Petroleum geochemistry of the Lower and Middle Jurassic in Atlantic margin basins of Ireland and the UK
Published:January 01, 2001
Iain C. Scotchman, 2001. "Petroleum geochemistry of the Lower and Middle Jurassic in Atlantic margin basins of Ireland and the UK", The Petroleum Exploration of Ireland’s Offshore Basins, P. M. Shannon, P. D. W. Haughton, D. V. Corcoran
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Potential hydrocarbon source rocks of Lower and Middle Jurassic age have been reported from outcrop, shallow boreholes and exploration wells in Atlantic margin basins of the UK (Hebrides, West of Shetlands and flanking the NE Rockall Trough) and, recently, in the continuation of this trend offshore Ireland (Slyne, Erris and Porcupine basins). Previously these organic-rich mudrocks were considered to be of little economic importance, due largely to their perceived limited areal distribution and low maturity. However, recent geochemical studies of oils and shales from exploration drilling of thèse basins shows the Lower and Middle Jurassic to have considerable potential as effective hydrocarbon source rocks, supplanting the Late Jurassic-Early Cretaceous Kimmeridge Clay Formation equivalents as the only viable oil source rock in the region.
Flanking the Atlantic margin in the Irish and UK sectors, rich oil source potential occurs in two transgressive mudrock cycles of Lower Jurassic age. These are the Sinemurian-Pliensbachian interval and the overlying Toarcian section, present in basins such as the Solan, Minch, Hebrides, Slyne, North Celtic Sea, St George’s Channel and Central English Channel. The Middle Jurassic source rocks have a more limited areal distribution and occur in the Faroe-Shetland, Solan, West Lewis, West Flannan, Hebrides, Slyne and North Porcupine basins with oil source potential in regressive marginal marine to lacustrine facies mudrocks.
Geochemical studies were undertaken on mudrocks from the Lower and Middle Jurassic sections in Atlantic margin basins (outcrop, shallow borehole core and exploration well cores and cuttings samples) and on oils from drill stem test and shows (core and cuttings extracts). Detailed analyses using GC, GC-MS and carbon isotopes allowed both characterization of the source rocks and oil-to-source correlation. Biomarker and carbon isotope studies of oils from the Faroe-Shetland Basin (Foinaven and Schiehallion fields), the Porcupine Basin (Connemara accumulation), the Wessex Basin (Wytch Farm and Kimmeridge oil fields) and wells in the Slyne Basin show strong correlations to the various source rock developments in the Lower and Middle Jurassic. The mixed biodegraded Foinaven and Schiehallion oils have a major waxy component and correlate with lacustrine Middle Jurassic source rocks in the Solan and West Lewis/West Flannan basins. Middle Jurassic sourcing of the Connemara oils is also suggested, while oils in the Slyne Basin appear to have been largely sourced by the Lower Jurassic Pabba Shale Formation. Oils in the Wessex Basin (Wytch Farm and Kimmeridge) appear to have been sourced by Hettangian-Sinemurian mudrocks and those in the North Celtic Sea Basin by Toarcian source rocks.
The results from this study, in combination with previously published data, show that rich, effective oil-prone source rocks occur in both the Lower and Middle Jurassic of the Atlantic margin basins offshore Ireland and the UK. These source rocks can be correlated with indigenous oils, indicating the existence of a previously under-evaluated petroleum system.
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The Petroleum Exploration of Ireland’s Offshore Basins
Ireland is virtually encircled by sedimentary basins which developed in response to periods of rifting and thermal subsidence. These offshore basins have been the focus of intermittent phases of exploration since drilling of the first well in 1970 and, to date, 136 wells have been drilled. Most of the drilling so far has concentrated on structural traps, but recent exploration has begun to focus on a variety of stratigraphic traps, with greater emphasis on results obtained from studies of the Atlantic margin basins.
The Petroleum Exploration of Ireland's Offshore Basins contains a set of 27 papers on a wide range of topics relating to recent exploration of the Irish offshore sedimentary basins. These papers address aspects of the structural and stratigraphic evolution, thermal history, petroleum systems, reservoir geology and sea-bed processes in the Irish offshore area. Although the main focus is on petroleum systems and those issues bearing on exploration risk, the exploration effort has yielded fundamental new insight into the wider development of starved passive continental margins.
The volume will be of interest to oil industry explorationists and researchers focusing on NW European sedimentary basins and the evolution of the Irish Atlantic margin.