Published balanced structural interpretations of the Fuegian thrust‐fold belt (southernmost Andes) fail to address aspects such as basement shortening in the thrust‐fold belt or its structural connection with the central belt of the orogen. We tackle this deficiency by constructing four serial balanced cross sections based on surface geologic information, 2‐D reflection seismic lines, and well logs, which depict the first‐order structural geometry and kinematics from the thrust front to the internal thrust‐fold belt. The interpretation reveals three main tectonostratigraphic packages separated by major detachments: basement (Paleozoic‐Upper Jurassic), Cretaceous, and Paleogene, all variably involved in thrusting along strike. Minimum shortening in the Paleogene cover ranges between 16% and 43%, and significant style changes are observed across and along the thrust wedge. We show that basement thrust sheets have a major role in the construction of the thrust‐fold belt; some of them were emplaced below the thrust front during late Oligocene or early Miocene times. The kinematic evolution of the thrust‐fold belt indicates that the active detachments depended on the existence of high pore pressure to propagate forward. Our model shows that this pore pressure buildup was controlled by overburden, mean rock impermeability, and strain rate; the variable interaction among these factors controlled the propagation and style of the thrust wedge.

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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.