Determination of Fault Populations below the Limit of Seismic Resolution for Reservoir Models

Abstract

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 {minus}0.5 to {minus}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 extrapolations are valid are determined by specific scaling laws. Calculated faultmore » 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.« less