Control of clay mineral properties on hydrocarbon generation of organo-clay complexes: Evidence from high-temperature pyrolysis experiments

In clay-rich mudstones, organic matter (OM) is intimately associated with clay minerals to form organo-clay complexes, which play crucial roles in affecting hydrocarbon generation of OM. Different clay minerals show diverse nature and evolution paths, which would affect clay-OM interactions and further the hydrocarbon generation processes and mechanisms. In this way, smectite-octadecanoic acid complex (Sm-OA) and illite-octadecanoic acid complex (I-OA) were prepared for confined pyrolysis experiments to analyze the co-evolution of mineralogy and pyrolytic behaviors. According to the clay-OM interaction, hydrocarbon generation process was divided into three stages. During smectite illitization, the collapse of interlayer space induces the desorption of OA, which contributes to the yield of resin. The Brønsted and Lewis acid sites of clay minerals are considered to promote the hydrogenation and decarboxylation of desorbed OA, respectively, which enables the transformation of resin into saturates and gaseous hydrocarbons. Therefore, the hydrocarbon generation in Sm-OA is a process that OM desorbs from clay minerals and transforms into saturates and then into gaseous hydrocarbons. For I-OA, the quantitative calculation of hydrogen reveals that the lack of inorganic hydrogen supplied by clay minerals compels OM to evolve along the opposite path, generating high-molecular-weight asphaltene and releasing organic hydrogen to ensure the proceeding of cracking reaction. In this case, the desorbed OM converts into asphaltene and then to gaseous hydrocarbons. Conclusions are obtained that the differences in clay and bulk mineralogies determine the different contributions of desorption, hydrogenation, and decarboxylation to hydrocarbon generation in Sm-OA and I-OA, leading to variations in composition of pyrolytic products. Sm-OA is beneficial to the formation of liquid hydrocarbons, while I-OA is conducive to formation of gaseous hydrocarbons. In other words, the differences of mineral properties control the hydrocarbon generation processes and mechanisms.