Changes

[[file:Geological_time_spiral.png|thumb|400px|The geologic time spiral.<ref name=USGS_2008>United States Geological Survey, 2008, Joseph Graham, William Newman, and John Stacy, [http://pubs.usgs.gov/gip/2008/58/ The geologic time spiral—A path to the past] (ver. 1.1): U.S. Geological Survey General Information Product 58.</ref>]]

[[file:Geological_time_spiral.png|thumb|400px|The geologic time spiral.<ref name=USGS_2008>United States Geological Survey, 2008, Joseph Graham, William Newman, and John Stacy, [http://pubs.usgs.gov/gip/2008/58/ The geologic time spiral—A path to the past] (ver. 1.1): U.S. Geological Survey General Information Product 58.</ref>]]

Petroleum reservoirs may contain any of the three fluid phases—water ([[brine]]), [[Oil as an energy source|oil]], or [[natural gas|gas]]. The initial distribution of phases depends on [[depth]], [[temperature]], [[pressure]], [[composition]], [[migration|historical migration]], type of [[geological trap]], and [[reservoir heterogeneity]] (that is, varying rock properties). The forces that originally distribute the fluids are gravity, 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 reservoirs may contain any of the three fluid phases—water ([[brine]]), [[Oil as an energy source|oil]], or [[natural gas|gas]]. The initial distribution of phases depends on depth, [[temperature]], [[pressure]], [[composition]], [[migration|historical migration]], type of geological [[trap]], and [[reservoir heterogeneity]] (that is, varying rock properties). The forces that originally distribute the fluids are gravity, 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, hydrocarbons 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, hydrocarbons 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.