Composition changes of hydrocarbons during secondary petroleum migration

Abstract

We investigate secondary migration of hydrocarbons with significant composition difference between the source and oil pools in the Cooper-Eromanga Basin, Australia. The secondary migration period is significantly shorter than the time of the hydrocarbon pulse generation, so neither adsorption nor dispersion of components can explain the concentration difference. The filtration coefficients, obtained from oil compositions in source rock (Patchawarra Formation) and in the reservoir (Poolowanna Formation and Hutton Sandstone), monotonically increase as carbon number increases. The monotonicity takes place for heavy hydrocarbons (n > 10). Loss of monotonicity for light and intermediate hydrocarbons can be explained by their evaporation into the gas phase. The evaporation of light and intermediate hydrocarbons into the gas phase is supported by their concentrations in oil, which are higher in source rock than in trapped reservoir oil. The paper proposes deep bed filtration of hydrocarbons with component kinetic retention by the rock. Introduction of the component capture rate into the mass balance transport equation allows matching the concentration difference, and the tuned filtration coefficients are in the common range. The results suggest that deep bed filtration controls the final reservoir oil composition during secondary migration in the Cooper-Eromanga Basin petroleum system, which was not previously considered.