There are 2 basic types of natural fractures that need to be studied when characterising naturally fractured reservoirs - whether it be a geothermal or hydrocarbon reservoir. These are extension and shear fractures.
Fractured reservoirs are often dual porosity systems where the fractures provide an additional porosity to the matrix. Nelson provides a valuable classification of fractured reservoirs based upon relative contribution of matrix and fractures to porosity and permeability.