Laser-induced fluorescence microscopy—application to possible high rank and carbonate source rocks

Abstract

It is difficult to obtain useful fluorescence information from coals and source rocks of high maturity by means of conventional fluorescence microscopy due to the low fluorescence intensity of the macerals. A laser-induced fluorescence microscopy (LIFM) designed by the authors was applied to target the problem. An investigation was made by means of this technique on a suite of coals having a range of mean random vitrinite reflectance (VRo)=0.3–2.5%, and some high maturity source rocks of early Palaeozoic age. The results show that LIFM greatly improves several functions of conventional fluorescent microscopy. LIFM is very sensitive to organic matter in sediments and for the suite of Chinese coals and possible source rocks investigated, considerably extends the maturity range over which visible fluorescence can be observed. The range is extended from about 1.40% to 2.0% VRo for vitrinite of normal composition, from 1.30% to about 1.50% VRo for alginite and other liptinites, and from about 2.5% to 3.2% VRo for organic inclusions and asphalt-absorbed minerals. With LIFM, it was found that the organic matter in some early Palaeozoic carbonates and argillaceous limestones is distributed very unevenly. In these rocks, which are believed to have been significant source rocks, some microregions are very rich in organic matter or contain a large number of organic inclusions although the total organic carbon (TOC) of the whole rock is low. A microchannel network for petroleum expulsion and migration, comprised of grain boundaries, bedding planes and suture textures containing hydrocarbons was revealed in carbonate strata. LIFM of organic matter in rocks should extend the application of organic petrology in petroleum exploration.