Rift-related magmatism influences petroleum systems development in the NE Irish Rockall Basin, offshore Ireland

Large volumes of hydrocarbons reside in volcanically influenced sedimentary basins. Despite having a good conceptual understanding of how magmatism impacts the petroleum system of such basins, we still lack detailed case studies documenting precisely how intrusive magmatism influences, for example, trap development and reservoir quality. Here we combine 3D seismic reflection, borehole, petrographic, and paleothermometric data to document the geology of borehole 5/22-1, NE Irish Rockall Basin, offshore western Ireland. This borehole (Errigal) tested a four-way dip closure that formed to accommodate emplacement of a Paleocene-to-Eocene igneous sill-complex during continental breakup in the North Atlantic. Two water-bearing turbidite sandstone-bearing intervals occur in the Upper Paleocene; the lowermost contains thin (c. 5 m), quartzose-feldspathic sandstones of good reservoir quality, whereas the upper is dominated by poor-quality volcaniclastic sandstones. Paleothermometric data provide evidence for anomalously high temperatures in the Paleocene-to-Eocene succession, suggesting the poor reservoir quality within the target interval likely reflects sill-induced heating, fluid flow, and related diagenesis. The poor reservoir quality also likely reflects the primary composition of the reservoir, which is dominated by volcanic grains and related clays derived from an igneous rock-dominated, sediment source area. Errigal appeared to fail due to a lack of hydrocarbon charge; i.e. the low bulk permeability of the heavily intruded Cretaceous mudstone succession may have impeded vertical migration of sub-Cretaceous-sourced hydrocarbons into supra-Cretaceous reservoirs. Breakup-related magmatism did however drive formation of a large structural closure, with data from Errigal at least proving high-quality, Upper Paleocene deep-water reservoirs. Future exploration targets in the NE Irish Rockall Basin include: (i) stratigraphically trapped, Paleocene-to-Eocene deep-water sandstones that onlap the flanks of intrusion-induced forced folds; (ii) structurally trapped, intra-Cretaceous deep-water sandstones incorporated within intrusion-induced forced folds; and (iii) more conventional, Mesozoic fault-block traps underlying the heavily intruded Cretaceous succession (e.g. Dooish). Similar plays may exist on other continental margins influenced by break-up magmatism.