Decades ago, geologists knew there were vast oil and natural gas resources locked in shale rock deep beneath the earth’s surface over much of North America. They just didn’t know that these hard-to-produce resources would one day become the pathway towards greater U.S. energy independence.
Shale is the underground source of a tremendous amount of the world’s oil and natural gas. Composed primarily of clay, quartz and organic material, shale is characterized by thin, parallel, horizontal layers. These layers are formed as cumulative deposits of sedimentary rock (sand, silt, mud, decaying plants and animals and other microorganisms) are compressed over long periods of time (millions of years), a process known as compaction. Compaction occurs when older sediments are covered by progressively younger sediments. Shale forms beneath bodies of very slow-moving water such as lakes, lagoons, river deltas and deeper portions of seas. Over time, these areas are not only compacted but also subjected to geologic forces that can uplift and bend them into layers of rock that reside on what is today dry land.
Petroleum & natural gas formation
Coal beds, sometimes called coal seams, are generally found at depths beneath aquifers but above most conventional oil and natural gas reservoirs. Coal beds contain methane, which is stored within the coal by a process called adsorption. It is in a near-liquid state inside the pores within the coal.
Oil and gas reservoirs can occur at any depth, and many drilling targets are reservoirs below 10,000 feet. The depth of organic materials and temperatures over time determine whether organic matter transforms into natural gas or oil. Anticline reservoirs slope downward on both sides from a common crest, and form as rocks are compressed by tectonic plate movement deep within the earth’s crust. The low density of gas and oil causes them to migrate to the highest parts of the folded rocky layers where they are trapped under a low-permeability rock layer. This would be one example of what geologists call “seals,” which effectively allow oil and gas to accumulate instead of escape upward.