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.