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.
Summary
We report here a new microscopic technique for imaging and identifying sedimentary organic matter in geologic materials that combines inverted fluorescence microscopy with scanning electron microscopy and allows for sequential imaging of the same region of interest without transferring the sample between instruments. This integrated correlative light and electron microscopy technique is demonstrated with observations from an immature lacustrine oil shale from the Eocene Green River Mahogany Zone and mid‐oil window paralic shale from the Upper Cretaceous Tuscaloosa Group. This technique has the potential to allow for identification and characterization of organic matter in shale hydrocarbon reservoirs that is not possible using either light or electron microscopy alone, and may be applied to understanding the organic matter type and thermal regime in which organic nanoporosity forms, thereby reducing uncertainty in the estimation of undiscovered hydrocarbon resources.
Microscopic solid bitumen is a petrographically
defined secondary
organic matter residue produced during petroleum generation and subsequent
oil transformation. The presence of solid bitumen impacts many reservoir
properties including porosity, permeability, and hydrocarbon generation
and storage, among others. Furthermore, solid bitumen reflectance
is an important parameter for assessing the thermal maturity of formations
with little to no vitrinite. While the molecular composition of solid
bitumen will strongly impact associated parameters such as the development
of organic matter porosity, hydrocarbon generation, and optical reflectance,
assessing the molecular composition of solid bitumen in situ within shale reservoirs can be challenging due to the small grain
sizes (often ≤1 μm in diameter) and the inherent heterogeneity
of shale formations. Here we employ the recently developed atomic
force microscopy based infrared spectroscopy (AFM-IR) technique to
investigate solid bitumen molecular composition in situ within shale samples from the Late Cretaceous Eagle Ford Group.
These samples possess sulfur-rich type II kerogens that span a natural
thermal maturity gradient from early oil generation to the dry gas
window. The application of AFM-IR allows for the rapid collection
of thousands of compositional measurements from solid bitumen with
∼50 nm resolution. Our results indicate that (i) solid bitumen
from the lower Eagle Ford displays both intra- and intergranular variation
in the relative abundance of CH2, CC, and CO
moieties present; (ii) this molecular variation tends to, but does
not always, decrease with an increase in thermal maturity; and (iii)
the solid bitumen composition between samples, from an atomic ratio
perspective, is more similar than analysis of bulk kerogen isolates
would indicate. These findings are discussed with perspective toward
understanding the impact of thermal stress on the composition of secondary
organic matter within the Eagle Ford Shale and highlight the growing
awareness that organic matter heterogeneity within petroliferous mudrocks
extends down to the nanoscale regime.
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