Microfractures and macrofractures in cores of the Mobil Blakeney Kruger No. 1 well, Ozona Gas Field, West Texas were analyzed to test the hypothesis that the microfractures are the microscopic representation of the associated macrofracture system. Cores taken from subvertical beds in the hanging wall of a reverse fault contain carbonate- and quartz-filled fractures at high angle to bedding. More than 300 mechanical apertures of macrofractures were measured visually along fracture-perpendicular scanlines in these subvertical beds. Similarly, microfracture aperture data were collected from SEM-CL (Scanning Electron Microscope Cathodoluminescence) photomicrograph mosaics along scanlines of the same orientation as the scanlines in the cores. The macrofracture apertures range from less than one tenth of a millimeter to nearly one centimeter, and the microfracture apertures range from tenths of a micron to tenths of a millimeter. The size distributions of microfracture and macrofracture apertures are each well modeled using a power-law. Normalization of the aperture size distributions by the lengths of the scanlines allows direct comparison of the fracture frequencies of both data sets (Figure 1). The results represent over four orders of magnitude in fracture aperture and strongly suggest that the microfractures and macrofractures are expressions of the same fracture system at two different scales of observation. This implies that the prediction of macrofracture frequency based on microfracture data is possible, at least up to the scale of fractures that span the mechanical layer. We currently are using a similar methodology in other fractured sandstone reservoirs to evaluate the general applicability of the technique. The systematic application of this methodology may help generate more realistic models of fractured reservoirs for fluid flow simulation.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah