Rocks are a lot like people… they have a past, they are made up of smaller bits and pieces, they respond to stress in different ways — some deform, some break, some change in structure and make-up. Some rocks are straight forward, like a homogeneous sandstone, and other rocks are more complicated like a really old gneiss (pronounced “nice”) or a metaconglomerate.
Rocks are roughly categorized by how they came into being: did molten rock make its way into the earth or on to the earth? Igneous! Did layers of sediments get covered over and compressed (lithified)? Sedimentary! Were the pressure and temperature just too much and the rock’s mineral crystals changed? Metamorphic!
The rock I work with the most is a sedimentary rock called shale. According to some that is not a real name for rock. Shale is a catchall term to describe a lithified chunk of extremely tiny sediment. Depending on whether you are speaking with a geologist, a sedimentologist (a geologist who specializes in sedimentology), a petroleum engineer, a civil engineer, or a soil mechanic for that matter, you will get a different definition of precisely what a shale is and what it is not. Or if it even exists.
The term “shale” was originally used to describe a very fine-grained, laminated, clayey rock. It can also describe a formation, such as the Bakken Shale, Pierre Shale or Marcellus Shale. It can also be and has been used to describe any very fine-grained rock regardless of lamination or clay content.
There are any number of rock names used to describe what is typically referred to as shale: mudstone, claystone, siltstone, argillite, marl, and lutite to name a few. These names bear specific reference to measurable attributes: grain size, clay type and amount, lamination or bedding planes in the rock, and the amount and type of organic content in the rock.
Why does defining a rock by grain size, clay type, lamination, or even organic content matter? Scientists like precision, which requires the definition of terms. Engineers like repeatability, which means predictability, which means that a process can be created and then refined for the best possible outcome. Researchers like definitions and predictability because we use these to generate a hypothesis. If the outcome diverges wildly from the hypothesis, we get very excited and want to figure out why this is.
In the picture above you can see the Marcellus Shale in outcrop behind me. The visible horizontal layers means it is laminated. You cannot determine how clay-rich the shale is just by looking at it, a mineralogy study needs to be done for that. As for grain size, very-fine-grained rock grains are less than 0.0625 mm. Too tiny for the human eye to distinguish. The blacker a shale is the higher the total organic content (TOC). The higher the TOC, the greater the chance for hydrocarbons.
And this, my friends, is when it gets interesting.