cretaceous and Paleocene strata in the southernmost Taranaki Basin is postulated to be in part due to the limited coverage of effective top seals above these reservoirs. Effective top seals are formed by fine grained, clay prone sediments, which are deposited mainly in marine environments. This study presents paleogeographic maps for the Latest Cretaceous North Cape Shale and Early Eocene E Shale, which are probable and proven top seals respectively for hydrocarbon reservoirs in the offshore Taranaki Basin of New Zealand.

The aim of this thesis is to define the spatial distribution of thickness and permeability for two primary seal units formed during two marine flooding events (transgressions) of Latest Haumurian and Waipawan ages. These seals have been characterised using a selection of 12 petroleum exploration wells along with seismic reflection data from 20 2D seismic surveys and five 3D surveys. Top and base seal horizons identified from wells were mapped in seismic data and interpreted over the study area, resulting in an improved understanding of seal-rock spatial distributions. Petrographic and mercury injection capillary test results were collated from samples within the two primary seal intervals to identify their common properties and further analyse what constitutes an effective seal in this part of the basin.

The North Cape Shale shows more widespread distribution than the E Shale and can be seismically traced further south, whereas the E Shale is thicker but is limited to a narrower distribution, changing facies type to a coastal plain/non-marine facies near the north western boundary of the study area. Risk of topseal absence increases at both intervals towards the southern boundary of the study area.