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.