Exploration in the offshore Golfo San Jorge Basin requires an understanding of the regional trends of the Lower Yacimiento El Trébol Fm. Sandstones. These reservoir rocks yield the best oil shows in the offshore and they are among the most prospective and prolific onshore reservoir on the Golfo San Jorge Basin's north flank. Detailed correlation enabled identification of three lower order sequences in this stratigraphic interval. These sequences are recognizable on seismic sections and well logs. The Lower Yacimiento El Trébol interval was regionally correlated from the offshore wells to the producing intervals at Escalante and El Trébol fields. No major truncations or stratigraphic wedges were identified on seismic lines or well logs, although the individual sequences that comprise this package can thin against some persistent structural highs. In addition, these sequences were correlated in 144 wells all around the north flank of the basin; sand thickness was measured from the SP curve in these wells. Net sand and sand percent maps were created. These maps demonstrate that lower Yacimiento El Trébol sandstones are concentrated in sand fairways in the Comodoro Rivadavia area and in the area south of the Marta wells. Conversely, there are many parts of the basin where Lower Yacimiento El Trébol Formation is represented by silts and shales (west and south of Comodoro Rivadavia and between Comodoro Rivadavia and Marta wells). Sand rich fairways, represented by gross thickness isopachs, map as trends that are perpendicular to the basin edges. Lower Yacimiento El Trébol production is found in these thick sand areas, while in thin sand areas, production comes from the underlying Comodoro Rivadavia Fm. Introduction The sandstones at the Lower Member of the Yacimiento El Trébol Fm. have produced over 20 MMm3 of oil as one of the main reservoirs at northern flank of San Jorge Basin. In this paper we present a synthesis of the main characteristics of these reservoirs. The Upper Cretaceous Lower Member of the Yacimiento El Trébol Fm. is defined by two regional surfaces; the lower one is a regional unconformity at the top of Comodoro Rivadavia Fm. The upper limit is a regional shale that records a basin wide flooding event. Based on log trends and markers of regular continuity, three sequences have been recognized in this unit. These sequences begin with sharp based sand packages with Christmas tree pattern and evolve upwards to shale intervals with some isolated sandstones. The stratigraphic maps of these sequences show sand concentrations with a distributary pattern 40 km wide centered at the west of the Campamento Central field. These sands pass laterally to shales to the west, east and south. A new sand concentration occurs at the location of Marta and Kaiken wells at the offshore of the basin. Sand thickness variograms show the greater continuity in NNW-SSE direction and indicate provenance from the north. Cores at the Marta wells show sandstone packages arranged in upper fining trends and interpreted as high suspension to traction fluvial channels followed by highly burrowed silt packages. These silts become dominant at the top of the interval. Cores at El Trébol field show up fining sandstone packages interpreted as fluvial meandering channels.
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.
TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThe depositional environment in the west section of the Golfo San Jorge Basin is characterized by fluvial sand-shale sequences in which the reservoirs rarely have thickness that exceeds 10m, while average thickness is 4m. The acquired seismic has a tuning thickness of 17m, so that the wavelet effect and thin bed interferences make the seismic derived attribute analysis not ideal for lateral and vertical delineation of sandstone complexes. The seismic limitations of only describing interfaces without discriminating lithologies, is overcome inverting 3D seismic traces into pseudo logs of Acoustic Impedances by applying a constraint sparse spike algorithm. Both well data (lithology and compaction) and horizons (structural framework) help to constrain the process, thus the reflectivity series found meets the criteria of minimizing the residual between the synthetic model and the real 3D seismic trace. An Acoustic Impedance volume is derived from the reflectivity series, (the frequency spectrum is limited to the seismic bandwith lacking the low frequencies corresponding to compaction) and is called bandlimited. Impedance logs from key wells were generated, filtered to a range between 0-7 Hz and then interpolated (honoring both the stratigraphy and the structural framework) to generate a low frequency model of impedances. By merging the low frequency model with the bandlimited volume a total Acoustic impedance volume is obtained.Well data analysis describes that Acoustic impedances not only at well frequency but also at seismic frequency are visualized as maximum for sandstone packets. The acoustic impedances derived from the inversion process are sensitive to the presence of a stack of several sand units and hence can be used to map lateral variations of petrophysical reservoir properties. We have attempted to quantify the petrophysical properties of such sand sequences using extensive well control. To do so we first low-pass filtered the well logs to exhibit vertical variations consistent with those of the 3-D seismic data. The low-pass filtered logs were further analyzed via lithology-based cross-plots to ascertain their effective petrophysical properties. This procedure successfully provided us a quantitative model to predict vertical and lateral variations of petrophysical properties from seismic-derived acoustic impedances. We have also developed a similar procedure based on attributes extracted from the reflectivity data generated as by-product of the inversion process. This effectively allowed us to delineate the lateral and vertical petrophysical behavior of useful sand units for subsequent infill and step-out development operations. We have successfully tested this methodology on a number of new wells in the area.
TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThe depositional environment in the west section of the Golfo San Jorge Basin is characterized by fluvial sand-shale sequences in which the reservoirs rarely have thickness that exceeds 10m, while average thickness is 4m. The acquired seismic has a tuning thickness of 17m, so that the wavelet effect and thin bed interferences make the seismic derived attribute analysis not ideal for lateral and vertical delineation of sandstone complexes. The seismic limitations of only describing interfaces without discriminating lithologies, is overcome inverting 3D seismic traces into pseudo logs of Acoustic Impedances by applying a constraint sparse spike algorithm. Both well data (lithology and compaction) and horizons (structural framework) help to constrain the process, thus the reflectivity series found meets the criteria of minimizing the residual between the synthetic model and the real 3D seismic trace. An Acoustic Impedance volume is derived from the reflectivity series, (the frequency spectrum is limited to the seismic bandwith lacking the low frequencies corresponding to compaction) and is called bandlimited. Impedance logs from key wells were generated, filtered to a range between 0-7 Hz and then interpolated (honoring both the stratigraphy and the structural framework) to generate a low frequency model of impedances. By merging the low frequency model with the bandlimited volume a total Acoustic impedance volume is obtained.Well data analysis describes that Acoustic impedances not only at well frequency but also at seismic frequency are visualized as maximum for sandstone packets. The acoustic impedances derived from the inversion process are sensitive to the presence of a stack of several sand units and hence can be used to map lateral variations of petrophysical reservoir properties. We have attempted to quantify the petrophysical properties of such sand sequences using extensive well control. To do so we first low-pass filtered the well logs to exhibit vertical variations consistent with those of the 3-D seismic data. The low-pass filtered logs were further analyzed via lithology-based cross-plots to ascertain their effective petrophysical properties. This procedure successfully provided us a quantitative model to predict vertical and lateral variations of petrophysical properties from seismic-derived acoustic impedances. We have also developed a similar procedure based on attributes extracted from the reflectivity data generated as by-product of the inversion process. This effectively allowed us to delineate the lateral and vertical petrophysical behavior of useful sand units for subsequent infill and step-out development operations. We have successfully tested this methodology on a number of new wells in the area.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.