6.3.2.3 Fluorescence

There are numerous fluorescent compounds in crude petroleum. A number of them will fluoresce in the visible spectrum. Generally, dense, low gravity oils will fluoresce at the longer, infrared, red and orange visible wavelengths, while the lighter, high gravity oils will fluoresce at shorter, yellow or blue, visible wavelengths. Unfortunately, crude oil is not the only material found in cuttings that will fluoresce under ultraviolet light. Some minerals and sample lithologies will also fluoresce. The fluorescence colors emitted are dark and the intensity is low. Below is a table of fluorescence of some common minerals and lithologies.

Table 2. Common Minerals and Lithologies and Their Fluorescence Color

Some mud additives may also exhibit traces of fluorescence. Pipe dope, the heavy metalized grease used to lubricate and seal threaded tool joints of drill pipe and drill collars, is a source of fluorescence contamination. Pipe dope has a very bright gold, white or bluish-white fluorescence normally indicative of a light, high gravity oil or condensate. In natural light, it has a heavy, viscous appearance and a blue-black or brown metallic color, while high gravity oil or condensate is transparent and gold in natural light.

A reservoir containing heavy, low gravity oil might be passed over because bright white fluorescence of pipe dope might mask the darker color of the oil. In addition, low gravity crude has is dense and dark brown in natural light. It is important to check all samples under ultraviolet light, regardless of whether oil is suspected.

Examination of mud, drill cuttings and cores for hydrocarbon fluorescence under ultraviolet light often indicates oil in small amounts, or oil of light color which might not be detected by other means. All samples should be examined. Color of fluorescence of crude ranges from brown through green, gold, blue, yellow, to white; in most instances, the heavier oils have darker fluorescence. Distribution may be even, spotted, or mottled, as for stain. The intensity range is bright, dull, pale, and faint. Pinpoint fluorescence is associated with individual sand grains and may indicate condensate or gas. Mineral fluorescence, especially from shell fragments, may be mistaken for oil fluorescence, and is distinguished by adding a few drops of a solvent.

Hydrocarbon fluorescence will appear to flow and diffuse in the solvent as the oil dissolves, whereas mineral fluorescence will remain undisturbed.