John Millett

Effects of igneous intrusions on the petroleum system: a review

Igneous intrusions feature in many sedimentary basins where hydrocarbon exploration and production is continuing. Owing to distinct geophysical property contrasts with siliciclastic host rocks (e.g., higher Vp, density and resistivity than host rocks), intrusions can be easily delineated within data sets including seismic and CSEM profiles, provided igneous bodies are larger than the detection limit of the geophysical methods. On the other hand, igneous bodies affect geophysical imaging in volcanic basins. Recent analyses of 3D seismic data, supported by field observations and lab-based experiments, have provided valuable insights into the prevailing geometries of intrusions, i.e. (1) layerdiscordant dykes, (2) layer-parallel sills and (3) saucer-shaped intrusions. Where emplaced, intrusive bodies affect all five principal components of a given petroleum system: (1) charge, (2) migration, (3) reservoir, (4) trap and (5) seal. Magmatic activity may positively or adversely affect any of these individual components, for instance by locally enhancing maturation within regionally immature source rocks, typically 30-250% of the intrusion thickness, or by causing compartmentalization of source and reservoir rocks. Site-specific evaluations, including the timing and duration of the magmatic event are needed to evaluate the overall effect of intrusions on a given sedimentary basin’s petroleum system, and these are highlighted by case studies from different volcanic basins.

Frontier exploration and the North Atlantic Igneous Province: new insights from a 2.6 km offshore volcanic sequence in the NE Faroe–Shetland Basin

The Lagavulin exploration well 217/15-1Z penetrated a c. 2.6 km thick volcanic sequence dominated by extrusive basaltic rocks spanning the Paleocene–Eocene boundary in the NE Faroe–Shetland Basin. The well comprises one of the thickest drilled sequences through the North Atlantic Igneous Province. Integrated analysis of drill cuttings and wireline-log data reveals key volcanic lithofacies: (1) tabular lava flows; (2) compound lava flows; (3) hyaloclastite; (4) volcaniclastic rocks. The volcanic facies reveal two major sub-aqueous to subaerial sequences consistent with lava delta progradation. These sequences are separated by a volcanic hiatus represented by extensive reddened soils, which preceded the re-submergence of the area. Emergence followed by submergence of the first lava delta is interpreted to record an intra-T40 transient uplift event near the Paleocene–Eocene boundary. Basalts from the lower c. 1.3 km have low TiO2 (<1.5 wt%) and low Zr/Y (2–3), with olivine-phyric picrites towards the base (Mg# 70–82; olivine Fo85–91). The hiatus correlates precisely with a change to high-TiO2 (2.5–3.2 wt%), high-Zr/Y (>4) compositions, which dominate the upper sequence. The associated change in lava geochemistry, transient uplift and volcanic hiatus appears consistent with a transient pulse of hot buoyant plume material passing beneath the area. Supplementary material: All raw geochemical data and supplementary analyses are available at http://www.geolsoc.org.uk/SUP18888.

Rifting and mafic magmatism in the Hebridean basins

Basalt dykes from the regional dyke swarm of the British Palaeogene Igneous Province (BPIP) were emplaced parallel to structural lineaments linking onshore and offshore volcanic edifices. Basalts that underwent minimal interaction with the crust have Mg# 60–75, εNd58 c. 8, 206Pb/204Pb c. 17.5, δ18O 5.9 ± 0.3‰, and 87Sr/86Sr <0.7040. Basalts with convex-upwards REE profiles ([La/Sm]N <1; [Sm/Yb]N >1) were generated by limited extents of melting (<10%) in the garnet–spinel transition of the upper mantle. Basalts with LREE-depleted ([La/Sm]N <1) or flat REE profiles require substantial (up to 20%) melting of spinel lherzolite. Modelling of major element compositions and olivine equilibration temperatures indicates that the mantle potential temperature was a maximum of 1530 °C beneath the BPIP at 58–60 Ma. Magmatism occurred at the periphery of a mantle thermal anomaly (proto-Iceland plume; TP ≤ 1560 °C) centred beneath western Greenland. The distribution of BPIP magmas was controlled by extensional tectonism driven by plate boundary forces resulting from plate reorganizations in the northern hemisphere starting at c. 62 Ma. The well-known mildly alkaline lava piles of Skye and Mull represent volcanoes on the flanks of the resulting rift system.

Large-scale magmatic pulses drive plant ecosystem dynamics

The 6.6 km gross thickness of the Palaeogene lava field of the Faroe Islands Basalt Group was erupted in the initial phases of North Atlantic rifting. Thin interlava sedimentary rocks yield palynofloras that vary in composition and diversity with the duration of the interlava period. Long-term trends in plant ecological succession occur within the record, each reflecting initially rapid and subsequently slowing eruption tempo. TiO2 and MgO plots derived from the basalt lava flows show corresponding fractionation trends. These link melt column processes to vegetation ecosystem dynamics via controls on eruption tempo, thermal support and substrate disturbance. Supplementary material: Further data are available at www.geolsoc.org.uk/SUP18538.

Igneous seismic geomorphology of buried lava fields and coastal escarpments on the Vøring volcanic rifted margin

Voluminous igneous complexes are commonly present in sedimentary basins on volcanic rifted margins, and they represent a challenge for petroleum explorationists. A 2500 km2 industry-standard 3D seismic cube has recently been acquired on the Voring Marginal High offshore mid-Norway to image subbasalt sedimentary rocks. This cube also provides a unique opportunity for imaging top- and intrabasalt structures. Detailed seismic geomorphological interpretation of the top-basalt horizon, locally calibrated with high-resolution P-Cable wide-azimuth data, reveals new insight into the late-stage development of the volcanic flow fields and the kilometer-high coastal Voring Escarpment. Subaerial lava flows with compressional ridges and inflated lava lobes cover the marginal high, with a comparable structure and size to modern subaerial lava fields. Pitted surfaces, likely formed by lava emplaced in a wet environment, are present in the western part of the study area near the continent-ocean boundary. The prominent Voring Escarpment formed when eastward-flowing lava reached the coastline. The escarpment morphology is influenced by preexisting structural highs, and these highs are locally bypassed by the lava. Volcanogenic debris flows are well-imaged on the escarpment horizon, along with large-scale large slump blocks. Similar features exist in active volcanic environments, e.g., on the south coast of Hawaii. Numerous postvolcanic extensional faults and incised channels cut into the marginal high and the escarpment, and we found that the area was geologically active after the volcanism ceased. In summary, igneous seismic geomorphology and seismic volcanostratigraphy are two very powerful methods to understand the volcanic deposits and development of rifted margins. Our study demonstrates great promise for further understanding the igneous development of offshore basins as more high-quality 3D seismic data become available.

3D structure and formation of hydrothermal vent complexes at the Paleocene-Eocene transition, the Møre Basin, mid-Norwegian margin

AbstractThe mid-Norwegian margin is regarded as an example of a volcanic-rifted margin formed prior to and during the Paleogene breakup of the northeast Atlantic. The area is characterized by the presence of voluminous basaltic complexes such as extrusive lava and lava delta sequences, intrusive sills and dikes, and hydrothermal vent complexes. We have developed a detailed 3D seismic analysis of fluid- and gas-induced hydrothermal vent complexes in a 310  km2 area in the More Basin, offshore Norway. We find that formation of hydrothermal vent complexes is accommodated by deformation of the host rock when sills are emplaced. Fluids are generated by metamorphic reactions and pore-fluid expansion around sills and are focused around sill tips due to buoyancy. Hydrothermal vent complexes are associated with doming of the overlying strata, leading to the formation of draping mounds above the vent contemporary surface. The morphological characteristics of the upper part and the underlying feeder structure (condu...

A fresh approach to ditch cutting analysis as an aid to exploration in areas affected by large igneous province (LIP) volcanism

Abstract Where hydrocarbon exploration targets occur within basins affected by large igneous province (LIP) sequences, an understanding of the volcanic stratigraphy is essential in compiling accurate basin models at all scales. Ditch cutting samples are one of the most commonly available sources of data yielding information in the context of LIP stratigraphy such as phenocryst load, degree of secondary precipitation and lava composition, largely unattainable by remote sensing. Where core data is limited or absent, cuttings provide the only means of accessing such data along with valuable inference of volcanic facies development and down-hole conditions. Interpretations based on cuttings data are widely used in industry and, as such, a repeatable and well-defined methodology for the analysis and designation of volcanic facies from cuttings is an important requirement for regional and individual play modelling. Such an approach has not been common practice to date. We propose a system of basic percentage-based cuttings analysis and ternary classification specifically tailored to LIP sequences, and argue for the benefits of a coherent and transparent basin-wide approach. The classification system is further developed into a log-style output for easy integration and comparison with other down-hole geophysical and biostratigraphic data.

Mafic intrusions, hydrothermal venting, and the basalt-sediment transition: Linking onshore and offshore examples from the North Atlantic igneous province

AbstractThe emplacement of large intrusive complexes into sedimentary basins can have profound effects on host sedimentary rocks including deformation, thermal aureole metamorphic reactions, alteration of fluid-flow pathways, and the formation of associated hydrothermal vent complexes (HVCs). These processes can in turn have major implications for petroleum systems on the local and regional scale, and can contribute to global climate change due to the production and outgassing of greenhouse gases, such as CH4 and CO2. Imaging these features and assessing their implications from seismic data beneath extrusive volcanic cover is challenging due to heterogeneities in the volcanic pile and at the basalt-sediment transition. We have evaluated combined field and laboratory petrophysical data from the Isle of Skye, Scotland, where we identify a seismic-scale example of extensive intrusions interacting with the base-basalt transition. We have also evaluated a unique onshore example of a linked sill and associated ...