DEVONIAN SHALE

There is, however, yet another source of natural gas Devonian shale. Devonian is the name of the geological age some 350 million years ago when ferns and forests had just been invented (mammals were still 300 million years away). In those days, there was a mountain range west of the present Atlantic coast line, and rivers flowed westward to a great compound delta into the sea, which was then in the Ohio-Kentucky region, at times merging with the seas in the Michigan and Illinois basins. The delta was in what is now the Appalachian Basin, and the rivers deposited mud, silt and sand, which became Devonian shale; every year, the rivers would also wash organic material, mostly derived from plants, into this sea, and these sediments became deep brown layers called Brown shale within the Devonian shale. The couplets of dark (organic) and light (inorganic) shale are reminiscent of annual tree rings, and even more of the annual layers in Colorado and Wyoming oil shale, except that the organic layers in oil shale are kerogen, whereas Brown shale is coal like matter. The couplets in Devonian shale are very thin - there are as many as 46 to an inch of thickness.

This is what resulted from stratification in a stagnant condition, preventing the organic matter from being oxidized and destroyed. Over the next hundreds of millions of years, it was covered with about 3,000 ft of overburden and turned into coal; also, the carbohydron molecules, or some of them, were cracked in much the same way as in a refinery, until they were down to the simplest form methane, the main ingredient of natural gas. Almost all of it is trapped in the thin layers of Brown shale.

And it's still there in the Appalachian region, over an area of more than 200,000 square miles stretching from New York to Alabama. Natural gas is being produced from it in several places, mainly to both sides of the West Virginia Kentucky border. The total amount of readily recoverable gas is 15 to 25 trillion cf. of the order of the entire present annual US production (19.5 Tcf in 1976). And as always with a resource, there is more at a price.

Curiously, the Brown shale contains not only methane, but also uranium. The reasons are unknown, except that it got there at the time of the deposition, not later. The uranium content is only between 5 and 7 per mille, which is not good enough for commercial exploitation (as yet), but it does have the advantage of indicating the precise location of the Brown shale on gamma-ray logs of drilled wells.