Geochemical comparison of core extracts and oil samples in reservoir

A geochemical study was undertaken to compare the geochemical parameters obtained from the analysis of different sample types of oils collected from the same horizons, in particular produced oil samples and oil extracted directly from reservoir cores in the laboratory and analysed by slightly different methods. It is not easy to carry out a major comparison because of the scarcity of horizons from which both DST oil samples and cored reservoir samples are available. BP was only able to locate such samples from three wells (Kirklington-2, onshore England; Qusahwira-1, Middle East and 26/28-1, offshore Ireland). The composition and concentration (petroleum/rock percentage) of the oil in these three wells varied widely. Some molecular maturity parameters, e.g. methylphenanthrenes and sterane isomerisation ratios, suggest that the extraction process mobilises lower maturity material from the cores. In some cases the parameters contradict one another, possibly indicating measurement of different components of the petroleum or inherent uncertainties in the analysis. Source-type indicators are observed for some of the oils and their respective core extract samples, from the same intervals in the same wells, which suggest derivation from different types of organic material. The results show that it is not possible to characterise an oil definitively by examination of the core extract, that the differences may not be systematic and that comparison of oil and extract data from different wells or horizons may be misleading. This study demonstrates some of the differences that need to be taken into account during interpretation.