Changes

* Because many oils have a high fraction of saturate molecules<ref name=ch11r34 /> it is possible that over 50% of the mass of the oil and gas may be removed.

* Because many oils have a high fraction of saturate molecules<ref name=ch11r34 /> it is possible that over 50% of the mass of the oil and gas may be removed.

* Condensates and dry gases are also affected by biodegradation.<ref name=ch11r36>Walters, C. C., 1990, Organic geochemistry of gases and condensates from Block 551A High Island South Addition offshore Texas, Gulf of Mexico, in D. Schumacher, and B. F. Perkins, eds., Gulf Coast Oils and Gases—Their Characteristics, Origin, Distribution, and Exploration and Production Significance: Proceedings of the Ninth Annual Research conference, GCS-SEPM, October 1990, p. 185–198.</ref>

* Condensates and dry gases are also affected by biodegradation.<ref name=ch11r36>Walters, C. C., 1990, Organic geochemistry of gases and condensates from Block 551A High Island South Addition offshore Texas, Gulf of Mexico, in D. Schumacher, and B. F. Perkins, eds., Gulf Coast Oils and Gases—Their Characteristics, Origin, Distribution, and Exploration and Production Significance: Proceedings of the Ninth Annual Research conference, GCS-SEPM, October 1990, p. 185–198.</ref>

* Most biodegraded oils are characterized by higher viscosity and lower API gravity than unaltered petroleum, but biodegraded high-wax oils may have lower viscosity.

* Most biodegraded oils are characterized by higher viscosity and lower [[API gravity]] than unaltered petroleum, but biodegraded high-wax oils may have lower viscosity.

* Sulfur and nitrogen concentration increases in most biodegraded oils.

* Sulfur and nitrogen concentration increases in most biodegraded oils.

==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., 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 gravity|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 [[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.

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< 76¡C). Basin-peripheral tar sands may result from degrading of oil as the migration pathway intersects the surface. These indicate where and in which formation migration occurs. Look downdip from tar sands for possible productive accumulations on the migration pathway.

==See also==

==See also==