A method for estimation of the density of fault displacements below the limits of seismic resolution in reservoir formations



C. Childs, J. J. Walsh & J. Watterson

Abstract - The fracture density per unit volume in a reservoir formation can be measured directly from seismic data for fractures with displacements greater than the limit of seismic resolution. Plots of displacement size vs. cumulative number are produced from seismic interpretations digitized and processed using software developed for the analysis of fault geometries and displacements.

Data from several North Sea oilfields have been analysed and show that the cumulative fault displacement distributions are non-linear but systematic and are a function of (i) the variation in displacement on individual faults, which varies with fault size, and (ii) the size population of faults within the measured volume. On logarithmic plots of displacement size vs. cumulative number, the oilfield data distribution curves have straight central segments with slopes between -0.5 and -0.1. The differences in slope probably represent real differences in the type of fault population. Straight segments of the population curves can be extrapolated from the lower limit of seismic resolution, about 10ms or 15m, to lower displacement values for estimating the densities of fractures with displacements too small to be resolved seismically. Extrapolation to displacements as small as 1cm is justified by numerically modelled population curves and by the empirical demonstration that fault data, from coal mines and other sources, show systematic distributions for displacements from 50m down to 1cm. Fault displacement distributions appear to be similar in extensional and compressional regimes. Fault displacement distributions plots can be used to determine the limits of seismic resolution and could form the basis of a method for testing the quality of seismic interpretations.

The theoretical model developed for fracture density distribution is based on theoretical displacement distributions on single faults, combined with a "dead" fault population model that is based on earthquake populations and a fault growth model


In: North Sea Oil and Gas Reservoirs II (edited by Buller, A. T. et al.) Graham & Trotman, London, 193-203, 1990.