Near-surface destruction
| Exploring for Oil and Gas Traps | |
| |
| Series | Treatise in Petroleum Geology |
|---|---|
| Part | Predicting the occurrence of oil and gas traps |
| Chapter | Predicting preservation and destruction of accumulations |
| Author | Alton A. Brown |
| Link | Web page |
| Store | AAPG Store |
Oil and gas in near-surface accumulations and in seeps can be destroyed by three processes that may act concurrently:
- Biodegradation
- Water washing
- 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.
Biodegradation[edit]
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.[1] These factors aid biodegradation:
- Availability of oxidant and nutrient
- Inoculation of the reservoir by a microbial community that can degrade the oil
- Temperature below approximately temperature::170°F[2]
Geochemical signatures of biodegradation[edit]
The geochemical signatures of biodegradation are very distinctive. 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[edit]
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.
- 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.[3]
- Because many oils have a high fraction of saturate molecules[2] 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.[4]
- 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.
Water washing[edit]
Water washing is the dissolution of light molecular species from oil and gas into water.[5] 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[edit]
Where the reservoir is exhumed or where the seal is breached near the surface, light molecular species will evaporate into the atmosphere due to their high vapor pressure. The volatile hydrocarbons are presumably oxidized in the atmosphere. This is devolatilization. The process selectively strips the oil of components up to a carbon number of about 15 or so. This process can account for destruction of up to 50% of the mass of the oil and essentially all gas and condensate. Most devolatilized oils have viscosity so high that conventional recovery may be uneconomic.
Example: near-surface loss[edit]
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[6] with stratigraphic trapping due to tarsealing and sand pinch-outs on the homoclinally dipping east side of the field.[7] 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.
Predicting near-surface destruction[edit]
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[edit]
References[edit]
- ↑ Palmer, S., 1991, Effect of biodegradation and water washing on crude oil composition, in R. K. Merrill, ed., Source and Migration Processes and Evaluation Techniques: AAPG Treatise of Petroleum Geology—Handbook of Petroleum Geology No. 1, p. 47–54.
- ↑ 2.0 2.1 Tissot, B. P., and D. H. Welte, 1984, Petroleum Formation and Occurrence, 2 ed.: New York, Springer-Verlag, 699 p.
- ↑ Horstad, I., S. Larter, and N. Mills, 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
- ↑ 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.
- ↑ Lafargue, E., and C. Barker, 1988, Effect of water washing on crude oil composition: AAPG Bulletin, vol. 72, p. 263–276.
- ↑ 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.
- ↑ Nicholson, G., 1980, Geology of the Kern River field, in Kern River Oilfield Field Trip: AAPG Pacific Section Guidebook, p. 7–17.
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