Petroleum Expulsion Part 2. Organic Matter Type and Maturity Effects on Kerogen Swelling by Solvents and Thermodynamic Parameters for Kerogen from Regular Solution Theory

The swelling of kerogen in several well-defined solvents was measured to derive thermodynamic parameters specified by an extended Flory−Rehner and Regular Solution Theory framework. These parameters are needed to assess the importance of the selective solubility of individual petroleum compounds into kerogen as a mechanism for the chemical fractionation that occurs during expulsion. The swelling of kerogen by a set of organic solvents was quantified following exposure to 30, 90, and 150 °C temperatures for 24 h in a closed system. The organic solvents were selected to reflect the prevalent hydrocarbon and heteroatom structures in petroleum. A total of 13 different, well-characterized Type II and Type IIIC kerogens were studied that included a maturity suite for each kerogen type. The swelling results were pooled and analyzed using an analysis of variance (ANOVA) statistical method to determine if swelling differences were related to the temperature, solvent, kerogen type, and level of kerogen maturity. There was no statistical difference in the mean volumetric swelling ratio ( Q v ) at 30 and 90 °C (2.1%) and only a slight increase in the mean Q v at 150 °C (3.0%) after exposing kerogen for 24 h to the solvents used in this study. The weak temperature dependence indicates that these kerogen−solvent systems have nearly equilibrated after 24 h at 30 °C. Statistically significant differences in the mean Q v were found among different kerogens and nonpolar/slightly polar solvents with varying solubility parameter (δ) values. Average swelling data for Type II and Type IIIC kerogens were interpreted to derive thermodynamic parameters (solubility parameter, cross-link density, and the volume fraction of the kerogen network V eq that minimizes its elastic strain energy, termed “native swelling”) by optimizing the match between the experimental data and theoretical predictions. These parameters enable general predictions of swelling behavior of these kerogen types in other solvents and complex solvent mixtures. To verify the model, the swelling of kerogen by 50:50 n- hexadecane/solvent mixtures was calculated and compared to experimental data. Good agreement was observed between the Q v and compositionally based swelling ratio ( Q c ) for all kerogens.