Oil-water contact (OWC) – surface separating oil deposit or gas deposit oil rim from contacting confined strata water in the reservoir. There is no clear-cut boundary between water and oil; there is always a transitional zone or inter-penetration zone in OWC. It may be of different thickness (from parts of meter up to 10–15 meters) depending on the height of water capillary rise, on collecting properties of water-bearing and oil-bearing deposits, as well as on physical and chemical parameters of water and oil. OWC surface has very complicated morphology and only conventionally may be considered horizontally flat. As a rule, in case of strata waters pressure gradient, OWC is angled in the direction of pressure decrease. OWC position is defined by well testing, by a set of geophysical methods or by calculations based on strata water and oil pressure and density measurements. OWC position is normally calculated from wellhead or in absolute values from sub-sea.