Fluorescence microscopical rank studies on liptinites and vitrinites in peat and coals, and comparison with results of the rock-eval pyrolysis

Abstract

A series of peat and coal samples ranging in rank between the stages of brown coal and high-volatile bituminous A coal were studied for the relationships between vitrinite reflectance, fluorescence properties of different liptinite macerals and of huminite/vitrinite and results of elemental analyses and of the Rock-Eval pyrolysis. The coalification of liptinites is characterized by a decrease of fluorescence intensity and a red shift of fluorescence colour, both varying for the different liptinite macerals as well as for the different rank stages. Most liptinites run through a stronger red shift at a rank stage of about 0.5% Rm and thereafter — much more severely — at about 0.8 – 0.9% Rm. Both fluorescence “jumps” are obviously connected with the neo-formation of bitumen out of liptinites. In contrast to sporinite and cutinite, suberinite and a great part of resinite (the fluorescence properties of the latter liptinite maceral varying strongly at the same rank stage) change their fluorescence drastically already at low rank stages (0.35–0.45% Rm). The primary (inherited) fluorescence intensity of fluorescing huminities/vitrinites decreases drastically up to the stage of sub-bituminous C/B coal (0.45% Rm) where it is almost lost. Thereafter a new but weaker fluorescence of vitrinites begins to develop, due to the generation of liquid bituminous products formed from liptinites and perhydrous vitrinites. This secondary fluorescence extinguishes (at least as visible fluorescence) at the stage of low-volatile bituminous coal (ca. 1.8% Rm). The secondary vitrinite fluorescence is combined with the occurrence of the secondary maceral exsudatinite and expulsions of fluorescing liquids from fissures during irradiation. Parallel with the strong decrease of primary fluorescence intensity, the spectral fluorescence maximum ( λ max) of huminite/vitrinite shifts very rapidly from 520 nm (green) to 660 nm (red) between the peat stage and the boundary between sub-bituminous coal and bituminous coal (ca. 0.5% Rm). Thereafter the red shift is less pronounced. This trend is similar for the spectral quotients Q and Qmax which seem to pass even a short reversal (slight decrease) at 0.5–0.6% Rm, i.e. at the stage corresponding to the boundary between sub-bituminous coal and bituminous coal (Braunkohle/Steinkohle according to the German classification). The changes of fluorescence properties of the coal macerals are explainable by the neo-formation and successive fragmentation of petroleum-like products in coal. Comparison with the results of elemental analyses and of the Rock-Eval pyrolysis made on the same rank series of coal (and on additional coal samples) revealed that — due to CO 2 - and H 2O-release during coalification — O/C decreasesmore than H/C up to a rank stage varying between 0.4 to 1.0% Rm (depending on the petrographic composition). Thereafter H/C decreases more than O/C, corresponding to a loss of hydrocarbons mainly. The temperature of maximal hydrocarbon release during the Rock-Eval pyrolysis ( Tmax) increases gradually with increasing vitrinite reflectance. This Tmax can be used as a rank parameter for coals. The amount of hydrocarbons and tars released by the Rock-Eval pyrolysis (S2) increases up to about 0.7% Rm, with maximal values of more than 300 mg/g org. C (mean maximal values 220 mg/g org. C), then it decreases. This maximum separates an earlier coalification stage with loss of CO 2 and H 2O from a later stage with loss of hydrocarbons mainly. The amount of hydrocarbons released during pyrolysis varies widely at a given rank, due to the different petrographic composition. Coals with high amounts of liptinite, with exsudatinite and “oil” expulsions from fissures tend to the highest yields of hydrocarbons. The Rock-Eval data support the idea of oil generation in coals and allow a quantitative estimation of hydrocarbon generation either in nature or during pyrolysis.