Construction and geological application of comprehensive evaluation indices for biomarker characteristics: A case study of the Yanchang Formation in the Ordos Basin, China

In response to the issue of numerous, redundant, and complexly interpreted biomarker parameters in molecular geochemistry, this paper proposes a novel comprehensive index evaluation method. Focusing on the Yanchang Formation in the Ordos Basin, factor analysis is employed to screen biomarker parameters that are most closely related to organic matter maturity, parent material type, and water salinity. Based on the selected parameters, three comprehensive indices—the Maturity Index (MI), Parent Material Index (PMI), and Water Salinity Index (WSI)—are constructed through linear combinations. Applications of these indices to different stratigraphic layers and structural blocks of the Yanchang Formation demonstrate their ability to represent the geochemical characteristics of organic matter more concisely and clearly, with higher accuracy and reliability than single biomarker parameters. Research findings based on these indices reveal that the maturity of organic matter in the Yanchang Formation gradually increases with burial depth, that the Chang 7 oil group exhibits a significant contribution from aquatic organic matter, and that deeper oil groups, such as Chang 9 and Chang 10, are associated with relatively higher water salinity. Additionally, the planar distribution patterns of maturity, parent material type, and water salinity across different blocks are complex, reflecting the heterogeneity of geological conditions. Overall, the comprehensive index evaluation method proposed in this paper provides a new analytical framework for molecular geochemistry research and has important implications for oil and gas exploration under complex geological settings.