Fraction of pore volume occupied by oil at any location in a reservoir at any time during its life. Remaining oil saturation has no petrophysical significance beyond an assessment of the amount of oil remaining in the rock. Considered a snapshot of the reservoir, this assessment depends on the location in the reservoir and time of the measurement. This saturation is measured without regard to the displacement process or processes that produced it.
Remaining oil saturation (ROS) may be calculated from 1 − Sxo, where Sxo is the invaded zone, or flushed zone, water saturation.