--> Pore Pressure Evolution And Distribution Across A Tectonostratigraphically Complex Basin, Taranaki Basin, New Zealand

AAPG Asia Pacific Region GTW, Pore Pressure & Geomechanics: From Exploration to Abandonment

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Pore Pressure Evolution And Distribution Across A Tectonostratigraphically Complex Basin, Taranaki Basin, New Zealand

Abstract

The Taranaki Basin is New Zealand’s only hydrocarbon producing basin, and lies onshore and offshore in the central-west of the North Island. The polyphase nature of the Taranaki Basin has led to a complex pore pressure history, generating significant variations in present day vertical and lateral distribution of overpressures. Cretaceous to Early Miocene Formations can be found both normally pressured (near or at hydrostatic) and significantly overpressured (>1500 psi/10 MPa) at the same depth in separate parts of the basin.

The basin can be split into two distinct regions: the Western Stable Platform and the intensely deformed Eastern Mobile Belt. The Eastern Mobile Belt (EMB) is characterised by a number of rapidly subsided fault bounded grabens which have distinct tectonostratigraphic histories, and subsequently pore pressure regimes. The grabens in North Taranaki within the EMB contain up to 8 km of predominantly fine grained sediments. These grabens act as independent pressure compartments with laterally sealing faults and thick competent top seals, which inhibit lateral and vertical drainage leading to maintenance of significant overpressures of up to 4000 psi (28 MPa), calculated to be 730 psi (5 MPa) below fracture pressure. In the Southern Taranaki, thin sand stringers within thick undercompacted mudstones are stratigraphically isolated, and display formation pressure increases of 1705psi (11.8MPa) in only 154mTVD, which poses a significant drilling hazard. In comparison, the Western Stable Platform has remained relatively quiescent since the Cretaceous, and is characterised by progradational deposition on a structurally undisturbed, regionally subsiding sea floor, producing a hydrostatic pressure regime.

One dimensional burial history modelling has shown that distinct tectonic regimes present within the basin have generated a huge variation in the onset and subsequent development of pore pressures across the Taranaki Basin. The Maui Graben in the Southern Taranaki experienced initial slow subsidence and low sedimentation rates but these increased from the mid-Miocene driving the formation of limited overpressures, although this has been laterally drained to stratigraphically shallower depths through high permeability pathways in the Paleogene stratigraphy producing the near hydrostatic conditions observed today. These long drainage pathways can stretch great distances, as shown by the depletion of Palaeocene reservoirs in the Tui Field from production in the Maui Field 20km away. In contrast, the Manaia Graben of the EMB has experienced early rapid and continued burial since the Late Cretaceous; though the absence of significant intervals of fine-grained strata within Palaeocene and Eocene has impeded the early generation of shallow overpressure through disequilibrium compaction. Subsequent rapid Pliocene subsidence and exceptionally high sedimentation rates has generated late overpressure in the Oligocene to early Miocene Otaraoa and Taimana Formations, which act as seal to Eocene reservoirs.

This study will allow for the accurate spatial definition of overpressure, which is crucial to implementation of effective well design, drilling safety, determination of hydrocarbon column heights and for exploration migration analysis. Furthermore, this knowledge will enable the prediction of anomalously high shallow overpressures and crucially where shallow onset of low vertical effective stress could allow for the preservation of excellent reservoir quality to significant depths.