INTRODUCTION

Modern well-logging methods locate petroleum-bearing formations without the aid of odor, taste, and visual observations of cores, or drill cutting; however, these methods are occasionally useful.

Rock samples are analyzed using a petrographic binocular microscope to qualitatively provide a description of the core materials along with any indication of oil or gas.

Fresh samples can indicate the presence of hydrocarbons by a distinctive aromatic odor, indicating that more careful analyses of other samples or logs from the zone should be made.

Crushed samples of the cores, or cuttings, can be extracted with a light solvent such as pentane, carbon tetrachloride, or ether. The extract is then examined visually for change of color and for fluorescence to provide another qualitative indication of the presence of petroleum in the formation.

Fluorescence of the samples, or a vial of the solvent extract of samples, is examined in a box under ultraviolet light. The aromatic ring compounds in crude oils fluoresce with a white to yellow glow. This property has been proven to be a very sensitive indicator of the presence of crude oil in rocks (fresh, and aged in storage), muds, and solvents. It also has been used for the qualitative analyses of lighter fractions of oils and lubricants.

Ultraviolet light has a higher frequency than visible light and, therefore, is just outside of the visible light spectrum. Under ultraviolet light, however, many molecules absorb some of the ultraviolet light energy, causing their electrons to jump to a higher energy level orbit.