Tectonics, sedimentary facies development and maturity modelling in the Great South and Taranaki Basins, New Zealand

This thesis primarily investigates the regional evolution of rift faulting, sedimentary facies development and petroleum systems in the Great South Basin (GSB), together with a detailed study of coaly source rocks in the Taranaki Basin (TB). The TB source rock study serves as an analogue for similar rocks in the GSB, where there was insufficient data to study coaly source rocks in detail. These investigations primarily use seismic reflection and well data along with some outcrop and laboratory (geochemical, geophysical and geochronological) information. The thesis comprises four main chapters (excluding the Introduction and Conclusions chapters) that address thematic topics related to the geological history of Gondwana in the GSB and TB during, and subsequent to, Cretaceous rifting. Chapters 2, 3 and 5 have already been published in peer-reviewed journals and can read as stand-alone text. The results can be applied to similar basins elsewhere in New Zealand and worldwide to understand better their tectono-sedimentary evolution and petroleum systems. GSB initiated in the mid-Cretaceous from continental rifting at the southeastern margin of Gondwana (Chapter 2). Fault-growth analysis in the GSB revealed three stages of Cretaceous rift faulting, which collectively occurred over ~22 Myr (~105 to 83 Ma). The dominant fault set trends NE (NW‐SE extension) with a minor fault set trending NW that formed along basement terrane boundaries. Stage 1 (~105–101 Ma) faulting occurred mainly in the central GSB with rift depocentres mostly on, or close to, NW trending basement terrane boundaries. These pre-existing basement boundaries represent zones of weakness that locally promoted early localisation of NW faults and retarded the propagation of NE faults. During Stage 2 (~101–90 Ma), regional extension increased, with NE-trending faults increasing in length, number, displacements, and spatial distribution, and the influence of the basement fabric gradually diminishing. Finally, during Stage 3 (~90–83 Ma) the length, number and displacements of faults decreased. Rift faulting is interpreted to reflect the embryonic stages of Gondwana breakup. Analysis of seismic facies and paleogeographic evolution in the GSB provides an improved understanding of the interplay between normal faulting and the spatial distribution of sedimentary facies in the syn- and post-rift Cretaceous–Paleocene interval (Chapter 3). During the initial syn-rift (~105–101 Ma), an isolated northeast- trending graben formed, with resultant alluvial fan/fan delta, fluvial, coastal and lacustrine sediment fill. In the later stages of syn-rift (~101–90 and 90–83 Ma), marine transgression occurred, with deposition of shelfal, shoreface and marginal marine facies and increased connectivity between grabens. With marine transgression across the hinterland the sediment supply significantly reduced in the northern part of the basin leading to underfilled grabens (with respect to sediment supply). In the west, where the sediment supply was higher, rift topography was quickly filled. The NW-trending basement terrane boundaries controlled the development of accommodation space during the initial stages of graben formation. Later in the syn- and post-rift stages, these terrane boundaries formed long-lived sediment input points for the basin and controlled the position of repeated large deltaic depositional units. A multi-1D modelling study (Chapter 4) was carried out to understand the thermal history and maturity in the GSB, and to highlight key petroleum prospectivity risks. The rapid burial and the heating effects of rifting combined with insulating properties of the porous sediments caused temperatures to rapidly increase in the basin throughout the Cretaceous and Paleocene. In areas of continued deposition of sediments, similar temperatures are maintained throughout the Eocene. During decreasing subsidence and lowered sedimentation rates from Oligocene, the basin is predicted to cool. This burial history led to oil and gas generation/expulsion in the Cretaceous and peak expulsion during the Paleocene–Eocene with a reduction in expulsion of oil and gas since the Oligocene. One of the main risks in the basin is the timing of hydrocarbon expulsion with respect to the development of an effective seal rock. Significant volumes of early- generated oil and gas during the Cretaceous are likely to have been lost from the system due to poor or ineffective seals overlying reservoirs prior to Paleocene times. Mature source rock, effective seal during periods of peak hydrocarbon expulsion and reservoir rocks are interpreted to be present in many parts of the basin including the eastern part of the Central Sub-basin, areas east and south of this sub-basin, and the Kawau Sub- basin and therefore, these regions are considered to be most prospective in the basin