Filming for a BBC Scotland documentary on Oil Shales by Carol Cotterill

In April of this year, BBC Scotland will air a programme presenting the debate around shale gas and fracking. The Scottish Government currently has a moratorium in place (as of January 2015) preventing the granting of planning consents related to unconventional oil and gas developments, including fracking, in Scotland. The moratorium will stay in place for an unknown length of time, until evidence gathering and a consultation process has been concluded.

                

BBC Scotland film crew debate a wide angle shot

Bob gets fitted with a microphone

Shale gas and fracking is a contentious issue in many countries. Avoiding the political discussions surrounding this issue in Scotland, BGS’ Bob Gatliff, Director of Energy and Marine Geoscience, met with a film crew from BBC Scotland on a windy and cold beach in South Queensferry to go back to geological basics about what shales actually are. I went along to take some photos and to quiz Bob about shale formation and quality.
 

Weathered West Lothian Oil Shales at surface

Shales are fine grained sedimentary rocks, composed of silt and clay-sized mineral particles (<1/16 mm) which typically accumulate at the bottom of large bodies of water, such as oceans, shallow seas and lakes. Marine organisms such as algae and plankton die and settle-out along with these inorganic silts and muds, giving the accumulating sediment an organic content.  In general, the more organic material the shale contains, the darker the colour. It is this organic content that is important for the generation of hydrocarbons. However, organic content isn’t the only important factor – maturity of the shales is also a prime factor to consider. Maturity of the shales is determined by the depth of burial, length of burial time and the temperature to which the shales are exposed. Bob described this process as being a bit like a pressure cooker heating the shales to just the right temperature and pressure. Oil is the first product generated at lower pressures and temperatures (~110°C at 4-5 km depth), with higher temperatures resulting in the creation of natural gas (~180°C at 6-9 km depth).

Historically, “conventional” oil and gas reservoirs have been exploited by drilling. Conventional reservoirs form when shales have reached maturity under the right temperature and pressure, and the resultant oil or gas has migrated out of the shale into overlying rocks that are more porous (e.g. sandstones). Where these sandstones are covered by a cap or sealing unit, such as a mudstone, further upward migration of the oil or gas is prevented, forming a reservoir. The larger pore spaces of the sandstone make the oil and gas more accessible for extraction as the fluids flow more easily out of the rock. Recently, new technology developed in the USA now means that “unconventional” reservoirs can be accessed and drilled, extracting oil and gas directly from the shales through artificial fracturing, thereby unlocking some of the largest natural gas reserves in the world.

Bob describes the West Lothian Oil Shales to the BBC presenter

So what link does that have to a chilly spot on a beach in South Queensferry? Bob took us along the beach a short way to where the West Lothian Oil Shales outcrop at the surface to explain how it formed, and the general principles behind oil and gas formation in shales.

The Midland Valley is a basin that lies between two faults, the Highland Boundary and the Southern Upland faults, both of which span Scotland from east to west, encompassing Glasgow and Edinburgh. 

Midland Valley location

During the Carboniferous Period (~358.9 ± 0.4 to 298.9 ± 0.15 million years ago), Scotland was located just south of the Equator, with a hot, humid climate, seasonal rains and a large organic source from vegetation. Sediments and organic matter slowly filled the Midland Valley basin, depositing the West Lothian Oil Shales in a perennial lake with adjoining rivers and a shallow marine delta environment. Over the course of time, these sediments were buried at depth and heated, forming shales and providing the perfect conditions for the development of natural oil and gas.

Pumpherston Oil Works, commercial postcard, c.1922. Copyright Almond Valley Heritage Trust. www.scottishshale.co.uk

 Pumpherston is a small village situated about one mile north of Mid Calder in West Lothian. 220 houses were built, starting in 1885, by the Pumpherston Oil Company to house the workers for the Pumpherston Oil Works – a refinery established in 1882 to exploit the West Lothian Oil Shales. The Pumpherston Oil Works was one of about 120 refineries which operated up until 1964, extracting oil from the various shale seams within the West Lothian Oil Shale Formation. The presence of these prolific oil shales in Scotland formed the basis for the oil industry in Europe. Bathgate Chemical Works, established in 1851, is believed to be the first site in the world where mineral oils were processed on an industrial scale. James Young, a Scottish chemist, held a patent from 1851 – 1864 that created a monopoly for production of oils from cannel coal.

Bob Gatliff in front of the West Lothian Oil Shale outcrop

The shales which form the West Lothian Oil Shale Formation now lie at a maximum depth of ~3,500m beneath sea level in the Firth of Forth, reaching a maximum thickness of ~1.1km. However, for a shale to be effective as an “unconventional” source, it also needs to have the right physical properties to allow the rock to deform in a brittle manner. This ensures that when a shale is cracked open through hydraulic pressure, the shale does not behave plastically, sealing the crack up again and so preventing flow. Tests undertaken on some of the Midland Valley samples show that these shales are more clay-mineral rich and carbonate poor than other unconventional source rocks in the UK, and so might behave in a more plastic manner.

One final question was asked by the BBC team

 “Are we reacting before we have got the scientific answers to accurately address this issue?”

I think only time will tell us the answer to that one. But to see the documentary in its entirety, please keep a look out on the BGS website where we will post the date and time that it will be shown once it has been scheduled.

References:

For more detailed information on The Carboniferous Shales of the Midland Valley please see Monaghan, A.A. 2014. The Carboniferous shales of the Midland Valley of Scotland: geology and resource estimation. British Geological Survey for Department of Energy and Climate Change, London, UK.

Thanks also to the Museum of the Scottish Shale Oil Industry (www.scottishshale.co.uk)

By Carol Cotterill