Seismic Mapping of Reservoirs at the Golfo San Jorge Basin

Abstract

Reservoir maps are amongst the most important data necessary for oil exploration and development. Reservoir mapping through seismic data at San Jorge Basin has always been difficult due to the low thickness of the individual sandstones. The techniques of seismic reservoir mapping based on the integration of the top and base reflection amplitudes and the seismic isopach map allowed us to map successfully reservoirs in the order of system tracts in those cases where sand and shale thickness were beyond the tuning thickness. The technique involves a detailed well tying through synthetic seismograms at the control wells. This allows to obtain the phase of the seismic data and to identify the seismic response of the different lithologies. Based on this tying the top and base of the reservoir interval are carefully picked. From these picks the reflection amplitudes at the top and base of the interval and their isopach map are extracted, quality controlled for tuning and calibrated. The final reservoir thickness is obtained from the integration of the data and it is quality controlled against the existing wells. This methodology allowed us to map reservoirs at the Salamanca and Yacimiento El Trébol Fm. at the North Flank of San Jorge Basin and at Mina El Carmen Fm. at the north and west flanks of the basin. As a summary, this technique allows us to map reservoirs at the system tract level at the northern flank of Golfo San Jorge Basin in those cases where there is poor well control and results are not interfered by tuning. Although there are more sophisticated techniques (seismic inversion, geostatistical inversion), this one let us do a fast mapping of the reservoirs. On the other hand, the tying and picking involved in this technique can be applied as an input to the most sophisticated techniques. Introduction The seismic mapping of reservoirs at the northern flank of Golfo San Jorge basin has always been challenge due to the low thickness and great areal and depth distribution of the sandstones (fig. 1). Although there was a great improvement in the seismic data quality and resolution, the thick of the sandstones (1 to 5 meters) do not allow to map the base and the top of each sandstone. For this reason the efforts on seismic reservoir mapping were historically focused on the application of seismic attributes, preferentially seismic amplitude, to wide seismic windows. Results obtained with this method were ambiguous due to the limited definition of the problem this situation can be improved with a more detailed approach to the problem. Another difficulty was that in some sequences the closely interbedding of sandstones and shales don't allow to have seismic response due to the tunning effect. This tunning is a fundamental factor since although the individual sandstones are below the seismic definition the sandstone shale packages produce high amplitudes that could be misinterpreted as a increase in reservoir thickness. Our main objective was to apply the conventional seismic reservoir mapping techniques in a maturely drilled area as to validate this metodologies as an exploratory tool at the less drilled areas in the basin. Methodology The tools and techniques proposed here where applied successfully by other authors (Brown, Meckel and Natt, Neff); their application in the exploratory nature areas of this basin was a test that allowed us to understand the methodology its main drivers and limitations and to propagate the results to other parts of the basin. The techniques are based on the measurement of the cero phase of a 3D final migration seismic amplitude as the only seismic attribute. The analysis of this attribute was done at two scales the qualitative and the quantitative. At the qualitative analysis classification of geometric attributes permit to arrive to geologic models (channels, meanders, crevasse splays). At the quantitative analysis we are allowed to correlate reservoir attributes (reservoir thickness) to the seismic response.