What controls the reservoir quality of andesitic arc derived volcano clastic sandstone reservoirs: An example from the Mohakatino Formation, Taranaki Basin, New Zealand

Abstract

The mid-Miocene Mohakatino Formation of the Taranaki Basin, New Zealand, is a submarine fan succession sourced dominantly from an offshore, submarine andesitic arc. It is composed of volcaniclastic sandstones, siltstones and mudstones, characterised by a prominent andesitic detritus. The Mohakatino Formation acts as the reservoir rock for the sub-commercial, tight oil producing Kora Field in the Northern Taranaki Basin, New Zealand. This formed due to a combination of proximal tuffs on the flanks of the Kora Volcano and sector collapse of the eastern flank of the volcano within the Mohakatino Volcanic Centre off the west coast of the North Island, and this mass wasting material was deposited distally along the present day Awakino coastline.
The hydrocarbons present in the Kora Field are the product of the thermal maturation of the Late Paleocene Waipawa Formation kerogen, and matured by the residual heat of the Kora volcano. While the Mohakatino Formation does not have significant quantities of hydrocarbons present, the sandstones do have high porosity and permeability with potential that the Mohakatino Formation could have acted as commercially viable reservoir rock in the proximal regions to the Kora volcano. In the medial to distal reaches of the submarine fan systems the reservoir quality for the Mohakatino Formation is of poorer quality and is controlled by sedimentary processes and breakdown of the more volatile and friable components of the andesitic volcaniclastics due to transportation process of the submarine high density turbidity currents.
The reservoir quality is preserved in these proximal regions by early stage authigenic clay precipitation from the breakdown of volcanic and hypabyssal rock fragments which prevented quartz overgrowths and later stage pore filling clay formation. Pore filling calcite that forms in shallow marine rocks was prevented from precipitating by the acidic nature of the proximal rocks, preserving the reservoir quality. Furthermore, rapid burial of the proximal Mohakatino Formation by ~2 km over ~2 Ma by the Giant Foresets Formation contributed to sealing the pore network to create a closed system, occluding the porosity, and which allowed for the reactive materials to be broken down and the resulting authigenic minerals to be retained in the rock. While these factors combined preserved some of the reservoir quality, relative to typical clastic sandstone reservoirs the reservoir quality is still low. The proximal deposits are found to have relatively coarser grain sizes due to their increased weight and density causing them to fall out of water column and be deposited first, and together combined to give relatively good primary porosity. The turbidity currents and tuffs at their furthest extent had lost the majority of their energy and coarsest material, and so when they reached what is now the now Awakino Coastline the deposited material was of a relatively fine grainsize and better sorted than that of proximal. These combined features led to reduced primary porosity and therefore a decreased reservoir quality of the distal Mohakatino Formation rocks from their inception, and is therefore a key control on the potential for these rocks to act as a reservoir.