ABSTRACT: Characterizing Fluid Inclusion Oils via UV Fluorescence Microspectrophotometry--A Method for Projecting Oil Quality and Constraining Oil Migration History

T. F. Tsui

Oil-bearing and gas-bearing inclusions occur in diagenetic minerals (calcite, dolomite, and quartz) cementing or lining pores or fractures. Such inclusions are microscopic samples of oil and gas trapped during the growth of cements from hydrocarbon-bearing aqueous fluids migrating through the fractures and pores. Analysis of the inclusion oils in cements from outcrop, cutting, and core samples allows inferences about hypothetical reservoired oils. Comparison between inclusion oils and reservoir oils can likewise be useful in reconstructing the history of oil migration in a basin.

Oil fluoresces when irradiated by ultraviolet light. Fluorescence colors change with increasing oil gravity from dull brown to yellow, to white, to blue. Ultraviolet (UV) fluorescence microspectrophotometry, using the wavelength of 366 nm for excitation, is a method of characterizing the fluorescence color of oil in individual micron-sized inclusions. The empirical relationship between UV fluorescence and API gravity for a variety of cogenetic produced crude oils from a basin provides the calibration required to interpret the UV fluorescence of the oil inclusions in that basin. This technique is a nondestructive means of estimating gross properties (density or API gravity, and some geochemical characteristics) of inclusion oils.

Such correlations were used to estimate the API gravities of fluid inclusion oils within sedimentary rocks from the western United States, North Sea, and Alaska. Comparisons between inclusion oils in subsurface samples and oils from drill-stem tests led to recognition of a single migration episode in the Viking graben of the North Sea and, possibly, two migration episodes in the North Slope of Alaska. Also, analysis of inclusion oils in outcrop samples allowed estimation of potential oil quality in an incompletely explored basin in the western United States example.

Definitive determination of organic geochemistry of inclusion oils can be potentially obtained by using multiple wavelengths for excitation. This technique, however, is largely untested. Until it is readily available, fluorescence under one excitation wavelength offers a means for rapid preliminary determination of the quality of inclusion oils and inferences about reservoired oils.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990