Identification and Geochemical Significance of Novel <i>des</i> -A-Triterpenoids and Mononuclear-Aromatic <i>des</i> -A-Triterpenoids in Tertiary Terrestrial Oils from the Northern of the South China Sea

A series of mononuclear-aromatic des-A-triterpenoids (MADATTS) and des-A-triterpenoids (DATTS) were simultaneously detected in terrestrial oils from the Baiyun Sag, Pearl River Mouth Basin, South China Sea. Besides five previously reported MADATTS, i.e., C23 mononuclear-aromatic des-oleanane (C23MADAO), C23 mononuclear-aromatic des-ursane (C23MADAU), C24MADAO, C24MADAU, and C26MADAU, this study identified a novel molecular marker in these oil samples, namely C26 mononuclear-aromatic des-oleanane (C26MADAO). C26MADAO elutes before C26MADAU in m/z 187 and 352 → 187 chromatograms, with a molecular ion at M+·352, base peak ion at m/z 187, and diagnostic ion fragment at m/z 199, 213, and 337. MADATTS with the same carbon number and identical substitutions on the B-ring exhibit strong positive correlations in their chromatographic peak abundance but show no evident correlation with the DATTS counterparts that share corresponding similar molecular skeletons, indicating that the formation mechanisms of the two series of compounds are different. In addition, the chromatographic peak abundance of both MADATTS and DATTS shows strong correlations with the C29 diasteranes/C29 regular steranes (C29-DiaSt/C29-St) and C30 rearranged oleananes/C30 hopane ((I + II + III)/C30H), which possibly reflects that their formation process is jointly controlled by terrestrial organic inputs and the catalysis of acidic clay minerals. Based on the sedimentary facies characteristics, conventional biomarker parameters, and distribution patterns of MADATTS and DATTS, this study finely reclassifies the Baiyun Sag oils into five groups and provides a comprehensive interpretation of their origins across different subregions.