Abstract
The organic matter (OM) in shale is closely associated with clay minerals, and its maturation is usually accompanied by the diagenesis of these minerals, especially smectite illitization. However, the effect of mineral transformation and its accompanying change of mineral-OM interactions in shale on hydrocarbon generation is still unclear. To investigate this question, smectite-rich immature shale was selected to carry out hydrous pyrolysis. Organic geochemistry and mineralogy of pyrolysates at different temperatures show that the maturation of OM is accompanied by the transformation of bulk and clay minerals. Based on the change in hydrocarbon yield, Rock-Eval parameters, and mineral composition, hydrocarbon generation in this study is divided into three stages: 25-300, 300-400, and 400-500 °C, which are the result of the synergistic evolution of clay minerals and OM. Multistage hydrocarbon generation can be attributed to the mineral transformation-induced desorption of mineral-bound soluble OM (SOM), decarboxylation and hydrocracking of kerogen promoted by solid acids, and cross-linking and cracking reactions of free SOM and residual kerogen under high temperatures. Although different from the classical hydrocarbon generation model of kerogen, this multistage hydrocarbon generation is consistent with the characteristics of the saline lacustrine source rocks in nature. The mineral transformation-induced desorption of SOM is a new pathway for petroleum formation, which can well explain the formation of low-mature oils in nature. In addition, the release of mineral-bound and kerogen-bound biomarkers results in two reversals of isomerization ratios. Considering mineral transformation and mineral-OM interactions can help us better understand and refine the hydrocarbon generation theory of OM.