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The Taranaki Basin is an active‐margin basin that has been significantly affected by Miocene subduction tectonics along the Pacific‐Australian plate boundary. We have analyzed its present‐day thermal state using 354 bottom‐hole temperatures (BHTs) from 115 wells distributed throughout the basin. The measured temperatures were corrected using an exact solution to Bullard's equation rather than the Horner approximation, thereby allowing for recovery dependence on well diameter and correction for some BHTs at early time after circulation had ceased. Thermal conductivity measurements were completed on 256 samples from eight wells, and matrix conductivities were determined for six end‐member lithologies by inversion. Formation conductivities are based on the conductivity and relative proportion of each end‐member component. Corrected BHTs, in situ thermal conductivity, and estimates of sediment heat production were combined to compute the present‐day, steady state heat flow. The average heat flow is 60 mW m−2, but important geographic variations are present: heat flow on the Western Platform is remarkably consistent at 53–60 mW m−2, attesting to its relative stability since the Late Cretaceous; heat flow in the southern part of the basin is 65–70 mW m−2 due to as much as 3 km of late Miocene erosion; on the southern onshore and to the south of the peninsula, heat flow is 50 ± 3 mW m−2, possibly due to the heat sink effects of crustal thickening; heat flow is highest at 74 mW m−2 on the northern peninsula adjacent to the Taranaki volcanic zone, suggesting a causal relationship between Quaternary volcanism and high heat flow.

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International audienceRock—Eval HI values for coals vary with rank and do not give a direct measurement of oil potential. However, oils from coals are characteristically paraffinic and can be considered to derive from a polymethylene (PM) component, so the PM content should provide an estimate of the paraffinic oil potential. A trend apparently representing lignin evolution has been identified on the Van Krevelen diagram which permits the relative proportions of carbon in lignin and PM to be determined for coals that approximate a mixture of these two components, such as the members of the New Zealand (NZ) Coal Band. On the basis of this compositional model, HI values can be calibrated to provide an alternative estimate of the paraffinic oil potential. A maximum in HI is generally reached in coals near the onset of oil generation, at Rank(S) 12 (R. ca. 0.7%), from which it is suggested that the PM contri¬bution can be obtained using the formula HIpm = 1.15HIma„-172 for the suite of NZ coals examined. The onset of oil expulsion can be identified from a variety of geochemical measurements, and occurs in the Rank(S) range ca. 12.0 — 14.5 (Ro ca. 0.7-1.1%) for coals with paraffinic oil potentials exceeding ca. 40 mg HC/g TOC. Data from Taranaki Basin coals correlate well with the theoretical relationship between BI/HIPM and HIpm, using bitumen index (BI = S 1/TOC) values of 10 mg HC/g TOC at the start of oil generation (i.e. bitumen inherited from diagenesis) and 40 mg HC/g TOC at the onset of oil expulsion, suggesting the HIpm model is reasonably accurate for members of the NZ Coal Band. Kinetic modelling of paraffinic oil generation from vitrinite-rich coals may be best approximated by consideration of PM degradation alone

The East Coast Basin of New Zealand lies in the frontal arc of the Hikurangi subduction zone. Overpressure forms a major hazard for exploration in the basin. Fluid pressures of 12.8 MPa at 600 m depth have been encountered, equivalent to 90% of lithostatic pressure, and mud weights of up to 19.5 ppg are required to control formation fluids at subsurface depths of only 1400 m. The distribution and magnitude of overpressure shows no relation to present-day depth. Lithostratigraphy controls the distribution of overpressure and widespread lowpermeability bathyal mudstones form seals to overpressure across the basin. The transition from normal pressure to overpressure is not associated with any single stratigraphic unit. The Neogene evolution of the plate boundary has given rise to rapid Miocene sedimentation (400 m Ma 1 ) in areas of the basin and has caused disequilibrium compaction. Late Neogene compression has subsequently uplifted the overpressured, undercompacted sediments by up to 3000 m at rates of 1000 m Ma 1 . Lateral tectonic compression associated with the plate boundary has also caused undrained shear of thick mudstones, leading to extremely high overpressures in deformed sediments independent of depth.