Water contact
Water contact is a term used in the hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....

 industry to describe the elevation above which fluids other than water can be found in the pore
Porosity or void fraction is a measure of the void spaces in a material, and is a fraction of the volume of voids over the total volume, between 0–1, or as a percentage between 0–100%...

s of a rock.

For example, in a traditional hand-excavated water well
Water well
A water well is an excavation or structure created in the ground by digging, driving, boring or drilling to access groundwater in underground aquifers. The well water is drawn by an electric submersible pump, a trash pump, a vertical turbine pump, a handpump or a mechanical pump...

, the level at which the water stabilizes represents the water table, or the elevation in the rock where air starts to occupy the rock pores.

In most situations in the hydrocarbon industry the term is qualified as being an oil-water contact (abbreviated to "OWC") or a gas-water contact ("GWC"). Often there is also a gas-oil contact ("GOC").

In an oil or gas field
Oil field
An oil field is a region with an abundance of oil wells extracting petroleum from below ground. Because the oil reservoirs typically extend over a large area, possibly several hundred kilometres across, full exploitation entails multiple wells scattered across the area...

, hydrocarbons migrate into rocks and can be trapped if there is a permeability
Permeability (fluid)
Permeability in fluid mechanics and the earth sciences is a measure of the ability of a porous material to allow fluids to pass through it.- Units :...

 barrier to prevent upward escape. Gas and oil are lighter than water, so they will form a bubble at the high end of the "trap" formed by the impermeable barriers. A simple physical model of this would be a coffee cup held upside down underwater with an air bubble occupying the highest portion of the cup's interior. The base of the bubble is the water contact.

Capillary action
Capillary action
Capillary action, or capilarity, is the ability of a liquid to flow against gravity where liquid spontanously rise in a narrow space such as between the hair of a paint-brush, in a thin tube, or in porous material such as paper or in some non-porous material such as liquified carbon fiber, or in a...

 can obscure the true water contact in permeable media like sandstone
Sandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains.Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow,...

. Capillary pressure prevents the hydrocarbons from expelling all of the water in the pores, which creates a transition zone between the fully saturated hydrocarbon levels and the fully saturated water levels.

In poorly porous intervals, the oil-water, gas-water or gas-oil contacts can similarly be obscured, which makes estimation of hydrocarbon reserves difficult. Descriptions of the well's petrophysics will then often further qualify to delineate a gas-down-to, oil-up-to, oil-down-to and water-up-to line, clearly showing the uncertainties involved.
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