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 This contribution presents a detailed geological study of a well-exposed igneous petroleum system at Sierra de Cara Cura, Neuquén Basin, Argentina, which consists of sills, dykes, laccoliths and hybrid intrusion morphologies. The size of the exposed intrusive complex (19 km in north–south direction) is similar to those of sill-clusters hosting producing oil fields of the nearby Río Grande Valley. Most sills (>80%) are preferentially emplaced in the organic-rich rocks such as Vaca Muerta and Agrio Formations. Sills are extensively fractured and represent potentially good fractured hydrocarbon reservoirs. We document several fracturing mechanisms that can lead to heterogeneous fracture distribution. Laccoliths represent very different igneous reservoirs with a typical zonation: the core of the laccoliths are massive and poorly fractured, whereas the rims consist of a breccia formed during laccolith emplacement. The Vaca Muerta and Agrio Formations are thermally altered by the cooling of the sills. The deep parts of the outcropping laccoliths likely altered thermally the shale formations they were emplaced in. Hydrogen Index (HI) and Transformation Ratio (TR) of the organic matter are the best parameters to determine the thickness of the thermal aureole. The Sierra de Cara Cura appears as a world-class field analogue of igneous petroleum systems.

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 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 Shale reservoirs host ubiquitous multi-scale natural fractures created by factors such as pore pressure build-up after petroleum generation and palaeo crustal stress changes during tectonic episodes. Natural fractures are among postulated drivers of stimulated shale well productivity and related variability. Quantifying the aforementioned role and optimizing recovery call for litho-structural assessments. In this interdisciplinary study, natural fracture density (fractures per unit length) for North and South American shales (Marcellus, Eagle Ford, Haynesville, Barnett, Fayetteville and Vaca Muerta) was estimated from published observations of outcrops, cores and borehole images. Associated production for the latest horizontal wells, drilled in the most productive locations (sweet spots), was normalized by lateral length and reservoir thickness. It was found to correlate positively with the density of small-scale natural fractures. Durations of transient linear flow, diagnosed from production data, were play-specific, negatively correlated with small-scale natural fracture density and led to realization of picodarcy matrix permeability. Conversely, large-scale (tectonic) fractures limit stimulation efficiency and pose environmental/induced seismicity risks. Therefore, stimulation-driven reactivation of small-scale fractures facilitates drainage and enhances well productivity. Relatedly, reservoir flow regimes and production decline curves are intricately controlled by interplay of natural fracture density and matrix permeability. Variability of these parameters calls for acreage-tailored stimulations.

Abstract Charles Darwin's research during the second voyage of HMS Beagle is examined within the context of Charles Lyell's ideas on crustal movement. Darwin's pre-voyage training is summarized and the impact on his own subsequent theorizing of his commitment early in the voyage to a Lyellian theoretical framework is analysed. Two sites studied by Darwin which he interpreted as strong support for Lyell's theory of vertical crustal mobility are examined: the first is Signal Post Hill in the Cape Verdes, visited in 1832; the second is Agua de la Zorra in Argentina, visited in 1835. Darwin's work at both sites was key to his first theory of globally-balanced elevation and subsidence as an explanation for the structure and distribution of coral reefs. The case is made that both sites are of international geoheritage importance and that their protection should be assured with enhanced access and interpretation.