Dim spot

In reflection seismology, a dim spot is a local low amplitude seismic attribute anomaly that can indicate the presence of hydrocarbons[1] and is therefore known as a direct hydrocarbon indicator. It primarily results from the decrease in acoustic impedance contrast when a hydrocarbon (with a low acoustic impedance) replaces the brine-saturated zone (with a high acoustic impedance) that underlies a shale (with the lowest acoustic impedance of the three), decreasing the reflection coefficient.

The diagram above shows the acoustic impedance relationship that results in a dim spot.

Occurrence

For a dim spot to occur, the shale has to have a lower acoustic impedance than both the water sand and the oil/gas sand, which is the opposite situation required for a bright spot to occur. This is possible because compaction causes the acoustic impedances of sands and shales to increase with age and depth but it does not happen uniformly – younger shales have a higher acoustic impedance than younger sands, but this reverses at depth, with older shales having a lower acoustic impedance than older sands.[2]

Similarly to bright spots, not all dim spots are caused by the presence of hydrocarbons and therefore they should not be treated as conclusive evidence hydrocarbon accumulations.

gollark: ARM is just a horrible mess because the vendors apparently don't want to agree on any standards and frequently just blatantly ignore the GPL.
gollark: Android is even breaking Termux and all that in android... 10 or whatever, I forgot their exact versioning scheme.
gollark: Phone software is so annoyingly annoying.
gollark: If it's bricked, it can't do much and you can probably fix it. If it's been haxxed, it can actively "work against you".
gollark: Probably hacked is worse.

See also

References
  1. Schlumberger: Oilfield Glossary
  2. Brown, Alistar. R., (2010), “Dim Spots in Seismic Images as Hydrocarbon Indicators”, AAPG Search and Discovery Article #40514.
  • Hilterman, Fred J., (2001), “Seismic Amplitude Interpretation”, SEG.
This article is issued from Wikipedia. The text is licensed under Creative Commons - Attribution - Sharealike. Additional terms may apply for the media files.