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Bureau Seminar, February 13, 2009

From meters to microns: Subseismic fault populations and fault zone architecture

John Solum

Features less than a few meters in size cannot be directly imaged from seismic data but small structural features (small faults, deformation bands, etc.) and smaller-scale components of larger structures (e.g., fault zone architectural elements) can significantly impact fluid flow and therefore reservoir performance.  This presentation will focus on two topics: 

1) Improving statistical predictions of subseismic fault populations by incorporating mechanical stratigraphy. It appears that in some settings fault size distributions deviate from a power law relationship at scales that roughly correspond to reservoir thickness.  This appears to be due to the difference in the mechanical properties of the reservoir and its surrounding units.  Moreover, statistical predictions of subseismic fault populations are in part a function of seismic resolution since it is not possible to resolve discrete faults at low resolutions.

 2) Incorporating clay authigenesis into predictions of fault rock composition and fault behavior.  Fault-related clay authigenesis has been recognized in faults from a wide variety of settings from plate boundary faults like the San Andreas to thrust faults with offsets of a few meters.  This phenomenon has impact on both fault mechanics (stick-slip vs. creeping behavior) and fault hydrologic properties (permeability, transmissibility, etc.).  Authigenic changes in mineralogy such as smectite to illite can lead to an increase in fault strength, and potentially increase rupture interval.  Similarly since smectite and illite have different rate dependent frictional properties authigenesis can cause a change from stick-slip to creep or vice versa.  Finally, authigenesis appears to strongly influence slip surface continuity and therefore has the potential to influence cross-fault permeability.


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