Fluorescence of organic matter and thermal maturity assessment

Abstract

One of the aims of the organic petrologist is to study the thermal changes of the sedimentary organic matter through its consequences on a microscopical scale, i.e. on the maceral scale. Different fluorescence properties of organic constituents were studied on several natural or artificial series, each of them exhibiting both specific organic matter content and thermal history: Green River Shales, Uinta basin, U.S.A. (type I); Toarcian Shales, Paris basin, France (type II); Sancerre borehole, Paris basin, France (type II); Mahakam basin, Indonesia (type III); Haltenbanken graben, North Sea (type III); Cretaceous basin, Sudan; Sbaa basin, Algeria. In each case, the results were compared to different optical and geochemical evolution parameters. Vitrinite reflectance ( R m) behaves as a rank parameter and follows a monotonous evolution, systematically increasing with diagenesis. For its part, the fluorescence of macerals is strongly influenced by their bitumen fraction. When a maceral reaches a peculiar thermal evolution state (maturity threshold), and produces its own thermal effluents, important changes in its fluorescence properties occur. We show that along a thermal series each fluorescent maceral exhibits its own threshold. This threshold is different from that of the whole organic content, as defined by global geochemical parameters. For these reasons, the liptinite fluorescence parameters do not parallel the monotonous evolution of vitrinite reflectance; they account for oil generation and thus behave as maturity parameters. Special attention must be paid to additional phenomenon affecting the organic matter (oxidation, oil migration, etc.), since their resulting effect on fluorescence properties can at times hide the thermal effects, or, at others, can be sometimes mistaken for them. The experienced use of such pinpoint tools leads to an accurate knowledge of the mechanisms which govern the thermal maturation of the sedimentary organic matter. Considering the difference between the rank and maturity concepts, and taking into account the proper maturity state of several organic constituents, they allow the detailed determination of the maturity state of a source rock in relation to its microtexture.