The composition, origin and fate of complex mixtures in the maltene fractions of hydrothermal petroleum assessed by comprehensive two-dimensional gas chromatography

► Petroleum from different hydrothermal systems are analyzed by GC × GC. ► We define what are simple, complex and unresolved complex mixtures in GC × GC data. ► We demonstrate that UCMs are a complex array of configurational isomers. ► We provide a metric for determining the molecular complexity of a chromatogram. ► Subtracted GC × GC chromatograms illustrate the evolution of the reservoir.