Diversified roles of mineral transformation in controlling hydrocarbon generation process, mechanism, and pattern

Organic matter (OM) is intimately associated with minerals in clay-rich mudstones, leading to widespread organic-mineral interaction during hydrocarbon generation in argillaceous source rocks. What we are concerned is the effects of the different mineral properties on hydrocarbon generation process and mechanism during mineral transformation. In this way, pyrolysis experiments with smectite-octadecanoic acid complexes (Sm-OA and Ex-Sm-OA) were conducted to analyze correlation of mineralogy and pyrolysis behaviors. Based on organic-mineral interaction, hydrocarbon generation process was divided into three phases. At 200-300 ℃, collapse of smectite led to desorption of OM, resulting in high yield of resin and slight increase in saturates. Subsequently, enhanced smectite illitization at 350-450 ℃ was accompanied with large amounts of saturates and a mere gaseous hydrocarbon. Featured by neoformed plagioclase, ankerite, and illite, 500 ℃ saw plenty of asphaltene and methane-rich gaseous hydrocarbons, revealing cracking reactions of OM. Noteworthy is that saturated and gaseous hydrocarbons in Ex-Sm-OA were considerably more than that in Sm-OA during second and third phases. Quantitative calculation of hydrogen revealed organic hydrogen provided by cross-linking of OM could not balance hydrogen consumed by cracking reactions, but supply of inorganic hydrogen ensured cracking could readily occur and consequently greatly promoted hydrocarbon generation. Further investigating characteristics of mineralogy and pyrolytic products, as well as effects of solid acidity on hydrocarbon generation, we concluded desorption of OM and decarboxylation promoted by Lewis acid were dominated at 200-300 ℃, resulting in low-degree hydrocarbon generation. While high yield of saturated and gaseous hydrocarbons in second and third phases, together with occurrence of ankerite, indicated predominance of decarboxylation and hydrogenation promoted by Lewis and Brønsted acid, respectively. Variations in organic-mineral interactions indicated (1) the controls of mineral transformation on hydrocarbon generation process and mechanism include desorption, decarboxylation, and hydrogenation reactions; (2) clay minerals acted as reactants evolving together with OM rather than catalysts. These findings are profoundly significant for understanding the hydrocarbon generation mechanisms, organic-inorganic interactions, and carbon cycle.