The concept of the centroid was described by Traugott (1997), and has also been referred to as “lateral pressure transfer” (Yardley and Swarbrick, 2001). The concept assumes that there is a midpoint or centre-point (the centroid) along a dipping sequence of permeable reservoirs and sealing non-reservoirs where the pore pressure in both is the same (pressure equilibrium). At this point, the predicted pore pressure (in the sealing non-reservoir) is assumed to be equal to the measured pore pressure in the porous reservoir.

In a hydraulically-connected formation, deeper overpressured areas are connected to shallower areas by a permeable layer. As a result, fluid flow along such a pathway can cause pressures to increase at the crest of structure – a common drilling target.

The relationship between predicted and measured pore pressure can be complicated, and an understanding of the relevant geological factors involved (compaction disequilibrium, seal breach, geologic and structural setting, etc.) is important. The measured pore pressure in the reservoir will usually follow the hydrostatic gradient of the formation fluid. The pressure in a hydraulically-connected formation can be calculated based on the difference in the heights of the fluid columns.

The centroid method can be used to predict the pore pressure in a porous reservoir (or hydraulically-connected formation) using formation fluid density and fluid column height, and can be calibrated to measured formation pressures in the well bore.