Applications of Total Scanning Fluorescence to Exploration Geochemistry

ABSTRACT A total scanning fluorescence technique is described for correlation (oil/oil and oil/source rock) and surface geochemical prospecting studies. The fluorescence system acquires a total fluorescence spectrum of emission, excitation and intensity for wavelengths between 200 and 800 nm using a computer controlled UV-Spectrofluorometer (Perkin-Elmer 650-40). The resulting matrix of intensity values, obtained at specific emission/excitation wavelengths, can be viewed in a three-dimensional or contour presentation. Similarity indices are calculated to compare spectra for correlation studies. The method can also be used as a regional evaluation tool for surface geochemical prospecting. Shallow surficial (>2 meters) sediments collected over oil, condensate, and gas provinces exhibit distinct fluorescence signatures. INTRODUCTION Fluorescence spectroscopy is a technique that has had wide application in characterizing hydrocarbon mixtures. Ultraviolet (UV) fluorescence is inherently more selective for aromatic compounds than conventional absorption measurements and at least an order of magnitude more sensitive. Fluorescence methods are particularly useful for the detection and measurement of organic compounds containing one or more aromatic functional groups. A number of workers have used fluorescence techniques to estimate petroleum hydrocarbons in marine waters 1, 2,3,4,5 and sediments. 6, 7 Since all oils contain a significant amount of aromatic compounds, with one to four (or more) aromatic rings and their alkylated analogues, oils exhibit distinctive fluorescence "fingerprints". These "fingerprints", used in conjunction with other analyses, can provide significant information for typing oils, shale extracts and sea bottom sediment extracts. Conventional fluorescence analyses have traditionally used fixed emission/excitation wavelengths or fluorescence emission spectra (at a fixed excitation wavelength) to characterize aromatic mixtures, qualitatively and quantitatively. Gordon et a1. 2 and Wakeham7 used a synchronous scanning technique developed by Lloyd8 to "fingerprint" aromatic compounds in marine sediments. The excitation and emission monochromators are simultaneously varied with the excitation wavelength offset 20-30 nm lower than the emission wavelength in synchronous methods. This technique was an improvement over simple emission spectroscopy for analyzing complex mixtures because larger ring number aromatic compounds generally fluoresce at successively higher excitation wavelengths. Synchronous scanning fluorescence approximates ring size distributions for aromatic mixtures. Fixed wavelength and synchronous scanning fluorescence suffer from non-selectivity and are generally ineffective in structural elucidation of mixtures. Despite the ability to select both the excitation and emission wavelengths, conventional fluorescence methods have limited applicability and are difficult to interpret because spectra of complex mixtures cannot be satisfactorily resolved. In an attempt to overcome these problems, a methodology for total scanning fluorescence was developed. A three-dimensional, contour and tabular presentation of the data is possible. A total scanning fluorescence system has several advantages over simpler scanning methods:the acquisition of multiple fluorescence spectra is faster;the amount of fluorescence data per sample is greatly increased;the stored data can be extensively manipulated by computer; andindividual excitation spectrum can be retrieved from the total fluorescence spectrum and analyzed for the intensity and wavelength of maximum excitation and/ or emission fluorescence