Predicting preservation and destruction of accumulations
| 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 |
Some petroleum accumulations are likely to persist for hundreds of millions of years with relatively little alteration or dilution. Other accumulations, however, may be destroyed. Several processes deplete or destroy hydrocarbon accumulations. In many prospects it is not enough just to know that a trap is present in a basin where hydrocarbons were generated and migrated. We also must know that the trap was preserved over time. It is imperative that explorationists know destructive processes and how to determine the age of an accumulation.
The problem
Petroleum may have accumulated at a prospect sometime in the past but may not be preserved in economic quantities, even where trapping geometry is still intact. Or a trap may still contain petroleum, but the [oil and/or gas has been diluted or altered so that accumulations are no longer economic.
From an explorationist's point of view, these accumulations are destroyed. The problem is to determine where destruction of accumulations is likely and what mechanisms are likely to lead to destruction of accumulations in different geological settings.
Destructive processes
Petroleum can be destroyed as a result of the following processes, each of which is discussed in this chapter.
| Process | Description |
|---|---|
| Spillage | Trapping geometry changes so petroleum spills below the sealing lithology. |
| Leakage | Lack of integrity of sealing lithology allows petroleum to leak through the seal. |
| Destruction | Petroleum is destroyed, altered, or diluted with nonhydrocarbon gases. |
| Cementation | Reservoir quality drops below economic limits. |
Determining age of accumulations
Accumulations should be dated to evaluate the potential importance of accumulation destruction in an area of interest. Leakage, spillage, petroleum destruction, and cementation are more likely to alter the size and quality of old accumulations than young accumulations. Young accumulations with active petroleum charge are more likely to be affected by displacement of oil by later gas charge. Accumulation preservation is a function of tectonic setting, trap type, depth of burial, and seal type.[1]
Three methods help us determine the age of accumulations.
- Dating the generation of the trapped hydrocarbons
- Dating the formation of the reservoir, seal, and trap
- Directly measuring entrapment by radiometric means
In dating the generation of the trapped hydrocarbons, we use geohistory models to determine when the oil or gas charged an accumulation. If the migration distance is short, this date is an estimate of the age of the accumulation. Oil and gas may remigrate later due to structural growth, so these dates may overestimate the true age of the accumulation. For example, by this approach Sho-Vel-Tum trend oil fields in southern Oklahoma accumulated from the Atokan (early Pennsylvanian), when generation began in the Ardmore basin, to Permian, when oil generation ended in the Ardmore and Anadarko basins. Because no significant tectonic events have changed the structure of these fields since trapping, these accumulations are at least length::250 m.y. old—maybe as old as length::300 m.y.
In dating the formation of the reservoir, seal, and trap, the age of an accumulation can be no older than the age of the reservoir, seal, or trapping geometry. For example, offshore Gulf of Mexico accumulations in Pleistocene reservoirs can be no older than the Pleistocene.
Direct radiometric measurements are difficult to perform because most reservoirs do not have datable material that formed during charging. The Groningen gas field (Permian, the Netherlands) has been dated as pre-Late Jurassic by the retardation effect of gas on radiometrically datable illite cements in the reservoir.[2]
References
- ↑ Mcgregor, D. S., 1996, Factors controlling the destruction or preservation of giant light oilfields: Petroleum Geoscience, vol. 2, p. 197–217., 10., 1144/petgeo., 2., 3., 197
- ↑ Lee, M., J. L. Aronson, and S. M. Savin, 1985, K/Ar dating of time of gas emplacement in Rotliegendes sandstone, Netherlands: AAPG Bulletin, vol. 69, p. 1381–1385.
See also
External links
| find literature about Predicting preservation and destruction of accumulations |
|
|
|
|