East Coast Basin, New Zealand
The East Coast Basin (ECB) is a petroliferous Neogene forearc basin located on the eastern margin on the North Island, New Zealand.
The ECB contains five main tectono-stratigraphic components (Fig. 2) including the:
- Hikurangi Plateau: an igneous province ranging from 15 km to 35 km thick that is currently subducting underneath the ECB as part of the Pacific plate
- Torlesse terrane: a meta-sedimentary basement terrane representing the former accretionary subduction complex of Gondwana prior to rifting
- Convergent to passive margin sequence: representing sediments related to the final stages of Cretaceous convergence, then subsidence as the western margin of Zealandia rifted from Gondwana. This component contains the most prospective source rocks in the basin, including the Waipawa Black Shale and the Whangai Formation
- Neogene apron fill: containing localized and fragmented sub-basins within the ECB associated with the present day tectonic configuration
- Accretionary subduction complex: a small, young, subduction complex comprised of accreted Quaternary and Neogene sediments.
The basin has undergone several phases of faulting in the Neogene as part of a convergent margin, which has caused significant structural complexity and inhibited successful exploration since the late 19th century. Blair Burgreen’s research tests development of the petroleum system in the basin through utilization of a structurally robust model.
The structural model is based on an offshore 2D regional seismic line that runs through Hawke Bay and incorporates the Neogene structural history. The line is interpreted based on ties to well data (Hawke Bay-1), onshore geology, and offshore drill cores. The 2D line is palinspastically reconstructed using Dynel 2D through an iterative process.
Although the 2D structural model is non-unique, it provides several testable structural scenarios including rapid sediment burial related to accommodation from extension, structural thickening related to low-angle thrust events, and uplift and erosion related to high-angle thrust events. Therefore, these types of events may act as analogs to better understand their influence in other areas of the basin.
The structural model also provides a framework for understanding pore pressure development
in the basin. The smectite-rich Eocene Wanstead Formation acts as a regional seal, and underlying sediments are typically highly overpressured, often approaching lithostatic pressures. Neogene sediments are more variably overpressured due to heterogenous sand, silt, and clay lithologies. Basin and petroleum system modeling tests the impact of faults in compartmentalizing pressure cells, disequilibrium compaction from effective seal rocks, and horizontal stress due to tectonic compression. Because sediment compaction is based on effective stress, not burial, it is essential to track pore pressure and effective stress in the basin through time to predict porosity (Fig. 3).
Prospectivity of the basin is highly dependent on the burial history, and therefore the structural history. Each structural regime is assessed separately for both total generation of hydrocarbons and timing of critical moment (when 50% of the source rock has undergone transformation). Numerous scenarios are modeled to assess model sensitivities related to uncertainties in source rock kinetics, paleo-heat flow, timing of structural events, and source rock quality. Parameters were calibrated to well and outcrop data. Heat flow is poorly constrained, and is presently very low due to subduction of the Pacific Plate. Paleo-heat flow is calibrated to vitrinite-inertinite reflectance and fluorescence data, apatite fission track data, Tmax data, and bottom hole temperature data from wells.