Identification of Several Tetramethylated and C ₄ ‐Alkyl‐Substituted Biphenyls in Petroleum and Source Rocks

Biphenyl (BP) and its alkyl‐substituted derivatives, distinguished by rotational flexibility between covalently linked phenyl rings, constitute significant constituents of petroleum and sedimentary organic matters. Through comprehensive gas chromatography–mass spectrometry (GC–MS) analysis and co‐injection validation with authentic standards, this study unambiguously identifies four novel isomers, that is, the 2,2′,5,5′‐tetramethylbiphenyl (TeMBP), 3,3′,5,5′‐TeMBP, 4‐butanebiphenyl (BBP), and 3,3′,4,4′‐TeMBP, in a set of petroleum and source rock samples. Quantitative comparisons reveal distinct substitution patterns across depositional environments, that is, marine‐derived oils exhibit progressive enrichment of mono‐, di‐, tri‐, and tetramethylbiphenyls relative to their lacustrine counterparts. Notably, the 3,3′‐dimethylbiphenyl (3,3′‐DMBP) demonstrates preferential accumulation in sulfidic oils associated with carbonate source rocks deposited under anoxic, sulfur‐rich conditions. Thermal maturity analysis indicates systematic compositional evolution: The BP concentrations increase with thermal stress across the oil generation window, while meta ‐ and para ‐substituted derivatives, for example, 3‐methylbiphenyl (MBP), 4‐MBP, 3,5‐DMBP, and 3,5,3′‐trimethylbiphenyl (TMBP), become predominant in high‐mature source rock extracts and condensates. This thermal resilience highlights BPs’ geochemical significance as proxies for high‐maturity petroleum systems.