Print this pageSealing Behavior of Normal Faults in 
Fault
Block II,
Wilmington Oil Field, California
AN, LINJI Y., IRAJ ERSHAGHI, CHRISTOPHER C. PHILLIPS, and DONALD D. CLARKE
Fault Block II in Wilmington oil field is bounded by the Wilmington fault to
the west and the Cerritos fault to the east. Fault block II is further divided
by the Ford fault into Fault Block IIA and Fault Block IIB. The sealing property
of these boundary faults are evaluated in the Tar zone interval, a Lower
Pliocene Middle Repetto formation, as part of a DOE cost-share project for
vigorous reservoir characterization and subsequent steam flood. Oil-water
contacts indicate that the Wilmington fault slowly leaks oil near the southern
end from the T2, T5 and T7 sub-subzones in Fault Block IIA into the same
sub-subzones in Fault Block I. The Wilmington fault also leaks oil from the T2,
T5, and T7 sub-subzones in Fault Block I into the S sand (a wet sand immediately
above Tar) in Fault Block IIA through a paleochannel near the crest of the
Wilmington anticline. Ford fault leaks oil from T2, T5 and T7 sub-subzones in
Fault Block IIA to S sand in Fault Block IIB, also through the paleochannel. The
northern segment of Cerritos fault leaks in T1 sub-subzone and gradually becomes
sealing with depth. All other portions of the boundary faults are sealing
structures indicated by zigzagged lower and lateral reservoir boundaries, and
lack of communication between injection and production wells across the fault.
Well log examination, core analysis, sandstone analysis and Allan diagram
construction indicate that juxtaposition of permeable sand against impermeable
shale is not enough to explain fault sealing in the studied area. All the
evidence indicates that a fault itself behaves like a layer of impermeable shale
and such a behavior becomes stronger as fault displacement increases. A gouge
zone model is therefore considered to be most appropriate.
The future sealing ability of each fault is predicted from fault sealing
strength. Fault sealing strength at any portion of a fault is evaluated by
applying two criteria: (1) If a permeable layer juxtaposes against an
impermeable layer, the fault seal is strong; (2) if all the other conditions are
the same, a fault with larger displacement has a stronger fault seal than the
one with smaller displacement. The strong and weak sealing portions on the fault
planes are directly identified on the Allan diagrams constructed for each
fault
.