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.
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 [3], 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. [4] 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.
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