--> --> Jurassic North Atlantic paleogeography & Porcupine Basin rifting: implications for source and reservoir development offshore Atlantic Ireland

AAPG Europe Regional Conference, Global Analogues of the Atlantic Margin

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Jurassic North Atlantic paleogeography & Porcupine Basin rifting: implications for source and reservoir development offshore Atlantic Ireland

Abstract

The Porcupine Basin may be divided into a low-subsidence, relatively well-explored North and a high-subsidence, lightly-explored South. Plays in the South Porcupine include pre-rift (Middle Jurassic) tilted fault blocks, syn-rift (Upper Jurassic) structural-stratigraphic traps and post-rift (Lower Cretaceous and Paleocene) submarine fans. Pre-rift reservoir quality is of critical importance to the success of the Middle Jurassic play and is addressed here with reference to regional paleogeographic maps. Our thesis is that the Aalenian regional uplift postulated for the North Sea area (Underhill & Partington, 1993), and subsequently identified in north Eastern Greenland, the Norwegian Shelf and Denmark (discussed in Surlyk & Ineson, 2003) may also have extended to the west of Ireland. Subsequent onlap of this uplift is characterised by paralic sediments in proximal settings (e.g. the Pentland Formation of the North Sea, the Great Estuarine Series of the Hebrides and the paralic facies of the North Porcupine, sometimes known as the Bantry Group) and shallow marine sediments at the periphery (e.g. the Broom and Tarbert Formations of the northern North Sea, and the Sandnes and Garn Formations of the southern North Sea). On this basis, we suggest the presence of shallow marine Middle Jurassic sediments in the Southern Porcupine – such sediments potentially providing improved poroperm and lateral continuity in comparison with their paralic equivalents. There is some support for shallow marine development in core from the Goban Spur (62/7-1) well. Recent high quality 3D seismic imaging suggests at least two phases of Upper Jurassic rifting in the South Porcupine Basin. Initial rifting occurs on NNE-SSW normal faults terminating against WNW-ESE relays. Many of these faults dip away from the basin. Later in the Jurassic, as subsidence progresses, new N-S down-to-basin faults develop, often soling-out on the top of the pre-rift. New seismic reprocessing supports the development of sandstones in these growing half-grabens, which we postulate are the equivalent of the Tithonian-Berriasian sandstones of the Flemish Pass. The restricted basins associated with Upper Jurassic rifting enabled a range of source rock facies to develop. Our understanding of these source rocks is being transformed by current studies sponsored by the Irish Petroleum Affairs Division and Nalcor Energy. Here, we add proprietary interpretations of drop core data over the Padraig Basin which demonstrate the presence of the Bisnorhopane biomarker. This suggests an active Upper Jurassic source rock system in the vicinity of Padraig, but also supports an affinity between oils all along the Rockall Margin including the 12/2-1 (Dooish) discovery, and extending into the West and East Shetland Basins and the northern extremes of the Jeanne d’Arc Basin offshore Newfoundland. Broadly contemporaneous oils from the Porcupine and Flemish Pass have quite different geochemical signatures to Dooish. The paleogeographic implications of these data will be explored.