Total Scanning Fluorescence Characteristics of Coals and Implication to Coal Rank Evaluation

Total Scanning Fluorescence (TSF), as a kind of new fluorescence technique, has great significance and wide application in identifying hydrocarbon inclusions of reservoirs, hydrocarbon migration pathways and palaeo-current oil-water interfaces. Total scanning fluorescence (TSF) is characterized by high efficiency, requiring low sample amount and high accuracy. Vitrinite reflectance (Ro) is one of significant parameters for determining coal ranks, it cannot only reflect coalification features, but also provide a favorable indicator for coal ranks. In order to establish a relationship between vitrinite reflectance (Ro) and the characteristic parameters derived from total scanning fluorescence (TSF), fourteen coal samples (coal powder without separating macerals) collected from Qinshui basin and Huaibei coalfield are tested by TSF technique and vitrinite reflectance (oil immersion), respectively. It shows that TSF parameters are related to vitrinite reflectance value, although TSF parameters and fluorescence intensity of coals differ in Qinshui basin and Huaibei coalfield. Research indicates that more factors should be taken into consideration for coal sample TSF testing in the future so as to obtain an empirical formula relationship between Ro and TSF parameters.


Introduction to Total Scanning Fluorescence (TSF) Technique
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.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 [1].Fluorescence analysis technique is significant in identifying hydrocarbon inclusion, hydrocarbon migration pathway, and palaeocurrent wateroil interface [2][3][4].It includes three items, quantitative grain fluorescence (QGF), quantitative grain fluorescenceextract (QGF-E), and total scanning fluorescence (TSF).TSF technique is used in this study.TSF which developed from 1980s, was used to test aromatic hydrocarbon and concentration.This method of obtaining three-dimensional fluorescence data is scanning emission spectra continuously at different excitation wavelength, then plotting fluorescence "fingerprints" in the form of Ex-Em contour or Ex-Em-IF contour using Surfer software.Conventional fluorescence analyses are traditionally used fixed emission/excitation wavelengths or fluorescence emission spectra (at a fixed excitation wavelength) to characterize aromatic mixtures qualitatively and quantitatively.Fixed wavelength and synchronous scanning fluorescence suffer from nonselectivity 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.Compared with the traditional fluorescence technique, total scanning fluorescence system has several advantages over simpler scanning methods: (1) the acquisition of multiple fluorescence spectra is faster; (2) the amount of fluorescence data per sample is greatly increased; (3) the stored data can be extensively manipulated by computer; (4) individual excitation spectrum can be retrieved from the total fluorescence spectrum and analyzed for the intensity and wavelength of maximum excitation and/or emission fluorescence [5,6].
Vitrinite reflectance is one of the significant parameters for determine coal ranks.It cannot only reflect coalification degree, but also provide a favorable indicator for coal ranks and oil maturity around the world.It is obvious that R o is an important part for coal rocks analysis, reservoir evaluation, and hydrocarbon system research.Early in 1960s, Former soviet and America have established national standard for R o testing, and International Standard Organization published R o testing standard (draft) in 1980s.So far, there have been at least ten kinds of methods for testing vitrinite reflectance [7].Most of vitrinite reflectance testing need to microscopic observation and statistics, so the results are always depending on the experiments operators' experiences.With the development of technique, the testing procedures are greatly simplified, and testing time is shortened obviously.However, some subjective factors of testing procedure are inevitable.This paper analyses the fluorescence characteristics of coal samples and manages to apply TSF technique into coal ranks testing in order to explore a fast, effective, and accurate method to evaluate coal ranks.

Samples and Experiments Procedures
Coal samples were collected from coal-bearing region of north China, Qinshui basin and Huaibei coalfield.From the perspective of structural geology, Qinshui basin is situated in the transitional area of regional tectonism and deep thermology action, where coal reservoir experienced moderately tectonic deformation and distinct magma-thermal action, thus the metamorphic degree of coal reservoir is strong and the deformational degree is comparatively weak [8][9][10].The tectonic deformation in Huaibei coalfield is comparatively strong, and magma hydrothermalism is active; deformation and metamorphism of coal in this area is strong.Based on the understanding on regional geology, fourteen coal samples are collected.
R o measurements (oil immersion) and TSF testing are conducted in the key laboratory of basin structure and hydrocarbon accumulation, CNPC.R o measurements (oilimmersion) are conducted according to following procedures.Firstly, representative coal samples were polished to coal section, and observed by using of oil-immersion objective of DETA V4000 microphotometer, and then over 50 points are counted for each sample, and calculated the even value as the R o value.
Total scanning fluorescence testing is conducted by Varian Cary-Eclipse of the key laboratory of basin structure and hydrocarbon accumulation, CNPC.Varian Cary-Eclipse is composed of spectrophotometer, special sample platform, color filter, data collector and QGF, QGF-E, TSF software package.The first step is coal sample crushing, then putting the crushed coal into agate mortar, grinding coal sample into uniform coal powder, then weighing 0.1 g by high-precision electronic balance; put the coal powder into 50 mL beaker, and pour into 10 mL dichloromethane; put the beaker into supersonic bath for 10 minutes; extracting the liquid into a glass bottle and tightly capped, put it in room temperature for a week so as to subside suspended matter of coal powders (see Table 1).Then, set up the experimental parameters, excitation wavelength range is 200-340 nm, and offset is 30 nm.Based on the experimental results, the TSF Max, Max Em, R1 and R2 can be calculated (see Table 2).The excitation and emission monochromators are simultaneously varied with the excitation wavelength offset 30 nm lower than the emission wavelength in synchronous methods.

Results and Discussions
The results of vitrinite reflectance testing and TSF analysis are summarized (see Table 1).R o of coal samples covers a large scope, varying from 0.86-4.66(%).And TSF parameters are obtained by analysis software, TSF Max is the maximum fluorescence intensity which can reflect the components of different coals; Max Ex represents the maximum excitation (Ex) wavelength; Max Em represents the maximum emission (Em) wavelength; the R1 parameter is a ratio between TSF emission intensity at 360 nm and 320 nm at an excitation wavelength of 270 nm, R2 parameter are the an ratio between TSF emission intensity at 360 nm and 320 nm at a excitation wavelength of 270 nm, R1 and R2 are closely related to the API and maturity.Fingerprints can be obtained by using surfer software (see Figures 1 and 2).The coal samples are characterized by different deformational degrees, they are cataclastic structure coal, mortar structure coal, wrinkle structure coal, scaling structure coal, and mylonitic structural coal, respectively [11,12].On the whole, the TSF parameters of coal samples are relevant to vitrinite reflectance values.TSF results show that TSF R1 increases with the decreasing of R o (this trend is contrary to the variation trend of oil with R o value from 0.55-2%) (see Figure 3).Ex wavelength of coal samples from Qinshui basin is increasing with TSF parameter R1 distinctly; Ex wavelength of coal samples in Huaibei coalfield is similar; however, the trend is not as distinct as the  one in Qinshui basin.This indicates that the difference of geological background of coal samples should be taken into consideration when discussing the relationship between vitrinite reflectance and TSF parameter R1.
Furthermore, fluorescence intensity of coal samples increases with TSF parameter R1 (there are three anomalous samples from Huaibei coalfield).Ex wavelength decreases with the increasing of R o (two samples from Huaibei coalfield are anomalous).Ex wavelength and Em wavelength of coal samples show good correlation, Ex wavelength of coal samples increases with Em wavelength coal samples.Overall, TSF parameters R1 and R2 also have favorable correlation, TSF parameter R1 increases with TSF parameter R2.These anomalous samples may be relevant to the macerals content of coal sample because the coal samples did not separate the vitrinite, inertinite, and exinite.And the deformation characteristics may be other influencing factor; therefore, samples for TSF testing should be collected based on the similar geological background.

Conclusions
Research shows that fluorescence characteristics parameters of coals are related to R o values of coals in the study area.Vitrinite reflectance value of coal samples increases with TSF R1 of coal samples in Qinshui basin, while the relationship between vitrinite reflectance of coal samples and TSF R1 in Huaibei coalfield is not clear; fluorescence intensity of coals increases with coal ranks, and excitation wavelength increases with TSF R1.
In summary, TSF technique can reflect the coal maturity at some extent.TSF parameters and vitrinite reflectance have a good correlation, and fluorescence intensity of coals is a significant parameter.TSF technique may provide a fast, effective method to determine coal ranks; however, more different type coal samples should be conducted by TSF experiments, meanwhile more factors such as macerals and geological background of coal samples should be considered before the experiments so as to establish empirical formulas between TSF parameters and vitrinite reflectance value of coals.

Figure 1 :
Figure 1: TSF hydrocarbon fingerprint map of coal samples in Qinshui basin.

Figure 2 :
Figure 2: TSF hydrocarbon fingerprint map of coal samples in Huaibei coalfield.

Figure 3 :
Figure 3: The relationship between R o and TSF parameters.

Table 1 :
Laboratory procedure to clean coal for TSF analysis.

Table 2 :
Characteristic parameters derived from TSF experiment.