PETROPHYSICAL AND MECHANICAL PROPERTIES OF BURIED VOLCANIC ROCKS ON BANKS PENINSULA, NEW ZEALAND

Banks Peninsula is a 1200km² Miocene volcanic complex, that represents an accumulation of four separate volcanic groups: Lyttelton, Mount Hebert, Akaroa, and Diamond Harbour Volcanic Group, on the east coast of New Zealand’s South Island. Banks Peninsula presents a unique opportunity to study buried volcanics that can be used as an analog for offshore reservoir energy exploration. A core donated by the Christchurch City Council, following an investigative borehole in a wastewater project, provided samples to test relationships between petrophysical and mechanical properties for buried volcanic rock and the influence of alteration. As samples below 100 m were considered more altered than those above 100m depth, depth was used as a proxy for alteration. Samples of a dike and two lava flows, one above and one below 100 m, were chosen to be representative lithologies. Lava flows are separated into three lithofacies: upper and lower breccias, and middle coherent lava, typical of an a’a lava flow. Petrophysical and mechanical properties were tested in the laboratory: porosity, permeability, P- and S-wave velocities, and uniaxial compressive strength (UCS). Thin section analysis was conducted to corroborate or explain findings following testing. Testing and analysis highlights the strong influence of lithotype on petrophysical and mechanical properties rather than depth, and as a consequence, alteration. Brecciated lava samples had higher porosity and permeability, and lower uniaxial compressive strength than coherent lava and dike samples. Permeability values indicated that the brecciated margins of lava flows have some reservoir potential, but the effect of depth and unit size (volume of the potential reservoir) must still be taken into account. Brecciated samples, in general, had lower strengths than dense coherent lava and dike samples. These rocks can be used as an onshore analog for potential offshore reservoirs, making these findings important for future oil and gas or geothermal energy resource exploration.