SANDSTONE FRACTURE CLUSTERING AT THE OUTCROP AND MICROSCOPIC SCALES

The spatial arrangement of fractures is an important problem for fluid flow, intersection probability while drilling or tunneling, and the evolution of fracture patterns. Microscopic fractures have been shown to be useful surrogates for large-fracture intensity, timing, orientation, and openness, but it is less clear whether spatial clustering persists across scales of observation. We surveyed fractures along 1D scanlines at outcrop scale, using a hand lens, and at thin section scale, using scanning electron microscopy. Surveys were collected from four sandstones in variably deformed settings. Spatial arrangement was quantified using several approaches, including the coefficient of variation of nearest-neighbor fracture spacings, staircase plots of cumulative aperture versus distance, and normalized correlation count. These methods were generally internally consistent for each survey, and indicate that some fracture arrangements are indistinguishable from random and some are systematically clustered. No surveys we made had significant periodicity to fracture spacing. We find that microfracture clustering is a poor predictor for macrofracture clustering. However, a significant correlation exists between microfracture clustering and macrofracture intensity (number of fractures per unit length of scanline). We interpret that the growth of large fractures is part of a spectrum of outcomes that can result from interaction among microfractures via linkage, coalescence, and stress shadow processes.