Published 1996 as part of Memoir 66
Copyright © 1996 The American Association of Petroleum Geologists.  All Rights Reserved.

One method for detecting the presence of oil in shallow cores uses fluorescence of core extracts. Synchronous scanning of the extracts is performed using excitation wavelengths of 250-450 nm monitoring emission intensity over the same wavelengths. Several parameters are recorded that describe the fluorescence characteristics. First, the intensity of maximum emission is recorded along with the wavelengths at which this maximum was obtained: Max Ex is the excitation wavelength and Max Em is the emission wavelength. Second, a ratio (R₁) of emission intensity at 320 and 360 nm arising from excitation at 270 nm is recorded. To interpret these measurements in terms of oil density and maturity, fluorescence spectra have been compiled from about 130 oils representing a wide range of source rock types and thermal maturities from basins around the world. A large data set has been gathered for North Sea oils (about 56) which all have a marine siliciclastic source but vary greatly in thermal maturity. Several naturally biodegraded oils from the North Sea and other basins are included in the study. Bulk oil data and molecular parameter data have been acquired for many of the oils.

The prime effect on oil fluorescence characteristics is the thermal maturity of an oil. Oil fluorescence depends on the quantity and type of aromatics in oils, and since aromatics decrease with increasing thermal maturity, oil fluorescence intensity also decreases substantially. A light 50°-60° API oil from the North Sea is about 10 times less fluorescent than a 20°-30° API oil from the North Sea. There appears to be little effect of source type on oil fluorescence properties, but biodegradation may alter fluorescence. Shallow cores appear to have a constant background fluorescence signature with a Max Ex/Max Em of 320/370 nm. We attribute this to recent organic matter, but further work is needed to be certain of its origin. This background makes fluorescence an unreliable tool for detecting microseepage in shallow cores.