Volcanic rocks develop primary and secondary porosity and permeability, depending on both their lithology and the sequence of processes involved in their formation. Primary processes (welding, deuteric crystal dissolution, gas release, flow fragmentation, and crystal shattering) may lead to high porosity and permeability, the best example of which is a nonwelded ignimbrite with well-developed gas-pipe zones. Secondary processes (different types of alteration), however, tend to decrease primary porosity. However, certain secondary processes, such as dissolution and hydraulic fracturing, may contribute to enhance total porosity and permeability. These conclusions were developed through a systematic review of reservoir quality in volcanic rocks, integrating lithology and process interpretation with petrophysical data. The said information was taken from selected cores of volcanic rocks from the Serie Tobífera unit in the Austral Basin and the Precuyano unit in the Neuquén Basin, Argentina.
A clear understanding of both primary and secondary processes may serve to predict the quality of volcanic reservoirs and could be used as a guide for oil and gas exploration and development in many parts of the world.