Expulsion of petroleum hydrocarbons from shale source rocks

Abstract

There is compelling evidence that movement of petroleum from organic-rich shales into sandstones and limestones, occurs in the subsurface¹. This expulsion of material must occur if crude oil accumulations are to form, and it is termed primary migration². Earlier studies^3,4 suggested that this movement occurs with chemical fractionation: some components are more mobile than others. One of the agents of expulsion at early maturity stages is compaction, which is not only capable of forcing water, but also organic components out of the pore network of a source rock^2,5,6. Because the fractionation effects associated with expulsion provide clues to the transport mechanisms involved, we have made detailed quantitative comparisons of the hydrocarbons of shales of different thicknesses, the centre and edges of thick shales and sandstones adjacent to shales from two cored, interbedded shale and sandstone sequences from Spitsbergen Island, Svalbard, Norway. These observations confirm considerable movement of hydrocarbons in the subsurface, with lower molecular weight components being more mobile. Up to 90% the original amount of n-pentadecane may be lost from very thin shales sandwiched between sandstone horizons and from shales close to a lithological boundary with sandstone.