Fluid contacts are input in the SOLUTION section, under the keyword EQUIL (meaning Equilibration). Read the keyword details in the Eclipse manual. In addition, if you have input Capillary Pressure in the PROPS section, then Eclipse, or any other simulator will use this data (plus the fluid contacts) to set the initial fluid saturations in the model.

In a Capillary-Gravity Equilibrium initialization, the required input data to set the fluid distribution initially is: Pcow, Pcgo, fluid densities, and the fluid contacts.

Of course capillary pressure might or might not be included depending on whether capillary is important or not in your particular case, for example, Pcgo is frequently neglected since the transition zone between the gas zone and the oil zone is small due to the large density difference between gases and liquids, furthermore in high permeability gas reservoirs capillarity can be readily dismissed due to the combination of low capillary forces (due to large pore throats) plus large density difference between gas and water. However, in heavy oil reservoirs (API gravities close to 10) for instance, capillarity is often important due to the small density difference between those oils and water, this leads to significant transition zones.