Graham Yielding1,
Pete Bretan2,
Gary Marsden3
(1) Badley Earth Sciences, Lincolnshire, United Kingdom
(2) Badley Earth Sciences
(3) Amerada Hess, Aberdeen, United Kingdom
Abstract: Fault
-seal evaluation in exploration and production
environments
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