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JARR (Jurassic Analogues: Resources to Reserves)

The unconventional potential of Jurassic shales: fracked or fiction?

The British Geological Survey (BGS) and the Department for Energy & Climate Change (DECC) recently published a report on the potential for the Jurassic shales beneath south-east England to produce economic quantities of oil or gas. It suggested that there may be resources of between 2.2 and 8.5 billion barrels of oil trapped in the shales of the Weald Basin. There is considerable uncertainty over this figure, however, and the percentage of the oil that could be extracted economically is likely to be very low. It is vital therefore that further Jurassic shale research is carried out.

 

JARR (Jurassic Analogues: Resources to Reserves) is a project led by Durham University, focussed on building a better understanding of Jurassic shale systems. It is funded by the Natural Environment Research Council (NERC) and brings together researchers from Durham and Newcastle universities with partners from industry and the public sector.


The unconventional potential of Jurassic shales: fracked or fiction?

 

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.

The British Geological Survey report on the Jurassic shales of the Weald Basin is timely, and demonstrates some of the challenges facing those who wish to try and exploit shale gas or shale oil in the UK. Research being carried out by JARR is providing further insights into this topical and controversial issue.

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 sediment accumulated.

Studies from North America indicate that for a shale gas or shale oil 'play' to work, the shale needs to have more than 2% total organic carbon and less than 35% clay content. Most shales, however, contain much higher quantities of clay minerals and much small amounts of organic matter. This is demonstrated by the Jurassic shales of the Weald Basin. Of the samples analysed by the BGS (see Fig. 43 in the report), not one meets the North American criteria.

This "2 per cent TOC, 35 per cent clay" rule may be over-simplistic though. A new study of Jurassic shales led by JARR researcher Dr Jonny Imber, to be published shortly, shows that the natural fracturing behaviour of a shale can vary laterally over (geologically) small distances, even in shales of the same clay content.

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.

The available evidence strongly suggests that the Jurassic shales beneath the Weald do not contain large-scale, economically viable quantities of oil or gas. In a recent article for The Conversation, JARR researcher Professor Andy Aplin pointed out that “neither the rock nor the oil is of optimal quality…[and] only 1-2% [of the oil] might be recoverable.”

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. Borehole data can also be used to provide information for many other activities, such as water extraction and geothermal energy.

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.

Political, economic and environmental issues are important, but in the end, everything is dictated by the geology. If you don’t have the right rocks, nothing else matters. It is crucial that more decision-makers understand this. It is also crucial that the government and the petroleum industry continue to provide funding for academic shale research projects such as JARR. Put simply, we need facts before fracks.

Key points:

  • There is no shale gas potential in the Jurassic rocks of south-east England and even the shale oil potential may be very limited;
  • Shales can be very variable in their properties, both laterally and vertically;
  • Research carried out by JARR shows that assumptions about the impact of clay and organic content on the ‘frackability’ of shales may be incorrect;
  • Academic-industry research partnerships are vital to developing a better understanding of shales.

 

For further details on the JARR project, please contact:

Dr Howard Armstrong, Principal Investigator (h.a.armstrong@durham.ac.uk; 0191 334 2320)

Dr Liam Herringshaw, Postdoctoral researcher (l.g.herringshaw@durham.ac.uk, 0191 334 2348)