In situ deep and shallow measurements of electrical resistivity recorded with the Dual Laterolog in the basement of the Cajon Pass scientific drillhole are compared with laboratory measurements of resisvitity and porosity on fracture-free core samples. This comparison is the basis of a simple electrical model of the pore space of the rock from which both matrix and fracture porosity are derived. In parallel, the analysis of borehole electrical images obtained with the Formation Microscanner provides a means to identify the fractures that intersect the borehole wall, to evaluate their frequency, and to determine their orientation. Further processing of these images enables us to distinguish mineralized from fluid-filled fractures. The comparison of laterolog-derived fracture porosity, computed over a large volume of rock, with a pair of fracture intensity profiles obtained from the analysis of the two images of the borehole wall is the key of a new technique by which drillhole-related fractures (either due to hydraulic fracturing or to stress-induced borehole breakouts) and natural fractures (either open or mineralized) are discriminated. The fractures in the upper part of Cajon Pass basement are inferred from the analysis of in situ resistivity mesurements as mostly steep and mineralized. ¿ American Geophysical Union 1988