With few exceptions, petroleum reservoirs are water-wet - that is, the oil is not in contact with the rock grains because they are coated with a film of water. Most oil fields have 50% to 80% maximum oil saturation. Above 80% oil saturation, the oil can be produced with very little water mixed in; below 10%, the oil is not recoverable.
A hydrocarbon reservoir is divided into two or more zones. If only oil and water are present, the oil occupies the upper zone. Although water still lines the pores, this is the zone in which maximum oil saturation occurs. The oil zone is underlain by water along the oil-water contact, which is not a sharp line but a transition zone usually many feet thick. Oil saturation increases gradually from near 0% at the base of this zone to 50% to 80% at the top. The region of maximum oil saturation extends from the top of the transition to the top of the reservoir.
Natural gas is present in nearly all hydrocarbon reservoirs, dissolved in the oil (as solution gas) and to some extent in the water. In many situations, however, reservoir conditions and gas saturation allow undissolved gas (associated free gas) to accumulate above the oil zone as a gas cap. The wetting fluid in a gas cap is usually water but occasionally oil. The transition zone between oil and gas (the gas-oil contact) is thinner than the oil-water contact zone because of the greater difference in density and surface tension between gas and oil.
Free methane, the lightest hydrocarbon, remains in a gaseous state even under great pressure. Ethane, propane, and butane, which are gases at surface pressure and temperature, are often found in a liquid state under reservoir conditions.
Some reservoirs contain gas but not oil. This gas is called non-associated gas. The transition zone is a gas-water contact. The water almost always contains gas in solution, and water lines the pores in the gas zone.
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