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

¹ Hydro ASA, Research Centre, Box 7190, N-5020 Bergen, Norway; roald.farseth{at}hydro.com
² Eni Norge AS, Box 101 Forus, N-4064 Stavanger, Norway; eivind.johnsen{at}eninorge.com
³ Statoil ASA, Box 7200, N-5020 Bergen, Norway; sperre{at}statoil.com

Roald B. Færseth received a Cand. Real in structural geology from the University of Bergen in 1971. He worked as a researcher at the University of Bergen (1971–1977) and has been with Hydro ASA since 1977. He has worked extensively in the area of fault architecture in onshore and offshore areas, fault seal analysis, and the risking of fault seal capacity in prospect analysis.

Eivind Johnsen has worked with Eni Norge AS since 2003. He has a Cand. Scient. in structural geology from the University of Bergen (1998). From 1998 to 2003, he worked as an explorationist within operations and as a researcher at Norsk Hydro Research Centre. His main competence areas are seismic interpretation, fault architecture, fault seal analysis, and prospect analysis.

Susanne Sperrevik has worked with Statoil ASA since 2005. She has a Cand. Scient. in structural geology from the University of Bergen (1998) and a Dr. Scient. from the University of Bergen (2002). From 1998 to 2005, she worked as a researcher at Norsk Hydro Research Centre. Her main competence areas are fault seal analysis, structural restoration, seismic interpretation, and prospect analysis.

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.