COMPARISON OF PALAEO OIL CHARGES WITH CURRENTLY RESERVOIRED HYDROCARBONS USING MOLECULAR AND ISOTOPIC ANALYSES OF OIL-BEARING FLUID INCLUSIONS: JABIRU OIL FIELD, TIMOR SEA

Abstract

Geochemical techniques have been used to compare the composition of oil trapped in fluid inclusions from the Jabiru oil field with currently reservoired oil. The inclusion oil is preferentially enriched in polar compounds, probably due to an adsorption effect during trapping, but this has not affected the hydrocarbon composition of the trapped oil. Source characterisation using biomarker and gasoline range hydrocarbon parameters shows that the fluid inclusion oils have the same source affinity as the current production oil. This is corroborated by the carbon isotopic compositions of high molecuJar weight n-alkanes trapped in oil-bearing fluid inclusions, which are similar to those of the production oil. Both oils have maturities in the peak oil generative window, but aromatic hydrocarbon ratios demonstrate that the fluid inclusion oil is less mature (calculated reflectance [RJ = 0.84 per cent) than the currently reservoired charge (0.92 per cent Rc). Fluid inclusion abundance data and residual oil saturations indicate the Jabiru oil column was previously significantly larger, with subsequent leakage reducing the column to its present size. The geochemical data collected for the fluid inclusion oil suggests that it is representative of early charge to the Jabiru structure. The difference between the fluid inclusion oil and the production oil is thought to reflect continued charging of the trap with progressively more mature oil from the same or similar source rock facies. The change in the molecular composition of the oil in the Jabiru structure probably occurred by dilution of earlier, lower maturity charge with larger volumes of more mature oil.