Structure of the Nga Motu Dome and its potential for oil and gas resources

The Sugar Loaf Volcanics at New Plymouth are high-level remnant plugs associated with a deeper structural dome, the Nga Motu Dome. The Nga Motu Dome is nearly circular, with a diameter of about 7.5 km and a maximum height of 720 m. Radiometric ages of the Sugar Loaf volcanics suggest magma emplacement occurred in the Pliocene to early Pleistocene. The top of the dome was subsequently eroded and is now overlain by volcaniclastic units derived from the younger Pouakai and Egmont centres to the southeast. The region is known for oil and gas seeps, and about 250,000 bbl of oil were produced from shallow, upper Miocene levels in the adjacent onshore Moturoa Field from 1906 to 1972. The reservoir horizons targeted in this study are in the deeper structure of the Nga Motu Dome. To successfully drill a deep well into the traps associated with the Nga Motu Dome, an understanding of the emplacement history and structure is paramount. We utilize 2D and 3D seismic data to form a structural model of the dome. The present day high heat flow of 76 mW/m(2) near New Plymouth requires an igneous intrusion, when modeled by I D thermal models. The thermal models and seismic data limit the timing and thickness of the intrusions, likely to consist of at least two stages of Late Miocene and Pliocene age. Geochemical biomarkers suggest that the Moturoa-family oil source rocks are coaly units from the Eocene Mangahewa Formation. Thermal modeling in the vicinity of PEP 38464 predicts that hydrocarbon generation in the Mangahewa Formation started in the late Miocene, but was concentrated in the Pliocene to Recent. Most of the hydrocarbon generation post dates the formation of the Nga Motu Dome, and products may now be trapped at several stratigraphic levels in the anticlinal structure of the dome and onlapping strata.