Groundwater flow beneath Mt Taranaki, New Zealand, and implications for oil and gas migration

Abstract

Abstract The known area of oil and gas seeps on the Taranaki peninsula is restricted to the northeastern quadrant of the volcanic ring plain around Mt Taranaki, with the only confirmed oil seeps being on the coastline at New Plymouth. This paper investigates whether oil migration at <3 km depth could be influenced by the groundwater flow pattern. Up to 8000 mm of rain falls annually on the summit of Mt Taranaki. Assuming a conservative 15% of the precipitation on the Taranaki peninsula becomes infiltration, then a simple model of the horizontal flow regime indicates that the meteoric recharge on the volcanic cone and its ring plain is 3.8 million m 3 /day. The topography ensures that the flow regime is radial, with maximum flow velocities of up to 8 cm/day occurring in the northeast and southeast of the peninsula due to the convergence of groundwaters from Mt Taranaki and the eastern highlands. Vertical flow modelling incorporating a decrease in permeability of over five orders of magnitude with increasing depth, shows that the hydraulic gradient at 3 km depth is 20% of that at the surface. Sand layers at 2 km depth could be conduits for deeply circulating groundwater, with the timescale for deep circulation being of the order of 10 4 ‐10 5 years. Oil may be driven along the hydrodynamic gradient from beneath the summit area of Mt Taranaki. This gradient enhances oil migration if the sand units dip upwards from the centre of the peninsula. However, oil may also be driven downdip if the sand units have dips of up to c. 1°. Oil reservoirs are possible in groundwater flow stagnation areas where the dip of sandstone units locally increases, or towards the coastline where the horizontal hydraulic gradients significantly diminish.