However,the surface or "stock tank'oil as finally sold by the producer is different from the liquid volume that existed underground. The difference is due to the changes in the oil properties as the pressure is decreased from high underground pressure and temperature to surface pressure and temperature.However, the surface or “stock tank” oil as finally sold by the producer is different from the liquid volume that existed underground. The difference is due to the changes in the oil properties as the pressure is decreased from high underground pressure and temperature to surface pressure and temperature