APPLICATION OF EXCITATION-EMISSION FLUORESCENCE MICROSCOPY TO THERMAL MATURITY OF GEOLOGICAL SAMPLES

Conventional fluorescence microscopy of organic materials is used in hydrocarbon exploration to characterize thermal maturity via a progressive redshift in emission wavelength with increasing temperature and burial depth. Here we utilize confocal microscopy to obtain, for the first time, excitation-emission matrix (EEM) fluorescence data for Tasmanites algal cysts in a natural maturation sequence within Devonian age Huron Shale in the Appalachian Basin. Thin sections from three well-characterized (R_o 0.45-0.62%) samples of similar lithofacies were examined for changes in the EEM across a regional oil-in isograd. Fields of approximately 0.15 mm² containing multiple Tasmanites cysts were examined using a Leica^TM DM6000 inverted microscope with a SP5X confocal system, a white light (470-670 nm) laser source, and a 20x 0.7 NA dry objective. EEM were created with 10 nm steps in excitation and 5 nm steps over 490-785 nm in emission. In each sample, 10 regions of interest were selected within cysts to sum for an average Tasmanites EEM. EEM showed a redshift in emission with an increase in λ_max from ≈530 nm to ≈570-580 nm with increasing maturity. Conversely, the excitation wavelength for maximum emission is blue shifted from λ_max ≈490 nm to λ_max ≈475 nm with increasing maturity. Within individual cysts, EEM show that the emission of less intensely fluorescing segments is red shifted relative to more intensely fluorescing segments. This observation has implications for conventional spectral fluorescence measurements, where no distinction is made in selection of the region for measurement, or where brighter locations are selected to improve signal-to-noise ratios. Fluorescence emission intensity at the EEM maximum increased from low maturity to high maturity samples. Changes in the EEM and fluorescence intensity are interpreted to occur due to molecular aromatization and condensation within Tasmanites bodies. Future work will use microbeam techniques such as µ-FTIR and XPS to characterize compositional alterations associated with the demonstrated fluorescence-associated thermal maturity progression to attempt to identify the major fluorophores within Tasmanites.