The impacts of igneous systems on sedimentary basins and their energy resources Free

The world's energy needs are evolving as society is progressing through the energy transition (e.g. Gardiner et al. 2023). It is generally accepted that, for the foreseeable future, the world still requires hydrocarbons. However, the maturity of many of the world's hydrocarbon producing basins means that further exploration is being pushed into more extreme environments such as ultra deepwater continental margins (e.g. Fainstein et al. 2019) and also encounters more complex geological problems including basins heavily impacted by ancient igneous systems (e.g. Schutter 2003; Holford et al. 2012; Senger et al. 2017; Schofield et al. 2017a; Kilhams et al. 2022). At the same time, applied geological understanding must evolve to; (1) aid the search for gases such as hydrogen and helium, (2) deal with operational issues such as the need for pre-drill understanding of the CO₂ associated with hydrocarbons (e.g. Wycherley et al. 1999; Schutter 2003) and (3) to provide a confident geological framework for CO₂ sequestration (e.g. Underhill et al. 2023).

The scientific understanding of igneous rocks within sedimentary basins, specifically rifted margins, has evolved significantly over the past few decades thanks to high quality hydrocarbon industry data such as long offset 2D and 3D seismic data in areas such as the Brazilian Atlantic Margin, West of Shetland (United Kingdom Continental Shelf) and Australia (e.g. Holford et al. 2012; Hardman et al. 2018; Magee et al. 2018; da Costa Correia et al. 2019; Penna et al. 2019; Pérez-Gussinyé et al. 2024 ; Kilhams et al. 2021; Millett et al. 2022). For example, it is now possible to image and understand the development of seaward dipping reflectors (SDRs) plus oceanic transform zones and their relationship to basin development and heat flow (e.g. Lawrence et al. 2017; Paton et al. 2017; McDermott et al. 2018; Thomas et al. 2022), how igneous centres are preserved in the subsurface (e.g. Bischoff et al. 2017; Walker et al. 2021) and, with the help of more traditional outcrop studies, we can understand how networks of sills develop (e.g. Magee et al. 2016; Eide et al. 2017; Fiordalisi et al. 2024 ; Lombardo et al. 2024 ) and envisage how igneous systems influence the movement of volcanogenic or basinal fluids through sedimentary piles (Fiordalisi et al. 2024 ; Galland et al. 2024 ; Roelofse et al. 2020; Schofield et al. 2020a; Velayatham et al. 2024 ; Manton et al. 2022). Based on this data, exploration companies and International Ocean Discovery Program (IODP) expeditions have been able to drill boreholes which have further developed our understanding of both igneous features and their associated impact on source rocks, heatflow and charge systems (e.g. Archer et al. 2005; Rebori et al. 2024 ; Kilhams et al. 2022; Berndt et al. 2023; Lizarralde et al. 2023). This evolution in understanding covers everything from margin- to pore-scale (e.g. McDermott et al. 2018; Lima and De Ros 2019; Markwick and Paton 2024 ; Passey and McLean 2024 ). The result is that some areas of the world have a rich database of scientific literature and published industry material, but these lessons continue to be applied elsewhere if data becomes available. In addition, there continues to be cross-disciplinary learnings available from, for example, hydrocarbon exploration data to CO₂ storage opportunities or from methods in methane exploration to more traditional oil and gas extraction.

Although the community has been able to develop an enhanced understanding of how igneous systems impact sedimentary basins and their energy/sequestration opportunities, many questions remain and consequently this is an area of intense active research. This special publication, as a follow-up to a 2022 Geological Society conference (GS 2022), presents a collection of global research and state-of-the-industry manuscripts. These address the evolving science of, for example, the role of igneous systems in tectonics, structuration of the subsurface, heat flow, generation and migration of fluids (including hydrocarbons, helium and hydrogen) and the range of reservoir processes within more traditional reservoirs and igneous hosted examples (including opportunities for CO₂ storage and geothermal systems). The global setting of the specific contributions to the special publication are shown in Figure 1 with each mentioned in scientific context below. Figure 2 schematically illustrates the range of subsurface scenarios and applications covered within this special publication and the associated conference (GS 2022). Figure 3 illustrates the range of scales, from margin to pore, impacted by igneous processes within sedimentary basins as a summary of the following discussion.

The tectonic setting and evolution of a sedimentary basin is key to understanding its stratigraphy, including whether any (hydrocarbon) source rocks and potential reservoirs may be present and how these play elements may have been impacted by heat flow (and thus igneous events) through time (e.g. van Wees et al. 2009; Gardiner et al. 2019; Li et al. 2019; Pérez-Gussinyé et al. 2024 ). Global maps of the Earth's igneous record (e.g. Markwick and Paton 2024) are a key resource to understanding tectonic evolution, the distribution of igneous rocks and how they might have impacted the basins in which they are located.

Predominantly, in the pursuit of energy resources, researchers have focused on passive margin settings with a considerable publication base in the north and south Atlantic. For example, our understanding of seaward dipping reflectors (SDRs) and their relationship to basin evolution has evolved in places like the Vøring Basin of Norway (Planke and Eldholm 1994; Gernigon et al. 2020) and through high quality industry datasets offshore Brazil and Argentina (e.g. McDermott et al. 2018, 2019; Lovecchio et al. 2019; Harkin et al. 2020). In turn, this has progressed our understanding of how the rifts evolved through time such as in Greenland/Norway (Neumann et al. 2013; Peron-Pinvidic and Osmundsen 2018) and Angola, Namibia and South Africa with respect to southern Brazil, Uruguay and Argentina (e.g. Chauvet et al. 2021; Fiordalisi et al. 2024 ). Such work has provided insights into energy resource plays including tracing of source rocks and reservoirs across margins, this facilitates the understanding of how hydrocarbon discoveries in, for example, Namibia relates to prospectivity on the South American margin, or not (e.g. Garzanti et al. 2021; Sapin et al. 2021; Hodgson et al. 2023).

A detailed understanding of mantle heat flow through time and associated igneous events is important both to understand how rifts propagate (e.g. Lovecchio et al. 2024 ; Pérez-Gussinyé et al. 2024 ) but also how this might relate to energy resource plays (specifically the style of igneous systems present, source rock development and subsequent hydrocarbon charge). In any sedimentary basin an understanding of the timing of heat flow pulses enables better prediction of any igneous sequences present, the style of these systems (extrusive or intrusive igneous rocks for example) and how such events may have impacted energy resources (including possible hydrogen or helium plays) or potential CO₂ storage targets (e.g. Hutchinson et al. 2024 ; Millett et al. 2024 ; Olivares et al. 2024 ; Rosenqvist et al. 2024 ). For example, are large structural closures prospective or do they represent (deep intrusive or extrusive) igneous centres (e.g. Archer et al. 2005; Holford et al. 2012; McLean et al. 2017; Kilhams et al. 2021; Walker et al. 2021; Layfield et al. 2023)? Are extrusives associated with reservoirs (e.g. Hardman et al. 2018; Millett et al. 2024 ; Rosenqvist et al. 2024 )? How might the emplacement of extrusives impact the sedimentary pile below (Maharjan et al. 2024 )? Have intrusive systems interacted with source rocks and reservoirs (e.g. Iyer et al. 2018; Schofield et al. 2020a)? Can an understanding of the extent of extrusives and intrusives tell us about potential charge timing within a basin (e.g. Gardiner et al. 2019; Mangione et al. 2023)? The ability to make informed interpretations and predictions regarding the role of igneous rocks in sedimentary basins has evolved significantly thanks to high quality seismic data in combination with well penetrations, modern drilling techniques and more traditional outcrop work (e.g. Smart et al. 2024 ; Tsutsui et al. 2024 ).

As an example of the importance of such understanding in frontier (deepwater) hydrocarbon exploration, researchers have been able to develop models of source rocks in early rift settings associated with Seaward Dipping Reflectors (SDR) development (early thermal sag being associated with conceptual marine restriction; McDermott et al. 2018; Chauvet et al. 2021; Fig. 3a) or understand how transform and fracture zones might relate to source rock and reservoir development (e.g. Davison et al. 2016; Nemčok et al. 2016; Fig. 3b). Such concepts potentially open-up very large new deepwater hydrocarbon plays globally but also place further emphasize on accurately understanding dynamic rift evolution including heat pulses and associated igneous phases both in offshore and onshore domains (e.g. Schofield et al. 2020b).

Recent research efforts have shown the role of intrusives, especially sills, within sedimentary basins with a focus on understanding the thickness of igneous material at seismic and sub-seismic resolution (Mark et al. 2018). Alongside other elements, such as basement lithologies, this element of basin ‘overthickening’ has been shown to have a strong influence on hydrocarbon source rock maturation, fluid migration timing and basin heat retention (e.g. Gardiner et al. 2019; Mangione et al. 2023). Such work has placed emphasis on the ability to make pre-drill predictions of igneous sequences (Tsutsui et al. 2024 ).

Igneous rocks also influence fluid migration within sedimentary basins both by generating structuration but also, at smaller scale, through associated fracture networks and internal porosity (for example in basic intrusives) (e.g. MacDonald and Davies 2018; Rabbel et al. 2021). This can be positive for hydrocarbon exploration allowing migration routes to be mapped with some confidence (e.g. Tormore field in the Faroe Shetland Basin; Schofield et al. 2015) but can also lead to overpressure cells (creating potential safety concerns and drilling issues; Schofield et al. 2020a) or the bypass of hydrocarbons (e.g. Sun et al. 2019; Curtis et al. 2024 ). Such fluids can also be hydrothermal or similar leading to characteristic fluid escape features (e.g. Velayatham et al. 2024 ; Manton et al. 2022). Outcrop studies allow researchers to understand these processes at surface and make a link to the wider basinal hydrocarbon system (e.g. Fiordalisi et al. 2024 ; Galland et al. 2024 ; Rebori et al. 2024 ).

Both extrusive and intrusive processes generate and modify structuration within sedimentary basins (e.g. Fig. 3c). From the perspective of hydrocarbon exploration and CO₂ storage targets this potentially generates robust trapping geometries within the subsurface (after subsequent burial of extrusive systems). Extrusive igneous centres are commonly associated with carbonate reservoir targets as the volcanics form pinnacles or platforms on which ancient reefs grew (e.g. Courgeon et al. 2017; Ranger-1 well in Guyana, Trude et al. 2022). In theory, ancient extrusive centres, or transform faults, can also generate overburden traps with clastic reservoirs through differential compactional drape.

The deliberate search for large structures within igneous impacted basins can carry significant risk. Structures have been drilled which subsequently have been shown to be associated with large, interconnected networks of sills and/or laccoliths (e.g. Archer et al. 2005; Kilhams et al. 2022; Rochelle-Bates et al. 2024). In extrusive systems, lava deltas and structures associated with them often mis-identified as clastic or carbonate progradations (e.g. well 154/3-1 United Kingdom Rockall Basin; Broadley et al. 2019, 2020). Additionally, intrusives are often associated with forced folds which are considered a valid exploration target but have issues with fractures and fault systems (e.g. Schmeidel et al. 2017; Magee 2024 ; Fig. 3d). Detailed outcrop studies with integration of subsurface data can help to understand where such traps may or may not work. In this case the Neuquen Basin of Argentina is an excellent laboratory (e.g. Galland et al. 2024 ; Lombardo et al. 2024 ; Palma et al. 2024 ).

Reservoir intervals (carbonate, clastic and the igneous products themselves) within sedimentary basins can be influenced by igneous processes in a multitude of ways from defining depositional fairways to impacting porosity and permeability. Extrusives can act as fluid reservoirs themselves, an example being volcanic and epiclastic reservoirs associated with the Kora volcano in New Zealand (Bischoff et al. 2021) and possible CO₂ storage projects offshore Portugal (Pereira and Gamboa 2023). Intra-basaltic clastic reservoirs have proven to be an important hydrocarbon exploration target (e.g. Rosebank discovery in the United Kingdom Faroe Shetland Basin; Hardman et al. 2018; Fig. 3e). A deeper understanding of how these reservoir systems form, how they interact with the broader environment, the types of stacking patterns generated, and the internal reservoir quality will help to understand both further hydrocarbon exploration targets and their suitability for CO₂ storage (Snæbjörnsdóttir and Gislason 2016; Walker et al. 2022; Beresford-Browne et al. 2024 ; Maharjan et al. 2024 ; Millett et al. 2024 ; Rosenqvist et al. 2024 ). Intrusives can also act as their own reservoir systems due to internal fracturing (e.g. Palma et al. 2024 ), can influence the overburden and subsequent reservoir depositional routing (e.g. Egbeni et al. 2014) or can impact reservoirs through both contact metamorphism (e.g. Duffy et al. 2021) or by introducing hot fluids or exotics (e.g. mercury or nitrogen; Svensen et al. 2023) into a reservoir interval. In the case of the introduction of hot fluids this can cause mineralogical changes to clastic or carbonate reservoirs (e.g. Lima and De Ros 2019). Smaller sub-seismic sills may provide surprises and present challenges when predicting reservoir thickness in drilling offshore hydrocarbon wells such that, in some basins, cutting edge drilling technology may be required as safety, time and cost mitigation (e.g. Smart et al. 2024 ).

On a regional scale, the presence of igneous processes may lead to the development of volcaniclastic reservoirs and sealing units. The lithologies and associated porosity and permeability relationships associated with volcaniclastics can vary considerably and detailed mineralogical work is needed to understand their role as potential hydrocarbon or storage sites (e.g. Ólavsdóttir et al. 2015; Passey and McLean 2024 ; Fig. 3f). This forms part of a wider issue of provenance and reservoir quality within volcanic terranes where associated minerals such as feldspars and clays can lead to very low porosities and permeabilities (e.g. O'Neill et al. 2018).

There has been a recent push to actively explore for native hydrogen and helium accumulations for their role in decarbonization and industrial processes. For hydrogen this is also dependent on an understanding of heat flow through time and the subsequent interaction between specific host rocks and groundwater systems (Hutchinson et al. 2024 ; Olivares et al. 2024 ). For helium, the understanding of the distribution of suitable basement rocks plays a key role in exploration (Markwick and Paton 2024; Olivares et al. 2024 ). Both are dependent on a good understanding of the tectonic evolution of a particular sedimentary basin and, therefore, an understanding of the igneous processes are key.

From a hydrocarbon exploration perspective, there are a number of prospective basins around the world that may have been considered ‘too challenging’ by some companies due to the presence of igneous systems (a possible example being the Morandava basin of Madagascar; Tari et al. 2017). Furthermore, there are many examples where a lack of understanding of igneous systems has led to unpleasant surprises where their manifestation in the subsurface has not been adequately captured or accounted for. It is hoped that the examples within this special publication illustrate that, although there is an increased complexity associated with the presence of igneous rocks, the potential associated heat flow evolution, source rocks, traps and reservoir intervals can provide exploration opportunities in basins worldwide. Many such basins have a considerable uncertainty which can be mitigated by, for example, suitable outcrop studies, new seismic data or stratigraphic well test (sensu Schofield et al. 2017b; Kilhams et al. 2022). This understanding is also key to de-risking exploration activities and understanding the role of these igneous influenced basins in future CO₂ storage projects.

The editorial team extends its appreciation to all the authors and collaborators that contributed to this book. The time and effort taken represents considerable effort and perseverance. Everyone that contributed to the original 2022 Geological Society conference is also appreciated, the open and collaborative atmosphere formed the basis of this book, with special thanks going to: Samira Bashar, Becky Goddard and Andrea Schito. We thank various managers who allowed the editors and authors to spend time making a significant contribution. Ben Kilhams particularly thanks Nick Feast, Marc Gerrits, Richard Knight, Jim Pickens, Roald Rijnbeek, Paul Tricker who supported the time needed to deliver the conference and special publication and also offered sponsorship. Sincere thanks go to all the reviewers and contributors who took time and care to give excellent, constructive feedback: Stuart Archer, Alan Bischoff, Sierd Cloetingh, Michel de Sant-Blanquat, Óluva Eidesgaard, Geoffrey Ellis, Petter Frantzen, Andrew Green, David Hinds, Mads Huuse, Dougal Jerram, David Jolley, Garry Karner, Fernando Lebinson, Nicolas Lefeuvre, Nils Lenhardt, Craig Magee, Ben Manton, Ken McDermott, Avon McIntyre, Vanessa Mendoza, John Millett, Tim Minshull, Webster Mohriak, Sven Morgan, David Muirhead, Simon Passey, Matthew Pound, Emilio Rojas, Tobias Schmiedel, Nick Schofield, Kim Senger, Aimee Watling, Penny Wilson and others who wished to remain anonymous. Finally, the editors wish to thank the Geological Society Publishing House, especially Caroline Astley and Danielle Tremeer for their support.

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

BK: visualization (lead), writing – original draft (lead), writing – review & editing (lead); SH: visualization (supporting), writing – original draft (supporting), writing – review & editing (lead); DG: writing – review & editing (supporting); SG: writing – review & editing (supporting); LL: writing – review & editing (supporting); CM: writing – review & editing (supporting); ST: visualization (supporting), writing – review & editing (equal); DW: writing – review & editing (supporting).

This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

Data sharing is not applicable to this article as no datasets were generated or analysed during the current study.