研究目的

To assess if fluorescence spectroscopy via CLSM can be used as a robust thermal index and potentially assist with more accurate placement of petroleum resource assessment boundaries at the entrance to the oil window.

研究成果

Confocal laser scanning microscopy (CLSM) was used to characterize the fluorescence response of sedimentary organic matter, including marine alga Tasmanites from Devonian shales of the northern Appalachian Basin, and Gloeocapsomorpha prisca from Ordovician kukersite in the Williston Basin. CLSM experiments examined fluorescence emission as related to excitation wavelength, sample orientation and location within individual organic entities, and on chemical transects documented from prior analytical approaches. Results from varying excitation wavelength demonstrate the presence of multiple fluorophores in Tasmanites, and we interpret reduced Stokes shift at longer excitation wavelengths as due to preferential absorption by less polar functions. We suggest higher intensity emission at fold apices in organic matter may be due to photoselective alignment of the fluorophore transition moment with polarized excitation and that observed blue shifts in high intensity emission could be driven by decreased abundance of polar O-containing functions, although this could not be confirmed through preliminary TOF-SIMS analysis. Optical anisotropy of Tasmanites was observed through variations in fluorescence emission color when the bedding plane-parallel orientation was rotated, whereas no variation in emission color was observed from rotation of the bedding plane perpendicular orientation. CLSM analyses of organic matter chemical transects occurring from Gloeocapsomorpha prisca-rich kukersite into adjacent reservoir facies showed micro-scale increases in fluorescence red-shift, confirming increases in thermal maturity as documented by Raman analyses and solid bitumen reflectance in a prior study. These findings inform petroleum processes research by clarifying and improving usage of organic fluorescence properties as thermal indices through: (1) characterizing in situ changes in fluorescence emission occurring due to conversion of oil-prone sedimentary organic matter to petroleum, and (2) providing insight into the fluorescence properties of organic matter impacted by expulsion and primary migration of petroleum fluids. Further work is warranted to expand the use of CLSM for investigating the molecular properties of ancient sedimentary organic matter and this is a focus of on-going research within the USGS.

研究不足

Lack of a standard test method or protocols for instrument settings have long been a problem in conventional fluorescence spectroscopy of sedimentary organic matter. Notably, it has been difficult to attain reproducible measurements of fluorescence color, intensity or alteration. Despite that the current work shows promise for application of CLSM to fluorescence spectroscopy, similar limitations are apparent. For example, variation in emission λmax as a function of excitation wavelength indicates users with contrasting laser systems will determine disparate spectral properties for the same sample. Likewise, measurement from high intensity emission areas or certain orientations of sedimentary organic matter may show emission color bias relative to other locations or orientations. These observations indicate need for standardization and specific measurement protocols to ensure inter-laboratory reproducibility for CLSM-based fluorescence spectroscopy.