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Oil and gas in near-surface [[accumulation]]s and in seeps can be destroyed by three processes that may act concurrently:

Oil and gas in near-surface [[accumulation]]s and in seeps can be destroyed by three processes that may act concurrently:

* [http://www.oiltracers.com/services/exploration-geochemistry/oil-biodegradation.aspx Biodegradation]

* [[Biodegradation]]

* Water washing

* Water washing

* Devolatilization

* Devolatilization

==Biodegradation==

==Biodegradation==

Saturated fractions of oil and gas are readily biodegraded in the near-surface environment by a host of microbial communities; as [http://www.oiltracers.com/services/exploration-geochemistry/oil-biodegradation.aspx biodegradation] proceeds, other components of the oil can also be destroyed.<ref name=ch11r28>Palmer, S., 1991, [http://archives.datapages.com/data/specpubs/geochem1/data/a037/a037/0001/0000/0047.htm Effect of biodegradation and water washing on crude oil composition], in R. K. Merrill, ed., Source and Migration Processes and Evaluation Techniques: [http://store.aapg.org/detail.aspx?id=436 AAPG Treatise of Petroleum Geology—Handbook of Petroleum Geology No. 1], p. 47–54.</ref> These factors aid biodegradation:

Saturated fractions of oil and gas are readily biodegraded in the near-surface environment by a host of microbial communities; as [[biodegradation]] proceeds, other components of the oil can also be destroyed.<ref name=ch11r28>Palmer, S., 1991, [http://archives.datapages.com/data/specpubs/geochem1/data/a037/a037/0001/0000/0047.htm Effect of biodegradation and water washing on crude oil composition], in R. K. Merrill, ed., Source and Migration Processes and Evaluation Techniques: [http://store.aapg.org/detail.aspx?id=436 AAPG Treatise of Petroleum Geology—Handbook of Petroleum Geology No. 1], p. 47–54.</ref> These factors aid biodegradation:

* Availability of oxidant and nutrient

* Availability of oxidant and nutrient

[[file:predicting-preservation-and-destruction-of-accumulations_fig11-4.png|thumb|500px|{{figure number|1}}Whole-oil gas chromatographs of a heavily biodegraded oil (A) and its undegraded precursor (B) on an example from offshore Louisiana.]]

[[file:predicting-preservation-and-destruction-of-accumulations_fig11-4.png|thumb|500px|{{figure number|1}}Whole-oil gas chromatographs of a heavily biodegraded oil (A) and its undegraded precursor (B) on an example from offshore Louisiana.]]

The geochemical signatures of biodegradation are very distinctive. [[:file:predicting-preservation-and-destruction-of-accumulations_fig11-4.png|Figure 1]] shows whole-oil gas chromatographs of a heavily biodegraded oil (A) and its undegraded precursor (B) on an example from offshore Louisiana. Normal paraffins (sharp peaks in B) have been removed by bacterial action.

The geochemical signatures of [[biodegradation]] are very distinctive. [[:file:predicting-preservation-and-destruction-of-accumulations_fig11-4.png|Figure 1]] shows whole-oil gas chromatographs of a heavily biodegraded oil (A) and its undegraded precursor (B) on an example from offshore Louisiana. Normal paraffins (sharp peaks in B) have been removed by bacterial action.

==Predicting and recognizing biodegradation==

==Predicting and recognizing biodegradation==

The following characteristics can help us predict and recognize biodegradation.

The following characteristics can help us predict and recognize [[biodegradation]].

* Biodegradation occurs most rapidly in oil [[accumulation]]s exposed to active meteoric water circulation because the water supplies the oxidants or nutrients.

* Biodegradation occurs most rapidly in oil [[accumulation]]s exposed to active meteoric water circulation because the water supplies the oxidants or nutrients.

==Water washing==

==Water washing==

Water washing is the dissolution of light molecular species from oil and gas into water.<ref name=ch11r21>Lafargue, E., and C. Barker, 1988, [http://archives.datapages.com/data/bulletns/1988-89/data/pg/0072/0003/0250/0263.htm Effect of water washing on crude oil composition]: AAPG Bulletin, vol. 72, p. 263–276.</ref> Significant water washing requires rapid water flow under the [[accumulation]]. Light aromatic molecules are affected most severely. Severe water washing may remove at most 5–10% of the oil mass, so it does not lead to destruction of accumulations by itself. Water washing at shallow depths is usually accompanied by biodegradation and devolatilization.

Water washing is the dissolution of light molecular species from oil and gas into water.<ref name=ch11r21>Lafargue, E., and C. Barker, 1988, [http://archives.datapages.com/data/bulletns/1988-89/data/pg/0072/0003/0250/0263.htm Effect of water washing on crude oil composition]: AAPG Bulletin, vol. 72, p. 263–276.</ref> Significant water washing requires rapid water flow under the [[accumulation]]. Light aromatic molecules are affected most severely. Severe water washing may remove at most 5–10% of the oil mass, so it does not lead to destruction of accumulations by itself. Water washing at shallow depths is usually accompanied by [[biodegradation]] and devolatilization.

==Devolatilization==

==Devolatilization==

Kern River field (San Joaquin basin, California) is an [[accumulation]] of 4 billion bbl of original oil in place of 13°API, biodegraded, water washed, and devolatized oil at a subsurface depth of tens to hundreds of feet. The trap is a combination hydrodynamic/structural trap on the south and west sides<ref name=ch11r19>Kodl, E. J., J. C. Eacmen, and M. G. Coburn, 1990, A geologic update of the emplacement mechanism within the Kern River Formation at the Kern River field, in J. Kuespert, and S. Reid, eds., Structure, Stratigraphy, and Hydrocarbon Occurrences of the San Joaquin Basin California: Pacific Section SEPM Guidebook 64, p. 59–71.</ref> with stratigraphic trapping due to tarsealing and sand pinch-outs on the homoclinally dipping east side of the field.<ref name=ch11r27>Nicholson, G., 1980, Geology of the Kern River field, in Kern River Oilfield Field Trip: AAPG Pacific Section Guidebook, p. 7–17.</ref> Oil source is the same for undegraded, 34° oils farther downdip on the Bakersfield nose.

Kern River field (San Joaquin basin, California) is an [[accumulation]] of 4 billion bbl of original oil in place of 13°API, biodegraded, water washed, and devolatized oil at a subsurface depth of tens to hundreds of feet. The trap is a combination hydrodynamic/structural trap on the south and west sides<ref name=ch11r19>Kodl, E. J., J. C. Eacmen, and M. G. Coburn, 1990, A geologic update of the emplacement mechanism within the Kern River Formation at the Kern River field, in J. Kuespert, and S. Reid, eds., Structure, Stratigraphy, and Hydrocarbon Occurrences of the San Joaquin Basin California: Pacific Section SEPM Guidebook 64, p. 59–71.</ref> with stratigraphic trapping due to tarsealing and sand pinch-outs on the homoclinally dipping east side of the field.<ref name=ch11r27>Nicholson, G., 1980, Geology of the Kern River field, in Kern River Oilfield Field Trip: AAPG Pacific Section Guidebook, p. 7–17.</ref> Oil source is the same for undegraded, 34° oils farther downdip on the Bakersfield nose.

By assuming that asphaltene and resin volumes were just concentrated and not altered by near-surface processes, the amount of oil components lost in the near-surface environment can be calculated. An estimated 77% of the oil reaching the Kern River field was lost by near-surface processes, 92% of the saturates were lost, and 60% of the aromatics were lost. This means approximately 16 billion bbl of oil reached the vicinity of Kern River field, of which about 12 billion bbl were lost by near-surface processes as the field was charged.

By assuming that [[asphaltenes|asphaltene]] and resin volumes were just concentrated and not altered by near-surface processes, the amount of oil components lost in the near-surface environment can be calculated. An estimated 77% of the oil reaching the Kern River field was lost by near-surface processes, 92% of the saturates were lost, and 60% of the aromatics were lost. This means approximately 16 billion bbl of oil reached the vicinity of Kern River field, of which about 12 billion bbl were lost by near-surface processes as the field was charged.

==Predicting near-surface destruction==

==Predicting near-surface destruction==

Analyze low-gravity oils and bitumens to determine if the poor oil quality is due to biodegradation, maturation level, or source type. Water washing and biodegradation are usually associated with active aquifers, which can be determined from potentiometric maps. Temperature or geothermal gradient maps can outline parts of reservoir formations where biodegradation is likely to be active (T

Analyze low-gravity oils and bitumens to determine if the poor oil quality is due to [[biodegradation]], maturation level, or source type. Water washing and biodegradation are usually associated with active aquifers, which can be determined from potentiometric maps. Temperature or geothermal gradient maps can outline parts of reservoir formations where biodegradation is likely to be active (T

==See also==

==See also==