Changes

  | part    = Predicting the occurrence of oil and gas traps

  | part    = Predicting the occurrence of oil and gas traps

  | chapter = Predicting preservation and destruction of accumulations

  | chapter = Predicting preservation and destruction of accumulations

  | frompg  = 11-1

  | frompg  = 11-11

  | topg    = 11-30

  | topg    = 11-11

  | author  = Alton A. Brown

  | author  = Alton A. Brown

  | link    = http://archives.datapages.com/data/specpubs/beaumont/ch11/ch11.htm

  | link    = http://archives.datapages.com/data/specpubs/beaumont/ch11/ch11.htm

  | isbn    = 0-89181-602-X

  | isbn    = 0-89181-602-X

}}

}}

Although the presence of a petroleum phase may retard [[diagenesis]] in an accumulation, sooner or later [[reservoir quality]] decreases with increasing age and burial.<ref name=ch11r5>Bloch, S., 1991, [http://archives.datapages.com/data/bulletns/1990-91/data/pg/0075/0007/0000/1145.htm Empirical prediction of porosity and permeability in sandstones]: AAPG Bulletin, vol. 75, p. 1145–1160.</ref>

Although the presence of a petroleum phase may retard [[diagenesis]] in an [[accumulation]], sooner or later [[reservoir quality]] decreases with increasing age and burial.<ref name=ch11r5>Bloch, S., 1991, [http://archives.datapages.com/data/bulletns/1990-91/data/pg/0075/0007/0000/1145.htm Empirical prediction of porosity and permeability in sandstones]: AAPG Bulletin, vol. 75, p. 1145–1160.</ref>

==The process==

==The process==

As the pore volume within a trap decreases due to [[burial cementation]], several phenomena combine to destroy the economic accumulation.

As the pore volume within a trap decreases due to [[burial cementation]], several phenomena combine to destroy the economic accumulation.

# Petroleum is displaced from the [[trap]] at the [[spill point]] as the pore volume within the structural closure decreases. If [[porosity]] is lowered sufficiently, the [[accumulation]] may be subeconomic in size.

# Reduced porosity may result in lower [[permeability]] so that production rates are subeconomic, even where economic quantities of petroleum are still trapped.

# As pore size decreases in a petroleum-filled reservoir, the [[capillary pressure]] of the petroleum phase must increase to occupy the pore spaces (assuming no change in [[wettability]]). In low-permeability [[tight gas reservoirs: evaluation|tight sands]] or carbonates, the capillary [[displacement pressure]] in the reservoir rock may approach that of moderate-quality seals. As a result, a lithology that could seal an accumulation at shallow depth may no longer be effective at deeper depths because it differs little from the reservoir rock.

| Petroleum is displaced from the [[trap]] at the [[spill point]] as the pore volume within the structural closure decreases. If [[porosity]] is lowered sufficiently, the accumulation may be subeconomic in size.

| Reduced porosity may result in lower [[permeability]] so that production rates are subeconomic, even where economic quantities of petroleum are still trapped.

| As pore size decreases in a petroleum-filled reservoir, the [[capillary pressure]] of the petroleum phase must increase to occupy the pore spaces (assuming no change in [[wettability]]). In low-permeability tight sands or carbonates, the capillary [[displacement pressure]] in the reservoir rock may approach that of moderate-quality seals. As a result, a lithology that could seal an accumulation at shallow depth may no longer be effective at deeper depths because it differs little from the reservoir rock.

==Predicting spillage by cementation==

==Predicting spillage by cementation==

Reduced porosity can be predicted from empirical or statistical evaluation of porosity data for reservoir intervals with similar composition and burial history as the prospect.<ref name=ch11r31>Schmoker, J., W., Gautier, D., L., 1988, Sandstone porosity as a function of thermal maturity: Geology, vol. 16, p. 1007–1010, DOI: [http://geology.gsapubs.org/content/16/11/1007.full.pdf 10.1130/0091-7613(1988)016<1007:SPAAFO>2.3.CO;2].</ref><ref name=ch11r5 /> In general, porosity decreases with increasing age, depth, and temperature. Numerical modeling techniques are not yet refined enough to quantitatively predict prospect reservoir quality loss.

Reduced porosity can be predicted from empirical or statistical evaluation of porosity data for reservoir intervals with similar composition and burial history as the prospect.<ref name=ch11r31>Schmoker, J. W., and D. L. Gautier, 1988, Sandstone porosity as a function of thermal maturity: Geology, vol. 16, p. 1007–1010, DOI: [http://geology.gsapubs.org/content/16/11/1007.full.pdf 10.1130/0091-7613(1988)016<1007:SPAAFO>2.3.CO;2].</ref><ref name=ch11r5 /> In general, porosity decreases with increasing age, depth, and temperature. Numerical modeling techniques are not yet refined enough to quantitatively predict prospect reservoir quality loss.

==See also==

==See also==

* [[Trap spillage]]

* [[Trap spillage]]

* [[Changes in trapping geometry]]

* [[Changes in trapping geometry]]

* [[Changes in hydrodynamic configuration]]

* [[Hydrodynamics: change in configuration]]

* [[Consequences of spillage]]

==References==

==References==

[[Category:Predicting the occurrence of oil and gas traps]]  

[[Category:Predicting the occurrence of oil and gas traps]]  

[[Category:Predicting preservation and destruction of accumulations]]

[[Category:Predicting preservation and destruction of accumulations]]