The constant compromise between oil production and reserves requires differentiating gas from oil. For the detection of gas we have used the density porosity vs. neutron porosity plots, including all the environmental correction processes and the influence of the clay content in the evaluated layers. Nevertheless, it is not always an accurate method because of the nature of the stratigraphic column of the San Jorge basin which is composed of fine sand and shale layers of lenticular geometry, with different types of clay, mostly generated in a continental environment with marked volcanic participation that make forecasts unreliable. With the introduction of NMR technology we have obtained, among other parameters, a total porosity curve. From this data, a successful quicklook technique has been developed using the cross over density porosity vs. total porosity, which allows the identification of gas levels and to figure out the clay content for its use in the log analysis. This paper presents the method for gas detection at well site and its economic impact in logging costs. In some oilfields of the San Jorge Basin, Argentina, logging costs were reduced by 12 %. Introduction Frequently during well completion, it appears the necesity of knowing the productive layers of gas, by using a logging program that includes density and neutron. The continental setting originated clays in lenticular layers with mixed grains distribution, which hides the particular crossover of density porosity and neutron porosity in gas detection. In this complex environment, the use of SP and GR as clay indicators puts into evidence the lack of reliability on the environmental correction process applied to the porosity tools, in order to determine an effective porosity through the bulk porosity, and then to determine the real water saturation in a porous environment. At the time of the writing of this article, more than 700 wells of the San Jorge Basin have NMR logs and more than 7000 individual swabbings make up the database. It is an important reason to develop this methodology. Geological framework - Reservoir features Anticlinal Perales Area The San Bernardo Fold Belt southern prolongation, in Santa Cruz Province, Argentina is composed of several structures with a high hydrocarbon potential, as Los Perales, Las Mesetas, Aguada Bandera and Cerro Guadal among others (fig.1). The sediments that filled the late sag sequence of the San Jorge Basin are controlled mainly by their proximity to source material and the exposure of the basement. Also they are highly contaminated with pyroclastic material provided by the continental arc volcanoes active during the Upper Cretaceous. Lower Member of Bajo Barreal Formation lithofacies represent proximal fans with wide low relief areas of fluvial deposition (fig.2). Sheet deposits of sand bodies due to non channelized currents of torrential rain origin, are present in this part of the column (2). Lenticular conglomerates and sandstones deposited by a poor drainage system related to tectonically active local areas, are also developed and both lithofacies represent a regional progradation. Thickness of individual sand bodies - 9 to 24 ft - are beneath seismic resolution. The reservoir properties of these volcanic rich sandstones are strongly affected by diagenetic evolution that varies because of the variations in depositional environments and lithofacies. Anticlinal Perales Area The San Bernardo Fold Belt southern prolongation, in Santa Cruz Province, Argentina is composed of several structures with a high hydrocarbon potential, as Los Perales, Las Mesetas, Aguada Bandera and Cerro Guadal among others (fig.1). The sediments that filled the late sag sequence of the San Jorge Basin are controlled mainly by their proximity to source material and the exposure of the basement. Also they are highly contaminated with pyroclastic material provided by the continental arc volcanoes active during the Upper Cretaceous. Lower Member of Bajo Barreal Formation lithofacies represent proximal fans with wide low relief areas of fluvial deposition (fig.2). Sheet deposits of sand bodies due to non channelized currents of torrential rain origin, are present in this part of the column (2). Lenticular conglomerates and sandstones deposited by a poor drainage system related to tectonically active local areas, are also developed and both lithofacies represent a regional progradation. Thickness of individual sand bodies - 9 to 24 ft - are beneath seismic resolution. The reservoir properties of these volcanic rich sandstones are strongly affected by diagenetic evolution that varies because of the variations in depositional environments and lithofacies.

This paper (SPE 52939) was revised for publication from paper SPE 38735, first presented at the 1997 SPE Annual Technical Conference and Exhibition, San Antonio, Texas, 5-8 October. Original manuscript received for review 7 October 1997. Revised manuscript received 17 August 1998. Paper peer approved 16 September 1998. Summary This paper describes our experience with nuclear magnetic resonance (NMR) logs since their introduction in the San Jorge basin in June 1995. To date, more than 400 logs have been run by Western Atlas and Schlumberger using Numar's magnetic resonance imaging tool (MRIL) and combinable magnetic resonance (CMR) tools, respectively, and more than 2,000 zones have been perforated and tested. This has allowed us to characterize the response of the tools in shaly and tuffaceous sands, compare the response of the Numar and Schlumberger tools with each other and with cores, detect the limitations of present tool designs, and relate NMR pore-size distribution and permeability to oil production. P. 504

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TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThe Estancia San Justo Norte Area in the Lomas del Cuy Field was characterized using geostatistical methods. The distributions and thickness of sand bodies were determined within an area of over 20 square km, with 42 drilled wells, and 3D seismic interpreted by REPSOL-YPF. The intervals of interest were determined from well logs, correlated throughout the field and related with the 3D seismic data. The appropriate reflectors were identified in the seismic, and time windows were established so that these would match the selected intervals. A total of seven windows was used and within each of them 25 seismic attributes was extracted. On the other side, petrophysical evaluations were made from well logs obtaining results for gross, net and net pay thickness and average petrophysical properties within the intervals. Good correlations were found between petrophysical properties and certain seismic attributes in 6 of the 7 windows. In these cases, cokriging was used to map the petrophysical properties integrating the seismic data. As a result, petrophysical maps guided by seismic attributes were obtained for net thickness, net/gross, net porosity thickness and average net porosity. These maps proved very useful for determining new well locations. The results clearly indicate that the application of this technique is very effective for describing the fluvial type reservoirs with small lateral continuity, typical of the Golfo San Jorge Basin. Geostatistical methods were good predictors of sand body distributions in each of the analyzed intervals. This data is extremely valuable in the development of fields when it is integrated with the rest of the available information in the area.

This paper describes our experience with Nuclear Magnetic Resonance (NMR) logs since their introduction in the San Jorge basin in June 1995. To date, more than 200 logs have been run by Western Atlas and Schlumberger using Numar's MRIL and CMR tools respectively and more than 2000 zones have been perforated and tested. This has allowed us to characterize the response of the tools in shaly and tuffaceous sands, compare the response of the Numar and Schlumberger tools amongst themselves and with cores, detect the limitations of present tool designs and relate NMR pore-size distribution and permeability to oil production. Introduction The bulk of YPF's development drilling in 1995 and 1996 took place in the San Jorge basin in Southern Argentina, where more than 1600 wells were drilled in 1995-1996. Production and reserves per well in this basin are relatively low, and therefore it is essential to keep costs at a minimum while optimizing production per well. To this end, several new drilling, logging and completion technologies were tested to evaluate their contribution in increasing production and decreasing the overall development cost. One of the main problems during the completion of the well is the high number of dry zones. In some cases dry zones can be predicted by their low porosity, or no SP development, or no Microlog separation. But in many cases, zones that look "good" on logs are dry or have very little oil production. Since the lack of production can be due either to low permeability or formation damage, the effect of stimulating these zones is unpredictable: production can either stay the same, increase a little or increase by a factor of 10 or more. Under these conditions, a measurement with the potential to discriminate between low permeability and damaged zones can save a considerable amount of money in perforating and stimulation. In June 1995 Western Atlas run the first NMR log using Numar's MRIL tool. The same month Schlumberger ran their first NMR with the CMR tool. Following a brief evaluation period, we found that the NMR-derived permeability was a valuable information to help decide which zones should be perforated and/or stimulated. To date, more than 200 NMR logs have been run by Western Atlas and Schlumberger with MRIL and CMR respectively. The present rate is about 20 NMR logs per month, i.e., about one well in three is logged with MRIL or CMR. MRIL vs. CMR Comparisons between the Numar and Schlumberger tools were made in two wells. The agreement of NMR porosity and permeability from the two logs is generally good, as shown on the example of Fig. 1, which corresponds to the first well. The differences can be attributed to the different vertical resolution of the tools and the effect of rugosity or loss of pad contact on the CMR. NMR porosity Comparison with conventional porosity logs - The most striking observation on the first NMR logs was that NMR porosity was systematically lower than density porosity. In this basin it is known that the density log may not be adequate for porosity evaluation because of the very low density of some tuffaceous sands with a high zeolite content. In this case it is better to derive porosity from the sonic log, which is less lithology-dependent. However, in areas of shallow reservoirs (less than 1200 meters) the density log is preferred as the sonic may overestimate porosity because of lack of compaction. A neutron log is also run when there is a possibility that some of the sands be gas-bearing. P. 183