Solid organic matter (OM) plays an essential role in the generation, migration, storage, and production of hydrocarbons from economically important shale rock formations. Electron microscopy images have documented spatial heterogeneity in the porosity of OM at nanoscale, and bulk spectroscopy measurements have documented large variation in the chemical composition of OM during petroleum generation. However, information regarding the heterogeneity of OM chemical composition at the nanoscale has been lacking. Here we demonstrate the first application of atomic force microscopy-based infrared spectroscopy (AFM-IR) to measure the chemical and mechanical heterogeneity of OM in shale at the nanoscale, orders of magnitude finer than achievable by traditional chemical imaging tools such as infrared microscopy. We present a combination of optical microscopy and AFM-IR imaging to characterize OM heterogeneity in an artificially matured series of New Albany Shales. The results document the evolution of individual organic macerals with maturation, providing a microscopic picture of the heterogeneous process of petroleum generation.
Fractionation of
petroleum during migration through sedimentary
rock matrices has been observed across lengths of meters to kilometers.
Selective adsorption of specific chemical moieties at mineral surfaces
and/or the phase behavior of petroleum during pressure changes typically
are invoked to explain this behavior. Such phenomena are of interest
as they impact both the quality and recoverability of petroleum resources.
Given the current emphasis on unconventional (continuous) resources,
there is a need to understand petroleum fractionation occurring during
expulsion and migration at the nanometer to micrometer scale, due
to the fine-grained nature of petroliferous mudrocks. Here, we explore
organic matter compositional differences observed within kukersites
(petroleum source beds containing acritarch Gloeocapsomorpha
prisca) and the overlying carbonate reservoir layer from
the Ordovician Stonewall Formation using a suite of spectroscopic
methods, primarily through atomic force microscopy-based infrared
spectroscopy (AFM-IR). AFM-IR is capable of providing spatial resolution
approaching 50 nm and allows for assessment of the molecular fingerprint
of kukersite organic matter across transition zones from organic-rich
“source” layers into neighboring carbonate “reservoir”
layers ∼150 μm away. Results indicate that organic matter
composition begins to vary immediately following expulsion from source
layers, with loss of carbonyl groups and a concomitant decrease in
alkyl chain-length, as migration distance increases. These chemical
transitions correlate with a decrease in fluorescence intensity, increase
in solid bitumen reflectance, and increase in Raman aromaticity proxies
(D-G band separation) in the organic matter. Our findings are consistent
with the retention of polar compounds onto mineral grains during expulsion
and migration, following primary cracking and bituminization of the Gloeocapsomorpha prisca kerogen.
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