Drawing a blank in the fjords

fjord02
23.03.2007
Prosperity is being pumped from deep beneath the seabed off the Norwegian coast, invisible to the tourists who come to admire Norway’s fjords. What such visitors are unlikely to know is that some of these steep-sided inlets have been explored to see if they once hid oil or gas.

Text: Pål Espen M. Kilstad.

Residues of Jurassic rocks have been found at several locations in the fjords of western Norway, reports geologist and researcher Reidulv Bøe at the Norwegian Geological Survey (NGU). “They were also encountered when the Bjorøy tunnel south of Bergen was built. A number of fjords have been studied over the past 30 years to see if they contain interesting rocks.”

Drawing: Oddmund Mikkelsen

Drawing: Oddmund Mikkelsen

 

Demanding
The Kristin field, regarded by operator Statoil as both demanding and prolific, lies in the Norwegian Sea just 250 kilometres from the Trøndelag region of mid-Norway.

A platform with the capacity to produce 18 million cubic metres of gas and 126 000 barrels of condensate (light oil) has been installed on this development.

So the question is why petroleum resources are not to be found just a couple of hundred kilometres to the east, in the Trondheim Fjord.

“Oil may once have been present in rocks beneath this fjord,” observes Mr Bøe. “But anyone who hopes that hydrocarbons are still to be found there is doomed to disappointment.

“This is because strata which are both porous and permeable – allowing liquids to flow through them – are missing. These are the types we find further out on the continental shelf.

“A rock which contains petroleum can be compared with a sponge, with the pores filled by oil and gas. If we squeeze the sponge, these resources will be driven out.

” The only location in the Trondheim Fjord where porous and permeable strata can be found is the Beitstad Fjord, which is a small section of a side inlet.

But Mr Bøe explains that it is again very unlikely that source rocks – where oil and gas are created – have existed in the Trondheim Fjord region.

The commonest source rocks are grey or black shales with a high content of organic residues from plants and animals which were deposited on the seabed.

This layer of dead matter was later covered by clay, sand and gravel, and thereby compressed and converted to rock. Over several million years at the right pressure and temperature, part of the organic substances is converted to oil.

“Jurassic rocks in the Trøndelag area are unlikely to have been subject to sufficient pressure and temperature over a long enough period for that to have happened,” Mr Bøe says.

Another requirement lacking in the Trondheim Fjord is a cap rock – in other words, an impervious layer of shale, limestone or salt which stops oil or gas escaping from the reservoir.

Both the fjord and large areas of Trøndelag may once have had both reservoir and cap rocks. Seismic data indicate that such formations extended far inland from the continental shelf.

Erosion over several million years, and particularly during the ice ages, has scoured away what used to be upper layers of Norway’s fjord bottoms.

“The Beitstad Fjord has residues of the same rocks we find in oil and gas fields off Trøndelag,” notes Mr Bøe. “But they’ve never been drilled, so we don’t know exactly what’s there.”

Blocks of stone scraped from the bottom of the fjord during the last ice age have been found along the north-western side of this inlet, he adds.

“These suggest that oil once existed there. In any event, seismic data show that the sandstones in the fjord once contained hydrocarbons. But cap rocks were missing.”

Deepest
The Trondheim Fjord is one of the deepest in Norway, descending to 700 metres west of Rissa. Its bottom is covered with a 700-metre-thick layer of clay, porous but not particularly permeable.

“Drilling in the fjord has nevertheless been considered, not to find hydrocarbons but to obtain scientific information,” Mr Bøe explains.

Fjord could provide new data about conditions 160 million years ago – such as whether it was land or sea, where the rivers ran and what the climate was like.

“I’ve worked on several projects which have looked more closely at the Beitstad Fjord, primarily in connection with carbon dioxide storage,” says Mr Bøe.

“We concluded that the local rocks offer insufficient volume, the whole area is to shallow for this purpose, and the absence of cap rocks means the carbon di­oxide would escape.”


Topics: Geology