GEOCHEMICAL CHARACTERISTICS AND SOURCE ANALYSIS OF LIGHT OILS IN XIHU SAG, EAST CHINA SEA BASIN

Xihu Sag is one of the most important area for oil and gas exploration in the East China Sea Basin. Earlier studies have shown that light oils are derived mainly from coal-bearing strata. But whether they come from coal or mudstone have been not clear. Here we present new understandings on this challenging issue based on geochemical parameters from Xihu Sag. Systematic analysis on the light oils were employed, including GC, GC-MS, GC-MS-MS and stable carbon isotopic composition analysis. Based on the biomarker parameters and carbon isotope composition, the light oils can be divided into three oil families: A, B and C. Family A oils are generated from mudstone which are enriched in clay minerals and formed in the oxidizing environment, displaying higher rearranged drimane/8β(H)-drimane ratios (1.0 to1.76) ,8β(H)-homodrimane/8β(H)-drimane ratios (0.76 to1.41) and C₂₉ diasterane/C₂₉ regular sterane ratios (1.74 to 6.42), and relatively lighter δ¹³C values (-26.6‰ to -27.5‰). The precursor is mainly terrestrial higher plants with certain proportions of aquatic organisms. These oils are distributed in Huangyan tectonic belt which is located in the center of Xihu Sag. Family C oils are generated from coal that were deposited under oxidizing environment, typically characterized by higher Pr/Ph ratios (>4.0), higher contents of C₂₉ regular steranes and wax, lower drimane/8β(H)-drimane ratios (0.54 to1.0), 8β(H)-homodrimane/8β(H)-drimane ratios (0.38 to 0.88), and C₂₉ diasterane/C₂₉ regular sterane (0.94 to1.65), and relatively heavy δ¹³C values (-25.0‰ to -26.2‰). These light oils were produced in Pinghu slope belt which is in the edge of the sag. Family B oils, generated from both coal and mudstone, are a mixture of family A and C light oils, displaying intermediate biomarker characteristics between family A and C oils with medium disterane contents and δ¹³C values (-26.2‰ to -26.9‰). This oil family is distributed in the Pingzhong area of Pinghu slope belt. This systematic geochemical analysis provides a detailed understanding on the complexity of the source of light oils in Xihu Sag.