Episodic, compression-driven fluid venting in layered sedimentary basins

Abstract

Fluid venting phenomena are prevalent in sedimentary basins globally. Offshore, these localised fluid-expulsion events are archived in the geologic record by the resulting pockmarks at the sea-floor. Venting is widely interpreted to occur via hydraulic fracturing, which requires near-lithostatic pore pressures for initiation. One common driver for these extreme pressures is horizontal tectonic compression, which pressurises the entire sedimentary column over a wide region. Fluid expulsion leads to a sudden, local relief of this pressure, which then gradually recharges through continued compression, leading to episodic venting. Pressure recharge will also occur through pressure diffusion from neighbouring regions that remain pressurised, but the combined role of compression and pressure diffusion in episodic venting has not previously been considered. Here, we develop a novel poroelastic model for episodic, compression-driven venting. We show that compression and pressure diffusion together set the resulting venting period. We derive a simple analytical expression for this venting period, demonstrating that pressure diffusion can significantly reduce the venting period associated with a given rate of compression. Our expression allows this rate of compression to be inferred from observations of episodic venting. We conclude that pressure diffusion is a major contributor to episodic fluid venting in mudstone-dominated basins.