Effect of water pressure on bitumen and expelled oil biomarker evolution: insights from laboratory simulation experiments

Biomarkers are powerful tools to assess thermal maturity of oil and rock extracts and for oil-source rock correlation. While temperature effects on biomarker evolution have been widely studied, the impact of pressure remains largely underexplored. This study examines the effect of high water pressure (up to 900 bar) on biomarker maturation in expelled oil and extracted bitumen from pyrolysis experiments on a rock sample from the Campanian–Maastrichtian Duwi Formation, Red Sea Basin, Egypt. Extracted bitumens exhibited higher maturity under anhydrous conditions compared to low-pressure hydrous samples, and this was more pronounced at 350◦C. The extracted bitumen displayed higher biomarker maturity ratios than the corresponding expelled oil at 350◦C, likely due to prolonged interactions of the bitumen with the rock mineral matrix. In contrast, δ13C values were similar for extracted bitumen and the corresponding expelled oil. At 320◦C, high pressure reduced values of biomarker maturity ratios, particularly C31–C35 homohopane isomerisation, Ts/Tm, Ts/H30, and C29Ts/ H29 ratios, whereas sterane ratios remained unaffected. At 350◦C, pressure effects were less significant, with some anomalous variations, suggesting a non-systematic influence on biomarker maturation at higher temperatures. These findings demonstrate the complex role of pressure in biomarker evolution, emphasising the need to consider pressure in biomarker-based maturity assessment, particularly in overpressured basins and deep petroleum systems, where high pressure may be a dominant factor. Based on 12 source-related biomarker and isotopic ratios, chemometric analysis reveals that artificially generated oils and South Malak-1 oils from southwestern onshore Gulf of Suez are quite different, and both groups differ significantly from other natural oils from the central and northern parts of the basin. This may be due to facies variations and the effects of mixing from multiple source horizons under natural subsurface conditions.