Petrophysics (from the Greek πέτρα, petra, "rock" and φύσις, physis, "nature") is the study of physical and chemical rock properties and their interactions with fluids.

A major application of petrophysics is in studying reservoirs for the hydrocarbon industry. Petrophysicists work together with reservoir engineers and geoscientists to understand the porous media properties of the reservoir. Particularly how the pores are interconnected in the subsurface, controlling the accumulation and migration of hydrocarbons. Some fundamental petrophysical properties determined are lithology, porosity, water saturation, permeability, and capillary pressure.

The petrophysicists workflow measures and evaluates these petrophysical properties through well-log interpretation (i.e. in-situ reservoir conditions) and core analysis in the laboratory. During well perforation, different well-log tools are used to measure the petrophysical and mineralogical properties through radioactivity and seismic technologies in the borehole. In addition, core plugs are taken from the well as sidewall core or whole core samples. These studies are combined with geological, geophysical, and reservoir engineering studies to model the reservoir and determine its economic feasibility.

While most petrophysicists work in the hydrocarbon industry, some also work in the mining, water resources, geothermal energy, and carbon capture and storage industries. Petrophysics is part of the geosciences, and its studies are used by petroleum engineering, geology, geochemistry, exploration geophysics and others.

The following are the fundamental petrophysical properties used to characterize a reservoir:

The rock's mechanical or geomechanical properties are also used within petrophysics to determine the reservoir strength, elastic properties, hardness, ultrasonic behaviour, index characteristics and in situ stresses.

Petrophysicists use acoustic and density measurements of rocks to compute their mechanical properties and strength. They measure the compressional (P) wave velocity of sound through the rock and the shear (S) wave velocity and use these with the density of the rock to compute the rock's compressive strength, which is the compressive stress that causes a rock to fail, and the rocks' flexibility, which is the relationship between stress and deformation for a rock. Converted-wave analysis is also determines the subsurface lithology and porosity.

Geomechanics measurements are useful for drillability assessment, wellbore and open-hole stability design, log strength and stress correlations, and formation and strength characterization. These measurements are also used to design dams, roads, foundations for buildings, and many other large construction projects. They can also help interpret seismic signals from the Earth, either manufactured seismic signals or those from earthquakes.