Expulsion from hydrocarbon sources: the role of organic absorption

Abstract

The role of absorption of generated petroleum within solid organic matter in source rocks has been investigated. We conclude that absorption is a significant phenomenon and that selective absorption is one cause of observed compositional differences between migrated petroleum and petroleum (bitumen) extracted from source rocks. Evidence for absorption includes the frequently-noted correlation between extractable organic content and TOC. Absorption selectivity appears consistent with documented interactions between solvents and either coals or synthetic polymers. Preliminary modeling of selective absorption equilibrium gives compositions of absorbed versus free liquids that are consistent with observations. A method is described for including hydrocarbon absorption effects in predictive models of hydrocarbon generation and expulsion. A key part of this method is assessment of expulsion efficiency from source rocks, using geochemical measurements. When absorptive retention of generated hydrocarbon liquids is included in yield calculations, expulsion of oil from low Hydrogen Index sources (even with high TOC values) is delayed. This results in ultimately higher gas-oil ratios of expelled petroleum.