Characterization of compounds in unresolved complex mixtures (UCM) of a Mesoproterzoic shale by using GC×GC-TOFMS

•UCM present in the Mesoproterozoic Xiamaling shale, North China.•Detailed C10–C25 compounds were identified in UCM by GC×GC-TOFMS.•Results by reversed-phase column system are better than normal-phase column system.•UCM was formed by isomerization of organic molecules besides microbial activities.

Unresolved complex mixtures (UCM) in shales have important implications for organic matter formation and diagenesis. However, their origin is not well understood, one important reason for this is lacking the knowledge of their molecular compositions. Here, using the comprehensive two-dimensional gas chromatography (GC×GC) coupled with time-of-flight mass spectrometry (TOFMS), we conduct a pilot study based on a Mesoproterozoic black shale from North China. Detailed identification of compounds was made and a possible new origin was suggested. Results show that the separation of UCM hydrocarbons by a reversed-phase column system (polar/non-polar) is efficient for the identification of molecular composition of UCM. The UCM in the shale is composed mainly of C11–C27 compounds, including paraffin hydrocarbons, cycloalkanes and aromatics, in particular 1–3 cyclic paraffins with a short-chain alkyl group, multi-branched isoparaffins and their homologous series, which are indicative of isomerization of molecules intensively. This implies that the origin of the UCM might include the isomerization of organic molecules in addition to the commonly-thought microbial activities. This study improves the understanding of the geochemical compositions and origin of UCM in shales.