Resolving the Nanoscale Chemical Composition of Source Rock using Hyperspectral Photo-Induced Force Microscopy

D. Nowak, K. Park, D. Jacobi, S.L. Eichmann, N. Burnham
Molecular Vista Inc,
United States

Keywords: source rock, unconventional reservoirs, shale, hyperspectral, PiFM, AFM, nanoscale chemical analysis

Summary:

Unconventional reservoirs provide an essential source of energy in the modern economy. A better understanding of a specific reservoir composition will allow for more economically efficient production. Usually, Fourier transform infrared (FTIR) spectroscopy would be used to characterize organic matter, typically kerogen, found within the source rock. The FTIR detection will be limited spatially by the diffraction limit, several microns as demonstrated by micro-FTIR. In order study the nanoscale multicomponent source rock material, higher resolution is required. A parallel approach is to use the Atomic Force Microscopy (AFM) to investigate the mechanical moduli of the source rock to distinguish organic and inorganic phases at nanoscale resolutions. The mechanical properties can provide some useful, albeit limited, information about the source rock. Parameters such as mineralogy and thermal maturity also vary at the nanoscale. Mineralogy includes information about the sediment source, and the thermal maturity and kerogen type determine the hydrocarbons generated. Kerogen is chemically complex and changes in crosslink density, porosity, the molecular structure through maturation, unable to be answered by mechanical means. Recent advances in optical AFM hybrid instruments have led to the ability to realize nanoscale spectroscopic imaging. Utilizing the AFM probe as a near-field sensor characterization methods such as Raman and IR absorption measurements can be conducted within the diffraction limit with a resolution defined by the size of the probe. A recently developed near-field imaging technique, photo-induced force microscopy (PiFM), shows great promise to provide nanoscale chemical characterization of unconventional reservoir rock. PIFM is a broadband optical technique that integrates an AFM operating in non-contact mode with an excitation light source. Tuning the light source to a frequency which matches the absorbance of the material will induce a dipole-dipole force interaction under the AFM tip that can be detected mechanically. PiFM will use one of the mechanical modes of the AFM cantilever to measure the force of the dipole-dipole interaction with resolutions below 10 nm, while simultaneously recording topographic information. Recent advances in IR laser technology have provided the ability to create hyperspectral images. By rapidly scanning the output frequency of the laser a spectrum can be acquired in 100s of ms, allowing for a full spectrum to be obtained at each AFM pixel location in a reasonable amount of time. Here we demonstrate the application of PiFM to provide nanoscale hyperspectral characterization of ion milled unconventional reservoir rock from the Barnett Formation using the Vista-IR (Molecular Vista, San Jose, CA). Variations in mineralogy are distinguishable, and the organics might be sorted by either aromatic-rich or aliphatic-rich, a key indicator of the thermal maturity. At resolutions of 10s of nanometers, the spectral peaks are those of the individual materials rather than a convolution of the peaks, enabling the characterization with the fingerprint region. A nanoscale understanding of the rock in unconventional reservoirs could have significant implications determining hydrocarbon potential and provide new insights for production.