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.