Environmental and Botanical Insights from Biomarker Variability in Xihu Depression Light Crude Oils

Comprehensive organic geochemical analyses were conducted on 41 light crude oil samples obtained from diverse structural zones within the Xihu Depression. These analyses revealed a pronounced enrichment of biomarkers, highlighting substantial contributions from terrestrial higher plants. Notably, tricyclic diterpanes and their aromatized derivatives were identified as key molecular markers, primarily derived from angiosperms, gymnosperms, and ferns. In contrast, biomarkers indicative of inputs from aquatic lower plants were significantly less abundant.

Detailed geochemical profiling uncovered distinct spatial variations in biomarker composition across the structural zones, as evidenced by diagnostic ratios such as isopimarane/nC₂₀, isopimarane/(isopimarane + 16β(H) phyllocladane), 1,2,5-trimethylnaphthalene/1,3,6-trimethylnaphthalene, and 1,7-dimethylphenanthrene/(retene + 1,7-dimethylphenanthrene). These compositional variations, analyzed through multivariate statistical techniques, facilitated the classification of the crude oils into four distinct groups, reflecting variable contributions from angiosperms, gymnosperms, and ferns.

Furthermore, the study established a robust linkage between the saturated and aromatic hydrocarbon fractions of the crude oils, using high-resolution gas chromatography-mass spectrometry (GC-MS) to investigate their molecular signatures. A novel biomarker-based index, defined as (isopimarane + 1,7-dimethylphenanthrene)/(retene + 16β(H) phyllocladane), was developed to infer the source input of the light crude oils. Elevated values of this index were interpreted as indicative of a dominant gymnosperm-derived input. Notably, a positive correlation was observed between this index and the bulk δ¹³C values of the crude oils, further corroborating the critical role of gymnosperm resins in the hydrocarbon generation processes within the Xihu Depression. This integrated geochemical framework provides valuable insights into the source contributions and depositional settings of the region's petroleum systems.