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Oil and gas in near-surface accumulations 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:

* Biodegradation

* 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 accumulations 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.

* Because biodegradation apparently does not significantly affect asphaltenes and many high-molecular-weight aromatics, severe biodegradation does not destroy the oil entirely.

* Because biodegradation apparently does not significantly affect asphaltenes and many high-molecular-weight aromatics, severe biodegradation does not destroy the oil entirely.

* For aromatic oils, biodegradation results in loss of only 10–20% of the mass of the oil.<ref name=ch11r14>Horstad, I., Larter, S., Mills, N., 1992, A quantitative model of biological petroleum degradation within the Brent Group reservoir in the Gullfaks field, Norwegian North Sea: Organic Geochemistry, vol. 19, nos. 1–3, p. 107–117., 10., 1016/0146-6380(92)90030-2</ref>

* For aromatic oils, biodegradation results in loss of only 10–20% of the mass of the oil.<ref name=ch11r14>Horstad, I., Larter, S., Mills, N., 1992, A quantitative model of biological petroleum degradation within the Brent Group reservoir in the Gullfaks field, Norwegian North Sea: Organic Geochemistry, vol. 19, nos. 1–3, p. 107–117., 10., 1016/0146-6380(92)90030-2</ref>

==Water washing==

==Water washing==

Water washing is the dissolution of light molecular species from oil and gas into water.<ref name=ch11r21>Lafargue, E., Barker, C., 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., Barker, C., 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==

==Example: near-surface loss==

==Example: near-surface loss==

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., Eacmen, J., C., Coburn, M., G., 1990, A geologic update of the emplacement mechanism within the Kern River Formation at the Kern River field, in Kuespert, J., Reid, S., 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., Eacmen, J., C., Coburn, M., G., 1990, A geologic update of the emplacement mechanism within the Kern River Formation at the Kern River field, in Kuespert, J., Reid, S., 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 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.