The contact surfaces in reservoir segregating fluids with different densities (normally between gas, oil and water) formed by gravity and capillary pressure.

Oil Water Contact (OWC)	The lowest elevation above aquifer at which mobile oil occur
Gas Oil Contact (GOC)	The highest elevation below gas reservoir at which mobile oil occur The reservoir above this point is characterised by gas inflow only despite the presence of connate oil.
Gas Water Contact (GWC)	The highest elevation below gas reservoir at which mobile water occur
Free Water Level (FWL)	The highest elevation at which the pressure of the hydrocarbon phase is the same as that of water. It marks the depth point in which the pressure gradient in water-saturated and oil-saturated reservoir meet.
HC-Water Transition	This interval is characterised by commingled hydrocarbon and water production from the same reservoir. The height of the interval may vary from dozens of meters to fraction of a meter depending on capillary pressure.

Fig. 1. Fluid contact schematic: gas in yellow, oil in green, hydrocarbon-water transition in blue and free water in aqua.

Usually the fluid contacts are relatively flat but varying reservoir properties may lead to inclination of the contact surface.

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