RESERVOIR PROPERTIES OF PROXIMAL PYROCLASTIC ROCKS INTRUDED BY A TRACHYTIC DYKE, LYTTELTON VOLCANIC COMPLEX, NEW ZEALAND

Volcanic rock is increasingly viewed as having potential as geo-energy reservoirs, and could play an important role in the transition from fossil fuel to renewable energies. The extinct and deeply eroded Lyttelton Volcanic Complex, New Zealand, makes it an ideal location to study the reservoir properties (porosity and permeability) of volcanic rocks. Here, we present the results of two common lithofacies that outcrop in Lyttelton (and modern analogues such as Mt. Etna in Italy): a (i) proximal pyroclastic rocks of a small (<1km³) parasitic scoria cone, and a (ii) intrusive and host facies of a trachytic dyke. The facies were described in detail the field, and permeability and porosity measurements were made using a field (TinyPermIII), and lab PDP and steady-state permeameter, and pycnometer, respectively. Additional characterization of the macro-scale fractures of the dyke was performed through analysis of a drone-captured 3D model. At the scoria cone, six lithofacies were identified: 1. coherent lava; 2. vesicular lava; 3. non-welded scoria; 4. moderately-welded scoria; 5. fractured and welded scoria; 6. highly-welded scoria. At Rapaki Dyke four lithofacies were identified: 1. core dyke, 2. damaged dyke, 3. damaged scoria, and 4. undamaged scoria. Damaged lithofacies occur as zones of intense fracturing of the dyke and host pyroclastic (scoria) deposits immediately in contact with the intrusion (<5m). Results from the cone indicate that that lava facies can serve as seals in geo-energy reservoirs if not fractured (~ 0.05 mD permeability) while pyroclastic rocks are ideal for carriers (~1.65 mD avg. for highly-welded scoria) and/or reservoirs (x100 mD of non-welded scoria and less-welded facies), even at greater depth. All dyke and host rock lithofacies present low permeability (<0.15mD) indicating that the intrusion reduced the porosity and permeability of the scoria lithofacies. Variable dyke macro-fracture patterns present could possibly act as pathways for fluids. Our study demonstrates that detailed characterization of volcanic rock heterogeneity is necessary to understand how primary and secondary (i.e. depositional and post-depositional) processes can impact the reservoir potential of volcanoes and their associated lithofacies.