Northern Alps' may conceal fuel reserves

Mounts's Tongariro, Ruapehu and Ngarahoe. Photo / Chris McLennan.

The mountain ranges along the North Island are only a few million years old, but once the island had its own "Northern Alps".

Though the range of mountains has long since eroded away, researchers at Waikato University are attracting interest in them from explorers searching for new petroleum reserves which may have evolved in coal deposits buried by that eroded material.

Professor Peter Kamp is developing a computer model which shows that 40 million years ago New Zealand's continental crust stretched, thinned and began subsiding, allowing the sea to flood much of the country, and form marine sediments such as the Waitomo Caves.

But his work - in collaboration with Penn State University in the United States - shows that from 27 million years ago, the Pacific Plate in the Earth's tectonic structures began pushing up against and sliding both underneath and over the Australian Plate.

The rocks that became Canterbury lay east of their current position, and as they moved westwards the crust in the region now occupied by Hawkes Bay and Wairarapa became compressed, pushing up a mountain range which Professor Kamp refers to informally as the "Northern Alps".

But as the range went up, erosion wore away the elevated crust, he said. New findings from the model showed that there would have been 10 to 20 kilometres of crust eroded from those Northern Alps.

"That would have generated a lot of the sediment that we find in the Taranaki Basin and off the East Coast," he said.

The geological activity left Taranaki in a good position to have oil and gas because it had been on the margins of all the plate boundary activity, being deformed just enough to create petroleum-trapping structures, and receiving enough sediment to bury its coal to the right depth.

"The Canterbury shelf was in a similar position but on the opposite side of the plate boundary," Professor Kamp said. The Great South Basin was further away and though it didn't receive much sediment from the boundary zone, it was an area of interest to oil and gas companies.

These things were important considering oil and gas reservoirs generally required coal deposits to have been heated to between 100C and 130C - meaning it needed to have been buried between 4km and 5km deep.

The west coast of the South Island was too close to the plate boundary, and the east coast of the North Island was within the boundary itself.

"Two years ago I spoke with a guy from an American company which had a licence area on the east coast, and he had to make a decision about whether to drill a hole," Professor Kamp said. "That's a multi-million dollar decision, and he said, 'You're telling me we don't know where my rocks were 10 million years ago?'. That's the problem with the east coast region - 10 million years ago it didn't look anything like it does today."

- NZPA