The Cerro Piedra field, west of Las Heras (Province of Santa Cruz), is one of the fields located on the western flank of the San Jorge basin. The occurrence of both amplitude anomalies, as seen on the seismic section, and gas layers (penetrated by several wells) which partially account for such anomalies led us to carry out this study. Its main objective is to implement a methodology aimed at identifying gas zones, which can be extrapolated into adjoining fields. Well data, detail geologic correlations, velocity laws, VSPs (YPF.SC.CP.X-4 and YPF.SC.CP.e-21 wells), and 2D seismic sections are available to achieve the above objective. The study has focused on a gas layer of Tertiary age which occurs at a depth of about 500m below ground level and is approximately 30 m thick. The main interest of this layer concerns its fluid content and the fact that it is within the seismic resolvable limit. Geological and structural considerations The San Jorge basin is a dominantly extensional basin that has been largely filled by continental sediments. The evolution of this basin involves two well-differentiated tectosedimentary stages. The Upper Jurassic extensional phase, where the filling of the basin was characterized by lacustrine and marine sediments, was followed by an Upper Cretaceous regional thermal subsidence phase, and the basin was filled with the continental sediments of the Chubut Group (Homovc et al., 1994). For the study area, the sedimentary column also involves rocks of Tertiary age which are approximately 600 m thick. The gas layer on which this study focuses occurs within this sequence and pertains to the Patagonia Formation. As mentioned above, the origin of this basin is the result of an extensional phase that took place since the Upper Jurassic and gave rise to normal faults which in turn formed a series of half-grabens. Compressional stresses occurred during the Tertiary and encouraged inversion of early normal faults, giving rise to a fold belt. The Cerro Piedra anticline, which occurs in a tectonic environment featuring NNW-SSE trending faulting and folding, is one of the series of anticlines of the area. This deformation extends southwards into the Zabala Field and Los Sauces zone, forming overall positive and continuous structures of low complexity. The Cerro Piedra anticline is asymmetrical with respect to the sharper western flank which is related to the propagation of the high-angle reverse fault by which it is bounded. It is approximately 10 Km long and about 2500 m wide. Owing to the depth at which it occurs, our target layer is completely unrelated to the pay zone. According to several geologic well controls, our target layer consists of a fine- to medium-grained greenish gray sandstone with fair sorting. It contains quartz grains, dark lithic fragments, and minor amounts of glass. Friable rock structure is due to low rock compaction. No significant lateral velocity variations are observed for the depths of interest considered herein. This conclusion can be readily derived as the only two curves for velocity laws available in the area are overlapped. Surface Seismic Seismic data from this area consist of a set of 2D lines recorded during 1991–94 using vibros as an energy source. The map of the study area along with the structure map drawn on the top of the study layer and datumed to sea level is shown in Figure 1.
TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThe Cerro Piedra field, west of Las Heras (Province of Santa Cruz), is one of the fields located on the western flank of the San Jorge basin.
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