Natural gas is formed from the decomposition and pressurisation of algae, plankton and other organisms that were deposited many millions of years ago in swamps, lakes, river deltas, and seabeds. The gas is held in geological formations far below the Earth’s surface.
A reservoir is a subsurface pool of oil or gas contained in a rock formation.
Natural gas is found in several different types of rocks, including shales, sandstone and coal seams. Sandstone reservoirs include low-permeability rocks, or “tight gas” reservoirs.
Porosity and permeability are two most important characteristics of oil and gas reservoir rocks.
Porosity is a measure of a rock’s ability to hold oil or gas in tiny spaces within the rock. The more porous the rock, the more oil or gas it can hold.
Permeability is a measure of the interconnectivity between pores within a rock. It determines how well oil, gas or fluids to flow through the rock. High permeability makes it easier to extract oil or gas from a reservoir. Permeability can be artificially increased by applying hydraulic fracturing (“fracking”) to the reservoir.
Because shales have low permeability, shale gas wells are always hydraulically fractured (or “fracked”).