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

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Description: Large quantities of hydrocarbons reside in volcanically influenced sedimentary basins, many of which occur along continental margins. Despite the common assumption that magmatism negatively impacts petroleum system development, we actually have a poor understanding of its true role, largely due to a lack of studies utilising integrated subsurface datasets. In this paper we combine 3D seismic reflection, borehole, petrographic and paleothermometric data to document the geology of borehole 5/22-1, which drilled the Errigal prospect, NE Rockall Basin, offshore western Ireland. This borehole tested a large four-way dip closure (i.e. a forced fold) that formed to accommodate forcible emplacement of a Paleogene igneous sill-complex during North Atlantic continental breakup. The borehole was unsuccessful, with only very minor traces of dead hydrocarbons discovered in Upper Paleocene deep-water siltstones. Two water-wet turbidite sandstone-bearing intervals occur in the Upper Paleocene. The lower interval contains two c. 5 m thick, quartzose-feldspathic sandstones of good reservoir quality, and several thin (<4 m), very poor-quality volcaniclastic sandstones containing abundant pore-filling and pore throat-bridging clay minerals. In contrast, the upper interval is dominated by the very poor-quality volcaniclastic sandstones, derived from a volcanic terrain genetically related to the magmatism driving forced folding and trap formation; the poor reservoir quality in this interval reflects diagenetic degradation of the abundant volcanic grains. Paleothermometric data, although of modest quality and quantity, provide equivocal evidence for magmatism-related elevated temperatures in the Paleocene-to-Eocene succession, suggesting sill-induced contact metamorphism and fluid flow were not solely responsible for the poor quality of the contained reservoirs; petrographic analysis suggests the poor reservoir quality likely reflects the abundance of volcanic grains and related clays derived from the igneous rock-dominated, sediment source area. The reason for failure of the Errigal, which is located only c. 42 km NNW of the Dooish gas discovery, is unclear, but we speculate the low bulk permeability of the heavily intruded Cretaceous mudstone succession impeded vertical migration of sub-Cretaceous sourced hydrocarbons into supra-Cretaceous reservoirs. Although the failure of Errigal casts doubt on the prospectivity of this play type in this particular part of the NE Rockall Basin, breakup-related magmatism clearly drove formation of a large structural closure, with data from 5/22-1 at least providing evidence for the local development of reservoir-quality, Upper Paleocene, deep-water reservoirs and thick, Eocene topseals. Post-Cretaceous deep-water stratigraphic traps on the flanks of intrusion-induced forced folds represent potential future exploration targets, in addition to more conventional, rotated fault-block traps containing Mesozoic reservoirs.

License: Academic Free License (AFL) 3.0

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