Pressure Stratigraphy: A Useful Concept for Pressure Seal Analysis

Williams, Kenneth E.¹, Joel Gevirtz¹ (1) Knowledge Systems, Inc, Sugar Land, TX

Geopressure is a vertically-changing phenomenon. Therefore, it is useful to describe it in stratigraphic terms. The fundamental unit of pressure stratigraphy is a coupled pressure seal and pressure compartment; and is a concept for improving accuracy and speed of pressure seal analysis for well design, reservoir risk assessment, and prospect selection.

Seal/compartment pressure units are characterized by contrasting permeability. They are not necessarily dependent upon lithology or traditional notions of a stratigraphic boundary. Instead, they are the result of a complex interplay between the sources of overpressure and the dissipation of that pressure over geologic time.

Pressure within seal intervals can increase or decrease depending on the pressure in the adjacent compartments. When pressure increases at a faster rate than overburden is added, the change cannot be attributed to undercompaction disequilibrium. Instead, other unloading phenomena, such as hydrocarbon generation or clay diagenesis, must be present. The most common reason for this high rate of change across the seal may be pressure dissipation from formation water draining out of the compartment via bleed-off into a more permeable unit.

An example of a pressure stratigraphic unit is a shale/sand pair, where the concept provides insight into pressure mechanisms and the relationship between seal and compartment pressures. Pressure in the sand can rise when fluids enter the sand at a rate faster than the shales can accommodate. If the couple is reversed, then normal pressure is maintained or released because the sand can accommodate the dewatering of the shale.