Prediction of macrofracture properties using microfracture information, Mesaverde Group sandstones, San Juan basin, New Mexico

Abstract

A better understanding of relationships between microfractures and macrofractures is prerequisite to the extrapolation of observed microfracture properties as a basis for inferring properties of associated macrofractures. The hypothesis that micro- and macrofractures represent different size fractions of the same fracture sets, and consequently have linked attributes, was tested on sandstones of the Mesaverde Group, San Juan basin, New Mexico. The use of a cathodoluminescence detector attached to a Scanning Electron Microscope (SEM) allowed observation of microfractures not readily visible using conventional microscopic techniques. The orientations of microfractures reproduce the orientations recorded at the macroscopic scale from cores and outcrops. Fracture size distributions are best modeled using power laws, and extrapolation of microfracture frequencies to the scale of macrofractures successfully predicts the macrofracture frequencies. Fracture-size distributions change at the length scale of the thickness of the mechanical layer, typically by increasing the exponent of their power-law distribution. This change is independent of censoring biases and reflects changes in sampling topology and/or fracture growth mechanisms.