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Brown, Alton A.1
(1) Consultant, Richardson, TX

ABSTRACT: Potentiometric Mapping of Deep Aquifers Using Environmental Head

A scalar fluid potential is strictly applicable only in isothermal systems with constant fluid salinity. These conditions are not met in deep (>2 km) aquifers, where hydrodynamic patterns are important for migration and trapping of petroleum. Despite this limitation, freshwater potentiometric (head) maps are routinely constructed and used to interpret subsurface flow patterns in deep basins. If misapplied, freshwater head maps of deep aquifers may indicate anomalous flow patterns, such as flow toward the center of a deep, freshwater basin. An index such as the DFR indicates where head maps may mislead, but DFR does not adjust the data to show the correct flow pattern.
Environmental head maps approximate deep aquifer fluid flow patterns better than freshwater head maps in basins where density changes are mainly caused by geothermal heating. Environmental head is calculated using water density averaged from the datum to the elevation of measurement instead of using constant density at all depths. Environmental head assumes that the density change is a function of elevation, much like a negative compressibility.
Where aquifers have significant salinity variations, environmental head can be calculated to include the effects of both temperature and salinity, but heads are valid only where high-salinity water is static in the deeper parts of the basin. Hydrodynamic maps constructed from a head including salinity variations should be used only to determine if the saline fluid is hydrostatic. Density is not a simple function of elevation in hydrodynamic, variable-salinity aquifers, so the environmental head approximation cannot be applied.

 

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.