The impact of igneous intrusions on sedimentary host rocks: insights from field outcrop and subsurface data

When igneous intrusions are emplaced into sedimentary host rocks, they alter the physical properties of these sediments through direct conductive heating, chemical alteration by hydrothermal fluids (often sourced from the cooling intrusion) and fracturing by mechanical processes (Van Wyk 1963; Einsele et al. 1980; Chevallier et al. 2001; Senger et al. 2015, 2017). Igneous intrusions have been documented in numerous sedimentary basins which are prospective for petroleum exploration and production, with research utilizing an abundance of three-dimensional seismic reflection and well data to characterize their subsurface morphology (e.g. Davies et al. 2002; Smallwood and Maresh 2002; Thomson and Hutton 2004; Planke et al. 2005; Schofield et al. 2017). Previous work has also explored the implications these intrusions have for petroleum exploration in relation to seismic imaging and drilling challenges (Archer et al. 2005; Holford et al. 2013; Eide et al. 2017; Mark et al. 2018).

Igneous intrusions are generally thought to be detrimental to sandstone reservoir quality through contact metamorphism and mineralization of hydrothermal fluids, thereby reducing the porosity and permeability of potential reservoir sandstones (Parnell 2010; Holford et al. 2013; Rateau et al. 2013; Grove 2014; Grove et al. 2017). When intrusions are emplaced into organic rich sedimentary rocks, they can alter the thermal maturity and geochemistry, through direct conductive heating (Murchison and Raymond 1989; Bishop and Abbott 1995; Muirhead et al. 2017; Peace et al. 2017). Research on the impact of igneous intrusions on reservoir quality and source rock potential has largely been based on either entirely subsurface studies (Holford et al. 2013; Rateau et al. 2013; Aarnes et al. 2015) or field outcrop studies (Murchison and Raymond 1989; Bishop and Abbott 1995; Grove 2014; Senger et al. 2015; Eide et al. 2016; Grove et al. 2017; Muirhead et al. 2017). Studies which focus purely on subsurface data are restricted by data availability, whereas field outcrop data is more accessible, but often incomplete (e.g. due to erosion). Few studies have therefore combined both fieldwork and subsurface data (Senger et al. 2017).

To demonstrate the impact of igneous intrusions on sedimentary host rocks, we present a series of subsurface and field-based examples from numerous basins around the world (Fig. 1). Specifically, this paper addresses three main elements:

1. First how a variety of analytical techniques, including geophysical well logs, petrography and organic geochemistry, can be used to understand the impact of igneous intrusions on sedimentary host rocks in both the subsurface and outcrop.

2.   Secondly, to understand what controls the extent of host rock alteration and how different lithologies react to the emplacement of igneous intrusions across a range of depositional environments, tectonic settings and depths of burial.

3.   Finally, we highlight how alteration of sedimentary host rocks can have both detrimental and positive impacts on the petroleum system, specifically reservoir and source rock intervals.

Igneous intrusions emplaced into siliciclastic sedimentary rocks can cause the expulsion of pore water fluids, in addition to thermal, mechanical and chemical alteration of the host rock (Ahmed 2002; Schofield et al. 2012; Holford et al. 2013; Senger et al. 2014, 2015, 2017). In the most extreme cases, contact between igneous intrusions and siliciclastic sediments will result in pyrometamorphism, where sufficient transfer of heat from the magma results in partial melting of the host rock sediments and the formation of buchite (Holness 1999; Grove 2014). Conductive heating is the most commonly studied form of host rock alteration by igneous intrusions (McKinley et al. 2001; Muirhead et al. 2017; Peace et al. 2017). Convective heating, due to circulating systems of hydrothermal fluids, can alter host rock at much greater distances, the extent of which is controlled by the host rock permeability (Ingebritsen et al. 2010). This alteration by hydrothermal fluids is greater than that of conductive heating, which is typically constrained to the host rock-intrusion contact (Einsele et al. 1980; Holford et al. 2012, 2013; Schofield et al. 2017; Haile et al. 2018). Apatite fission track analysis reported by Schofield et al. (2018a) & Holford et al. (2013) demonstrated that convective heating can be recorded at distances up to 80 km from the intrusive complexes. The resulting system of circulating hydrothermal fluids leads to mineralization within pore spaces, including, cementation of quartz, precipitation of illite–smectite to illite, calcite and chlorite, reducing porosity and permeability (Summer and Ayalon 1995; Ahmed 2002; Holford et al. 2013). Hydrothermal systems can be particularly detrimental to reservoir quality, by preferentially flowing through host rock regions which possess the highest porosity and permeability (Einsele et al. 1980; Holford et al. 2013). Although in many cases the conductive and hydrothermal effects of intrusions are negative, in some cases reservoir properties can be enhanced. For example, secondary permeability can be created by fracturing of the host rock during emplacement and cooling of the intrusion (Holford et al. 2013; Senger et al. 2014, 2015). Fracture-forming processes include magma cooling, thermal contraction and mechanical disturbance of host rock, with the greatest abundance of fractures around the contacts of intrusions (Senger et al. 2014, 2015). Matter et al. (2006) documented a case study from the Palisades sill in the USA (New Jersey, New York) where the permeability of host rock around the intrusion was increased by thermal fracturing. Xu et al (2015), meanwhile documented an increase in porosity from 2.8 to 6.8% pu, caused by high-salinity hydrothermal fluids associated with an igneous intrusion, corroding a carbonate reservoir in the Tarim Basin, NW China.

The effects of igneous intrusions emplaced into claystones is typically easier to directly quantify than with sandstones, as the thermal effect of heating the claystone is recorded by thermal maturity markers such as vitrinite reflectance (Raymond and Murchison 1991; Bishop and Abbott 1995; Aarnes et al. 2010, 2011, 2015; Muirhead et al. 2017; Peace et al. 2017; Spacapan et al. 2018). Previous work has shown that the emplacement of igneous intrusions into organic-rich shales alters the thermal maturity of the host rocks resulting in elevated maturity in the immediate vicinity of the igneous intrusions (Holford et al. 2013; Aarnes et al. 2015; Muirhead et al. 2017; Spacapan et al. 2018). Previous studies have noted that thermal aureoles (host rock with altered physical properties) around igneous intrusions are between 0.5 to 5 times the intrusion thicknesses (Duddy et al. 1994; Holford et al. 2013). The impact an igneous intrusion has on organic-rich argillaceous sedimentary rocks is typically recorded by geochemical parameters such as vitrinite reflectance, total organic carbon (TOC %) (Aarnes et al. 2015), spore colour index (Pross et al. 2007) and Raman spectroscopy (Muirhead et al. 2017). The conversion of kerogen to hydrocarbon fluids occurs with burial as the temperature and pressure increases with depth. In the vicinity of an igneous intrusion, it is possible to observe the full maturity window within the aureole of the intrusion (Clayton and Bostick 1986; Bishop and Abbott 1995; Aarnes et al. 2015; Muirhead et al. 2017). The host rock contact will commonly be overmature (maturity is a measure of a rocks ability to generate hydrocarbons, with overmature meaning most hydrocarbons have been generated) adjacent to intrusions, with maturity decreasing with distance from the intrusion to ultimately reach the background thermal maturity (Clayton and Bostick 1986; Bishop and Abbott 1995). The generation of hydrocarbons in the aureole of igneous intrusions can have important implications for petroleum systems, as it can result in the maturation of organic material in source rocks that would have otherwise been immature (Muirhead et al. 2017; Spacapan et al. 2018). Alternatively, it can result in overmaturation of the source rock which can be detrimental to petroleum exploration. Studies have also suggested that the emplacement of numerous igneous intrusions into organic rich sediments can result in the expulsion of large volumes of greenhouse gases, such as CH₄ and CO₂, with potential climatic consequences (Svensen et al. 2004; Aarnes et al. 2010).

It should be further noted that intrusions can have some counter-intuitive effects on source rock maturation within sedimentary basins, in fact acting to delay source rock maturation and hydrocarbon generation (Gardiner et al. 2019). When the overlying sedimentary units above a source rock region are intruded by extensive igneous sheet intrusions, the thermal conductivity of the overlying unit can be increased by the presence of intrusions post-emplacement, as the unit is composed of original sedimentary host rock and more thermally conductive intrusions. This has the effect of reducing the underlying geothermal gradient and reducing the temperature of the underlying source rock post-emplacement, delaying the onset of maturation (Gardiner et al. 2019).

The extent of alteration (alteration of the background rock properties by the intrusions) in the host rock sediments is highly variable, ranging from 30 to 700% d.t. (d.t.; a dimensionless number based on the thickness of the dyke, calculated by dividing the thickness of the contact zone by the thickness of the intrusion) (Aarnes et al. 2010; Muirhead et al. 2017). The extent of alteration can be influenced by several factors including the temperature of the intrusion, the duration of magma flux through the intrusion (Grove 2014) the thickness of the intrusion, depth of emplacement and number of previous intrusions within the region. Additionally, host rock specific factors such as depositional environment of the host rock (lithology and porosity-permeability), burial and diagenetic history prior to intrusion, background geothermal gradient and volume of porewaters all impact the extent of alteration (Schutter 2003; Rohrman 2007; Aarnes et al. 2010, 2011; Holford et al. 2013).

This study examined igneous intrusions and the surrounding country rock within a variety of different sedimentary basins and geological settings. The following section details a brief geological overview of each of these areas, and the general stratigraphy within which the igneous intrusions are hosted.

The NE Atlantic Margin is a rifted continental margin with a complex, episodic rift history (Ritchie et al. 2011. Perched Permo-Triassic rift sub-basins are present within the inboard domain, such as the Inner Hebrides (Fyfe et al. 1993), including the Ise of Skye, where Locality 3 is located on the Trotternish Peninsula on the north of the island (Fig. 2). The Isle of Skye lies in the Hebridean Terrane, which is bounded by two major structural lineaments: the Outer Hebrides fault zone (Minch Fault) to the NW, and the Moine Thrust in the SE (Emeleus et al. 2005; Schofield et al. 2018a, b). During the Paleogene, Skye was subject to magmatic activity in the form of intrusive doleritic sills and dykes, eruption of basaltic lava (Skye Main Lava Series) and the emplacement of gabbros and granites (the Cuillins). On the Trotternish Peninsula, a series of thin dykes (2–4 m thick) are intruded into the Upper Jurassic Oxford Clay Formation which is of similar age and deposited in a comparable environment to the Upper Jurassic Kimmeridge Clay Formation (KCF), the primary source rock for the North Sea and FSB.

Further outboard the age of the rifting is younger, with Late Jurassic extension, and early Cretaceous hyper-extension- it is within this domain where the Faroe–Shetland and Rockall Basins reside, and where the bulk of the subsurface well data from this study is located (Fig. 3). The vast majority of the igneous intrusions along the margin are hosted within Cretaceous deepwater claystones.

The Australian Southern Margin, another rifted continental margin, records the complex break-up of Eastern Gondwana during the Mesozoic (Stagg et al. 1990). The Southern Margin comprises a series of W–E orientated basins along the Australian coast, including the Bass and Torquay Basins, which host further subsurface well examples exhibited within this study (Fig. 4). Both basins have evidence of multiple phases of igneous activity, spanning the Cretaceous–Miocene in age and consists of volcaniclastic sediments, extrusive lava flows and intrusive sills and dykes, the latter of which are largely intruded into fluvial deltaic lacustrine sedimentary rocks (Holford et al. 2012, 2017; Reynolds et al. 2017; Watson et al. 2019). Intrusions are hosted within a variety of different stratigraphic intervals, though this study mainly concerns intrusions within lacustrine to fluvial-deltaic sedimentary rocks ranging in age from Cretaceous to early Eocene. In the Bass Basin, proven reservoirs are Paleocene and Eocene fluvial-deltaic sandstones (Boreham et al. 2002). In the Torquay sub-basin, coeval fluvial-deltaic sedimentary rocks also form potential reservoirs, along with shallow marine late Eocene sandstones (Messent et al. 1999).

Locality 1 is located within the San Juan Basin NW, New Mexico, a structural depression formed during the Laramide Orogeny (late Cretaceous-Eocene) (Craigg 2001) (Fig. 5). The field locality includes a three-metre-thick mafic igneous intrusion hosted within coarse-grained, quartz rich fluvial sandstones of early Eocene age, part of the San Jose Formation (Smith et al. 1992), with intrusion emplacement radiometrically dated at late Oligocene (Aldrich et al. 1986).

Locality 2 is situated along the margin of the Raton Basin, in the NE of New Mexico (Fig. 6). The Raton basin is a Rocky Mountain foreland basin, formed during the Laramide Orogeny (Woodward 1987; Johnson and Finn 2001; Cooper et al. 2007). The locality consists of a nine-metre-thick mafic igneous intrusion into the Pierre Shale, which was deposited in a marine setting associated with the Interior Cretaceous Seaway in North America (Cooper et al. 2007; Berry 2018). The igneous intrusions at both localities were emplaced during the late Oligocene to middle Miocene following extensional deformation associated with the Rio Grande Rift (Aldrich et al. 1986; Baldridge et al. 1991).

This section notes the approaches for examining the alteration of host rocks by igneous intrusions within two sperate domains, within the (1) subsurface and (2) within outcrop. Table 1 lists the different methods used in the study to assess the impact of igneous intrusions on host rocks and highlights the uncertainties/potential errors in these methods.

The subsurface data used in this study includes well and seismic reflection data from the Northern Atlantic Margin, UK, the Bass Basin and Torquay basin, Southern Australia (Fig. 1). For each well that penetrated igneous intrusions, the intrusion thickness and host rock lithology has been recorded. Geochemical data, such as vitrinite reflectance, has been extracted from well reports and the geophysical log data for the intervals containing intrusions has been interpreted to determine the impact on porosity of host rock and the petrophysical character of altered host rock.

Mafic igneous intrusions have a characteristic geophysical log response, making them distinguishable from the sedimentary host rocks (Bell and Butcher 2002: Smallwood and Maresh 2002; Mark et al. 2018; Watson et al. 2019). They typically have higher compressional sonic velocities, densities and resistivity values and lower gamma values relative to the surrounding sedimentary rocks (Fig. 7). Mafic igneous intrusions also exhibit a recognizable blocky geophysical log motif, in comparison to the host rock sediments (Fig. 7). Although gamma ray logs record a sharp change when an intrusion is encountered, resistivity, compressional sonic and density logs often show a gradual variation towards the contact of an igneous intrusion, typically expressed by a bell-shaped log response (Fig. 7) caused by the values ramping up or down within the host rock sediments directly above and below the intrusive contact (Fig. 7 also see figure 3 in Mark et al. 2018). This ramping up of the values in the host rock prior to encountering the intrusion is interpreted as representing the altered zone (Planke et al. 1999, 2005; Smallwood and Maresh 2002; Mark et al. 2018). Based on this characteristic petrophysical response for igneous intrusions and the contact zone, it is possible to identify the extent of host rock alteration in the subsurface (Fig. 7).

In the absence of direct porosity measurements from subsurface samples (e.g. side wall core and core), porosity is determined from wireline data by calculating sonic and density-derived porosities (Rider and Kennedy 2011). Compressional sonic logs can be used to evaluate the porosity of formations as the compressional velocity is lower in a fluid than in rock; if the formation of interest has pore space which is fluid filled, the compressional energy will take longer to travel through the rock from the transmitter to the receiver, indicating higher apparent porosity (Rider and Kennedy 2011). The link between compressional velocity and porosity is expressed by the empirical relationship known as the Wyllie time-average equation (Wyllie et al. 1956). The Wyllie time-average equation calculates porosity by: (ø: fractional porosity of the rock), ($Δ$t: acoustic transit time (µsec/ft)), ($Δ$t_f: acoustic transit time of the interstitial fluids (µsec/ft)), ($Δ$t_ma: acoustic transit time of the rock matrix (µsec/ft)) (Wyllie et al. 1956).

Porosity can also be calculated using bulk density, which is the measurement of the density of both the grains forming the rock and the fluids contained within the pore space of the rock. Therefore if grain density and fluid density is known (or assumed), it is possible to calculate the porosity from the bulk density using the equation:(ø: porosity of the rock), (⁠$ρ$_ma: grain density (g cm⁻³)), (⁠$ρ$_f: fluid density (g cm⁻³)), (⁠$ρ$_b: bulk density of the formation of interest (g cm⁻³)) (Rider and Kennedy 2011). Where fluid samples are not available for laboratory analysis, the fluid density is calculated from the measured fluid pressure gradient, where the formation pressure in determined from wireline or downhole pressure tools (e.g. RFT, MDT or RCX) (Rider and Kennedy 2011) used down the open borehole. For this study, both sonic and density derived porosities were calculated depending on data availability for each well.

Within this study vitrinite reflectance has been collated from geochemical reports carried out on wells that encountered igneous intrusions, illustrating the impact of igneous intrusions on the thermal maturity of the host sedimentary rocks. These results show the thermal effect on sediments which are not in direct contact with the igneous intrusions but have been altered as a result of hydrothermal fluids.

The field data in this study includes field localities in New Mexico, USA (localities 1 and 2, No. 23 in Fig. 1) and the Isle of Skye, NW of mainland Scotland (locality 3) (No.6 in Fig. 1)

Permeability was determined using a portable air permeameter (Tiny Perm II, New England Research) on the host rock sedimentary rocks along several transects, with increasing distance from the intrusion-host rock contact (Fig. 5a).

The TOC% was calculated at the University of Aberdeen using a carbon-sulfer analyser on decarbonated samples (Fig. 6). The vitrinite reflectance equivalent was estimated from the methylphenanthrene ratio from the aromatic fraction after the claystone samples had undergone soxhlet extraction and chromatography to separate the soluble hydrocarbons (Peters et al. 2005) (Fig. 2). For the vitrinite reflectance equivalent (a proxy for VR%) the methylphenanthrene ratio (MPR(MP2/MP1)) was used by applying the following formula: Rm (%) = 0.95 + 1.10log₁₀ MPR (Radke 1988; Muirhead et al. 2017).

Samples were taken along transects perpendicular to the contact between igneous intrusions and host rocks, with care taken to prevent sampling across different horizons and laminae. Fresh samples were collected ensuring that results would not be affected by surface weathering. At locality 1 (a 3 m thick dyke), air permeameter measurements were taken at the contact, then along a transect perpendicular to the igneous intrusion (Fig. 5a). Up to 1 m (33% d.t.), measurements were taken every 10 cm, and between 1–6 m from the contact measurements were taken every metre. At locality 2 (9 m thick dyke), samples were collected for TOC% analysis along a transect every 5% intrusion thickness (%d.t.) up to 150% d.t. then every 10% d.t to 250% d.t. with two background samples taken at 300% d.t and 500% d.t (Fig. 6a). At locality 3 (3.8 m thick dyke), samples were collected for vitrinite reflectance estimation along a transect away from the intrusion every 10% intrusion thickness (%d.t.) up to 20% intrusion thickness then every 20% d.t. up to 120% d.t. (Fig. 2b)

Utilising the characteristic petrophysical log response for the contact metamorphosed zone above and below igneous intrusions, it is possible to infer the extent of host rock alteration associated with subsurface igneous bodies (Fig. 7). This approach is valuable as commonly the extent of host rock alteration can only be recorded by analysis of field outcrops, which are limited by factors such as exposure, surface weathering and later tectonic events.

From analysis of 30 wells from the FSB, Rockall (both of the NE Atlantic Margin) and the Bass Basin (South Australia), we have collated the thickness of host rock alteration inferred from geophysical logs to compare the extent of host rock alteration in the subsurface (Fig. 7). Analysis of these wells has identified 255 individual igneous intrusions (180 FSB, 74 Rockall and 1 Bass Basin), with each igneous intrusion having a petrophysical altered zone above and below the intrusion (Fig. 7). The following observations were recorded from the host rock alteration zones above and below igneous intrusions:

• 97% of the intrusions occur in claystone, with only limited examples in siliciclastic sandstone.

• The extent of contact metamorphism in the host rock adjacent to igneous intrusions is greater above the intrusions than below the intrusions, with an average of 57.3% d.t. (average thickness: 7.3 m) above intrusions and 47% d.t. (average thickness: 5.5 m) below intrusions (average intrusion thickness 17 m).

• The minimum extent of alteration is 0.15 m for a 1.2 m thick intrusion and the maximum extent of alteration is 106 m for a 182 m thick igneous intrusion. The minimum extent of alteration when expressed in terms of percentage intrusion thickness is 2–249% d.t..

• Thick intrusions have the greatest extent of alteration, but when this data is expressed in percentage of intrusion thickness, the greatest extent of alteration occurs in intrusions which are within the <20 m thickness range.

Within the FSB and Rockall Trough, located on the NE Atlantic Margin (Fig. 3), one of the most prolific targets for petroleum exploration are Paleocene deep-marine sandstones, with major oil fields such as Schiehallion and Foinaven consisting of stacked turbidite reservoirs (Fig. 3). These turbidites were deposited in a deep marine environment, with coarse sediments transported into the basins and deposited as basin floor and slope sands (Lamers and Carmichael 1999). The Paleocene succession in both the FSB and the Rockall Trough is also intruded by igneous intrusions of the Faroe–Shetland Sill Complex (FSSC), with seismic data revealing multiple igneous intrusions occurring near these reservoirs (Rateau et al. 2013; Schofield et al. 2017). Despite these intrusions typically occurring within claystone and with a preference for the Cretaceous sedimentary strata of the FSB and Rockall Trough (Mark et al. 2018), there are examples from well data showing igneous intrusions within turbidite sandstone sequences.

Wells 164/28-1A (Rockall) and 214/28-1 (FSB) both contain igneous intrusions which have been emplaced into lower Paleocene turbidites which are prospective petroleum reservoirs. The 214/28-1 well, drilled in the FSB (Fig. 3) penetrated a 48 m thick igneous intrusion at 3992 mTVD which had been emplaced between a package of siliciclastic sandstone and claystone (Fig. 8a). The extent of alteration constrained by sonic-derived porosity is 15 m and corresponds to 31% d.t. (Fig. 8b). There is an 8 pu decrease in the porosity in the sandstone in the altered zone compared to the background porosity of 10%. Well 164/28-1A was drilled in the Rockall Trough (Fig. 3) and encountered two igneous intrusions, including one intrusion at 2912 mTVD which was two metres thick and a second at 2928 mTVD which was six metres thick (Fig. 9a). These intrusions were both emplaced into a package of siliciclastic sedimentary rocks (Fig. 9a), which resulted in alteration of the host rock, recorded by a decrease in porosity below the top intrusion and above the deeper intrusion. For both the 2 m and 6-metre-thick intrusion, the porosity of the host rock drops gradually as it approaches the intrusion-host rock contact, with a maximum reduction in porosity of 10 pu for the thicker intrusion and 8 pu for the thinner intrusion (Fig. 9b). For both intrusions, the porosity of the host rock returns to background porosity at one meter from the intrusion (Fig. 9b).

The Benbecula discovery (well 154/01-1) in the Rockall Trough (Fig. 3) encountered gas within lower Paleocene turbidites. The medium grained, arkosic reservoir sandstones exhibit porosities of 15 to 22% (Fig. 10). The sandstone core is visibly bleached to white colour. Apatite fission-track analysis undertaken on the turbidite sandstones shows they experienced elevated palaeotemperatures in the region of 240°C compared to the surrounding sediments, which have temperatures of 80°C at this depth of burial (Fig. 10) (Schofield et al. 2017).

In the FSB, Lower Cretaceous shallow marine delta fans of the Victory Formation are present in the eastern periphery of the basin (Goodchild et al. 1999). These sandstones were deposited within a paralic to shallow marine environment, with the sediments postulated to be sourced from the Rona Ridge during early Cretaceous rifting (Goodchild et al. 1999; Ritchie et al. 2011; Stoker 2016). These Lower Cretaceous delta fans generally occur as isolated packages along the Rona Ridge (Fig. 3). The Lower Cretaceous play in the FSB has had some exploration success with discoveries such as the Victory, Achmelvich, Glendronach and the Edradour gas discoveries (Fig. 3). Igneous intrusions within the FSB preferentially intrude Cretaceous sedimentary rocks, based on observations from both well penetrations and seismic reflection data (Mark et al. 2018), therefore any Lower Cretaceous play is susceptible to igneous intrusions potentially impacting the reservoir quality. In exploration well 207/01a-4, Victory Formation sandstones are intruded by a 213 m mafic dolerite intrusion at 1990 mTVD (Fig. 11a). Adjacent to the intrusion, the porosity of the sandstone has been reduced based on density log data (Fig. 11b). Within a three-metre window above the intrusion, the porosity gradually decreases from a background value of 20–28% down to 2% at one metre from the intrusion contact (Fig. 11b). In contrast to the previous examples, the porosity actually increases slightly within the first metre of host rock away from the intrusion (Fig. 11b).

Source rock quality is dependent on properties such as TOC%, hydrogen index, thickness, aerial extent, thermal maturity and kerogen type. The thermal maturity of a source rock can be elevated by igneous intrusions, but it is also possible for TOC% to be affected by the emplacement of igneous intrusions if the carbon is thermally altered (Clayton and Bostick 1986). The field outcrop study from New Mexico (Locality 2), demonstrates the impact of a 9 m thick mafic igneous intrusion on the TOC% of a marine claystone (Fig. 6). The TOC% of this claystone is reduced by as much as 97% in a region 7 m (80% d.t) away from the intrusion and the TOC% only returns to the background TOC% (1 to 1.1%) at 9 m away from the contact (100% d.t) (Fig. 6b). Within the initial 5 m from the intrusion, the TOC% of the host rock is as low as 0.03% with the majority of the organic carbon destroyed through thermal alteration (Fig. 6b). Within this zone, the host rock sediments are also visibly indurated and much more consolidated compared to the unaltered host rock sediments, which is highly friable (Fig. 6).

Within the FSB, where Cretaceous claystones have been intruded, the thermal effect of the igneous intrusions is sufficient to make the claystones thermally mature for petroleum generation compared to Cretaceous sequences at similar depths which have no igneous intrusions (Fig. 12). Figure 12 compiles vitrinite reflectance data (Ro%) from several wells drilled along the Atlantic Margin and demonstrates the increase in Ro% in the vicinity of the igneous intrusions. In general, the Ro% of Cretaceous samples is between 0.5–1.5% Ro, increasing gradually with depth and generally immature or within the early oil window, though adjacent to igneous intrusions the Ro% can be as high as 7% (i.e. overmature).

While the low TOC % Cretaceous claystones (averaging 1.4%; Svensen et al. 2004) have limited importance for the petroleum system in the Faroe–Shetland Basin, seismic reflection data from the FSB and Rockall Trough indicates that the KCF primary source rock may be intruded (Fig. 13). Therefore, to understand the impact of intrusions on the thermal maturity of the KCF, analysis of equivalent relationships in other basins, such as the Minch Basin, Isle of Skye has been undertaken. On the Isle of Skye, the Upper Jurassic Oxford Clay outcrops on the Trotternish peninsula. This claystone was deposited in an anoxic or restricted marine environment, similar to the KCF in the Atlantic Margin (Fig. 2). The Oxford Clay on Skye is immature for hydrocarbon generation due to the limited depth of burial (<1 km), but in the proximity of a 3.5 m dyke, the thermal maturity of the sediments is elevated up to 2 m away from the igneous intrusion, rising from 0.6 to 2.1% Ro (i.e. within the oil generation window) at the contact with the igneous intrusions (Fig. 2b).

Field outcrops of fluvial sedimentary host rocks intruded by a mafic dyke in New Mexico (Locality 1) provide insights into the impacts on permeability (Fig. 5). This fluvial sandstone outcrop is intruded by a 3 m thick dyke and is visibly indurated and bleached in the immediate intrusion-host rock contact zone (within ∼20–50 cm perpendicular to the intrusion contact) (Fig. 5b). Permeability measurements were recorded along four separate transects. Within the first metre from the intrusion-host rock contact, the permeability of the fluvial sandstone is reduced by as much as 80–90%, decreasing from an average of 3.1–4.9 Darcys in the background, to as low as 80 mD in the contact zone >5 cm from the intrusion (Fig. 5b).

The Koorkah-1 in the Bass basin (Fig. 4) encountered a 35 m thick mafic intrusion emplaced into the Early Eocene fluvial-deltaic sandstones (Fig. 14a) (Watson et al. 2019). The impact of the intrusion on the host rock sediments is recorded by the density derived porosity, which shows a gradual decrease in the sandstones from 20% at 3 m from the intrusion interface to 5% towards the contact (Fig. 14b). The porosity increases marginally to 8% within the first 50 cm of the intrusion-host rock contact zone (Fig. 14b). The Flinders-1 exploration well drilled in the Bass basin (Fig. 4) also encountered an igneous intrusion within fluvial-deltaic sediments. The Flinders-1 end of well report notes that the porosity of the sediments below the intrusion showed a gradual increase in porosity with depth ranging from 4–18%, comparable to the Koorkah-1 porosities.

Further west along the Australian Southern Margin in the Torquay sub-basin, fewer exploration wells have been drilled and to date no wells have encountered igneous intrusions (although volcaniclastic material has been recorded). The presence of igneous intrusions is inferred from seismic data (Fig. 15). Seismic reflection data over the Wild Dog-1 exploration well reveals the presence of a large igneous intrusion (>5 km in diameter) close to the drilled location of the Wild Dog-1 well (Fig. 15). Despite the Wild Dog-1 well not penetrating this igneous intrusion, vitrinite reflectance data from the fluvial-deltaic sediments and later shallow marine sediments have notably elevated palaeotemperatures compared to the background trend (Fig. 15a) (Holford et al. 2012). Figure 15b shows that the intrusion propagates steeply up a fault to the same stratigraphic interval as the Eastern View Group and Boonah Fm sediments encountered in Wild Dog-1 and intrudes these sediments >500 m down dip from the well location.

The examples outlined above illustrate the degrees of host rock alteration resulting from direct contact with igneous intrusions, or from hydrothermal fluids sourced from igneous intrusions. There appears to be no ‘rule of thumb’ regarding the spatial extent of alteration, which can be highly variable for different lithologies. Across all lithologies in this study, the greatest extent of alteration ranges from 2% d.t. to as much as 250% d.t. Generally, the greatest lateral extent of alteration is seen within argillaceous claystone sediments, whereas siliciclastic sandstone sediments tend to have thinner alteration zones. This is likely a result of the higher thermal conductivity of siliciclastic sandstone sediments and higher convective heat loss (due to higher permeabilities) (Allen and Allen 2013). The lower thermal conductivity of claystones results in the igneous intrusions retaining heat, resulting in a larger alteration zone. Vitrinite reflectance and TOC% measurements corroborate these findings, showing larger alteration zones in claystones relative to sandstones (Figs 6, 12).

Petrophysically, thinner intrusions have relatively larger alteration zones on average compared to the thicker intrusions when the data is normalized to percentage intrusion thickness. These results contradict previous work, which has demonstrated that thicker intrusions tend to be associated with thicker alteration zones (Aarnes et al. 2010 and references therein). The difference in extent of host rock alteration around igneous intrusions can be the result of various factors, including background geothermal gradient, emplacement history (duration of heating e.g. instantaneous emplacement or prolonged magma input), prior igneous activity, magma temperature and host rock characteristics (Aarnes et al. 2010, 2015; Senger et al. 2014; Muirhead et al. 2017). Similarly, the observation that smaller intrusions generate relatively larger alteration zones could be explained by magma cooling times. The intrusions which had their alteration halos measured are all part of the same FSSC complex (excluding one Bass basin intrusion) and due to the intrusions being predominantly dolerites, they will have similar magma temperatures (Gibb and Kanaris-Sotiriou 1988). These dolerite intrusions are also emplaced at similar depths and into similar host rock lithologies, therefore it is unlikely the geothermal gradient or host rock lithology is controlling the extent of alteration. The variable extent of alteration can also be explained by these thin intrusions acting as prolonged magma conduits resulting in the thin intrusions remaining molten for longer (Schofield et al. 2017).

Alternatively, the observed alteration could be greater adjacent to the thin intrusions as the zone of contact metamorphism is being overprinted by numerous, thin igneous intrusions which are in the vicinity of the main intrusions. Mark et al. (2018) and Eide et al. (2017) demonstrated from field outcrop data and subsurface examples that igneous intrusions can splay and bifurcate at the margins of the larger parent sills, creating anastomosing networks of thin interconnected intrusions. Individual intrusions within these anastomosing networks would likely have a limited effect on the host rock, but the cumulative heating effect of the entire network would likely result in greater extent of alteration. Furthermore, Mark et al. (2018) demonstrated that the number of igneous intrusions encountered by wells in the FSB could be underestimated due to the vertical resolution limits of the wireline data, therefore intrusions less than one metre thick would not be identified in well data but could contribute to the heating effect, although this is likely to have a small impact.

Most altered host rock examples in this study occur in argillaceous claystones. Where intrusions are hosted in lithologies such as sandstone or siltstone, such as the Bass Basin, the alteration zone is smaller (average 32% d.t. for sandstone and siltstones v. 48% d.t. for claystones). A possible explanation is that with greater burial, sandstones and siltstones have higher porosity and permeability than claystones and therefore more pore fluids, which allows for pore-water vaporisation and rapid cooling of the intruding magma, resulting in less direct conductive heat transfer to the sediments (Kokelaar 1982; Aarnes et al. 2010; Ingebritsen et al. 2010; Schofield et al. 2010). Continued heating of pore fluids by igneous intrusions can establish a convection dominated system, which is efficient at heat transfer (Kokelaar 1982; Busby-Spera and White 1987; Schofield et al. 2010). The higher permeability of sandstones and siltstones with respect to claystones allows convecting hot fluids to be transported, transferring heat away from the intruding magma more efficiently, reducing the effect on the immediate host rock sediments. The results presented in this study make frequent reference to the conductive and convective heating impact of the igneous intrusions, either through hydrothermal fluids or precipitation of pore blocking fluids. It should also be noted that there could be some mechanical impact on the host rock properties (e.g. porosity reduction) through compaction related to the intrusion emplacement (Dobb et al. 2024; Schofield et al. 2012). In this study several different processes have been used to estimate the impact of igneous so it is important to acknowledge that it is uncertain that all the processes will respond in a similar way.

In both the 207/04-1Z (FSB) and Koorkah-1 (Bass Basin) examples (Figs 11, 14), the porosity reduction in the host rock sediments away from the intrusion is not uniform. In the contact zone directly adjacent to the igneous intrusions, there is a slight increase in the porosity. This slight increase in host rock porosity in the immediate contact zone is interpreted to represent fracturing of the host rock sediments at the contact with the igneous intrusion. Senger et al. (2014, 2015) also observed increases in fracturing in the immediate contact zone between the intrusion and the host rock sediments and attributed this fracturing to a combination of magma cooling, thermal contraction, magma emplacement and mechanical disturbance of the host rock, with fracturing of the host rock sediments occurring syn-emplacement. Senger et al. (2015) postulated that fracturing within the contact zone around intrusions would be important for fluid flow, potentially mitigating the impact of intrusions compartmentalizing a basin as these fracture networks could focus petroleum migration (Rateau et al. 2013; Schofield et al. 2017, 2020).

A critical aspect of petroleum exploration is the identification of a source rock interval, which typically consists of an organic-rich shale or claystone which is of sufficient thickness and lateral extent (Magoon and Dow 1994). In sedimentary basins with numerous igneous intrusions, the evaluation of the source rocks through basin modelling becomes more complicated due to the localized addition of hot igneous bodies, affecting the thermal maturity and compartmentalizing the source rock and impeding migration (Rateau et al. 2013; Schofield et al. 2018b; Senger et al. 2017).

Igneous intrusions can clearly have important impacts on source rocks (e.g. maturation and compartmentalization), though there are relatively few studies directly examining the interactions between intrusions and primary source rocks. Previous field-based and subsurface studies have commonly looked at sedimentary rocks which, despite being-organic rich, are not the primary source rock intervals. This is primarily due to data availability, as mature source rock regions are seldom penetrated. Within the FSB, for instance, the vast majority of intrusions are within non-prospective, low TOC % Cretaceous claystones. However, within the Judd sub-basin (Fig. 3) (in the southern FSB), it is possible to identify multiple igneous intrusions within the KCF on seismic data (Fig. 13), providing some of the first clear evidence that this stratigraphic unit is heavily intruded in the FSB (Fig. 13). Figure 13 also demonstrates how the numerous overlying igneous intrusions result in poor sub-sill seismic definition, making it difficult to resolve the presence of the KCF.

The outcrop examples from Skye and New Mexico and subsurface examples of intruded Cretaceous sedimentary rocks from the FSB, demonstrates the impact that these igneous intrusions could have on the KCF, resulting in elevated thermal maturity and poorer source rock quality. Furthermore, igneous intrusions can often form laterally extensive, interconnected networks that transgress multiple stratigraphic units. Similar to impacts on reservoir quality, such extensive igneous networks could compartmentalize source rocks and potentially impede the migration of petroleum from source rock to reservoir.

It is worth noting that although igneous intrusions in the FSB could have a negative impact on the KCF, there are numerous, sizeable petroleum discoveries indicating there is a working petroleum system in the FSB. In contrast to the general consensus that the presence of igneous intrusions is detrimental to a sedimentary basins's petroleum system, our work suggests that the emplacement of igneous intrusions into source rock intervals can be beneficial (echoing Muirhead et al. 2017; Senger et al. 2017; Spacapan et al. 2020) If the source rock is immature for petroleum generation, the emplacement of numerous igneous intrusions can raise the thermal maturity to the point that hydrocarbons are generated (Monreal et al. 2009; Aarnes et al. 2011; Muirhead et al. 2017). In the Neuquén basin, Argentina the cumulative impact of multiple igneous intrusions has generated commercial volumes of petroleum from a source rock that would have been otherwise immature for generation of HC's (Monreal et al. 2009). A similar symbiotic relationship could be active in the FSB which has yet to be fully investigated.

When considering the impact of igneous intrusions on source rock intervals, it is critical to understand the timing of events. For the intrusions to have a detrimental impact, the source rock interval must already be in the oil or gas window prior to emplacement, with subsequent intrusion emplacement resulting in overmaturation. In contrast, emplacement into an immature source rock could be beneficial for a petroleum system. The impact of compartmentalization would also depend on whether the main phase of petroleum migration from the source rock has already occurred.

This study applies field and subsurface datasets to highlight the impacts of igneous intrusions on the sedimentary host rocks they are emplaced into and the relevance of this for petroleum exploration. This study demonstrates that igneous intrusions can markedly reduce reservoir quality through porosity and permeability reduction. The emplacement of igneous intrusions into source rock intervals can be detrimental to their generative potential depending on the background thermal maturity, with the heating effect caused by the emplacement of igneous intrusions resulting in localized overmaturation of source rocks. In contrast, there is potential that igneous intrusions could also cause localized generation of petroleum if the source rocks which they are emplaced into are immature. Figure 16 provides a summary of the critical properties of reservoir and source rock intervals which would be altered by igneous intrusions. Additionally, pervasive igneous intrusions are likely to cause compartmentalization of basin fills, which on regional scales could affect migration of petroleum from source rocks to reservoirs, and on local scales is likely to impact fluid flow and create pressure barriers within a reservoir.

Critically, this study demonstrates that the extent of alteration caused by igneous intrusions in host rock sedimentary rocks is highly variable and difficult to predict. The examples outlined in this study show variable extents of alteration but typically, larger alteration zones are recorded in claystone sequences than in sandstones. This is likely due to the thermal conductivity of sandstones being much greater as opposed to claystone. Additionally, the development of convective systems within siliciclastic sequences can carry hot fluids away from the intrusion-host rock contact, transferring heat over greater distances.

PGS are thanked for allowing the author access to the MegaSurveyPlus data and for allowing permission to publish this work. Seismic interpretation was carried out using IHS Kingdom software. Well log analysis was carried out using Schlumberger Techlog software. Well data was obtained from the UK Nort Sea Transition Authority (NSTA) National Data Repository (NDR).

NJM: methodology (lead), writing – original draft (lead), writing – review & editing (lead); NS: funding acquisition (lead), supervision (lead); DAW: investigation (supporting), methodology (supporting), writing – review & editing (supporting); SH: conceptualization (supporting), investigation (supporting), writing – review & editing (supporting); SP: investigation (supporting), project administration (supporting), writing – review & editing (supporting); DM: investigation (supporting), methodology (supporting)

The lead author's PhD is funded by JX Nippon Exploration and Production (U.K.) Limited as part of the Volcanic Margin Research Consortium Phase 2.

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.

All data generated or analysed during this study are included in this published article (and if present, its supplementary information files).