Changes

* Devolatilization

* Devolatilization

The solid fraction of oil unaffected by these processes ultimately is recycled in the erosional regime. Because all three processes result in oil with higher viscosity sulfur, and nitrogen, the processes may reduce the economic value of the accumulation before the accumulation is actually destroyed.

The solid fraction of oil unaffected by these processes ultimately is recycled in the erosional regime. Because all three processes result in oil with higher [[viscosity]], [[sulfur]], and [[nitrogen]], the processes may reduce the economic value of the accumulation before the accumulation is actually destroyed.

==Biodegradation==

==Biodegradation==

* 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.

==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< 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.

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==

[[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]]