Abstract: There is a tight relation between the width of rock fracture and its surface morphology. The description of a fracture surface morphologyis essential to the study of mechanic feature and the flow conductivity .As a pioneer, by means of photoelectric three dimension surface automatic measurement system, the authors measured the surface topography of natural fractures from five sedimentary rocks. The number of fractal dimension and coefficient C are got by spectrum analysis of a composed surface. The results show that the characteristic rule size plays a major roll to determine the fracture width. These parameters are essential to investigate the closure of a fracture under the reservoir conditions.

Key words: Rock, Fracture geometry, Surface property, Frequency spectrum analysis, Surface morphology, Characteristic rule