Oil Staining and Fluorence
Fluorescence is the light emission of material exposed to light of a shorter wave length and higher energy. Hydrocarbon benzol rings fluoresce when they are hit by ultraviolet light. This effect is used in hydrocarbon detection. Organic compounds without benzol ring compounds will not fluoresce.
Examination of mud, drill cuttings and cores for hydrocarbon fluorescence under ultraviolet light can indicate oil in small amounts. Any lithologies (Sandstone or Limestone) and not just those of the primary target reservoir should be checked for fluorescence.
Colors of fluorescence range from brown to green, gold orange blue, yellow or white. Typically lighter oils have lighter fluorescence. The distribution of the fluorescence may be spotted, patchy or dotty. The intensity may range from bright over dull and pale to faint.
| Weight | API Gravity | Color |
|---|---|---|
| Heaviest | 2 - 10 | Non Fluorescent - Dull Brown |
| 10 - 18 | Yellow Brown - Gold | |
| 18 - 45 | Pale Yellow - Gold | |
| Lightest | Over 45 | Blue White - White |
There are a few crude oils that do not fluoresce under ultraviolet light. They are usually of heavy gravity and are biodegraded. The biodegradation forms a double bonded molecule that does not fluoresce. However, on the addition of a solvent the bond is broken and an instant bright cut fluorescence is obtained. This is referred to as a solvent cut test and such a test is to be performed on a sample regardless if it fluoresces or not.