The relationship between rifting history and petroleum systems in the North Atlantic

Oil-source correlation studies along the north-east Atlantic Ocean margin west of Britain and Ireland indicate source intervals ranging in age from Middle to Late Jurassic and in facies from lacustrine/fluvio-deltaic to marginal marine form the dominant regional Jurassic petroleum system of the NE Atlantic. This contrasts with the common assumption of a ubiquitous marine Late Jurassic rich, oil-prone source rock as seen, for example, in the syn-rift North Viking Graben and Haltenbanken sections.

The distribution of Jurassic source rocks along the Atlantic Ocean margin is strongly related to the development of rift basins associated with the break-up of Pangaea. Marine flooding of the Permo-Triassic rifts in the earliest Jurassic resulted in the widespread deposition of source rocks through much of the basin system. The earliest source rocks of Hettangian-Sinemurian age occur in the more southern parts of the basin system with a Tethyan fauna. Younger source rocks with a boreal provinciality occur in the Sinemurian- Pliensbachian and in the Toarcian, the Toarcian marking the maximum extent of the transgression with widespread deposition of rich, marine source rocks through much of the basin system.

Regional uplift in the Middle Jurassic led to a major marine regression and the end of widespread marine source rock deposition. Non-marine lacustrine and fluvio-deltaic source rocks were deposited during both the Aalenian and in the Bathonian-Callovian of the Hebrides and Slyne Basins and continued into the Oxfordian and Kimmeridgian with increasing marine influences becoming more apparent. These source rocks are very widespread through the basin system including the Porcupine and Faroe-Shetland Basins, the Jeanne d'Arc and Flemish Pass Basins on the Canadian conjugate margin and the Lusitania Basin of Iberia.

Late Jurassic tectonics had a strong influence on source rock deposition with thick, marine oil-prone shales being deposited in rifting basins such as the North Viking Graben, Haltenbanken and Barents Sea (Kimmeridge Clay/Draupne/Mandal/Spekk Formations). Again marine flooding of these actively rifting basins appears to have been critical in the development of these source rocks. However, the Atlantic Margin basins do not show an equivalent thick Late Jurassic source rock development with just a relatively thin transgressive “skim” of rich source rock overlying the thick Middle- Late Jurassic section, seen in the Porcupine and Faroe-Shetland Basins. Elsewhere marginal marine or non-marine deposition took place, with equivalent aged lacustrine source rocks deposited in the North Celtic Sea Basin.

Widespread regional uplift at the end of the Jurassic ended source rock deposition throughout western Britain and Ireland basin systems, heralding the onset of major plate-break up between Canada, Iberia and Ireland as well as the Bay of Biscay in the early Cretaceous. A major new rift basin system developed and open marine conditions in these often hyper-extending basins unified the marine systems of the proto NE Atlantic, flooding the former restricted basin systems which promoted the formation of Jurassic source rocks. Occasional phases of oceanic anoxia in the Hauterivian-Barremian and later in the Aptian and Turonian-Cenomanian deposited thin but widespread rich marine black shales with source potential, primarily gas-prone. To date, these represent the only documented non- Jurassic source rocks along the margin, with the exception of a local Carboniferous source for gas off western Ireland.