The Agrio fold-and-thrust belt is located between 37°S and 38°S latitude in the eastern part of the Neuquén Andes. The belt can be divided into a western inner sector and an eastern outer sector. The inner sector is characterized by a thick-skinned deformation style. The dominant structures are large anticlines produced by the inversion of half-grabens formed during the Triassic-Jurassic extension that initiated the Neuquén Basin. The outer sector is characterized by thin-skinned structures; recent studies have shown that these structures have been reactivated in a thick-skinned style. A long-standing question has been whether the deformation in this belt occurred in a continuous pulse or in two independent pulses. The analyses of synorogenic deposits, crosscutting relationships between magmatic rocks and sedimentary formations, and new single-crystal 40 Ar/ 39 Ar ages from volcanic rocks presented here indicate a minimum age of 102 Ma for the beginning of deformation in this belt and that deformation occurred in at least two pulses, one during the Lower to Middle Cretaceous, and a second one in the middle Miocene, with different degrees of propagation into the foreland.

Abstract The Andes of the Neuquén Mesozoic basin have experienced multiple episodic tectonic events as a consequence of the changes of the plate tectonic boundary configuration. Each episode of deformation has overprinted the previous one, making it difficult to unravel the Andean tectonic history. The first deformation event took place in the uppermost Cretaceous with the formation of the Agrio fold-and-thrust belt. This event was related to the shallowing of the subducting plate recorded by the migration of the volcanic arc toward the foreland. During the late Oligocene–early Miocene, an extensional event, related to the steepening of the subducted plate, affected only the hinterland region causing the opening of the Cura Mallín basin. This basin was closed during the late Miocene, together with the development of a new fold-and-thrust belt that reactivated the previous structures. During the Late Tertiary, two more episodes of extension and compression affected the Andean area. The trip focuses on the field evidence that documents this complex history of evolution by looking at evidence of the sequence of the distinct tectonic events.

Abstract The Andean Fold and Thrust belt between 36° and 39°S can be divided in two sectors. The Eastern Sector corresponds to the Agrio Fold and Thrust Belt (FTB) characterized by a major exhumation during the Late Cretaceous, and minor deformation during the late Eocene and Late Miocene. The Western Sector corresponds to the main cordillera and is characterized by a complex evolution that involves periods of out-of-sequence thrusting with respect to the previously deformed outer sector, and pulses of relaxation of the compressive structure. Cretaceous uplift constituted an orogenic wedge that extended to the inner sectors of the Agrio FTB. Eocene compression was mainly concentrated within the Western Sector but may have reactivated the pre-existing structures of the Agrio FTB, such as the Cordillera del Viento. Late Miocene minor compressional deformation occurred in the retro-arc area and extended into the foreland area. This deformation event produced the closure of a short-lived intra-arc basin (Cura Mallín Basin, 25–15 Ma) at the innermost sector of the FTB. The Pliocene and Quaternary, between 37°30′ and 39°S, have been periods of relaxation of the inner part of the FTB and fossilization of the Agrio Fold and Thrust Belt. Localization of episodic late Oligocene–Early Miocene and Pliocene to the present extensional structures in the intra- and inner retro-arc is controlled by pre-existing Jurassic half-grabens related to the formation of the Neuquén Basin. The Jurassic rift seems to be controlled by deep crustal–lithospheric discontinuities derived from a Proterozoic–Palaeozoic history of amalgamation in the area, now deeply buried under multiple episodes of Mesozoic–Tertiary synorogenic and synextensional sedimentation.

The Neuquén Embayment, which developed along the eastern foothills of the southern Central Andes, has a complex history of intraplate deformation. The Paleozoic basement fabrics exerted a major influence in Mesozoic and Cenozoic deformation. The most important feature is an E-W–striking fault system that is related to a late Paleozoic fabric and is associated with the Huincul basement high, which truncates the basin. This fabric is interpreted as being the result of the accretion of the Patagonia terrane with Gondwana during the Early Permian. Two-dimensional (2D) and three-dimensional (3D) seismic coverage and subsurface information identify different sectors in the Neuquén Embayment that record alternating episodes of contraction and extension during the Jurassic and Cretaceous. The deformation history east of the thrust front of the Agrio fold-and-thrust belt is characterized by periods of (1) transpression and almost orthogonal contraction to the continental margin, (2) extension, and (3) relative quiescence, which alternates in different sectors. The earliest shortening occurred in the Early Jurassic when the main stress was oriented in the N-NW sector. The stress rotated to the northwest up to Valanginian times, when a more orthogonal orientation to the continental margin became dominant and prevailed after the Cenomanian. After a period of quiescence in the Neuquén Embayment associated with very oblique subduction during the Paleogene, the final contractional deformation took place in the late Miocene, with a west-east orientation of the main stress, and was followed by Pliocene extension. The changing stress patterns correlate with differences in convergence vectors between the Aluk, Farallon, and Nazca oceanic plates and the Gondwana or South American continental plates. The Aluk stage from the Jurassic to the Valanginian was characterized by tectonic inversion that is shown by shortening and right-lateral strike-slip structures that are concentrated in the Huincul system and more subtle deformation in the Chihuidos and Entre Lomas systems. The early Farallon stage was distinguished by reduced inversion and displacement in the Huincul system and a general retreat of deformation after the Valanginian. The change to late Farallon stage was characterized by a prominent tectonic inversion of the Entre Lomas system, which resulted from the inception of the formation of the Agrio fold-and-thrust belt in the retroarc area. This belt developed during most of the Late Cretaceous, when the embayment showed a general quiescence. The Nazca stage was characterized by the main episode of uplift, tectonic inversion of the older half-grabens, and important strike-slip faulting that was followed by local collapse of some structures during the Pliocene.

The southern Central Andes of Argentina between 35° and 39°S latitude can be divided into two sectors with contrasting geological histories. The boundary between the sectors coincides with the Cortaderas lineament. North of the Cortaderas lineament, the Andes record a foreland expansion of arc magmatism that is associated with contractional deformation in the Malargüe fold-and-thrust belt, and subsidence of the Río Grande foreland basin between 15 and 5 Ma. The peak expansion of deformation into the foreland occurred as late Miocene magmatic arc rocks erupted more than 500 km east of the trench and the San Rafael basement block was uplifted in central Mendoza. This stage was followed by the collapse of the basement uplift by normal faulting, the retreat of the magmatic arc, and the eruption of widespread late Pliocene to early Pleistocene within-plate lava flows in the Payenia region. Extensive Quaternary calderas and rhyolitic domes along the axis of the main Andes reflect crustal melting associated with basaltic underplating. In contrast, the structural evolution of the sector south of the Cortaderas lineament is dominated by the Late Cretaceous development of the Agrio fold-and-thrust belt, which underwent minor reactivations in the Eocene and the late Miocene. The post-Miocene Guañacos fold-and-thrust belt that has since developed along the axis of the main Andes concentrates neotectonic contraction. Arc magmatism in this sector is largely restricted to the axial area of the Andes. Both the sectors north and south of the Cortaderas lineament show evidence of an important episode of extension during the Oligocene to early Miocene, and for renewed extension in the Pliocene and the Pleistocene. The contrasting geological histories north and south of the Cortaderas lineament reflect differences in the geometry of the subducting plate, variations in crustal rheologies inherited from a more restricted distribution of Mesozoic rifts in the northern than the southern segment, and variations in the trench roll-back velocity through time.

The fold-and-thrust belt of the Neuquén Andes between 37°S and 37°30′S can be divided into two sectors that show contrasting evolutionary styles. The eastern, or outer, section contains the relatively well-known Agrio and Chos Malal fold-and-thrust belts, which were active from the Late Cretaceous to the late Miocene. The western, or inner, sector contains the Guañacos fold-and-thrust belt, which is the subject of this paper. In contrast to the eastern-sector belts, contractional deformation in the Guañacos fold-and-thrust belt began during the late Miocene and continues today. The Guañacos belt is particularly noteworthy for being out-of-sequence, in that the youngest deformation occurs in the western sector of the deformational belt in the Pleistocene volcanic arc, in contrast to the usual situation in the Andes, where the youngest brittle deformation occurs in the eastern sector. In detail, the Guañacos belt is located proximal to the present volcanic arc and coincides with the maximum heights of the Neuquén Andes. This thrust belt formed in response to tectonic inversion of an Oligocene-Miocene intra-arc rift. Detailed structural traverses in three valleys and observations in two other valleys through this 60-km-wide thrust belt show that the youngest tectonic activity is concentrated in the easternmost 40 km. Over the last 5 m.y., folds and thrusts in the Guañacos belt have affected Pliocene to lower Pleistocene volcanic arc rocks as well as Quaternary deposits that are immediately east of the Upper Pleistocene to Holocene centers of the Southern volcanic zone. Among the volcanic rocks incorporated into the Guañacos fold-and-thrust belt are those of the Pliocene to Quaternary Trohunco caldera and the Los Cardos–Centinela volcanic center. The inversion of the Tertiary extensional structures in the Guañacos belt is considered to be mechanically linked with the La Laja strike-slip fault system in the intra-arc in Chile.

Abstract Oil seeps related to igneous rocks in the Neuquén Basin have been known since pre-Hispanic times (sixteenth century) and have been explored in the southern Mendoza province since the late nineteenth century. In the 1980s, YPF (Yacimientos Petrolíferos Fiscales) began the exploration of igneous rocks as hydrocarbon reservoirs in the Río Grande Valley area. The possible productivity of these ‘unconventional’ reservoirs was recognized by studying outcrops and well data of sills and dykes emplaced in different formations of the fold and thrust belt of the northern Neuquén Basin. Mud-logging control and evaluations with drill stem tests (DST) were decisive to define these reservoirs as prospective. From petrographical reports on samples from outcrops and cores, six lithological types could be distinguished in the igneous units of this region. Recent works confirm that this volcanism belongs to two predominant cycles: from the late Oligocene to the Miocene (‘Molle’) and from the Middle to upper Miocene (‘Huincán’). Although in igneous reservoirs it is difficult to forecast the estimated ultimate recovery (EUR), sills that crosscut the source rocks of the Vaca Muerta and Agrio formations demonstrate surprisingly high production rates, although the number of wells for the complete development is always difficult to establish.

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 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 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.