Fluid inclusion record of early oil preserved at Jabiru field, Vulcan Sub-Basin

Abstract

Oil trapped in fluid inclusions in a sample of Plover Formation sandstone from the main oil zone in Jabiru-1A has been subjected to detailed geochemical comparison with production oil from the well. Fluorescence microscopy of the sandstone showed that predominantly blue-fluorescing oil inclusions occur in 69% of quartz grains, both within quartz overgrowths and on healed fractures within detrital quartz grains. There are some notable similarities and differences between the fluid inclusion oil and the production oil. Similarities include the n-alkane profiles (both maximising at n-C15 with slight odd over even predominance at high molecular weights), n-alkane to isoprenoid ratios and Pr/Ph ratios (2.7±0.04). Hopane and sterane biomarker maturity ratios are at or close to equilibrium values, typical of peak oil generation conditions. The fluid inclusion oil differs from the production oil in containing relatively lower amounts of rearranged hopanes, Ts, C29Ts, C27 steranes and diasteranes. These differences are interpreted to be due to lower maturity rather than source rock facies variation. Aromatic hydrocarbon ratios confirm a small but consistently lower maturity for the fluid inclusion oil. For example, the calibrated methylphenanthrene index shows an equivalent reflectance of 0.84% for the fluid inclusion oil, compared to 0.92% for the production oil. These geochemical data suggest that the oil trapped in fluid inclusions in Jabiru-1A is from the same source rock but was generated at a lower maturity than the average of the oil now in the reservoir, and was trapped soon after initial charge mainly by thin quartz overgrowths. Further charge to the Jabiru structure was of progressively higher maturity oil.