Predicting Fluid Production of Tuffaceous Sandstones in the San Jorge Basin With Array-Type Induction Logs
Authors: Khatchikian Alberto , Breda Eduardo 
Published: December 2007

Abstract
Conventional log interpretation is a poor predictor of fluid production in the San Jorge basin in Southern Argentina. This was thought to be due to unpredictable variations of formation water resistivity. In this paper we show that the real reason is a combination of overestimation of the porosity and underestimation of the Archie exponent m in tuffaceous sandstones, and that a method based on the ratio of deep to shallow induction curves from array-type induction logs can be used to estimate fluid content as it is insensitive to those factors. This method was successfully used in more than 200 wells using Schlumberger's (AIT) and Western Atlas (HDIL) array-type induction tools with over 2000 zones that were perforated and tested.

Introduction
Formation evaluation in the San Jorge basin is difficult because of the thinness and lack of continuity of the reservoirs, which complicates well-to-well correlation, and the shaly-tuffaceous nature of the sandstones, which complicates the evaluation of porosity and water saturation. Because of this, historically the usual practice was to run a minimum logging program and decide which intervals to perforate on the basis of visual examination of the logs, sidewall cores and correlation. In order not to miss potentially productive zones, usually 10 to 20 sands are perforated and tested, of which only a few produce hydrocarbons. Many perforated intervals are dry, either because of poor permeability or formation damage. Formation evaluation in the San Jorge basin Porosity evaluation - Previous studies have shown that in tuffaceous sandstones the sonic log is less sensitive to variations in lithology than the density log. For this reason, sonic is the log of choice in the deeper wells. In shallow wells (1200 meters or less), density is the standard porosity log as it is difficult to match sonic velocity with porosity in unconsolidated sandstones, even when using the so called "empirical" relations. But due to the volcanic material content of many sandstones, grain density may vary from 2.4 to 2.7 g/cm3, introducing a large factor of uncertainty on the porosity. The example shown on Fig. 1 is a sandstone with an apparent density porosity of 30% and relatively clean according the density-neutron response. However, the laboratory measurement of a rotary sidewall sample at 2185.8 meters indicates that the true porosity is 20.7% and that the grain density is 2.45 g/cm3. Water saturation - The quick-look method Rwa was, until recently, the standard fluid predictor in this basin. Unfortunately, this method is too optimistic in tuffaceous sandstones and pessimistic in shaly sands and therefore is not a reliable fluid predictor. Fig. 2 shows a water-producing, tuffaceous sand with an Rwa of about 4.0 ohm. m, i.e., roughly ten times the resistivity of the formation water. Note the anomalous SP caused by streaming potential, typical of low permeability tuffaceous sandstones.
Rwa is too high in tuffaceous sandstones because they tend to have lower permeability and higher values of m than quartz sandstones. Therefore, Rwa will be too high and computed water saturation too low if standard values of a and m are used. The example of Fig. 3 illustrates this effect. Using standard values for a and m (a = 0.62, m = 2.15), the average computed water saturation in the interval 1291–1299 meters is 60% (Fig. 3a). On the other hand, using a = 1, m = 2.5, which was estimated from a Pickett plot (Fig. 3c), the average water saturation reaches 100% (Fig. 3b). P. 257^