Rocks filled with heavy oil do not comply with established theories for porous media. Heavy oils demonstrate a blend of both purely viscous and purely elastic properties, also referred to as viscoelasticity. They have a non-negligible shear modulus that allows them to support shear-wave propagation depending on frequency and temperature. These oils behave as solids at high frequencies and low temperatures, and as fluids at low frequencies and high temperatures. The solid-like properties of heavy oils violate Gassmann's equation, the most common and widely used fluid-substitution technique in the industry.