A study of the gross compositions of oil-bearing fluid inclusions using high performance liquid chromatography

A high performance liquid chromatography (HPLC) method for assessing the compositions of crude oils and fluid inclusion oils is described. The necessary increase in instrumentation sensitivity was accomplished using a narrow bore Waters Spherisorb column with 5 mu m particle diameter. This HPLC method has advantage over the latroscan TLC-FID technique previously described as the <C-16 fraction is measured by the HPLC method. However, due to uncertainties in the calibration procedure, the HPLC method is only semi-quantitative. Aliphatic hydrocarbons were calibrated taking into account the molecular weight distribution of a typical oil derived by gas chromatography. Aromatic hydrocarbons were calibrated using standards according to retention times, but the response factors span two orders of magnitude, dependent on ring size. Polar compound response factors are strongly standard dependent. The gross composition of a suite of 12 inclusion oils and 17 co-occurring crude oils from various basins in Australia and Papua New Guinea have been analysed. The inclusion oils at the Jabiru and Tirrawarra wells are enriched in polar compounds relative to the co-occurring crude oils from these wells. This is attributed to an adsorption effect during trapping. However, many inclusion oils do not appear to be polar enriched, which may relate to trapping of inclusion oils in sealed fractures as well as on overgrowths. Inclusion oils tend to be relatively depleted in aromatic hydrocarbons, which could reflect a trapping phenomenon, with preferentially less aromatic hydrocarbons and/or more waxy aliphatic hydrocarbons being trapped in fluid inclusions. Alternatively, this may be a characteristic of the different chemistry of oils that are preserved early in the charge history of a reservoir. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.