Abstract Accurate permeability values, required in reservoir characterization, can be derived from logs using adequate petrophysical constants with their cut-off for different lithologies and environments, specially in those fields where good relationship between porosity and permeability exists. This methodology has been successfully applied in Tordillo Formation in Entre Lomas Block, in the ENE of the Neuquen Basin, Argentina. Tordillo Formation, of 150 m thick section, is composed of sandstone and conglomerate units interbedded with siltstones layers deposited under braided fluvial and eolian environments. Taking advantage of the logs that most of the wells at any field have, many authors, mentioned in Reference 1, have given various techniques to estimate permeability. In the majority of the cases, the evaluations show erratics behaviours. To improve the quality of the results, core data (capillary pressures, formation factor, porosity, etc.) have been used to determine the irreducible water saturation and the different petrophysical constants in order to calibrate the permeability equation. This adjusted expression yields results which have good correlation degree with those permeabilities derived from direct and indirect measurements as core data and well test analysis. Later, numerical simulation studies have confirmed the validity of the described method. Introduction Study Area. Entre Lomas Block is located at the northeast of the Neuquen Basin. Oil was discovered in 1960. The Block, with an extension of 742.5 Km2, is divided by the Neuquen and Rio Negro interprovince border (Figure 1). The stratigraphic column (Figure 2), present in Entre Lomas Block, shows the three main productive Formations: Punta Rosada (Aalenian-Bajocian), Tordillo (Kimmeridgian), the most prolific, and Quintuco-Vaca Muerta (Tithonian-Valanginian). Tordillo Formation appears productive in two main structures, Charco Bayo (CB) and Piedras Blancas (PB). They are under exploitation since the late 60's. A secondary recovery project that started in 1975 was performed in several stages. The original pattern was inverted seven spot, but later a half of those patterns have been changed into a staggered line drive system. Both structures are not completely developed neither close to the water contact nor inside the field. The future locations should be chosen carefully due to the risk of finding a thin pay zone or hitting a secondary water bank. Up to day almost 330 wells of the CB and PB structures have been drilled. Almost 270 were located with a well spacing of 88 acres. The rest of the wells can be defined as interspaced. Currently 205 are still on production, 80 are injectors. By December 1996, the oil production rate of Tordillo Formation was 1,180 m3/d and the cumulative produced volume was 17,234 Mm3.