Abstract Cooling subvolcanic igneous intrusions are known to have a tremendous impact on fluid flow in the shallow Earth's crust. However, the long-term post-cooling legacy of subvolcanic intrusions on fluid flow has received much less attention. Here we describe geological examples in the Andean foothills, Argentina, showing that igneous intrusions have long-term effects on fluid flow after their emplacement and cooling. The case study consists of ∼11 Myr-old eroded andesitic intrusions of Cerro Alquitrán and Cerro La Paloma, northern Neuquén Basin, Argentina, at the rims of which large volumes of bitumen are naturally seeping out at the Earth's surface. The intrusions exhibit laccolithic shapes with steep-sided contacts with the host rock. Near the intrusive contacts, the andesite is intensely broken along concentric breccia bands and fracture bands, interpreted as resulting from syn-emplacement brittle magma deformation, which represent high-permeability pathways for the migrating bitumen. Organic geochemical analyses of the bitumen show that the seeping oils were generated from incipiently mature Vaca Muerta sections located in a regional kitchen to the west, implying a lateral migration of ∼10–20 km. The Cerro Alquitrán and Cerro La Paloma intrusions are demonstrative examples highlighting how extinct subvolcanic intrusions have long-term consequences for subsurface fluid circulations in sedimentary basins.

Abstract Volcanic plumbing systems emplaced in sedimentary basins may exert significant mechanical and thermal effects on petroleum systems. The last decade of research has evidenced that igneous intrusions may enhance thermal maturation of organic matter in source rocks and lead to both small- and large-scale structures that can deeply impact fluid migration or trapping. This contribution describes how the emplacement of a whole intrusive complex generated a dome structure of the overburden, which is the main trapping structure of a large producing oilfield. Our case study is the lower Miocene Cerro Bayo de la Sierra Negra (CBSN) intrusive complex, Neuquén Basin, Argentina, associated with the El Trapial oilfield where the main trapping structure is a large domal antiform centred on the CBSN complex. This study integrates the large subsurface dataset produced during the development of the El Trapial oilfield. More than 1200 vertical wells (producers and injectors) have been drilled in the flanks of CBSN complex. In addition, five 3D seismic cubes have been acquired over the years that have been merged and reprocessed into a single volume. Such a dataset allows a detailed characterization of both the structure affecting the Mesozoic strata and the geometry of the intrusive complex. Igneous rocks have been recognized along the entire stratigraphic section. Sill intrusions appear to concentrate in the shale units and the stacking of them has a direct impact on the doming structure generation. Our study allowed us to establish a direct correlation between the distribution of the intrusions and the extent, amplitude and style of doming, showing that the dome structure results from the emplacement of the intrusive complex. We also show that part of the doming is related to intrusions emplaced in the Mesozoic formations of the Neuquén Basin, whereas the other part of the doming is related to deeper structures not imaged on the geophysical data. We estimate that the amplitude of the doming reaches up to c. 500 m. The voluminous subsurface data, combined with exposed outcrops, makes the CBSN complex a world-class case study for showing how the shallow plumbing system of a volcanic complex may control the growth of large-scale trapping structures for various fluids, such as drinkable water, geothermal fluids and hydrocarbons.

ABSTRACT The Late Jurassic–Early Cretaceous Vaca Muerta Formation deposited in the Neuquén Basin has an exceptional development, in the thickness of its sequences and in the volume of its sedimentary record. These marine deposits, which constituted the source rock of conventional oil and gas for more than a century, continue to be an important source of unconventional hydrocarbons as a producer of oil- and gas-shale and have a large potential future. When the exceptional characteristics of the Vaca Muerta Formation are compared to other basins of Late Jurassic to Early Cretaceous age along the same continental margin, they stand out even more. The basin development and preservation parallel to the continental margin are discontinued by important processes of subduction erosion. The analysis of the thickest and widest part of the basin (the Neuquén Embayment) shows that its large development is related to the combination of two rift systems, a north–northwest system truncated by an almost east–west system. These two extensional systems were developed in the hanging-walls of sutures between different crustal terranes that formed the basement of the Neuquén Basin. The paleogeographic development and the structure of these basement terranes are the control that conferred to the Neuquén Basin and its Embayment their unique characteristics.

ABSTRACT The Vaca Muerta Formation deposited in a retro-arc setting, favored by an under filled depocenter and uplifted peripheral zones. The onset of the deposition corresponds to a rapid flooding of oceanic water penetrated through sudden ruptures along the incipient Andean arc, as a consequence of the Late Jurassic extension during the thermal subsidence stage. The Vaca Muerta Formation forms part of the Lower Mendoza Subgroup, and is defined as the organic-rich calcareous mudstone (av. 5% TOC) deposited above the sharp contact of the continental Tordillo and the Quebrada del Sapo formations; the upper contact with the Quintuco Formation is gradational and diachronous. The Vaca Muerta Formation spans the early Tithonian Virgatosphinctes andesensis to the early Valanginian Lissonia riveroi ammonite Zones. In association with classical stratigraphic characteristics of this unit, this chapter presents a cyclostratigraphic and magnetostratigraphic study displaying an unprecedented accurate time scale for the Vaca Muerta deposition, which took place from Chron M22 to Chron M15 in a timespan of ~10 My (4.86 My within the Tithonian and 5.27 My within the Berriasian).

ABSTRACT This chapter summarizes and integrates some of the most relevant published results related to the geomechanics of the Vaca Muerta Formation and its relationship with structural geology, stratigraphy, and petrophysics. The Vaca Muerta unconventional play is overpressured in most parts of the Neuquén Basin. Direct measurements are not feasible because of the low permeability of these rocks. Indirect measurements used to assess the range of pore pressure derive from diagnostic fracture injection tests (DFIT) taken over several days, and the initial reservoir pressure estimated from production models of horizontal wells. The Neuquén Basin underwent several compressional tectonic episodes. The main direction of the maximum horizontal stress changed from northwest–southeast in the Middle Jurassic, to approximately east–west of present day, associated to the Andean tectonics. Maximum and minimum horizontal stress directions, a critical parameter to design horizontal wells, has been regionally characterized using abundant high-confidence indicators, such as wellbore breakouts, wellbore-induced fractures, and microseismic analysis. The chapter presents and discusses the estimation techniques of overpressure and minimum horizontal stress, the overpressure source mechanism, the overburden variations at the base of Vaca Muerta, a regional characterization of rock strength and elastic properties, and the current understanding of the stress regime and the hydraulic fracture behavior. Significant progress has been made during the past five years in the geomechanical characterization of the Vaca Muerta Formation. As drilling, coring, testing, and production data become available, more comprehensive databases are built and uncertainty is reduced. Whereas a few years ago analogs from other basins were the only available information, today, local databases of triaxial and other rock mechanic laboratory tests, together with well logs, are successfully used to elaborate local correlations.

ABSTRACT Utilizing a basin-wide data set of three-dimensional seismic volumes and the application of principles of seismic stratigraphy and seismic geomorphology allowed identifying numerous depositional elements within the Vaca Muerta–Quintuco system, a set of clinoforms whose topsets belong to the Quintuco Formation, whereas the bottomsets and foresets belong to the Vaca Muerta Formation. Within the topsets, small circular geobodies clustered near the prograding shelf margin, averaging 200–800 m (656–2625 ft) in diameter and up to 75 m (246 ft) in height. These features comprise small carbonate buildups defining carbonate factories trending strike parallel. Identification of intervals where these geobodies are abundant is important because wells drilled through them have experienced either drilling mud admission or gas influx. In addition to these biogenic carbonate mounds, the topsets show elongated oolitic grainstone shoals oriented orthogonal to coeval shelf margins, in some cases measuring up to 22 km (14 mi) long and 5 km (3 mi) wide. The foresets (slope deposits) become progressively enriched in total organic carbon (TOC) and porosity downdip—key variables for a self-sourced unconventional reservoir. These deposits commonly comprise mudstone and marlstone, interbedded with limestones. In the lower foresets to toesets, strike-parallel, high–seismic-amplitude, and high-energy calcareous deposits are embedded in organic-rich mudstones. In some instances, these amplitude anomalies, drilled and cored by a few wells, show both well-defined linear geobodies along the toeset and evidence of bottom currents in cores (thicker limestone beds, ripples, bioturbation, and occasional centimeter-scale soft-sediment deformation). Identification of such geobodies is critical, as there is evidence from ongoing development drilling that these may act as hydraulic-fracture barriers and can also affect well performance as evidenced by increased water production. The bottomsets consist of low–amplitude-parallel, “railroad track” reflections that extend for tens of kilometers, characterizing the classic basin center Vaca Muerta play. Within these deposits, no major mappable geobodies are observed other than localized compressional ridges of mass-transport deposits (MTD) near the toesets. In some areas of the basin, the Vaca Muerta Formation was deposited directly on top of a preexisting non-marine paleo-aeolian dune topography which had a direct impact on the stratal geometries and the bottomset facies of the Vaca Muerta Formation. Acoustic impedance (AI) from seismic inversion show well-defined Vaca Muerta low-impedance seismic facies, which relate to the presence of predominantly fine-grained, organic-rich (~5%), porous (~11%) mudstones and marlstones. Within the high-TOC Vaca Muerta interval, most AI three-dimensional (3-D) volumes throughout the basin show an average of five to six seismic facies with discrete AI values that can be directly correlated to rock types, depending on their position within the clinoform (i.e., topset, foreset, bottomset). These seismic facies correlate to facies associations with distinct petrophysical and geomechanical properties at core/outcrop scale, as measured by lab studies. Understanding this relationship and its distribution in space is critical to predicting optimum horizontal well landing zones and sweet spots.

ABSTRACT The Vaca Muerta Formation (Late Jurassic–Early Cretaceous) bears a high-quality, oil-prone kerogen deposited under mostly anoxic, marine conditions and constitutes a world-class source rock with outstanding geochemical characteristics for the generation of petroleum (oil and gas) throughout the Neuquén Basin. The formation has been identified as the main source for the majority of the hydrocarbon pools found in conventional reservoirs of the basin, but in the last ten years, it has also acquired significance as a self-sourced unconventional reservoir target for both oil and gas. An extended database that comprises several tens of thousands of samples, including cuttings, cores, sidewall cores, and outcrops of the Vaca Muerta Formation from wells and outcrop sections along the entire basin was evaluated. This allowed formulating patterns of organic richness, hydrocarbon source quality, and distribution of free hydrocarbons in six reference areas of the basin. These reference areas are defined based either on the impact of the sedimentary rock on generated hydrocarbons or on the significant thermal maturity differences. The areas are: Malargüe, Chihuido-Lomita, Northeast Platform, Embayment, Huincul Arch, and Picún Leufú. More than 300 oils and organic extracts from Vaca Muerta and nearly 500 mud and production gas samples completed the data set to understand the key features of the fluids occurring in the prospectable areas for unconventional resources (shale oil and shale gas). Collected and evaluated analytical data include total organic carbon (TOC), programmed pyrolysis, visual kerogen analyses, bulk chemical composition of fluids, gas chromatography (GC) and gas chromatography–mass spectrometry for biomarker fingerprints (GCMS), stable carbon isotopes, bulk and compositional kinetics, and x-ray fluorescence (XRF). Moreover, insights into the essential processes of the Vaca Muerta unconventional petroleum system including kerogen-related issues and basin-scale processes are discussed in terms of source rock kinetics, modeling of burial/exhumation histories, porosity development, and overpressure occurrence. The Embayment area stands out as the most attractive for unconventional development to produce middle-to-light oil with low sulfur content along with gas condensate westward. An analogous pattern is observed in the Chihuido-Lomita and Huincul Arch areas, with similar source rock characteristics but with overall lower thermal maturity compared with the Embayment area. However, in the transition to the Northeast Platform, the northeastern fringe of Chihuido-Lomita and Embayment areas present middle-to-heavy, mostly sulfur-rich oils, with limited gas potential, hence requiring higher permeability to yield commercial production. The Malargüe area is characterized by overall mid maturity and limited quality of oil (middle to heavy), predominantly sulfur-rich. Finally, the Picún Leufú area is conditioned by a source rock with low potential because of thinner organic-rich intervals and low-to-middle thermal maturity.

ABSTRACT This chapter summarizes the abundant marine paleobiological markers recorded in the Vaca Muerta Formation illustrating their richness, biodiversity, and application for age dating and paleobiogeographical global correlation. The markers described include ammonites, calcareous nannofossils, calcisphaeres (calcareous dinoflagellate cysts), calpionellids, coprolites, dinoflagellates, radiolarians, saccocomid microcrinoids, and trace fossils. Traditionally, the ammonites have been the best known and most relevant macroinvertebrate fossil group. Nevertheless, in the past decade, the exploration of the Vaca Muerta unconventional play has boosted the research on the micropaleontology of the area determining an extensive growth in knowledge and reported here.

ABSTRACT The objective of this chapter is to introduce the case of the first large-scale oil shale development in Argentina. The work is focused on the reservoir characterization of the liquid-rich shale of the Vaca Muerta Formation in the Loma Campana block that was carried out to optimize the drilling campaigns executed since the oil discovery in 2010 and was supported by a comprehensive data acquisition program accomplished between 2010 and 2012. Data gathered involved three-dimensional (3-D) prestack depth migration (PSDM) seismic reprocessing, core and cutting analyses, mudlogging and e-logs, geomechanical, petrophysical, and geochemical studies, diagnostic fracture injection test (DFIT), microseismic monitoring in vertical and horizontal wells, wellhead pressures and temperatures, downhole pressures captured via retrievable gauges, time-lapsed production logging surveys, tracer tests, pressure–volume–temperature (PVT) analyses, and production records. According to the regional stratigraphic framework, the Vaca Muerta Formation in Loma Campana begins with two aggradational transgressive–regressive (T–R) sequences that represent the target of the current development. The analysis and interpretation of the data showed that the Vaca Muerta Formation in Loma Campana is 80–300 m (262–984 ft) thick, gradually thickening northwestw ard. The most common lithofacies include organic-rich fossiliferous calcareous mudstones, silty mudstones, heterolithic marlstones, wackestones, and ash beds. The total organic carbon (TOC) varies between 2.3% and 7.6% (average values from static model) decreasing upward, and the mineralogic composition consists of 33% siliciclastic material (quartz and plagioclase), 46% carbonates (calcite and dolomite), 18% clays (illite and interbedded illite/smectite), and 3% pyrite (average values, dry weight w/w). The local structural framework is characterized by a north–northeastern gently dipping monocline (depths ranging from 2800 to 3100 m—total vertical depth or TVD [9186–10,171 ft]). In a mainly strike-slip regime, the maximum horizontal stress (SH max ) is in the range of N90° and N110°, and Vaca Muerta’s pore pressure gradient is around 0.87 psi/ft. Oil density ranges from 38° to 60° API and gas–oil ratio (GOR) from 90 to >3500 m 3 /m 3 (500–20,000 scf/bbl) with increasing maturity westward. Well productivity has improved year after year because of operational experience and subsurface knowledge. Standard lateral length has evolved from 1000 m (3300 ft) in 2012 to 2500 m (8200 ft) in 2019. The main drivers to continue improving well estimated ultimate recovery (EUR) are: increasing precision in well placement and continuous optimization of stimulation design. By the end of 2019, the production from the Vaca Muerta play reached 56,000 BOE/d with 548 active producing wells (93% of the production from 184 horizontals), becoming the second largest oil field in Argentina. The development of the Loma Campana block is a milestone for the oil and gas industry in Argentina; the results obtained in terms of production, cost reduction and scale economy led to break the industry’s inertia, to develop new infrastructure, to position the Vaca Muerta Formation on the world map of unconventional plays, and to attract new investments and partners.

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 “Andean” configuration of the Neuquén Basin has been classically used in most paleogeographic reconstructions of the existing literature as a consequence of the compressive deformation episode of the Andean orogeny, which resulted in a foreland stage initiated during the Late Cretaceous. The configuration displays its classic structural elements and morphostructural domains, as well as the origin of fold-and-thrust belts. Conversely, based on a great amount of preexisting data, it has been noticed that the present-day “Andean” configuration greatly differs from that of the basin during the deposition of the Vaca Muerta Formation, when a stage of subduction-related thermal sag with some punctuated episodic inversions took place, in a retro-arc basin context. This chapter analyzes the paleoconfiguration of the basin during the time in which the Vaca Muerta-Quintuco system was deposited (early Tithonian to early Valanginian). The basin paleoconfiguration during that interval clearly represents a first-order geological control on the stratigraphic architecture of the Vaca Muerta Formation (clinoform internal and external geometries, progradation patterns, extent), as well as on the mineralogical composition and thicknesses of the organic-rich facies.

ABSTRACT Following an enthusiastic period when organic-rich mudstones were stimulated irrespectively of their intrinsic properties, it is now recognized that each self-sourced unconventional play presents its own challenges and requires specific evaluation and stimulation techniques to reach commercial efficiency. The Vaca Muerta Formation is no exception. This world-class unconventional play has a continuous organic-rich section of significant thickness, reaching 500 m (1640 ft) in some places, which enables numerous possible stacked landing zones. Nevertheless, the stratigraphic column exhibits variations in the reservoir properties, including mineralogy, pore system, and organic content, coupled with variations in thermal maturity. Additionally, the formation has a highly layered nature with occurrences of ash beds, bed-parallel veins of fibrous calcite (aka beefs), and alternations of thin carbonate beds and thicker mudstone beds. In the Vaca Muerta Formation, the pore system is complex and includes pores hosted within the inorganic matrix and different types of pores generated within the organic matter. This complexity has a fundamental impact on the characterization of the play. Presence, amount, connectivity, size, and shape of these pores vary with compaction, thermal maturation, mineralogy, and diagenesis. The estimation and characterization of the water volume in Vaca Muerta Formation is also a challenge. The presence of variable volume of movable water is suspected.