Volumetric imaging is used to determine the three-dimensional topology of natural fracture networks in coal subjected to reservoir conditions. Computerized X-ray tomographic data and bulk porosity measurements are combined with image processing techniques to quantify the porosity distribution of three-dimensional fracture networks in two coal cores. Under in-situ conditions, the porosity of the fracture networks in the cores is less than one-tenth of a percent. In addition, the porosity varies by ¿10% over depth intervals of approximately 16 mm. ¿ American Geophysical Union 1995