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Petroleum [[reservoir]]s may contain any of the three fluid phases—water (brine), oil, or gas. The initial distribution of phases depends on depth, [[Wikipedia:Temperature|temperature]], [[Wikipedia:Pressure|pressure]], composition, [[migration|historical migration]], type of geological [[trap]], and [[Geological heterogeneities|reservoir heterogeneity]] (that is, varying rock properties). The forces that originally distribute the fluids are [[gravity]], [[Capillary pressure|capillary]], [[molecular diffusion]], [[thermal convection]], and pressure gradients. It is generally assumed that reservoir fluids are in a static state when discovered or, more correctly, that fluids are moving at a very slow rate relative to the time required to extract the fluids (10 to 50 years). Clearly the fluids may still be in a dynamic state in terms of [[:file:Geological_time_spiral.png|geological time]].

Petroleum [[reservoir]]s may contain any of the three fluid phases—water (brine), oil, or gas. The initial distribution of phases depends on depth, [[Wikipedia:Temperature|temperature]], [[Wikipedia:Pressure|pressure]], composition, [[migration|historical migration]], type of geological [[trap]], and [[Geological heterogeneities|reservoir heterogeneity]] (that is, varying rock properties). The forces that originally distribute the fluids are [[gravity]], [[Capillary pressure|capillary]], [[molecular diffusion]], [[thermal convection]], and pressure gradients. It is generally assumed that reservoir fluids are in a static state when discovered or, more correctly, that fluids are moving at a very slow rate relative to the time required to extract the fluids (10 to 50 years). Clearly the fluids may still be in a dynamic state in terms of [[:file:Geological_time_spiral.png|geological time]].

Because gravity is the dominant force in distributing fluids through geological time, [[hydrocarbon]]s migrate upward and are trapped against impermeable cap rock. Gas overlies oil, which overlies water. However, because the reservoir pores are usually saturated completely by water before hydrocarbon [[migration]] and because capillary forces acting to retain water in the smallest pores exceed gravity forces, an initial ([[connate]]) [[water saturation]] will always be found in hydrocarbon-bearing formations. The connate water saturation may vary from 5 to 50% with the hydrocarbons still having sufficient mobility to produce at economical rates.

Because gravity is the dominant force in distributing fluids through geological time, [[hydrocarbon]]s migrate upward and are trapped against impermeable [http://www.oxforddictionaries.com/us/definition/american_english/cap-rock cap rock]. Gas overlies oil, which overlies water. However, because the reservoir pores are usually saturated completely by water before hydrocarbon [[migration]] and because capillary forces acting to retain water in the smallest pores exceed gravity forces, an initial ([[connate]]) [[water saturation]] will always be found in hydrocarbon-bearing formations. The connate water saturation may vary from 5 to 50% with the hydrocarbons still having sufficient mobility to produce at economical rates.

This article, along with the ''See also'' articles, reviews the physical and thermodynamic properties of gas, oil, and reservoir brine. As commonly done, the phase and volumetric behavior of petroleum reservoir fluids is referred to as ''PVT'' (pressure-volume-temperature). Two important general references on PVT are Katz et al.<ref name=pt10r18>Katz, D. L., 1959, Handbook of Natural Gas Engineering: New York, McGraw-Hill.</ref> and Society of Petroleum Engineers.<ref name=pt10r30>Society of Petroleum Engineers, 1981, Phase behavior: Dallas, TX, SPE Reprint Series No. 15.</ref>

This article, along with the ''See also'' articles, reviews the physical and thermodynamic properties of gas, oil, and reservoir brine. As commonly done, the phase and volumetric behavior of petroleum reservoir fluids is referred to as ''PVT'' (pressure-volume-temperature). Two important general references on PVT are Katz et al.<ref name=pt10r18>Katz, D. L., 1959, Handbook of Natural Gas Engineering: New York, McGraw-Hill.</ref> and Society of Petroleum Engineers.<ref name=pt10r30>Society of Petroleum Engineers, 1981, Phase behavior: Dallas, TX, SPE Reprint Series No. 15.</ref>