Worldwide reserves of hydrocarbons are declining; therefore we must investigate unconventional reservoirs such as buried volcanoes. Three examples of submarine Eocene-Oligocene basaltic monogenetic cones were studied in outcrop near Oamaru, New Zealand. The cones at Gees Point and Kakanui South consist of similar volcanic and sedimentary facies typical of single monogenetic basaltic volcanoes. However, the Cape Wanbrow complex is a stack of six overlapping monogenetic cones and has both similar and different facies due to its size and eruptive complexity. Cool-water carbonates were deposited on the shallow platform formed by doming associated with the volcanism meaning the highly porous limestones would not exist in this basin without the presence of the volcanoes. All cones are onlapped and overlain by a series of subaerially exposed carbonates, calcareous glauconite sandstones and mudstones.

Porosity and permeability was tested on samples from all five volcanic and six sedimentary facies associated with the cones. The results are plotted on porosity-permeability graphs showing significant overlap between volcanic and sedimentary facies types, each of which have three groupings corresponding to cementation history. This shows that while primary sedimentary and volcanic processes are different, they can result in similar porosities and permeabilities. For example, the low porosity and permeability of the Crystal Lapilli Tuff Facies is the result of being highly cemented by calcite. This is similar to porosity and permeability values of the calcite cemented and micritic Bryozoan Packstone. The sparingly cemented Bedded Ash Tuff Facies has high porosity- permeability values similar to those of the uncemented Bryozoan Grainstone Facies.

Reservoir evaluations have determined that the small, single, monogenetic cones tend to be highly cemented and thus are not likely to be suitable reservoirs. However, the larger volcanoes and stacks of cones tend to be less cemented with higher porosity and permeability values suggesting they may be effective reservoirs. The Bryozoan Grainstone Facies deposited on the platform away from the cones themselves is the most suitable reservoir target due to its very high porosity (48.6-50.6%) and permeability (1080-3176 mD).

The outcrop case studies are compared to seismic data containing buried volcanoes of similar composition and eruptive style from the Canterbury (New Zealand) and Bass Basins (South Australia). The comparison indicates that the large stacked volcanoes may act as prospective reservoirs. These are generally overlain by potential seal sediments deformed into dome shapes forming potential traps. In conclusion, both volcanic and associated sedimentary rocks can act as reservoirs and should therefore not be discounted in exploration.