The Influence of Fault Zones on Crustal-Scale Fluid Transport

LOGAN, JOHN M., Texas A&M University, College Station, TX

Crustal-scale fluid transport is restricted by hydrologic conditions, fault zones, and horizontal "seals" of low permeability. Vertical fault zones may impede horizontal fluid flow to such an extent that when combined with horizontal constraints chambers of abnormal fluid pressures develop. These "seals" are not absolutely impermeable but have sufficiently low values that substantial pressure differences can be maintained for geologic times. Paradoxically, fault zones may also act as conduits that enhance fluid migration. Separate portions of the same fault may act in both fashions simultaneously, and single faults may change from one to the other with time.

Field studies demonstrate the potential decrease in permeability across fault zones. Measurements on subsurface core document the anisotropic permeability with low values at high angles to the zone combined frequently with significantly higher readings parallel to the zone.

Laboratory experiments under triaxial conditions show that mechanical shearing alone under dry conditions in the brittle and semi-brittle domains will significantly reduce permeabilities across the simulated fault. Discreet channels for flow parallel to the zone may remain, however. This anisotropy is a manifestation of the texturally heterogeneous character of the fault zones. In experiments with chemically active fluids, even at low temperatures, shearing further reduces permeabilities to the nanodarcy range, but much higher values are retained parallel to the zone. Relationships between pressure differential, measured permeabilities, and decay times can then be calculated.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)