Mapping ancient sedimentary organic matter molecular structure at nanoscales using optical photothermal infrared spectroscopy

Elucidating the molecular structure of sedimentary organic matter (SOM) is key to understanding petroleum generation processes, as well as ancient sedimentary environments. SOM structure is primarily controlled by biogenic source material (e.g., marine vs. terrigenous), depositional conditions, and subsurface thermal history. Additional factors, e.g., strain, may also impact the molecular structure of SOM. Multiple spatially resolved approaches exist for in situ evaluation of SOM, including Raman and infrared spectroscopies, as well as mass spectrometric methods. While these methods have enabled increased understanding of the occurrence and distribution of SOM functional groups, they suffer from disadvantages including low spatial resolution (infrared spectroscopy), limited molecular information (Raman spectroscopy), and sample destruction (mass spectrometric methods). Recent technological advances have resulted in infrared spectrometers capable of breaking the Abbe diffraction limit, greatly increasing the spatial resolutions accessible for an infrared measurement.

Here we utilize optical photothermal infrared spectroscopy (O-PTIR) to record maps of functional group distributions at 500 nm spatial resolution in Tasmanites (algal microfossils) from the Upper Devonian Ohio Shale. These data allow for discrimination between Tasmanites, adjacent SOM, and fine-grained minerals. Additionally, functional group distributions within Tasmanites were found to be generally homogenous, although slight variations exist between the body and fold apices (zones of greatest deformation) which may indicate strain-induced chemical reactions. The data presented here represent the first application of O-PTIR to study SOM, highlighting the promise of this analytical approach for future studies evaluating the molecular composition of geologic materials at sub-micron scales.