(1) Badley Earth Sciences, Lincolnshire, United Kingdom
(2) Badley Earth Sciences
(3) Amerada Hess, Aberdeen, United Kingdom
Fault seal in sand-shale sequences is broadly predictable, since sealing fault rock is generated by the sliding of different lithologies past one another. Potentially-sealing fault rocks include clay smears and cataclastic gouges; clay-rich fault rocks tend to form the better seals because they have smaller pore-throats. The first-order controls on fault-rock development are the lithologies (clay content) in the faulted sequence and the amount of offset on the fault. Input of this information to a simple algorithm (Shale Gouge Ratio, SGR) allows a prediction of the nature of the fault rock at each point on the fault plane.
In exploration/appraisal settings, the capillary entry pressure of the fault-zone material is the critical parameter in determining whether a fault can successfully form a side-seal to an accumulation. Published measurements on fault-gouge samples show how entry pressures vary with clay content. Observations of in situ pressure differences across faults provide evidence for static seal below the entry pressure. Together, these datasets define a 'seal failure envelope' for fault rocks of different compositions.
In production, the transmissibility (permeability/thickness) of the fault zone is more important. Measurements on fault-gouge samples show how permeability varies with gouge composition. Using the Shale Gouge Ratio, fault-zone permeability can be mapped across the fault surface, providing a geologically-driven estimate of the transmissibility for input to reservoir simulation models. Using this methodology on a producing North Sea field gave an excellent history match after only 1 day of analysis (compared to several months of earlier work).
AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana