ABSTRACT Natural fractures are abundant in the Vaca Muerta Formation and are important because they may affect hydraulic-fracture growth during well stimulation. They contribute to anisotropic mechanical behavior of the reservoir rock and may cause hydraulic fractures to arrest or divert along them by opening or shear. In the subsurface, the Vaca Muerta Formation contains bed-parallel veins (BPV) of fibrous calcite (beef) and bed-perpendicular, completely or partly calcite-filled, opening-mode fractures in multiple orientations. In outcrops of the Vaca Muerta Formation in the Agrio fold-and-thrust belt, BPV and bed-perpendicular fractures are also common. Fracture cement geochemistry (including stable isotopes) and fluid inclusion and clumped isotopic thermometry indicate that the outcrops are similar to the most mature parts of the Vaca Muerta reservoir and can be used as guides for this part of the basin. In outcrops near the Cerro Mocho area, two main bed-perpendicular, opening-mode fracture sets are oriented east–west (oldest) and north–south (youngest), and two additional sets (northeast–southwest and northwest–southeast) are locally present. Fluid inclusion microthermometry, combined with burial-history curves, indicates that BPV in the area of Loncopué formed in the Late Cretaceous during bed-parallel contraction and in overpressure conditions, whereas bed-perpendicular sets formed in the Paleocene. Similar ages were obtained for Puerta Curaco outcrop on the basis of clumped isotope temperatures, although BPV opening may have lasted until the Miocene in this area. BPV are the most common and some of the oldest types of fracture sampled by vertical cores, and stable isotope analyses indicate that they formed deep in the subsurface, probably under conditions similar to those inferred for outcrops. In cores of the Loma Campana block, bed-perpendicular fractures show orientations similar to those in outcrops, although the youngest, north–south set is generally missing. Without appropriate fluid inclusions for microthermometry or oriented cross-cutting relationships in core, fracture timing was established on the basis of a tectonic model. Our model indicates that in the Loma Campana block, fractures preferentially formed in east–west and northeast–southwest orientations in the Early Cretaceous, northeast–southwest in the Late Cretaceous, northwest–southeast in the Cenozoic, and east–west and east-northeast–west-southwest at present. Fracture timing and orientations from this tectonic model, fracture aperture from core, fracture height and length measured in outcrop, and fracture intensity from a geomechanical model calibrated with core and image logs were used to construct discrete fracture network (DFN) models of the subsurface and build specific reservoir development plans.

Abstract The Andean Orogeny in South America has lasted over 100 Ma. It comprises the Peruvian, Incaic and Quechuan phases. The Nazca and South American plates have been converging at varying rates since the Palaeocene. The active tectonics of South America are relatively clear, from seismological and Global Positioning System (GPS) data. Horizontal shortening is responsible for a thick crust and high topography in the Andes, as well as in SE Brazil and Patagonia. We have integrated available data and have compiled four fault maps at the scale of South America, for the mid-Cretaceous, Late Cretaceous, Palaeogene and Neogene periods. Andean compression has been widespread since the Aptian. The continental margins have registered more deformation than the interior. For the Peruvian phase, not enough information is available to establish a tectonic context. During the Incaic phase, strike-slip faulting was common. During the Quechuan phase, crustal thickening has been the dominant mode of deformation. To investigate the mechanics of deformation, we have carried out 10 properly scaled experiments on physical models of the lithosphere, containing various plates. The dominant response to plate motion was subduction of oceanic lithosphere beneath continental South America. However, the model continent also deformed internally, especially at the margins and initial weaknesses.

Abstract Thin-skinned gravitational gliding of sediments above a detachment layer of salt or shale is common on passive margins. Changes in surface slope result in a domain of extension upslope and a domain of contraction downslope. Contractional domains tend to occur under present-day deep water and are thus not well understood. In the deep-water Santos Basin, Brazil, a contractional domain contains a suite of salt-cored structures. Angular folds (chevron and box folds), as well as concentric folds, are common in the upper part of the Aptian evaporite sequence, which appears to comprise alternating layers. In general, angular and concentric folds form by flexural slip during shortening of mechanically layered sequences. Their occurrence in the Santos Basin is evidence in favor of horizontal contraction. The lower part of the Aptian evaporite sequence appears to be mostly rock salt. It has been squeezed out from under synclines into spaces created by growing anticlines. In places, the layered evaporite sequence has been thickened or even repeated across thrust faults and ramp anticlines. An overlying sequence of open-marine sediments has been condensed or eroded over anticlines but forms local depocenters. These depocenters are asymmetric (of foreland style) next to isolated thrusts but symmetric in synclines or between thrusts of opposite vergence. The structural styles have been reproduced in physical models, properly scaled for gravitational forces, in which salt is represented by silicone putty and sediments are represented by sand. The models were shortened horizontally by a screw jack. The experiments illustrate the importance of horizontal contraction and syntectonic sedimentation in shaping salt-cored structures. They have been used to establish criteria that may be diagnostic of construction.