--> ABSTRACT: Calibration of Pore Pressure Plots in the Gulf of Mexico, by J. R. Alnes and T. Lilburn; #91021 (2010)

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Calibration of Pore Pressure Plots in the Gulf of Mexico

ALNES, JOEL R., and TONY LILBURN

Pore pressure plots derived from resistivity data are used to identify top of geopressure, pressure release zones, and to plan mud densities needed to safely and efficiently drill wells in overpressured environments. The method to calibrate pore pressure plots described is applicable to Tertiary clastic basins where the main cause of geopressure is aquathermal pressuring.

A contributing cause of aquathermal pressures in the Gulf of Mexico is clay diagenesis. Dehydration of smectite (beginning in earnest at about 200 degrees F) causes water to be discharged from intermolecular positions to pore spaces. This increase in fresh water reduces salinity of both the shales and the more permeable rocks adjacent to those shales. The rate of change in salinity is proportional to the percentage of smectite-rich clay in the area. Because of this, resistivity plots need to be calibrated for specific regions.

Calibration of pore pressure plots is achieved by graphing temperature-adjusted minimum shale resistivities versus the ratio of actual to predicted pore pressures for a group of wells in an area. This graph will provide a linear relationship between actual and predicted pore pressures relative to water resistivities. Low salinity waters will cause pore pressure estimates to be low; high salinity waters will cause pore pressure estimates to be high.

Through calibration of pore pressure plots, changes in pore pressure gradients can be better understood. Examples of applications of this increased understanding include mapping of likely hydrocarbon migration routes and predictions of pore pressures in undrilled fault blocks. 

AAPG Search and Discovery Article #91021©1997 AAPG Annual Convention, Dallas, Texas.