Thermodynamic and Structural Characterization of Bulk Organic Matter in Chinese Silurian Shale: Experimental and Molecular Modeling Studies

Silurian
marine shale in Sichuan Basin is the most significant
target zone for shale gas resources in China. In this work, a combined
experimental and molecular simulation study was conducted to characterize
the thermodynamic and structural properties of the organic matter
in Silurian shale. Organic geochemistry experiments and Fourier transform
infrared (FTIR) spectroscopy were performed to provide structural
parameters for the main skeleton of the organic matter. A realistic
molecular model of the organic matter under typical reservoir conditions
was generated by molecular dynamics simulations based on the experimental
results and documented analytic data. The thermodynamic and structural
properties of the organic matter model were discussed in detail. Clear
correlations are found among geochemistry properties and structural
parameters of organic matter, independent of organic matter type and
maturity. Aromatic units in the organic matter are highly condensed,
and the interunit linkages are mainly short methylene groups. Ether
groups are the dominant oxygenated compounds, while aromatic sulfur
is the main form of organic sulfur. Reasonable consistencies are found
on results of compositions of the organic matter fractions and physical
density between simulated and available experimental data. The isothermal
compressibility and thermal expansion coefficient correspond to the
general range of a liquid. In addition to micropores, the organic
matter contains a large amount of ultramicropores, which contribute
a lot to the high porosity and specific surface area. The porous network
is highly connected with few dead pores. Interestingly, the introduction
of bitumen fractions has little effect on the spacing of polyaromatic
units, but it aggravates the relative slippage of polyaromatic units.
Also, separation of lighter compounds is observed in the structure.
The carbon dioxide molecules are closer to the oxygenated groups,
while the nitrogen molecules and methane molecules are closer to the
sulfur functional groups and nitrogen functional groups. This proposed
organic matter model can serve as a starting point for further theoretical
investigations on gas adsorption and transport mechanisms, representative
of the organic matter in Silurian shale at molecular scale.