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Zhurina, Elena1, Brann Johnson2 
(1) Marathon Oil Company, Houston, TX
(2) Texas A&M University, College Station, TX

ABSTRACT: Inference of the Spatial Distribution of the Permeability of a Fault from Analysis of 3-D Quasi-Steady State Hydraulic-Head Data

We have measured and analyzed the 3-D, quasi-steady state, hydraulic-head distribution in the vicinity of a fault that impedes cross-fault flow in a siliciclastic aquifer/aquitard system. Our analysis demonstrates systematic variation of fault-rock permeability with a five order of magnitude change over a 50 m dip section and up to two orders of magnitude variation along a 15 m strike section. These variations correlate closely with structural and stratigraphic position. 
The 20 m displacement normal fault partially offsets the 50m thick aquifer, which consists of fluvial sandstones with a few discontinuous mudstone interbeds that grades upward into laterally continuous, interbedded sandstone and mudstone strata of the aquitard. We used eight Westbay multilevel monitoring systems with 94 zones to measure head distribution near and across the fault. Head differences across the fault vary systematically and range from 0.1 to 0.9 m. 
The quasi-steady state data are analyzed using a 3-D finite element model incorporating a high-resolution geological model. The spatial distribution of the fault-rock permeability is obtained utilizing both forward modeling and geophysical inversion. 
Modeling and pump test data show flow focused through the lower portion of the fault with minor flow across the up-dip regions where the fault cuts the aquitard. The model calculates a fault-rock permeability of 10-30 md where the fault juxtaposes the aquifer (500 md) against itself. Where the aquifer is faulted against the aquitard, the fault-rock permeability ranges from 0.01 to 3.0 md. Within the aquitard, fault-rock permeability decreases from 0.01 to 0.0003 md up-section.


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