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Naturally Occurring Underpressure — A Global Perspective


Underpressure is defined as any pore-pressure below hydrostatic i.e., the pressure exerted by a column of water. Correspondingly, overpressure is defined as any pressure in excess of hydrostatic and is more widely documented in literature. Understanding the distribution and implications of underpressure is important for drilling safety, hydrocarbon exploration and the potential to miss pay. The injection of gas, such as carbon dioxide, can be complicated by underpressure as it influences storage capacity and fluid phase.

Underpressures of up to 1000 psi are common in sedimentary basins of North America, China, Russia and the Norwegian Barents Shelf and Svalbard. Whilst they are geologically distinct, all have undergone recent uplift.

A number of mechanisms have been hypothesized as main drivers of underpressure but fundamentally the phenomenon must relate to either reduction in fluid volume or an increase in connected pore volume. We investigate all potential mechanisms including the most widely cited of differential hydraulic flow, rock dilation, thermal effects, and differential gas flow.

Differential hydraulic flow is proposed to occur where a dipping reservoir has lower rates of meteoric recharge in up-dip areas than discharges from the system down-dip. Thermal effects include fluid volume reduction due to cooling. Differential gas flow occurs where initially overpressured gas accumulations displace water in tight formations which subsequently become underpressured during uplift. Rock dilation occurs due to unloading and subsequent elastic rebound and we also investigate the influence of fractures as recent studies in Svalbard indicate they may play an important role. In addition to our synthesis we investigate the present day and historical hydrostatic gradients and their implications.

The majority of these causal mechanisms are inferred from basins in North America. Other basins possessing underpressure have geological differences which enables us to rule out certain mechanisms. For example, on the Barents Shelf and Svalbard differential flow can be discarded as the entire underpressured target interval is subsea.

We investigate the geological characteristics of 19 underpressured basins from around the world on a case by case basis. We identify and compare where specific underpressure causing mechanisms are likely to occur and where they are geologically unfeasible.