Shale is a sedimentary rock – that is, rock formed over vast spans of time by the compression of sediments under water in a process known as “compaction.” Because calm conditions are required to permit the sediments to settle and accumulate undisturbed, sedimentary rock invariably forms on the bottoms of oceans, lakes or swamps.
A Muddy Beginning - Shale is principally a composite of many silicates commonly known as clay. The clay itself--an accumulation of microscopic particles formed from the decomposition of a mineral called feldspar--is a mud that, subjected to eons of unrelenting pressure from the water above it, transforms into stone, a process known as lithification.
Added Ingredients - Because the formation of shale--or of any other sedimentary rock, for that matter--takes many millions of years to accomplish, it is common for other minerals to settle and take up residence during the process. Professor Clive Neal of the Department of Civil Engineering and Geological Sciences at the University of Notre Dame calls those relative newcomers--typically quartz, mica and pyrite--"subordinate sand minerals." It is the presence of such foreign minerals and organic matter that gives shale its different colors--any of black, red or green.
Oil and a Peek at the Past - Neal says that shale is a "clastic"--meaning that it is made of of tiny fragments--sedimentary rock that is a potential source of oil because it can be rich in organic material. Geologists believe that the thick beds of shale under 17,000 square miles of northwestern Colorado may contain the largest untapped oil reserve in the world. Shale is the most common sedimentary rock, making up an estimated 60 percent of the class, and can be found virtually everywhere in the world. Because shale compacts in layers known as "laminations," it can be easily cracked apart into thin sheets. Those sheets can be a joy to naturalists, as they often contain the fossils of animals and plants that died many millions of years ago and whose remains sank to the ocean floor, to be gently covered and buried in the compacting clay.