Limestone and oil

Transcript

Dr Steve Hood

Well the Tikorangi Formation is a very important limestone rock in Taranaki Basin . It’s got similarities to what we’re seeing behind us in the quarry face here, which is the Ōtorohanga limestone. The difference with the Tikorangi is that it’s currently sitting 3 kilometres below the surface, to the east of Stratford. 

This Tikorangi is an oil reservoir rock. This is a section of the Tikorangi Formation. They’ve drilled down and extracted this length of core. So it’s muddier, finer and compositionally a little bit different than the Ōtorohanga. But very importantly, it’s of the same age. So the key characteristic that makes this limestone an important reservoir rock is the fact that, due to tectonic compression, the rock, already being cemented and hard, was brittlely fractured.

So here I have a section of core that intersects one of these fractures. One side of the fracture is seen here, and the fracture then introduced space or porosity – it’s called fracture porosity – into the rock. And it’s because of these major fracture systems that we see here that the otherwise tight non-porous limestone – there’s no holes or porosity in the limestone itself, they’ve all been plugged with cement during burial diagenesis and pressure dissolution – but these fractures have been introduced later on due to tectonic compression, and these fractures have again opened up space in the rock that have enabled it to act as a hydrocarbon reservoir.

And given we had a source of hydrocarbons further down – hydrocarbons being less dense migrates up – is able to move through the fractures, and if there’s a structure or a bowl feature with capping mudstones that are impervious, then the oil can be trapped in the fractures in a structure where you’ve got a seal over the top.

All of the reservoirs bar one in Taranaki Basin are sandstones, and sandstones have naturally occurring pores that haven’t been filled with cement in them. Now, the Tikorangi is the only carbonate or limestone reservoir in Taranaki and therefore the only one currently producing in New Zealand.

So we might see behind us in the quarry face some of the fracture-type features that we commonly get in hard competent limestones, which are called joints here in the quarry, but otherwise, outside of those joints or fractures, the limestone has no ability to host because the porosity has all been filled with cement during burial diagenesis.

Acknowledgements: McDonald’s Lime Limited Ministry of Economic Development Lloyd Homer, GNS Science Grant Pearson