Primary and Secondary Porosity Estimation of Carbonate Formations Using Total Porosity and the Formation Factor

Abstract

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractA technique for the primary and secondary porosity estimation is proposed. This technique is based on a resistivity model of carbonate formations with double porosity and the analysis of numerous core data. The resistivity model consists of an isotropic conductive matrix with a primary porous system and inclusions, which represent secondary porosity (vugs and fractures). Fracture systems are described by oblate ellipsoids of different sizes (which have the same orientation and aspect ratio) or by thin plates. These rock structures are characterized by an anisotropy of electrical properties. The components of the resistivity tensor have a nonlinear relationship with the matrix porosity and parameters of fracture systems (orientation and porosity). The model with spherical inclusions (Maxwell-Garnett approach) is applied to obtain the resistivity of vugular formations. The fluid type, saturation of matrix and secondary pores influence significantly the formation resistivity and anisotropy coefficient. When both pore systems are completely saturated by the same conductive fluid (the case of core analysis), the effective formation factor depends only on the matrix formation factor and the secondary porosity value. The statistical analysis of core data has shown that Archie's equation with cementation exponent m=2, is the best approximation to describe the matrix formation factor for carbonate formations in Mexico. For verification of the porosity separation technique, the fracture and vuggy porosities have been estimated for core with detailed description of pore structure.