porosites

Porosities, in geology and related disciplines, refer to the fraction of a rock, soil, or other material’s volume that is occupied by void spaces or pores. Porosity is typically expressed as a decimal or percentage of the total rock volume. The pore spaces may be filled with fluids such as water, oil, or gas, and they can be interconnected or isolated. When pores are interconnected and permit fluid flow, this is called effective porosity; total porosity includes both interconnected and isolated pores.

Porosities are categorized as primary, formed during sediment deposition and lithification, and secondary, developed later through processes such as dissolution, fracturing, or diagenesis. Other descriptors include micro-porosity (tiny pores within grains), fracture porosity (networks of cracks), and vug porosity (larger voids produced by dissolution).

Measurement and estimation of porosity occur through laboratory methods on core samples, including fluid immersion or gas-displacement techniques, as well as non-destructive methods such as NMR, CT scanning, and mercury intrusion porosimetry. In the field, porosity estimates derive from well logs (density, neutron) and empirical relationships like Archie's law for clean sandstones.

Porosity is influenced by mineralogy, grain size, sorting, cementation, compaction, diagenesis, and dissolution. While porosity indicates a rock’s storage capacity for fluids, it does not by itself determine permeability; pore connectivity and size distribution are critical for fluid flow.

Porosity informs resource exploration, groundwater hydrology, CO2 sequestration, and environmental engineering. Rocks exhibit wide porosity ranges: well-sorted sandstones often possess noticeable porosity; carbonates and shales vary widely; igneous and metamorphic rocks tend to be less porous.