  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.
