Abstract
The limited data available on the mechanism and the efficiency of the processes by which petroleum is expelled in the subsurface from source rocks into adjacent reservoir rocks are mostly qualitative1,2. We present here material balance-type comparisons of sample series extending from the centre portions of two adjacent hydrocarbon source rock units towards their outer edges3, which enable expulsion efficiencies to be determined. In certain parts of the shale the expulsion efficiencies for total extract and its saturated and aromatic hydrocarbon fractions, have much higher values than previously thought (reaching up to 80% for the total extract) which increase towards possible secondary migration avenues. The expulsion is associated with fractionation effects, such as those based on polarity differences. Also, expulsion efficiencies were found to be related to the degree of hydrocarbon saturation of the pore system: they are higher for a rich, oil-prone source rock unit compared with a poorer quality one, which had generated less hydrocarbons.
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