This report synthesises the results of the 21CXRM Palaeozoic project in the Irish Sea to
describe the Palaeozoic petroleum systems of that area.
One hydrocarbon play system dominates the basin system: Namurian organic-rich marine
shales (Bowland Shale Formation) generated oil and gas with a peak during maximum burial
of the system in late Jurassic/early Cretaceous time. These hydrocarbons passed to reservoirs
in the Triassic Ormskirk Sandstone (Sherwood Sandstone Group) by way of structures
generated during the Variscan Orogeny and Cenozoic inversion, resulting in the Morecambe,
Hamilton and other gas and oil fields
The Palaeozoic study of the wider Irish Sea area has assessed the potential for more
widespread petroleum systems situated outside the well-known play, particularly within the
Carboniferous.
Within the Main Graben system of the East Irish Sea Basin, Coal Measures strata were
partially removed following Variscan inversion and early Permian uplift. They are not rich in
coals, and not inferred to be a significant source rock. There is some potential in the
Millstone Grit and Yoredale sequences, as some shales (particularly those associated with
marine bands) are known to have high Total Organic Contents. The source rock potential of
shales within the Carboniferous Limestone sequence is poorly constrained by data. A
Devonian source rock is unproven and considered unlikely.
Potential Namurian source rocks, such as the Yoredale Group, have been largely eroded in
the Peel and North Channel basins, considerably reducing their prospectivity, although
terrestrial sequences of equivalent age in the Solway Basin may offer better potential.
The variable seismic data quality at Carboniferous levels and sparsity of deep well control
have led to challenges in interpretation, particularly of the deeper picks. The interpretation of
the surfaces contains a strong model-driven element, evidenced by the onshore relationships
and areas where seismic picks can be made with the greatest confidence. Based upon the
integration of regional seismic mapping with a limited well, source rock and reservoir
property dataset, the most prospective parts of the region, outside the Ormskirk conventional
gas play, are considered to be:
The thick Westphalian sequences preserved in the Eubonia Tilt-Block in Quadrant
109, outside the main Permian-Mesozoic graben system and unaffected by Cenozoic
inversion. The presence and quality of seals form a major risk as the Cumbrian Coast
Group seal is thin or absent and Carboniferous intraformational seals are required but
untested. Based on the limited dataset available in adjacent basins, reservoir quality is
also a significant risk.
A belt of Variscan inversion structures correlated with structures on the Formby
Platform, and Ribbledale Foldbelt onshore, from which hydrocarbons have leaked
into the overlying, Ormskirk-hosted Hamilton fields. The biggest risk here is whether
reservoirs remain unbreached at the Pre-Permian level, and retain good poroperm
characteristics at depths of about 2500 m.
A more speculative play lies in the extensive carbonate platform in Quadrant 109 and
surrounding the Isle of Man, in reefal facies with enhanced secondary porosity. Here,
source rock presence and migration pathways, reservoir properties and seal quality are
major risks
