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.
Mudstones can be extensively fractured - this blog looks at some characteristics of fractures inlcuding connected networks and plumose structures in the Cromlix Mudstone, near Stanley, Perthshire, UK.