Thermodynamic and microstructural properties of the lacustrine Chang-7 shale kerogen: Implications for in-situ conversion of shale

Abstract

In-situ conversion processes (ICP) represents an effective approach for the commercial exploitation of low- to medium-maturity shale oil. The thermodynamic and microstructural properties of kerogen, as the primary organic matter in shale, have important implications for the design and optimization of ICP. However, the thermodynamic and microstructural properties of the lacustrine Chang-7 shale remain unclear, and conducting ICP pilot tests continues to pose challenges. By employing elemental analysis, pyrolysis-gas chromatography/mass spectrometry (Py-GCMS), solid-state carbon nuclear magnetic resonance (¹³C NMR), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FTIR), the representative models for lacustrine Chang-7 shale kerogens with different organic matter types and maturity levels were established. Semiempirical quantum mechanics and molecular dynamics were leveraged to study thermodynamic and microstructural properties of kerogen. Subsequently, by integrating cluster analysis and partial least squares methods, quantitative correlations among kerogen structural parameters and thermodynamic, kinetic, and volumetric properties were identified. The findings suggest that low-maturity type I kerogen is predominantly consisted of long-chain aliphatic hydrocarbons, whereas the degree of aliphatic chain branching increases in type II₁ kerogen. Medium-maturity type II₁ kerogen exhibits the highest degree of condensation, but the length and degree of branching of its aliphatic chains are closely analogous to low-maturity type I kerogen. Between 273 K and 473 K, the ideal heat capacity of Chang-7 shale kerogen increases linearly by approximately 51%. The enthalpy of formation and ideal heat capacity of medium-maturity type II₁ kerogen are the highest. With increasing maturity and declining H/C ratio, the density of Chang-7 kerogen increases. Its matrix pore sizes are primarily concentrated at 0.1–0.2 nm, constituting >80% of all pores. Kerogen with long and abundant aliphatic chains, a moderate degree of condensation, high porosity, low activation energy, and moderate heat capacity is considered the preferred target. The findings offer substantial guidance for the ICP of lacustrine shale.