Petrophysical characterization of carbonates (SE of Spain) through full wave sonic data

Abstract

Carbonate reservoirs are characterized by strong permeability heterogeneity mainly linked to the predominant pore type. In this work, we present a case study focused on a shallow borehole (160 m depth) located at the eastern end of the Betic Cordillera, SE of Spain. This borehole sampled a marly seal (deep marine marls rich in planktonic organisms from Messinian – upper Tortonian) on top of a carbonate reservoir (algae limestone from Miocene - upper Tortonian). Laboratory analysis show that most of the samples from the reservoir correspond to grainstone and packstone limestone types, and that the quality of this reservoir is controlled mainly by its detritic composition and a high content of bioclasts. The aim of this work is to show the suitability of using standard geophysical logging to assess permeability supported by laboratory and Lugeon tests. Two approaches have been used based on Kozeny's equation (pseudo-k log) and Mari et al. (2011) work (Ik-Seis log). Both ways require obtaining specific surface which is an indicator of pore type. This calculation is based on porosity and Vp/Vs relations. Porosity has been obtained from resistivity and sonic logs constrained by laboratory measurements. Shear wave velocity has been inferred from Stoneley wave velocity since this formation is considered as slow (Vs lower than 1500 m/s). Pseudo-K and Ik-Seis logs have been scaled using the Lugeon tests results. Both logs reproduce the heterogeneities found in the in-situ test. Finally, an analysis of the dependence of different seismic attributes to the specific surface has been performed to check the suitability of using solely sonic measurements to assess permeability. This can be useful for boreholes were laboratory or field tests are not available.