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AAPG Bulletin

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AAPG Bulletin; Year: 2023; Issue: July
DOI: 10.1306/10242221068

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Figure 9. (A) The critical porosity, ϕ_c, represents the minimum porosity for fluid flow in a particular direction. The z is the bedding-perpendicular direction, and x–y represents the bedding plane. The critical porosity required for percolation (10%–12%) in the z direction is higher than the average porosity of 8% in mudrocks around the world. However, primary hydrocarbon migration occurs in the vertical direction, because of the complex interconnections of natural fractures, rock damage, and microcracks created during kerogen–oil conversion. Once all fractures are blocked with bitumen and pyrobitumen, mostly isolated clusters of the hydrocarbon emerge and migration is limited, leading to an overpressured system. Source: adapted from Keller et al. (2013a). (B) The source rock in contact with the highly permeable layer at the top. Oil starts flowing up through the microcracks created by the overpressure. The expulsion is modeled using a one-dimensional (1-D) pressure diffusion equation. z_av, y_av, x_av = average critical porosity in the z direction, y direction, and x direction, respectively.

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