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Quantitative analysis of slip-induced dilation with application to fault seal

¹ Shell International Exploration and Production, Inc., 3737 Bellaire Blvd., Houston, Texas 77025; scott.wilkins{at}shell.com
² Shell International Exploration and Production, Inc., 3737 Bellaire Blvd., Houston, Texas 77025; steve.naruk{at}shell.com

Scott J. Wilkins is a research geologist with the Structure, Traps, and Seals Team at Shell International Exploration and Production, Houston, Texas. He received his M.S. degree and his Ph.D. in geology from Florida International University (1999) and University of Nevada–Reno (2002), respectively. His work at Shell involves structural geology research and global technical consulting. He currently works in the fields of fault-seal analysis, fracture prediction, and structural restorations.

Stephen J. Naruk is currently a principal structural geologist for Shell International Exploration and Production, where he serves as functional lead for structural geology research and development. He received his M.S. degree and his Ph.D. in structural geology and tectonics from the University of Arizona.

A new quantitative model for calculating the volumes and rates of fault-zone fluid migration during dynamic slip events is developed. This model is based on observations from laboratory rock mechanics experiments of porosity evolution during frictional sliding and geophysical data that demonstrate the existence of propagating slip pulses that experience transient dilation during earthquakes. The nature of dilation during slip is dependent on the frictional behavior; that is, whether the fault experiences seismic or aseismic slip. The transient dilation that occurs during seismic slip is more efficient in transporting fluids along the fault than the relatively reduced dilation that occurs during aseismic slip. For aseismic faults, fluid-migration rates are primarily dependent on the intrinsic permeability structure of the fault rock during periods of no slip (static permeability). In contrast, both the static and dynamic permeabilities are important along faults that slip seismically. We infer that aseismic slip does not enhance migration significantly other than providing a transient, spatially restricted permeability increase that may act to initiate migration.

The proposed models are applied to estimate the maximum amounts of hydrocarbon leakage from seismically active faults that trap hydrocarbons in the Cusiana field, Colombia, and aseismic faults at Eugene Island Block 330 field, Gulf of Mexico. Model results are grossly consistent with observed hydrocarbon volumes and inferred along-fault migration rates from the two areas. The proposed models have broad implications for understanding the effects of faults on fluid migration during both exploration (geologic) and production time scales.