Upward movement of hydrocarbon-water contacts may leave a zone of [[residual saturation|residual hydrocarbon saturation]] below the present transition zone. The hydrocarbons might be interpreted as part of a transition zone from well log or [[Overview of routine core analysis#Residual fluid saturation|core analysis]], leading to an erroneous approximation of fluid contacts or rock properties. The presence of residual hydrocarbon saturation below an oil pool can sometimes be detected by the presence of two inflections in the plot of hydrocarbon saturation against depth, one at the present transition zone and another at the original transition zone. | Upward movement of hydrocarbon-water contacts may leave a zone of [[residual saturation|residual hydrocarbon saturation]] below the present transition zone. The hydrocarbons might be interpreted as part of a transition zone from well log or [[Overview of routine core analysis#Residual fluid saturation|core analysis]], leading to an erroneous approximation of fluid contacts or rock properties. The presence of residual hydrocarbon saturation below an oil pool can sometimes be detected by the presence of two inflections in the plot of hydrocarbon saturation against depth, one at the present transition zone and another at the original transition zone. |