Determination of Fault
Populations below the Limit of Seismic Resolution for Reservoir Models
WALSH, JOHN J., and JUAN WATTERSON, University of Liverpool, Liverpool, U.K., and GRAHAM YIELDING, Badley Ashton & Associates, Winceby, U.K.
Direct measurement of fault
displacement populations is possible only on the two disparate scales represented by seismic and by core data that are 2-3 orders of magnitude apart in terms of the
fault
displacement values recorded.
Seismic data from several offshore oilfields have been analyzed to determine the observable fault
surface area per unit volume and its distribution with respect to
fault
displacement values. On logarithmic plots of
fault
displacement vs. cumulative frequency, the data
distributions have straight central segments with slopes of -0.5 to -1.0. Differences of slope represent real differences in the type of fault
population. Extrapolation of the straight segments beyond the limit of seismic resolution gives estimates of the
fault
density at subseismic scales. Extrapolation has several justifications: (1) where available, measurements of
fault
displacements in core conform with the predictions made by extrapolation of seismic data from the same field; (2)
fault
data from coal mines and from outcrop show systematic distributions for displacements down to 1 cm; (3) numerical and analytical modeling of
fault
displacement populations indicates a near-linear distribution down to the smallest displacements in a population. The rock volumes for which extrapolat
ons are valid are determined by specific scaling laws.
Calculated fault
populations can be included in reservoir models either explicitly, as individual faults, or implicitly by adjustment of permeability values for defined scales of volume. In either case the effects of the specific sediment architecture and of the
fault
surface hydraulic properties must be taken into account.
AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)