Thermally driven organic-inorganic interactions in sedimentary basins: A review from source rocks to reservoirs

Abstract

Organic-inorganic interactions, ubiquitous in sedimentary basins, critically influence the genesis and evolution of petroleum and natural gases. These processes also modify the inorganic rock matrix as well as cause the formation of secondary pores. This review synthesizes evidences from thermal experiments and geological case studies to examine the genesis, evolution pathways and significance of thermally driven interactions in kerogen-rich source rocks and hydrocarbon reservoirs.

Several mechanisms have been proposed to elucidate the initiation of these interactions, including the carbonium-ion mechanisms, stepwise-oxidation of hydrocarbon by water in the presence of ferric iron-bearing minerals, Fischer-Tropsch-type (FTT) synthetic reactions, and free radical mechanisms. Recent observations of water microdroplets formed near oil-water interfaces at elevated temperatures suggest a potential non-catalytic free radical mechanism underlying the oil-water interactions in hot hydrocarbon reservoirs. In source rocks, clay minerals (smectite, S/I, and illite), calcite, and Fe/Mn-containing metal minerals significantly impact kerogen degradation through their Brønsted and Lewis acid sites, M²⁺-O surface groups, and partially filled d orbitals. In reservoir settings, kaolinite, illite, feldspar and carbonate minerals primarily influence hydrocarbon degradation. Water, present throughout these environments, facilitates the generation of low-molecular-weight hydrocarbons, organic acids, and CO₂ through external hydrogen and oxygen contribution. It also modulates minerals effects on the thermal evolution of kerogen, crude oil, and natural gases. Acids produced during kerogen maturation promote mineral alterations, including smectite illitization, feldspar alteration and calcite recrystallization. These processes, sustained by continuous acid generation from hydrocarbon oxidation, contribute to ongoing mineral transformation and secondary porosity development in deeply buried reservoirs, significantly affecting reservoir quality. At elevated temperatures, extensive organic-inorganic interactions, facilitated by inorganic-derived hydrogen and oxygen, influence deep hydrocarbon potential, natural gas isotopic composition, and the depth limit of liquid hydrocarbon preservation.

Despite these advances, uncertainties remain regarding the differential impacts of various inorganic species on organic reactions. Future research should focus on elucidating the detailed evolution pathways and associated reactions of thermally altered rocks with diverse organic-inorganic combinations, particularly in fine-grained shales and ultra-deep hydrocarbon reservoirs. Additionally, investigating the quantitative kinetics of coupled mineral alteration and organic reactions would address crucial knowledge gaps in our understanding of these processes.