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AAPG Bulletin

Abstract

AAPG Bulletin, V. 91, No. 9 (September 2007), P. 1231-1246.

Copyright copy2007. The American Association of Petroleum Geologists. All rights reserved.

DOI:10.1306/03080706051

Methodology for risking fault seal capacity: Implications of fault zone architecture

Roald B. Farseth,1 Eivind Johnsen,2 Susanne Sperrevik3

1Hydro ASA, Research Centre, Box 7190, N-5020 Bergen, Norway; [email protected]
2Eni Norge AS, Box 101 Forus, N-4064 Stavanger, Norway; [email protected]
3Statoil ASA, Box 7200, N-5020 Bergen, Norway; [email protected]

ABSTRACT

We introduce a methodology for quantifying the risk associated with a seal for fault-bounded prospects. Applying this methodology, the aspects of fault seal are confined within four main risk categories. The methodology allows comparable criteria to be applied in the risking procedure to reduce uncertainty in fault seal assessments. As a foundation for the methodology, we combine onshore and offshore data from large faults and demonstrate how architecture and the distribution of fault rocks may influence sealing capacity. Despite the variable and complex structure of fault zones, we have observed fault zone characteristics that appear in common to the faults investigated, and we consider these factors to be crucial in the risking of fault seal predictions. The fault zones in our database, typically bounded by external slip surfaces, represent two main categories: (1) a layer of shale smear entrained into the fault zone and derived from a thick shale source layer within the sequence offset by the fault and (2) fault zones characterized by internal slip surfaces, slivers of footwall and hanging-wall–derived material rotated along the fault zone and commonly enclosed in a matrix of shaly-silty fault gouge. This study highlights the disparity between the complexity of actual faults and the abrasion-type shale gouge ratio (SGR) algorithm currently used in the industry to estimate sealing capacity of faults, which assumes that the seismically derived throw is concentrated in a single fault plane. We discuss how this may influence sequence juxtaposition across a fault, the associated SGR values, and ultimately, the fault seal risking.

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