Evolution of nC₁₆H₃₄-water–mineral systems in thermal capsules and geological implications for deeply-buried hydrocarbon reservoirs

Organic-inorganic interactions between hydrocarbons and most minerals in deeply buried reservoirs remain unclear. In this study, gold capsules and fused silica capillary capsules (FSCCs) with different combinations of nC₁₆H₃₄, water (distilled water, CaCl₂ water) and minerals (quartz, feldspar, calcite, kaolinite, smectite, and illite) were heated at 340 °C for 3–10 d, to investigate the evolution and reaction pathways of the organic–inorganic interactions in different hot systems.After heating, minerals exhibited little alteration in the anhydrous systems. Mineral alterations, however, occurred obviously in the hydrous systems. Different inorganic components affected nC₁₆H₃₄ degradation differently. Overall, water promoted the free-radical thermal-cracking reaction and step oxidation reaction but suppressed the free-radical cross-linking reaction. The impact of CaCl₂ water on the nC₁₆H₃₄ degradation was weaker than the distilled water as high Ca²⁺ concentration suppressed the formation of free radicals. The presence of different waters also affects the impact of different minerals on nC₁₆H₃₄ degradation, via its impact on mineral alterations. In the anhydrous nC₁₆H₃₄-mineral systems, calcite and clays catalyzed generation of low-molecular-weight (LMW) alkanes, particularly the clays. Quartz, feldspar, and calcite catalyzed generation of high-molecular-weight (HMW) alkanes and PAHs, whereas clays catalyzed the generation of LMW alkanes and mono-bicyclic aromatic hydrocarbons (M-BAHs). In the hydrous nC₁₆H₃₄-distilled water–mineral systems, all minerals but quartz promoted nC₁₆H₃₄ degradation to generate more LMW alkanes, less HMW alkanes and PAHs. In the nC₁₆H₃₄-CaCl₂ water–mineral systems, the promotion impact of minerals was weaker than that in the systems with distilled water.This study demonstrated the generation of different hydrocarbons with different fluorescence colors in the different nC₁₆H₃₄-water–mineral systems after heating for the same time, implying that fluorescence colors need to be interpreted carefully in investigation of hydrocarbon charging histories and oil origins in deeply buried reservoirs. Besides, the organic–inorganic interactions in different nC₁₆H₃₄-water–mineral systems proceeded in different pathways at different rates, which likely led to preservation of liquid hydrocarbons at different depth (temperature). Thus, quantitative investigations of the reaction kinetics in different hydrocarbon-water-rock systems are required to improve the prediction of hydrocarbon evolution in deeply buried hydrocarbon reservoirs.