Unconventional Oil and Gas
Crude oil and natural gas that do not readily flow into a drilled well because the rock units in which they are contained either lack the porosity and permeability to allow the oil and gas to flow, or, the oil and gas are adsorbed onto or bound within the grains of the rock.
Unconventional oil and gas are produced from shale, tight sands, and coal beds, where liberating the oil and gas from the rock and moving it to the well are the challenges. These rock units must be stimulated by techniques such as hydraulic fracturing, horizontal drilling, steam flooding, injection of water, injection of carbon dioxide, or pressure reduction.
Unconventional oil and gas and conventional oil and gas do not differ in their chemical composition. They differ in the type of rock unit from which they are produced. Compare to "conventional oil and gas."