Science Spin January 2008

The Irish Offshore: An Oil & Gas Treasure Trove?

By Sean Duke

The ancient origins of the sandstone deposits that hold and contain the gas at the Corrib Gas Field, off the coast of Mayo, might not be a subject that grabs most people's attention. But, research into what some might regard as this obscure academic area has revealed information that could be of crucial importance to us all.

Dr Shane Tyrrell is a geologist and 'Research Fellow' in the UCD School of Geological Sciences. He is interested, among other things, in the origins of ancient sandstones, like those that host the Corrib Gas. He would like to find out where such ancient deposits came from, and from that then determine how they travelled to their current position, and what can be learned from that in terms of where ancient continents were located in relation to each other. His work is all about reconstructing the Earth's ancient geography.

Samples

The story begins with Dr Tyrrell obtaining samples from the Corrib Field courtesy of Shell Oil. This is nothing too unusual, as geologists that work in academic departments are often reliant on exploration companies to provide them with samples. The cost of drilling is far too high for any academic, or university to bear. In return, the academic will typically agree to make any data arising from the study of samples available to the industry partner, and this knowledge can then be used for further explorations.

In one such arrangement, Dr Tyrrell gained access to samples obtained from drill cores through sandstones at the Corrib. These sandstones were known to be Triassic in age, which means they were deposited about 230 million years ago. The Triassic period was very hot, far hotter than today, a time when there was no polar ice, and the continents we know today were joined together in one supercontinent called Pangea.

Dr Tyrrell wanted to know where did these Triassic sandstones come from and how did they get to the area around the Corrib? It was very much expected, said Dr Tyrrell, that the sandstone sediments would be traced to origins somewhere south of Corrib, perhaps in France or England. The thinking was that there were large scale rivers, at this time, draining over France and England, dumping sediments into the North Sea and the Irish Sea Basin. It was assumed that the Corrib deposits were just part of this.

Surprise

It came as a great surprise to Dr Tyrrell when the samples could not be linked to anywhere in the south, that the "scientific signature" in the individual sand grains proved conclusively that there were not part of any geological system to the south. The question now was, if they didn't come from somewhere to the south, where mountains existed in the Triassic, where were they from, and what did it all mean?

It is possible for geologists to pinpoint where individual sandstones come from by making use of the fact that the lead composition of sediments varies throughout the Earth's crust in a systematic way. The balance between different isotopes of lead differs depending on the location on the Earth's surface, and this fact is dependent upon factors like the age of the crust, and when a piece of crust was last melted.

Dr Tyrrell and his team were the first to apply laser analysis techniques to sandstone. Under this technique, a laser is used to drill a small hole into the sand grains, and a stream of gas carries particles out into a mass spectrometer (a machine used to separate and identify molecules based on their mass -- or how much matter is in the molecule) which is used, in turn, to measure the different isotopes of lead.

"By sampling with a laser we were able to measure lead isotopes in individual grains of the mineral K-feldspar, a common component in sandstones," said Dr Tyrrell. "Lead isotopes in K-feldspar grains have a very distinct fingerprint which they carry with them as they are released from rocks by erosion and are transported to where they form the sandstone under investigation. By linking the sand grains to their source, it is possible to track ancient rivers and pre-historic patterns of uplands and basins."

"Rocks from different regions on Earth can have distinct abundances of lead isotopes so we could tell that K-feldspar sand grains from the Corrib Gas Field could only have originated from Greenland and Canada," Dr Tyrrell concluded.

It had been expected that the Corrib sediments would have a lead isotope signature linking them in a continuous pattern with sediments to the south, as it was believed that rivers from there had run over land through the Corrib area. That was found not to be the case. It was clear the Corrib sandstones were from elsewhere -- far to the north.

"The lead signature is very, very, very different anywhere to the southeast of the Corrib -- very clearly different -- and anywhere to the northwest it is very distinctly different from anywhere to the south," Dr Tyrrell said.

Implications

This surprise finding was important on two levels. Firstly, it was a major step forward in terms of an academic understanding of the ancient position of continents, and ancient seas and rivers, in relation to one another in the Triassic period.

At the time the river deposits were laid down, the area around Corrib was, therefore, on land. This area was part of Pangea, and the Atlantic Ocean had not yet opened up. The fact that these deposits were laid down by a river system coming from the north, not the south, totally changes the perspective of those such as Dr Tyrrell that are trying to reconstruct what this part of the globe looked like 230 million years ago.

The finding was also important, on a more practical level, from an exploration point of view because it meant that the Triassic Sandstones at Corrib, were likely to be part of the same family of sandstones that lay further out to sea in the deeper waters of the Rockall Basin. They were all now likely to have been laid down at the same time.

If one sandstone held gas, could its relative further out to sea, not hold gas, maybe oil?

"It means that the same sorts of sands that we see in Corrib, probably occur in depth out here (Rockall Basin) as well," said Dr Tyrrell. "Now, whether they contain oil and gas we can't say, but they are the perfect place to find oil and gas."

The Corrib sandstones, given their origins in Greenland and Canada, must have been transported large distances to be deposited in the Irish offshore. Ireland was closer to Greenland and Canada in the Triassic, but still, the deposits must have been moved at least 500km and perhaps up to 1,000km, maybe even more, said Dr Tyrrell.

Origins

A large ancient river system emerged as the clear suspect. The sedimentary structures seen to be occurring every day in modern rivers can be studied by geologists. A very distinct set of structures build up at the bottom of a river that indicates clearly that the sediments were deposited by a river. Rivers will deposit sediments, for example, in an asymmetric pattern, as the flow is always in one direction. That is a river's telltale sign and this manner of deposition is reflected in the deposition structures.

These river-borne structures are seen in the ancient Corrib sandstones too, and the precise nature of the sediments indicate that the deposits were laid down by quite a powerful river system, perhaps on the scale of the Ganges or the Indus rivers today.

Treasure

The Triassic Sandstones of Ireland, which occur in slivers on land in Northern Ireland, but are mainly located in offshore waters, are the only sandstones (apart from the relatively small Kinsale Gas Field in the Celtic Sea) that have been proven to have significant hydrocarbon reserves -- at the Corrib Field. That makes them very important indeed, and, perhaps, in time, they will be regarded as a national treasure.

These rocks have been shown on land in the North to be a good and important aquifer for groundwater. If they hold water on land, and gas at sea, could they not also contain oil at sea? Many geologists would answer yes, and in time they could be proved right.

Certainly wherever the Triassic Sandstones occur, there is a good reservoir of one sort or another, said Dr Tyrrell. The geological reason for that is that these sandstones have lots of spaces between their grains. This space could host water, gas, or oil.

But, that's not all that these rocks could host. In the future, perhaps decades into the future, these rocks could be used to 'sequester' carbon dioxide. Ireland has a serious problem dealing with its emission of the greenhouse gas, carbon dioxide, so some have suggested using the Triassic Sandstones as a place to store the unwanted gas.

There are many technical obstacles to be overcome, said Dr Tyrrell, but this could be possible in future. But, if sequestering is to be attempted in Ireland, he said, the only rocks that could be used to perform this technical task on any meaningful scale would be the Triassic Sandstones.

Future

The next step that Dr Tyrrell and his colleagues would like to take is to take a more detailed look at how modern river systems behave in terms of how it transports its deposits. Achieving such a greater level of understanding could provide more insight into the workings of large scale ancient river systems, millions of years old.

Meanwhile, the search for oil and gas in the Irish offshore is set to intensify.