50 Shales of Grey

When it comes to the commercial viability of shale gas or shale oil, there are two fundamental questions:

1. Does the shale contain economic quantities of oil or gas?

2. Can the shale be hydraulically fractured to extract those hydrocarbons?

Both factors are dependent on the composition of the shale, which reflects the environmental conditions under which it formed.

An Upper Jurassic shale from Dorset, full of fossil burrows.

The BGS/DECC report on the Jurassic shales of the Weald Basin is very timely, and demonstrates neatly the challenges facing anyone who wishes to try and exploit shale gas or shale oil in the UK.

Political, economic and environmental issues are important, but in the end, everything is dictated by the geology. If you ain't got the right rocks, nothing else matters. More decision-makers and opinion-profferers need to realize this.

A simplified geological cross-section of the Weald Basin (from Wikimedia Commons).

Fine-grained sedimentary rocks - shales in the broadest sense - are composed of four main ingredients: clay minerals, quartz silt, calcium carbonate, and organic carbon. Higher amounts of quartz or calcium carbonate make a shale more brittle, whilst higher amounts of clays make it more ductile. The precise quantities of each are controlled by the environment in which the mud accumulated. Most shales, however, contain plenty of clay minerals and not a lot of organic matter.

A scanning electron micrograph of a Jurassic shale from the Yorkshire coast.
The black fragments are organic carbon, the flat, grey particles are clay minerals.

The truth of this is laid bare in the BGS/DECC report. Studies from North America indicate that for a shale gas or shale oil 'play' to work, the shale must have more than 2% total organic carbon and less than 35% clays. As I pointed out in my last post, not one of the samples from the Weald Basin analysed by the BGS meet those criteria. It's no wonder that the report says 'prospectivity would appear to be limited', and no-one should be getting excited about the potential for an oil boom in south-east England.

Out of gas.

Our research at Durham and Newcastle universities, much of it focussed on the Jurassic shales of the UK, indicates that this "2 per cent TOC, 35 per cent clay" rule is over-simplistic. A new study led by my colleague Jonny Imber, about to be published in the AAPG Bulletin, shows that the natural fracturing behaviour of a shale can vary laterally over (geologically) small distances.

The clay-carbon-quartz-carbonate ratios are important, but variations in the burial processes and tectonics across a region can give different geomechanical properties to the same shale in different places. It's a complicated story, which is precisely why more shale research is needed. We need to better understand how shales formed, how they vary, and how they behave.

This case is made rather eloquently by the BGS/DECC report. The available evidence strongly suggests that the Jurassic shales beneath the Weald do not contain large-scale, economically viable quantities of oil or gas. Nonetheless, we need more data to test this. The drilling of exploration boreholes might annoy local residents, but without the information the boreholes provide, we have to resort to geological guesswork, and that doesn't help anyone.

Furthermore, even if the Weald is not a shale oil or a shale gas prospect, the rocks are still very important to our understanding of what is. We can often learn more from failures than successes, and studying the properties of the Jurassic shales in southern England and why they aren't viable could help us understand why shales in other areas might be.

Are all these regions really prospective?

It is crucial that the government and the petroleum industry continue to provide funding for academic shale research. Put simply, we need facts before fracks.

0