Mining map of the Tarbrax area
Contributed, with thanks, by Jim Henry
Oil shale is an organic-rich, carbonaceous fine-grained sedimentary rock containing kerogen (a solid mixture of organic chemical compounds) from which liquid hydrocarbons, called shale oil, can be produced through destructive distillation or "retorting". Shale oil is distinct from tight or crude oil which occurs naturally in shales. Oil shales were formed in a variety of depositional environments including marine (oceans), lacustrine (lakes) and terrestrial (swamps and bogs) . Known oil shales are predominantly aquatic (marine, lacustrine) in origin.
There have been many different names used for oil shale over the years including: kerogen shale, kerosene shale, bituminous shale, bituminite, cannel coal, boghead coal and torbanite. Oil shales are found throughout much of the geological column and deposits are found in world wide. They have been a source of energy for many centuries, originally being burned like coal on open fires, then, much later, as the raw materials for an extraction process which separated out the carbon based and other valuable compounds.
In Scotland the oil shales occur in rocks of the Carboniferous period which were laid down between approximately 360 million and 300 million years ago. Most of the Scottish oil shales are found in the rocks of what were once known as the Lower and Upper Oil Shale Group and now known as the Strathclyde Group. Most of the these rocks occur in what is now West Lothian with a small extension into South Lanarkshire and isolated deposits occur in Midlothian and Fife. Other oil shales are found higher up the geological column often associated with coals.
The oil shales of West Lothian and South Lanarkshire are by no means consistent throughout the shale field and some are only what is best described as "locally well developed" or of a workable thickness in only one geographical area.
On initial inspection oil shales look like most other mudstones or shales. However if scraped with a metal implement they exhibit a chocolaty brown streak and, when struck with a hammer, it sounds a bit like hitting a bit of wood. Oil shales vary in chemical content and, unlike other minerals, they do not have a clearly visible edge, rather a mudstone and gradually becomes a bituminous mudstone then reverts gradually to a mudstone. The thickness of an oil shale won by the miner depended on the average gallons oil per ton the miner was seeking to obtain. For example a 0.5 metre section might yield 40 gallons per ton whereas a 1 metre section might yield 25 gallons per ton.
Oil shales strata is normally flat and in thin laminar sections. It is also found in what is described as a curly oil shale at Broxburn and at Pumpherston. The Broxburn Curly is merely "wavy" while the Pumpherston Curly is very highly contorted. It was noted that the curly oil shales were of higher quality than the "plain" shales and that this was a cause of their appearance. However a site investigation in the Pumpherston area encountered a section of ordinary mudstone with similar contortions to the Pumpherston Curly.
The Geological Survey memoir "Oil-Shales of the Lothians" contains a detailed account of geology in the shale districts. The first edition and the second edition of Oil-Shales of the Lothians are available online from www.archive.org. The third edition is available as a pdf on this website.
Further information about the geology in different areas of the West Lothian shale fields can be found in the following Geological Survey publications: