In hard formations (dolomite — limestone — quartzitic sandstone or even volcanics) the matrix is often of low to very low porosity and of extremely low permeability. Such formations are only considered as potential producers when fractures and fissures drain the hydrocarbons to the well bore. It is therefore extremely important to detect the presence of fractures, and several logging toots are able to give a good and reliable information about the location and even the orientation of those fractures. The combination Dual Laterolog-Microspherical log is one of the fracture indicators used nowadays. When analysing the process of mud or mud filtrate invasion in a fractured reservoir and observing die effect on the resistivity logs it was discovered that the Dual Laterolog itself was a fracture indicator and that the difference in conductivity between the two measurements — deep and shallow was a function of the volume of hydrocarbon displaced from the fracture system during the invasion process. It allows to compute a lower limit of the fracture porosity — the technique has been successfully applied in various formations and examples are presented.

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