Comparison of overpressure magnitude resulting from the main generating mechanisms

Overpressure is created by two main processes: (1) stress applied to a compressible rock and (2) fluid expansion. Both processes are most effective in fine-grained lithologies, such as mudrocks and chalks. Both processes involve ineffective fluid expulsion to create pressures in excess of hydraulic equilibrium, emphasizing the importance of permeability (a poorly known rock property in fine-grained sedimentary rocks) in controlling pore pressure in the subsurface. Overpressure generation and fluid expulsion can be modeled assuming Darcy flow though a pore matrix. The basin conditions favoring high-magnitude overpressure from stress are a high sedimentation (loading) rate and/or strong lateral compressive forces. A high sedimentation rate, as a means to create rapid increase in temperature, also favors high-magnitude overpressure from fluid expansion mechanisms. An alternative method to achieve a rapid increase in temperature is a thermal pulse associated with tectonic or magmatic processes. Magnitude of overpressure from applied stress is controlled by the rate of increase in stress, sediment and fluid compressibility, and the rate of fluid expulsion. Continuous deposition of fine-grained sedimentary rocks leads to onset of overpressure at the fluid retention depth (FRD), the point at which fluid expulsion is no longer fully effective. Overpressure magnitude in this setting commonly increases at approximately 12.0-12.6 MPa/km (0.95-1.0 psi/ft), that is, along a gradient subparallel with the lithostatic stress gradient, implying only minor fluid expulsion. The variable characteristic of each lithology creates differences in the magnitude of overpressure. Magnitude of overpressure from fluid expansion mechanisms is controlled by the rate of volume change, which can be shown to be slow for the burial rates and temperature gradients found in most basins. In addition the volume increase in all reactions except gas generation is small. Although gas generation has the capacity to create high-magnitude overpressure locally (up to tens of MPa), the magnitude is diluted where a large connected reservoir volume is involved. The process is therefore most likely effective only within gas-generative source rocks and in thin intraformational reservoirs where oil cracks to gas. Chemical processes involving fabric change, dissolution/reprecipitation, and solid to liquid transfer may also play a part in creating conditions favoring overpressure through fluid retention, but these cannot be quantified with existing data. Finally, overpressure related to hydraulic head and hydrocarbon buoyancy effects should not be ignored, but the magnitude of overpressure can be easily assessed.