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Petroleum Expulsion and Formation of Porosity in Kerogen


This study evaluates the magnitude and distribution of petroleum pressure within a source rock, drive mechanism for expulsion, and kerogen volume changes during transformation. Together, these concepts explain why kerogen is more porous in some source rocks than in others.

Petroleum pressures within a sheet-shaped source rock equilibrate rapidly (<1 My), and pressure distribution can be modeled as a pseudo-steady process. The vertical distribution of petroleum excess pressure within the sheet acquires a warped hyperbolic shape. Generation causes a symmetric petroleum excess pressure distribution that is zero at the top and base of the sheet and highest in the middle. This petroleum pressure is superimposed on a capillary pressure gradient controlled by water-petroleum density difference and fluid pressures at the top and base of the sheet of source rock. The capillary pressure gradient controls the direction of expulsion more than the magnitude of excess pressure.

Petroleum excess pressure increases with increasing petroleum generation rate, increasing source-rock thickness, and decreasing petroleum mobility. Modeled excess pressure does not exceed a few hundred psi except where source rocks are very thick (hundreds of m), generation rates are very high, or petroleum mobility is low (picodarcy/centipoise). For typical thin source rocks, excess pressures are on the order of a few psi. The excess pressure driving expulsion is too low to significantly reduce source-rock effective stresses during generation.

Density of oil-prone kerogen increases with decreasing hydrogen index and hydrogen/carbon ratio; therefore, kerogen volume changes with transformation can be modeled using initial hydrogen index and transformation ratio. The kerogen shrinks as its mass is converted to petroleum and its hydrogen content decreases. Where kerogen forms part of the framework supporting overburden stress, the source rock consolidates as fast as the kerogen shrinks (compaction equilibrium). The source rock thins during transformation, but generation creates negligible porosity in kerogen.

Porosity forms in kerogen only where effective stress remains low during and after generation. For example, the mechanical strength of rigid bodies shelter adjacent kerogen from high effective stresses. Kerogen porosity is more likely to form and be preserved in source rocks rich in silt-sized and coarser rigid grains than in clay-rich source rocks.