Improving Understanding of Compartmentalization through Linking Mechanical Stratigraphy to Statistical Predictions of Subseismic Faults

Solum, John G.¹; Naruk, Steve ¹; Thompson, Jill ²; Sneep, Rik ^3
1 Bellaire Technology Center, Shell Int'l E&P, Houston, TX.
² Shell Exploration & Production Company, Houston, TX.
³ Nederlandse Aardolie Maatschappij B.V., Assen, Netherlands.

Compartmentalization is a significant factor in underperforming reservoirs worldwide, and the source of this is often faults that are too small to resolve seismically. Despite their small size these faults can have permeabilities that are up to five orders of magnitude lower than undeformed reservoir. Prediction of subseismic faulting is commonly done by using larger, seismically resolvable fault populations to define a power-law size distribution (i.e., fault size distributions are assumed to be fractal), and assuming that this trends holds true for subseismic fault populations. The point at which the mapped fault population deviates from this trend line marks the point at which the fault population begins to be undersampled, with the degree of undersampling increasing as fault size decreases. This study shows that predictions of the number and size of subseismic faults is highly survey dependent, which high and low-resolution seismic surveys of the same area yielding dramatically different predictions. This occurs because discrete faults separated by ramps are lumped into a single fault on low-resolution surveys, changing the fractal dimension of the fault distribution that is used to predict the subseismic fault population. Moreover, the fault populations presented in this study do not follow a power law size distribution at all length scales, rather at some they are better fit by an exponential distribution, which results in far fewer predicted subseismic faults, and therefore more ramps. These ramps can significantly reduce compartmentalization. This deviation occurs when fault growth is affected by mechanical stratigraphy, and since a reservoir interval may be a mechanical unit, this finding has significant implications for defining reservoir compartments.

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009