Multistage Hydrocarbon Generation of Saline Lacustrine Source Rocks in Hydrous Pyrolysis: Insights from Clay Mineral–Organic Matter Interactions

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.