Pan contemplates Pandora's box by by j dial
As we've all heard, hydraulic fracturing releases natural gas contained in deeply buried shale formations. I wondered about the association between shale and natural gas. In researching that connection, I came across something unexpected. This section describes the shale-gas connection and the next reveals its largely understated threat. Finally I describe oversight.
Hiding in Plain Shale ...
We learn in grammar school that rock falls into three basic categories--igneous, metamorphic, and sedimentary. When you think about it, distinctions we make among rocks composed of mingled elements must be subjective, for the crust cooled from elements well shaken and stirred. Nevertheless, after those cooling periods (and there have been several) things happened, and on those events we base our distinctions. Igneous cool out of magma, forming crystals, and include basalt, granite, obsidian, quartz, and rhyolite; metamorphic have changed, in effect being born again by temperature and pressure, and include schist, slate , and marble; and sedimentary are crushed beneath the bulk of land or sea, and include chalk, coal, flint, and sandstone. Sedimentary rocks can be 'organic'--not because they're free of contagion, but rather because they contain debris from organic processes.
Shale is a sedimentary rock, formed by clay-sized minerals that blanketed the bottoms of ancient seas. Although gravel, sand, silt, or clay can make sedimentary rock, only clay makes shale. A claystone must have at least twice as much clay as silt and no more than ten percent sand.  Even with all that, however, a claystone may still not make the shale grade. What makes a claystone into shale is its fissility--its tendency to split into layers. Shale is usually soft and, with exposure, easily returns to clay. It can be seen along roadcuts resembling drunken parallels of flagstone (which is also sedimentary).
Geologically the Devonian period, about 400 million years ago, has been called the "age of fishes" because fish proliferated in that time of warmth and widespread seas. Fish did so well, in fact, that some grew legs and found novel new uses for them. It was during this time that many of the small, plankton-like lifeforms, far down the food chain that populated those seas, when that avoided being eaten, eventually died of other causes. Their little bodies fell through warm and sunlit waters into colder and oxygen-low waters far below, where, relatively free from decomposition, the bodies piled up like product of a busy serial killer. The end of that period, about 345 million years ago, was marked by a mass extinction, probably caused by glaciation or a meteor impact. In that extinction about 30 percent of all animals including fish perished, adding greatly to the bodily debris in the depths.
Over time and under bodies, pressure and heat began to build and the sediments compressed. Throughout the years layers coalesce; conditions change, adding layers of different composition that compress and heat these components into sedimentary rock. At times tectonic movements forced great plates of sediment over or under other plates. Eventually, what was once organic matter mixed with clay baked into hydrocarbons mixed with shale.
A large part of the US was once under water, and now we sit astride vast swaths of sedimentary rock of varying composition and thickness, buried at various levels. Sedimentary rock that began beneath oceans and contains metamorphosed life forms is known as "marine black shales".
Shales come tinted from the clays that made them. Moreover, the darker the shale the more organic matter went into it. And the more organic material that goes into shale, the hotter it gets.An Unwholesome Trilogy
Uranium, shale, and gas have a long history together. Almost all sedimentary rocks contain low-level radioactivity because of the radioactive isotope potassium-40 scattered throughout clays, feldspars, micas, and other common silicate minerals. Shales, containing their share of clay, tend to higher concentrations of potassium-40-bearing minerals. And shales that are rich in organics, which concentrate radioactive ions, have yet greater levels of radioactivity.
After the second world war the Atomic Energy Commission declared that the shale deposits, some of which are even now being fracked, are the largest uranium resource in the US. Dr. V. E. Swanson, author of Oil Yield and Uranium Content of Black Shales, estimates the "amount of uranium in these shales is extremely large reckoned in billions of tons of metallic uranium". But although the uranium in those shales is abundant, they are not being mined for it because even richer uranium deposits have been located. While those hold out, shale that used to form sea bottoms is safe from miners of uranium.
Uranium is a heavy metal, yet it has not sunk into the earth's core, where lurk most of the planet's heavy metals. Uranium failed to sink because its atoms did not fit into the crystal structure of the mantle underlying the crust. It is an incompatible element. Instead, uranium is found almost entirely on the earth's crust--dispersed in rocks and earth and dissolved in sea water, usually at a concentration of about 3 parts per billion. While dissolved in sea water and while conditions favor oxidizing , a positive state, uranium forms complexes with other substances and remains in solution. But when conditions cause the negative state of reducing, uranium drops out. Like a disaffected teen it is attracted to negative places.
Fast-moving water as in streams tends to contain a lot of oxygen whereas stagnant water, or water at the bottom of an ocean, contains less. Ocean floors have long been repositories of deceased life forms (even before Dexter). Bacteria that cause these bodies to decay consume oxygen and its level declines even more. Being low in oxygen, organic matter attracts and concentrates, among other things, dissolved uranium. When conditions are right, this brine-soaked clay coalesces into shale even as its carbon-based life forms convert into hydrocarbons. The richer in organic matter it is the darker it gets, producing marine black shale. What makes that shale black also makes it hot.
Just after the energy crisis in the early 70s the Department of Energy mapped out the extent of that shale gas in the US. They knew it existed and that it was huge, but as it was unattainable it was considered a "resource", something tantalizingly out of our reach. Until now, when higher prices and the new technology of fracking turned the lowly resource into an abundant, and attainable, "reserve".
There is a correlation between high gamma-ray response and total gas content in highly organic shales, so higher-than-normal radioactivity in shale means more gas-production potential. Although the correlation is not perfect it is very high; in fact, geologists use radioactivity levels to find natural-gas formations.  Some maps show thickness of shale formations and some show which shales have higher-than-normal radioactive signatures. If you're looking for natural gas, use the latter map.