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 Los Cavaos (Neuquén Basin, Argentina) is a large oil field in a fold-and-thrust belt setting, where voluminous igneous sill complexes are emplaced in different levels of the sedimentary sequence. In this field, the sills (horizontal igneous intrusions) are predominantly hosted in organic-rich units such as the Vaca Muerta and Agrio formations. The sills can be highly relevant elements of the petroleum system because they affect the thermal and maturation history of the hydrocarbon source rocks, and they also act as fractured reservoirs. In this chapter, we integrate new borehole data, core descriptions, petrophysical tests, and organic geochemistry to perform a two-dimensional (2-D) modeling of Los Cavaos oil field, a world-class case study of an igneous petroleum system in a fold-and-thrust belt. Especially, we focus on quantifying the implications of the sill complex on the hydrocarbon generation, migration, and accumulation. Our data and new modeling results show that (1) intrusions are the main reservoirs in the study area, with the highest amount of oil accumulation and reaching levels of oil saturation between 90% and 100%; (2) the reservoir quality of igneous intrusions is dominated by the combination of matrix and fracture framework properties; (3) zeolites are the most common type of cement in igneous reservoirs, and they have a relevant impact on reservoir quality as they preserve porosity; (4) intrusions released heat and modified the background geothermal gradient, promoting source rock maturation, especially during Miocene when most intrusions were emplaced; and (5) close to the sills, large hydrocarbon masses are generated and expelled in a short time (~300 years), whereas in areas without sills, the hydrocarbon generation and expulsion is gradual and occurs over a longer time (thousands of years). In Los Cavaos, oil production happens only from the sills emplaced in the Vaca Muerta Formation. The cumulative oil production of some sills in the Vaca Muerta Formation reached peaks of 1886 bbl/d during the first 10 years. This study concludes that sill complexes may be highly relevant components of petroleum systems, as they promote source rock maturation and act as fractured reservoirs. Our conclusions can thus have major implications on hydrocarbon exploration in other volcanic basins worldwide.

ABSTRACT The present-day structure of the Neuquén Basin is the result of the deformation of the sedimentary column through a poly-episodic tectonic history. The initial stage was dominated by extension with a general northeast–southwest direction during the Early Jurassic. The resultant structures were a widespread distribution of northwest–southeast striking grabens and half-grabens defined as the Rift stage. Subsequently, different stages of convergent tectonics took place with different directions of convergence vectors, reactivating and inverting the previous extensional structures and creating new ones. The most important stages were the Aluk stage (Early Jurassic-Early Cretaceous), the Farallon stage (Cretaceous), and the Nazca stage (Cenozoic). The tectonic pulses produced distinctive megastructures that affected the early Tithonian to early Valanginian Vaca Muerta Formation, such as the Chihuidos High, the Huincul High, and the Agrio fold-and-thrust belt. Also, multiple fault systems forming kilometer-scale features were created. The main fault systems affecting the Vaca Muerta–Quintuco system are classified according to the Anderson classification: normal, reverse, and strike-slip faults. The following fault families are identified: five normal fault families (northwest–southeast, east-northeast–west-southwest, north-northeast–south-southwest, north-south, and radial), five reverse (high-angle) fault families (east–west, northeast–southwest, north–south, north-northeast–south-southwest, and circular), two thrust (low-angle) fault families (north-northwest–south-southeast and north-northeast–south-southwest), and one strike-slip fault family (east-northeast–west-southwest). The geometrical characteristics, orientations, and distribution of these Vaca Muerta fault families are presented and the interpretation of their genesis is discussed. Second-order structures such as volcanic dikes and sills, soft-sediment deformations and accomodation/transfer zones that participate to the deformation of the Vaca Muerta-Quintuco system are also described because they also affect the unconventional self-sourced play. Finally, some structural aspects of the Chihuidos High, the impact of the inheritance on the Vaca Muerta–Quintuco structures (geometry, orientation, and distribution) and the Structural Risk Analysis are discussed.

ABSTRACT We have expanded previous trishear fault-propagation-fold forward models by allowing additional structural complexity in the form of multiple ramps and flats, variable propagation-to-slip ratio (P/S) and trishear angle, as well as multiple faults in a single section. The resulting forward models simulate characteristics of real structures very well. Our six-parameter grid search overcomes a longstanding obstacle to the application of trishear by providing a scientifically objective way of choosing the correct parameters to apply to real structures. By grid searching forward models with known initial parameters, we investigate the sensitivity of trishear to various parameters. These experiments highlight the importance of P/S in determining fold shape: A change in P/S of just 0.3 produces a change in fold shape that is equivalent to that produced by a 15-208 change in trishear angle. Application of the trishear model to contractional and extensional structures of the Neuquén Basin highlights its utility for predicting (1) strain and strain path, (2) fracture orientation and distribution, (3) fault-slip magnitude, and (4) fault nucleation point/děcollement depth. Our study of large basement-cored producing anticlines such as Filo Morado-Pampa Tril emphasizes an important point: Trishear and parallel kink-fold geometries can be compatible when applied at different scales. Trishear provides a bulk description of the deforming zone on the thickened, triangular eastern flank of the fold but makes no explicit prediction about how the strain is accommodated. In these structures, the strain is variably accommodated by tight folding, duplexing, and flow of evaporites and depends significantly on the thickness of the “competent” units in the section.