Pore Pressure Estimation From Velocity Data: Accounting for Overpressure Mechanisms Besides Undercompaction

TLDR

Some other pore pressure estimation approaches are examined to show how they have unknowingly accounted for overpressure mechanisms beyond undercompaction. The study presents a new method for estimating pore pressure from formation sonic velocity data. The method uses an effective‑stress approach, computing effective stress from velocity, subtracting it from overburden stress to obtain pore pressure, and introduces two velocity‑vs‑effective‑stress relations—one for normal pressure and undercompaction overpressure, the other for velocity‑reversal zones caused by fluid‑expansion mechanisms. The method is applied to the U.S.

Abstract

Summary A new method for estimating pore pressure from formation sonic velocity data is presented. Unlike previous techniques, this method accounts for excess pressure generated by both undercompaction, and fluid expansion mechanisms such as aquathermal pressuring, hydrocarbon maturation, clay diagenesis, and charging from other zones. The method is an effective stress approach; the effective stress is computed from the velocity, and the result is subtracted from the overburden stress to obtain pore pressure. To include multiple sources of overpressure, a pair of velocity-vs.-effective-stress relations are introduced. One relation accounts for normal pressure and overpressure caused by undercompaction. The second is applied inside velocity reversal zones caused by fluid expansion mechanisms. Example applications of the method are presented from the U.S. gulf coast, the Gulf of Mexico, and the Central North Sea. Some other pore pressure estimation approaches are also examined to demonstrate how these techniques have unknowingly accounted for overpressure mechanisms other than undercompaction. It is also explained how velocity-vs.-effective-stress data can be used to identify the general cause of overpressure in an area. For instance, the empirical correlation of Hottman and Johnson indicates that overpressure along the U.S. gulf coast cannot be due only to undercompaction.