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 Lava flows form important fluid reservoirs and have been extensively exploited for water aquifers, geothermal energy, hydrocarbon production and, more recently, for carbon storage. Effusive subaerial mafic to intermediate lava flows account for vast rock volumes globally, and form reservoirs with properties dictated by well-known lava flow facies ranging from pāhoehoe through several transitional forms to ‘a’ā lava. These variations in flow type lead to critical differences in the pore structure, distribution, connectivity, strength and fracturing of individual lava flows, which, alongside lava flow package architectures, determine primary reservoir potential. Lava flow margins with vesicular, fracture and often autobreccia-hosted pore structures can have porosities commonly exceeding 40% and matrix permeabilities over 10 −11 m 2 (>10 D) separated by much lower porosity and permeability flow interiors. Secondary post-emplacement physicochemical changes related to fracturing, meteoric, diagenetic and hydrothermal alteration can significantly modify reservoir potential through a complex interplay of mineral transformation, pore-clogging secondary minerals and dissolution, which must be carefully characterized and assessed during exploration and appraisal. Within this contribution, a review of selected global lava flow-hosted reservoir occurrences is presented, followed by a discussion of the factors that influence lava flow reservoir potential.

Abstract Soft sediment deformation structures may form when denser sediments or fluids are deposited on or flow over unlithified and less dense sediments. This study presents a seismic geomorphological study of the basal contact between an extrusive volcanic sequence and underlying sediments, defining the ‘Base Basalt’ surface, on the Mid-Norwegian Margin. This contribution focuses in particular on the development of geomorphological features related to the rapid loading of a several 100 m-thick lava delta package of hyaloclastite onto poorly consolidated sediments of the pre-volcanic sedimentary basin fill. Seismic horizons, sequence boundaries, volcanic facies units and attribute maps are used to characterize the seismic geomorphological features imaged within a high-quality 3D seismic cube. The ‘Base Basalt’ horizon and attribute maps reveal incised channels and a network of polygonal to irregular depressions and ridges described here as an ‘egg-box network’. More than 150 depressions, with a typical diameter of 1 km and a depth of 100 m, have been mapped. The deformation features, which are restricted to the base of the Lava Delta seismic facies unit, are interpreted to be the result of rapid loading of the Lava Delta onto poorly consolidated unlithified pre-volcanic sediments. This study presents new evidence for the dynamic nature of the transition between sedimentary basins and large-scale volcanism found along volcanic margins and basins associated with rapid volcanic deposition.

Abstract The NE Atlantic volcanic rifted margins include vast underexplored basin areas neighbouring mature petroleum-producing regions. We appraise the cross-border prospectivity of the outer and central Faroe–Shetland, Møre and southern Vøring basins and present insights from extensive new 3D seismic surveys. Regional seismic surfaces are used to compile a cross-border seismic profile highlighting key discoveries from the UK Rosebank field in the SW to the Norwegian Ormen Lange field in the NE. Cretaceous to Paleocene reservoirs remain the main exploration focus seaward of the platform areas, and the presence of several large untested structures presents important exploration targets in the mid-Norway region. Improved imaging of the areas affected by Paleogene igneous rocks reveals major untested sub-basalt structures including some regions on the marginal highs where the basalt cover has been entirely removed by erosion, revealing sub-basalt stratigraphy and structures with pre-Cretaceous potential prospectivity. The influence of igneous rocks on both discovered and prospective hydrocarbon systems is discussed. Neogene sand injectite fields and Quaternary glacial sand bodies are extremely well imaged in the Møre Basin, documenting shallow prospectivity supported by the presence of successful regional analogue plays. New 3D seismic data are revealing previously unseen prospectivity in frontier and underexplored regions.

ABSTRACT Fluid release structures resulting from the interaction of igneous intrusions with sedimentary basins form an important part of the evolution of large igneous provinces. Hydrothermal breccia pipes formed in the Karoo Basin in South Africa during emplacement of igneous sills in the Karoo large igneous province represent one of the best-exposed expressions of such venting structures. Earlier work has shown that degassing of thermogenic CO 2 and CH 4 through the breccia pipes may have contributed to the Early Jurassic environmental changes. Here, we present the first detailed analysis of the distribution of breccia pipes in the western parts of the Karoo Basin. We mapped 431 pipes in a 650 km 2 area using outcrop data. The pipes are rooted in contact aureoles around four sills emplaced in organic-rich Ecca Group shale, and thermal modeling of sill cooling and contact metamorphism gives a maximum temperature of 675 °C near the sill contacts, sufficient to convert a significant fraction of the organic carbon to gas. Model estimates indicate that metamorphism in the 650 km 2 area generated 75–88 Gt of CO 2 , depending on actual sill thicknesses and emplacement levels. When further up-scaled, an area of 7400–8700 km 2 (i.e., less than 2% of the area in the Karoo Basin intruded by sills) would be required to generate 1000 Gt of CO 2 . In order to characterize the degassing pipes, their geographical positions and diameters were analyzed using several point-pattern methods. The results showed that the pipes (1) have diameters in the 11–177 m range (average 44 m), (2) are spaced with an average nearest-neighbor distance of 452 m, and (3) are overall randomly spaced but with weak overdispersion at very small scales (<50 m) and weak clusters at larger scales (400–3000 m). In contrast to studies of volcanic pipe spacing, this study on breccia pipes does not indicate that the pipe spacing is controlled by any large-scale geophysical parameters such as crustal or basin thicknesses. Conclusions point to the pipes being formed following sill emplacement and pressure increase in the low-permeability organic-rich shale, followed by rapid carbon degassing, emphasizing their important role in the Early Jurassic climate change and oceanic anoxic event.

Abstract Formation micro-imaging (FMI) is a tool that produces micro-resistivity images of the sidewall of the well bore. FMI logging used in conjunction with conventional well logging techniques (e.g. GR, Gamma Ray/RES, Resistivity/NPHI, Neutron Porosity/SONIC, Velocity tools) allows detailed analysis of volcanic lithofacies variation and informs a robust interpretation of volcanic sequences. This methodology is of particular use where rock core data are limited or not present. Examples are presented from the Rosebank Field in the Faroe–Shetland Basin (West of Shetland, UK continental shelf) where the re-establishment of fluvial activity between phases of effusive volcanism resulted in a complex sequence of siliciclastic sedimentary rocks and basaltic lavas. We demonstrate how high-resolution FMI images through this sequence can differentiate internal basalt lava flow features, such as vesicular zones, brecciated intervals, sediment–lava contact relationships and joint/fracture networks. If FMI data exist through volcanic packages and if assessed and calibrated properly via core, sidewall core and field analogue comparisons, it can provide additional constraints on the interpretation and classification of reservoir (siliciclastic) and non-reservoir (volcanic) rocks.