Until about 2007, being overly fascinated with shale was regarded as a little, well, unconventional. Seals and source rocks were interesting, sure, but always took a back seat to reservoir characterization. But in many plays today, shale is the reservoir. And sometimes you need to know how to define something, because it affects how it is perceived, explored for, developed, and even regulated.
The Alberta Oil and Gas Conservation Regulations define shale as:
A lithostratigraphic unit having less than 50% by weight organic matter, with less than 10% of the sedimentary clasts having a grain size greater than 62.5 μm and more than 10% […] having a grain size less than 4 μm ~ Section 1.020(2)(27.1), as specified in ERCB Bulletin 2009-23
This seems strict enough, but it is ambiguous. ‘10% of the sedimentary clasts’ might be a very small volumetric component of the rock, if those ‘clasts’ are small enough. I am sure they meant to write ‘…10% of the bulk rock volume’. The Wikipedia entry for shale cites Blatt and Tracy (1965), giving a definition that also centres on grain size, but requiring the rock to be fissile:
A fine-grained clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and [silt-sized particles]… The ratio of clay to other minerals is variable. Shale is characterized by breaks along thin laminae or parallel layering […] called fissility. Mudstones, on the other hand, are similar in composition but do not show the fissility.
Arguably unreliable for technical definitions, dictionaries nonetheless try to describe how words are used and understood by most people. And there are specialist dictionaries, too. Here are some definitions:
A fissile rock that is formed by the consolidation of clay, mud, or silt, has a finely stratified or laminated structure, and is composed of minerals essentially unaltered since deposition ~ Merriam–Webster Dictionary
Soft finely stratified rock that splits easily, consisting of consolidated mud or clay ~ Concise Oxford Dictionary
A well-laminated argillaceous sedimentary rock [whose] fissility is related to the disposition of clay minerals [in] the rock. Shales do not form a plastic mass when wet, although they may disintegrate when immersed in water ~ Penguin Dictionary of Geology
What about sedimentologists? Here’s Potter et al. (2005):
Although the terms clay, mud and shale are widely recognized, their technical definitions and usage have long been troublesome and are not fully agreed upon. There are at least two reasons for this — the term clay is used both as a size and a mineral term, plus many clays, muds and shales are rich in silt-sized particles and thus span the clay–silt boundary.
Potter goes on to discuss some of the nuance here, then settles on mud as a generic fine-grained sediment, and mudstone as its lithified incarnation. But his point is clear: shale is, at best, a woolly term. This view is echoed by Merriman et al. (2003): ‘Problems have arisen with the precise definition of the terms shale, slate and clay.’ Those authors also call out flakiness as the diagnostic characteristic of shale (as opposed to blockiness for mudstone). They add, ‘shale, mudstone and slate are collectively referred to as mudrocks.’
It is interesting that none of the definitions mention organic matter, except to rule out coal. Many industry geologists today seem to use the word shale as shorthand for hydrocarbon plays that produce directly from source rocks, perhaps thinking of everything else as mudstone. The last thing the world needs is another definition, but we might summarize this notion as:
A tight, brittle rock composed mostly of mud-grade particles, containing substantial petroliferous kerogen.
So what is the geologist to do? The only safe thing to do is to be quite clear about what you mean when you talk or write about shale. If you aren’t, there’s a good chance that every member of your audience has a slightly different interpretation than the one you have in mind.
References & acknowledgments
Blatt, H and R J Tracy (1996). Petrology: Igneous, Sedimentary and Metamorphic. 2nd ed. Freeman, 281–292.
Merriman, R, D Highley, and D Cameron (2003). Definition and characteristics of very-fine grained sedimentary rocks: clay, mudstone, shale and slate. British Geological Survey, Commissioned Report CR/03/281N.
Potter, P, B Maynard, and P Depetris (2005). Mud and mudstones. Springer-Verlag, Berlin, DOI 10.1007/b138571.
Shale gas development — Definition of shale and identification of geological strata. Bulletin of the Energy Resources Conservation Board of Alberta, Canada, July 2009 (23).
Whitten, D and J Brooks (1972). Penguin Dictionary of Geology. Penguin, Harmondsworth.
A longer version of this essay first appeared as a blog post in March 2011, ageo.co/OIZcdo