Chemical changes of fluid inclusion oil trapped during the evolution of an oil reservoir: Jabiru-1A case study (Timor Sea, Australia)

This study of the Jabiru oil field, Timor Sea (Australia), shows how the variability of visual properties of fluid inclusions in palaeo-oil zones can be mapped to identify internal structures that result from fluid interactions in the reservoir. Variability in the attributes of oil inclusion assemblages (OIAs) is greater within an inferred palaeo-gas cap (Zone 1) and current residual zone (Zone 3) and less within the current oil zone (Zone 2). In Zone 3, where water imbibed, a higher proportion of OIAs with visual white fluorescing oil inclusions correlates with less mono-aromatic compounds and a higher abundance of polar compounds determined on a bulk extract of inclusion oil using High Performance Liquid Chromatography (HPLC). The CH2/CH3 ratios and water contents, determined by Fourier Transform-Infrared Spectroscopy (FT-IR), are higher. In Zone 1, where gas drained, there is a higher proportion of OIAs with visual yellow fluorescing oil inclusions, and these correlate with low HPLC mono-aromatic and moderate polar contents, and low FT-IR CH2/CH3 ratios and water contents. Interpretation of the grains with oil inclusions (GOI (TM)) and pressure-volume-temperature-composition (PVTx) fluid inclusion data suggests that a gas-saturated blue fluorescing palaeo-oil was present in the three zones at the same Pressure-Temperature fluid inclusion forming event PT1. Lighter gas-saturated blue fluorescing oil was present in Zone 2, but was rarer in Zones 1 and 3 at the later fluid inclusion forming event PT2. The yellow and white inclusion oils, trapped in Zones 1 and 3 respectively, are gas-depleted and interpreted to result from the interaction between non-movable residual oil around grains with the surrounding gas and water, respectively. Comparison between the PVTx data and the published burial curve suggests that most of the fluid inclusions were trapped during two different hot fluid flow events. The full oil column is interpreted to have been present during Early Mid Tertiary. The palaeo-oil column was reduced during the Late Miocene by imbibition of water in the present residual zone and drainage of gas at the top of the reservoir. (c) 2012 Elsevier Ltd. All rights reserved.