Porosity and Permeability in Sedimentary Rocks

Porosity and permeability are related properties of any rock or loose sediment. Porosity and permeability are absolutely necessary to make a productive oil or gas well.

Porosity of a rock is a measure of its ability to hold a fluid. Mathematically, porosity is the open space in a rock divided by the total rock volume (solid + space or holes). Porosity is normally expressed as a pecentage of the total rock which is taken up by pore space. For example, a sandstone may have 8% porosity. This means 92% is solid rock and 8% is open space containing oil, gas, or water. 8% is about the minimum porosity that is required to make a decent oil well, though many poorer (and often non-economic) wells are completed with less porosity.

Permeability is a measure of the amount of flow of a fluid through a rock. If it takes a lot of pressure to squeeze fluid through a rock, that rock has "low permeability" or "low perm.". If fluid passes through the rock easily, it has "high permeability, or "high perm."

Most oil and gas is produced from sandstones. These rocks usually have high porosity and are "high perm." Both porosity and permeability are needed for production. Porosity creates the spaces to hold the oil or gas. Permeability allows the oil and gas to flow out of the rock.

Permeability in petroleum-producing rocks is usually expressed in units called millidarcys (one millidarcy is 1/1000 of a darcy). A petroleum reservoir may have permeability in the range of a few millidarcys up to several darcys. Most oil and gas reservoirs produce from rocks that have 10-100 millidarcys.