Evolution and distribution of pore pressure across the Taranaki Basin, New Zealand

Abstract

The Taranaki Basin lies onshore and offshore in the central-west of New Zealand’s North Island. The polyphase nature of the basin has led to a complex pore pressure history, generating significant variations in the present-day vertical and lateral distribution of overpressures. Investigating pore pressure distributions at a basin scale can mask key details, so in this study eight new sub-basin areas and structural zones are designated, each displaying individual pore pressure trends defined by their stratigraphic architecture and structural development. Variations in the thickness and facies of Eocene-Oligocene stratigraphy both within and between sub-basins are shown to provide a first-order control on the magnitude, distribution and maintenance of overpressure across the Taranaki Basin. Fluid pressure compartmentalisation through sealing faults and stratigraphic architecture has been identified across the basin. Deep pore pressure transitions within Taranaki Basin are shown to be sealed by diagenetic, structural and stratigraphic mechanisms and generated by an increase in mudrock volume (reduced permeability) or gas generation.

A series of one-dimensional basin models are used to investigate the phases of overpressure generation by disequilibrium compaction across the Taranaki Basin: initially driven by the formation and early filling of a foreland basin in the eastern basin margin from ∼22 Ma, and subsequently by loading due to the progradation of a succession of sedimentary wedges across the continental shelf from ∼10 Ma to the present day. The plotting of vertical effective stress against porosity from reservoirs across each stratigraphic interval and depositional facies within the Taranaki Basin, identifies no clear relationships, primarily due to the late and deep onset of abnormal pressures.

Analyses focused on Palaeocene Farewell Formation and the F-Sands reservoirs, demonstrates that mechanical compaction is the principle porosity reducing mechanism during the first 3000m of burial. Dissolved CO2 in the pore fluids of the F-Sands and Farewell Formation are likely to have driven the dissolution of detrital grains, generating secondary porosity and enhancing permeability. Continued compaction of secondary porosity in the Farewell Formation has further reduced porosity towards maximum burial. The F-Sands and Farewell Formation can be characterised as geochemically closed systems, where dissolved solutes are being preferentially precipitated in fine-grained heterolithic siltstone beds.

This research presents the pore pressure evolution and distribution of the Taranaki Basin within a tectonostratigraphic context, demonstrating how multiphase overpressure generation and subsequent maintenance are controlled by a combination of sediment loading, uplift, lithofacies variations, fault zone permeability and mechanical compaction. This research has important implications for both hydrocarbon exploration in tectonostratigraphically diverse regions and research into the evolution of polyphase sedimentary basins worldwide.