Unconventional reservoirs (UNC) are considered those that do not produce at economic flow rates and cannot be cost-effectively produced without applying stimulation, fracturing, and recovery. They are located in predominantly extensive regional accumulations, which, in most cases, is independent of the stratigraphic and structural traps. This requires using special technology for extraction, either by its oil properties or the characteristics of the rock that contains it.
Today, these reservoirs represent an interesting source of income, because many of them are found in deposits that were considered to be exhausted or non-economic by traditional recovery methods, and it is estimated that they are present in large volumes. The recently exploited shale plays are typically constituted by a matrix of very fine grain rock (size clay, shale or marl might be), with varying proportions of clay, silica, and carbonate, which act as source rock, and reservoir seals at the same time. They have very low permeability and often require massive stimulation to produce hydrocarbon.
Generally, resource shale reservoirs must meet a series of requirements to make them economically viable. These conditions are:
Organic richness (> 2% COT for shale gas and shale oil variable)
Thermal maturity (> 0.7% Ro)
Thickness (> 30 m) and areal extent
Adsorption capacity (mainly in shale gas)
Fracturability (clay content < 40%)